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% ****************************************************
% * DO NOT MODIFY THIS FILE!!!!                      *
% * It was generated from gawktexi.in by sidebar.awk *
% * Edit gawktexi.in instead.                        *
% ****************************************************
\input texinfo   @c -*-texinfo-*-
@c vim: filetype=texinfo
@c %**start of header (This is for running Texinfo on a region.)
@setfilename gawk.info
@settitle The GNU Awk User's Guide
@c %**end of header (This is for running Texinfo on a region.)

@dircategory Text creation and manipulation
@direntry
* Gawk: (gawk).                 A text scanning and processing language.
@end direntry
@dircategory Individual utilities
@direntry
* awk: (gawk)Invoking Gawk.                     Text scanning and processing.
@end direntry

@ifset FOR_PRINT
@tex
\gdef\xrefprintnodename#1{``#1''}
@end tex
@end ifset

@ifclear FOR_PRINT
@c With early 2014 texinfo.tex, restore PDF links and colors
@tex
\gdef\linkcolor{0.5 0.09 0.12} % Dark Red
\gdef\urlcolor{0.5 0.09 0.12} % Also
\global\urefurlonlylinktrue
@end tex
@end ifclear

@ifnotdocbook
@set BULLET @bullet{}
@set MINUS @minus{}
@end ifnotdocbook

@ifdocbook
@set BULLET
@set MINUS
@end ifdocbook

@iftex
@set TIMES @times
@end iftex
@ifnottex
@set TIMES *
@end ifnottex
        
@c Let texinfo.tex give us full section titles
@xrefautomaticsectiontitle on

@c The following information should be updated here only!
@c This sets the edition of the document, the version of gawk it
@c applies to and all the info about who's publishing this edition

@c These apply across the board.
@set UPDATE-MONTH March, 2020
@set VERSION 5.1
@set PATCHLEVEL 0

@set GAWKINETTITLE TCP/IP Internetworking with @command{gawk}
@ifset FOR_PRINT
@set TITLE Effective awk Programming
@end ifset
@ifclear FOR_PRINT
@set TITLE GAWK: Effective AWK Programming
@end ifclear
@set SUBTITLE A User's Guide for GNU Awk
@set EDITION 5.1

@iftex
@set DOCUMENT book
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER @inmargin{@image{lflashlight,1cm}, @image{rflashlight,1cm}}
@set COMMONEXT (c.e.)
@set PAGE page
@end iftex
@ifinfo
@set DOCUMENT Info file
@set CHAPTER major node
@set APPENDIX major node
@set SECTION minor node
@set SUBSECTION node
@set DARKCORNER (d.c.)
@set COMMONEXT (c.e.)
@set PAGE screen
@end ifinfo
@ifhtml
@set DOCUMENT Web page
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER (d.c.)
@set COMMONEXT (c.e.)
@set PAGE screen
@end ifhtml
@ifdocbook
@set DOCUMENT book
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER (d.c.)
@set COMMONEXT (c.e.)
@set PAGE page
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@set DOCUMENT book
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER (d.c.)
@set COMMONEXT (c.e.)
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@end ifxml
@ifplaintext
@set DOCUMENT book
@set CHAPTER chapter
@set APPENDIX appendix
@set SECTION section
@set SUBSECTION subsection
@set DARKCORNER (d.c.)
@set COMMONEXT (c.e.)
@set PAGE page
@end ifplaintext

@ifdocbook
@c empty on purpose
@set PART1
@set PART2
@set PART3
@set PART4
@end ifdocbook

@ifnotdocbook
@set PART1 Part I:@*
@set PART2 Part II:@*
@set PART3 Part III:@*
@set PART4 Part IV:@*
@end ifnotdocbook

@c some special symbols
@iftex
@set LEQ @math{@leq}
@set PI @math{@pi}
@end iftex
@ifdocbook
@set LEQ @inlineraw{docbook, ≤}
@set PI @inlineraw{docbook, &pgr;}
@end ifdocbook
@ifnottex
@ifnotdocbook
@set LEQ <=
@set PI @i{pi}
@end ifnotdocbook
@end ifnottex

@ifnottex
@ifnotdocbook
@macro ii{text}
@i{\text\}
@end macro
@end ifnotdocbook
@end ifnottex

@ifdocbook
@macro ii{text}
@inlineraw{docbook,\text\}
@end macro
@end ifdocbook

@ifclear FOR_PRINT
@set FN file name
@set FFN File name
@set DF data file
@set DDF Data file
@set PVERSION version
@end ifclear
@ifset FOR_PRINT
@set FN filename
@set FFN Filename
@set DF datafile
@set DDF Datafile
@set PVERSION version
@end ifset

@c For HTML, spell out email addresses, to avoid problems with
@c address harvesters for spammers.
@ifhtml
@macro EMAIL{real,spelled}
``\spelled\''
@end macro
@end ifhtml
@ifnothtml
@macro EMAIL{real,spelled}
@email{\real\}
@end macro
@end ifnothtml

@c Indexing macros
@ifinfo

@macro cindexawkfunc{name}
@cindex @code{\name\}
@end macro

@macro cindexgawkfunc{name}
@cindex @code{\name\}
@end macro

@end ifinfo

@ifnotinfo

@macro cindexawkfunc{name}
@cindex @code{\name\()} function
@end macro

@macro cindexgawkfunc{name}
@cindex @code{\name\()} function (@command{gawk})
@end macro
@end ifnotinfo

@ignore
Some comments on the layout for TeX.
1. Use at least texinfo.tex 2016-02-05.07.
@end ignore

@c merge the function and variable indexes into the concept index
@ifinfo
@synindex fn cp
@synindex vr cp
@end ifinfo
@iftex
@syncodeindex fn cp
@syncodeindex vr cp
@end iftex
@ifxml
@syncodeindex fn cp
@syncodeindex vr cp
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@ifdocbook
@synindex fn cp
@synindex vr cp
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@c If "finalout" is commented out, the printed output will show
@c black boxes that mark lines that are too long.  Thus, it is
@c unwise to comment it out when running a master in case there are
@c overfulls which are deemed okay.

@iftex
@finalout
@end iftex

@c Enabled '-quotes in PDF files so that cut/paste works in
@c more places.

@codequoteundirected on
@codequotebacktick on

@copying
@docbook

“To boldly go where no man has gone before” is a
Registered Trademark of Paramount Pictures Corporation.

Published by:

Free Software Foundation
51 Franklin Street, Fifth Floor
Boston, MA  02110-1301 USA
Phone: +1-617-542-5942
Fax: +1-617-542-2652
Email: gnu@@gnu.org
URL: https://www.gnu.org/

Copyright © 1989, 1991, 1992, 1993, 1996–2005, 2007, 2009–2020
Free Software Foundation, Inc.
All Rights Reserved.
@end docbook

@ifnotdocbook
Copyright @copyright{} 1989, 1991, 1992, 1993, 1996--2005, 2007, 2009--2020 @*
Free Software Foundation, Inc.
@end ifnotdocbook
@sp 2

This is Edition @value{EDITION} of @cite{@value{TITLE}: @value{SUBTITLE}},
for the @value{VERSION}.@value{PATCHLEVEL} (or later) version of the GNU
implementation of AWK.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'', with the
Front-Cover Texts being ``A GNU Manual'', and with the Back-Cover Texts
as in (a) below.
@ifclear FOR_PRINT
A copy of the license is included in the section entitled
``GNU Free Documentation License''.
@end ifclear
@ifset FOR_PRINT
A copy of the license
may be found on the Internet at
@uref{https://www.gnu.org/software/gawk/manual/html_node/GNU-Free-Documentation-License.html,
the GNU Project's website}.
@end ifset

@enumerate a
@item
The FSF's Back-Cover Text is: ``You have the freedom to
copy and modify this GNU manual.''
@end enumerate
@end copying

@c Comment out the "smallbook" for technical review.  Saves
@c considerable paper.  Remember to turn it back on *before*
@c starting the page-breaking work.

@c 4/2002: Karl Berry recommends commenting out this and the
@c `@setchapternewpage odd', and letting users use `texi2dvi -t'
@c if they want to waste paper.
@c @smallbook


@c Uncomment this for the release.  Leaving it off saves paper
@c during editing and review.
@setchapternewpage odd

@shorttitlepage GNU Awk
@titlepage
@title @value{TITLE}
@subtitle @value{SUBTITLE}
@subtitle Edition @value{EDITION}
@subtitle @value{UPDATE-MONTH}
@author Arnold D. Robbins

@ifnotdocbook
@c Include the Distribution inside the titlepage environment so
@c that headings are turned off.  Headings on and off do not work.

@page
@vskip 0pt plus 1filll
``To boldly go where no man has gone before'' is a
Registered Trademark of Paramount Pictures Corporation. @*
@c sorry, i couldn't resist
@sp 3
Published by:
@sp 1

Free Software Foundation @*
51 Franklin Street, Fifth Floor @*
Boston, MA  02110-1301 USA @*
Phone: +1-617-542-5942 @*
Fax: +1-617-542-2652 @*
Email: @email{gnu@@gnu.org} @*
URL: @uref{https://www.gnu.org/} @*

@c This one is correct for gawk 3.1.0 from the FSF
ISBN 1-882114-28-0 @*
@sp 2
@insertcopying
@end ifnotdocbook
@end titlepage

@c Thanks to Bob Chassell for directions on doing dedications.
@iftex
@headings off
@page
@w{ }
@sp 9
@center @i{To my parents, for their love, and for the wonderful example they set for me.}
@sp 1
@center @i{To my wife, Miriam, for making me complete.
Thank you for building your life together with me.}
@sp 1
@center @i{To our children, Chana, Rivka, Nachum, and Malka, for enrichening our lives in innumerable ways.}
@sp 1
@w{ }
@page
@w{ }
@page
@headings on
@end iftex

@docbook

To my parents, for their love, and for the wonderful
example they set for me.
To my wife Miriam, for making me complete.
Thank you for building your life together with me.
To our children Chana, Rivka, Nachum and Malka,
for enrichening our lives in innumerable ways.

@end docbook

@iftex
@headings off
@evenheading @thispage@ @ @ @strong{@value{TITLE}} @| @|
@oddheading  @| @| @strong{@thischapter}@ @ @ @thispage
@end iftex

@ifnottex
@ifnotxml
@ifnotdocbook
@node Top
@top General Introduction
@c Preface node should come right after the Top
@c node, in `unnumbered' sections, then the chapter, `What is gawk'.
@c Licensing nodes are appendices, they're not central to AWK.

This file documents @command{awk}, a program that you can use to select
particular records in a file and perform operations upon them.

@insertcopying

@end ifnotdocbook
@end ifnotxml
@end ifnottex

@menu
* Foreword3::                      Some nice words about this
                                   @value{DOCUMENT}.
* Foreword4::                      More nice words.
* Preface::                        What this @value{DOCUMENT} is about; brief
                                   history and acknowledgments.
* Getting Started::                A basic introduction to using
                                   @command{awk}. How to run an @command{awk}
                                   program. Command-line syntax.
* Invoking Gawk::                  How to run @command{gawk}.
* Regexp::                         All about matching things using regular
                                   expressions.
* Reading Files::                  How to read files and manipulate fields.
* Printing::                       How to print using @command{awk}. Describes
                                   the @code{print} and @code{printf}
                                   statements. Also describes redirection of
                                   output.
* Expressions::                    Expressions are the basic building blocks
                                   of statements.
* Patterns and Actions::           Overviews of patterns and actions.
* Arrays::                         The description and use of arrays. Also
                                   includes array-oriented control statements.
* Functions::                      Built-in and user-defined functions.
* Library Functions::              A Library of @command{awk} Functions.
* Sample Programs::                Many @command{awk} programs with complete
                                   explanations.
* Advanced Features::              Stuff for advanced users, specific to
                                   @command{gawk}.
* Internationalization::           Getting @command{gawk} to speak your
                                   language.
* Debugger::                       The @command{gawk} debugger.
* Namespaces::                     How namespaces work in @command{gawk}.
* Arbitrary Precision Arithmetic:: Arbitrary precision arithmetic with
                                   @command{gawk}.
* Dynamic Extensions::             Adding new built-in functions to
                                   @command{gawk}.
* Language History::               The evolution of the @command{awk}
                                   language.
* Installation::                   Installing @command{gawk} under various
                                   operating systems.
* Notes::                          Notes about adding things to @command{gawk}
                                   and possible future work.
* Basic Concepts::                 A very quick introduction to programming
                                   concepts.
* Glossary::                       An explanation of some unfamiliar terms.
* Copying::                        Your right to copy and distribute
                                   @command{gawk}.
* GNU Free Documentation License:: The license for this @value{DOCUMENT}.
* Index::                          Concept and Variable Index.

@detailmenu
* History::                             The history of @command{gawk} and
                                        @command{awk}.
* Names::                               What name to use to find
                                        @command{awk}.
* This Manual::                         Using this @value{DOCUMENT}. Includes
                                        sample input files that you can use.
* Conventions::                         Typographical Conventions.
* Manual History::                      Brief history of the GNU project and
                                        this @value{DOCUMENT}.
* How To Contribute::                   Helping to save the world.
* Acknowledgments::                     Acknowledgments.
* Running gawk::                        How to run @command{gawk} programs;
                                        includes command-line syntax.
* One-shot::                            Running a short throwaway
                                        @command{awk} program.
* Read Terminal::                       Using no input files (input from the
                                        keyboard instead).
* Long::                                Putting permanent @command{awk}
                                        programs in files.
* Executable Scripts::                  Making self-contained @command{awk}
                                        programs.
* Comments::                            Adding documentation to @command{gawk}
                                        programs.
* Quoting::                             More discussion of shell quoting
                                        issues.
* DOS Quoting::                         Quoting in Windows Batch Files.
* Sample Data Files::                   Sample data files for use in the
                                        @command{awk} programs illustrated in
                                        this @value{DOCUMENT}.
* Very Simple::                         A very simple example.
* Two Rules::                           A less simple one-line example using
                                        two rules.
* More Complex::                        A more complex example.
* Statements/Lines::                    Subdividing or combining statements
                                        into lines.
* Other Features::                      Other Features of @command{awk}.
* When::                                When to use @command{gawk} and when to
                                        use other things.
* Intro Summary::                       Summary of the introduction.
* Command Line::                        How to run @command{awk}.
* Options::                             Command-line options and their
                                        meanings.
* Other Arguments::                     Input file names and variable
                                        assignments.
* Naming Standard Input::               How to specify standard input with
                                        other files.
* Environment Variables::               The environment variables
                                        @command{gawk} uses.
* AWKPATH Variable::                    Searching directories for
                                        @command{awk} programs.
* AWKLIBPATH Variable::                 Searching directories for
                                        @command{awk} shared libraries.
* Other Environment Variables::         The environment variables.
* Exit Status::                         @command{gawk}'s exit status.
* Include Files::                       Including other files into your
                                        program.
* Loading Shared Libraries::            Loading shared libraries into your
                                        program.
* Obsolete::                            Obsolete Options and/or features.
* Undocumented::                        Undocumented Options and Features.
* Invoking Summary::                    Invocation summary.
* Regexp Usage::                        How to Use Regular Expressions.
* Escape Sequences::                    How to write nonprinting characters.
* Regexp Operators::                    Regular Expression Operators.
* Regexp Operator Details::             The actual details.
* Interval Expressions::                Notes on interval expressions.
* Bracket Expressions::                 What can go between @samp{[...]}.
* Leftmost Longest::                    How much text matches.
* Computed Regexps::                    Using Dynamic Regexps.
* GNU Regexp Operators::                Operators specific to GNU software.
* Case-sensitivity::                    How to do case-insensitive matching.
* Regexp Summary::                      Regular expressions summary.
* Records::                             Controlling how data is split into
                                        records.
* awk split records::                   How standard @command{awk} splits
                                        records.
* gawk split records::                  How @command{gawk} splits records.
* Fields::                              An introduction to fields.
* Nonconstant Fields::                  Nonconstant Field Numbers.
* Changing Fields::                     Changing the Contents of a Field.
* Field Separators::                    The field separator and how to change
                                        it.
* Default Field Splitting::             How fields are normally separated.
* Regexp Field Splitting::              Using regexps as the field separator.
* Single Character Fields::             Making each character a separate
                                        field.
* Command Line Field Separator::        Setting @code{FS} from the command
                                        line.
* Full Line Fields::                    Making the full line be a single
                                        field.
* Field Splitting Summary::             Some final points and a summary table.
* Constant Size::                       Reading constant width data.
* Fixed width data::                    Processing fixed-width data.
* Skipping intervening::                Skipping intervening fields.
* Allowing trailing data::              Capturing optional trailing data.
* Fields with fixed data::              Field values with fixed-width data.
* Splitting By Content::                Defining Fields By Content
* More CSV::                            More on CSV files.
* Testing field creation::              Checking how @command{gawk} is
                                        splitting records.
* Multiple Line::                       Reading multiline records.
* Getline::                             Reading files under explicit program
                                        control using the @code{getline}
                                        function.
* Plain Getline::                       Using @code{getline} with no
                                        arguments.
* Getline/Variable::                    Using @code{getline} into a variable.
* Getline/File::                        Using @code{getline} from a file.
* Getline/Variable/File::               Using @code{getline} into a variable
                                        from a file.
* Getline/Pipe::                        Using @code{getline} from a pipe.
* Getline/Variable/Pipe::               Using @code{getline} into a variable
                                        from a pipe.
* Getline/Coprocess::                   Using @code{getline} from a coprocess.
* Getline/Variable/Coprocess::          Using @code{getline} into a variable
                                        from a coprocess.
* Getline Notes::                       Important things to know about
                                        @code{getline}.
* Getline Summary::                     Summary of @code{getline} Variants.
* Read Timeout::                        Reading input with a timeout.
* Retrying Input::                      Retrying input after certain errors.
* Command-line directories::            What happens if you put a directory on
                                        the command line.
* Input Summary::                       Input summary.
* Input Exercises::                     Exercises.
* Print::                               The @code{print} statement.
* Print Examples::                      Simple examples of @code{print}
                                        statements.
* Output Separators::                   The output separators and how to
                                        change them.
* OFMT::                                Controlling Numeric Output With
                                        @code{print}.
* Printf::                              The @code{printf} statement.
* Basic Printf::                        Syntax of the @code{printf} statement.
* Control Letters::                     Format-control letters.
* Format Modifiers::                    Format-specification modifiers.
* Printf Examples::                     Several examples.
* Redirection::                         How to redirect output to multiple
                                        files and pipes.
* Special FD::                          Special files for I/O.
* Special Files::                       File name interpretation in
                                        @command{gawk}. @command{gawk} allows
                                        access to inherited file descriptors.
* Other Inherited Files::               Accessing other open files with
                                        @command{gawk}.
* Special Network::                     Special files for network
                                        communications.
* Special Caveats::                     Things to watch out for.
* Close Files And Pipes::               Closing Input and Output Files and
                                        Pipes.
* Nonfatal::                            Enabling Nonfatal Output.
* Output Summary::                      Output summary.
* Output Exercises::                    Exercises.
* Values::                              Constants, Variables, and Regular
                                        Expressions.
* Constants::                           String, numeric and regexp constants.
* Scalar Constants::                    Numeric and string constants.
* Nondecimal-numbers::                  What are octal and hex numbers.
* Regexp Constants::                    Regular Expression constants.
* Using Constant Regexps::              When and how to use a regexp constant.
* Standard Regexp Constants::           Regexp constants in standard
                                        @command{awk}.
* Strong Regexp Constants::             Strongly typed regexp constants.
* Variables::                           Variables give names to values for
                                        later use.
* Using Variables::                     Using variables in your programs.
* Assignment Options::                  Setting variables on the command line
                                        and a summary of command-line syntax.
                                        This is an advanced method of input.
* Conversion::                          The conversion of strings to numbers
                                        and vice versa.
* Strings And Numbers::                 How @command{awk} Converts Between
                                        Strings And Numbers.
* Locale influences conversions::       How the locale may affect conversions.
* All Operators::                       @command{gawk}'s operators.
* Arithmetic Ops::                      Arithmetic operations (@samp{+},
                                        @samp{-}, etc.)
* Concatenation::                       Concatenating strings.
* Assignment Ops::                      Changing the value of a variable or a
                                        field.
* Increment Ops::                       Incrementing the numeric value of a
                                        variable.
* Truth Values and Conditions::         Testing for true and false.
* Truth Values::                        What is ``true'' and what is
                                        ``false''.
* Typing and Comparison::               How variables acquire types and how
                                        this affects comparison of numbers and
                                        strings with @samp{<}, etc.
* Variable Typing::                     String type versus numeric type.
* Comparison Operators::                The comparison operators.
* POSIX String Comparison::             String comparison with POSIX rules.
* Boolean Ops::                         Combining comparison expressions using
                                        boolean operators @samp{||} (``or''),
                                        @samp{&&} (``and'') and @samp{!}
                                        (``not'').
* Conditional Exp::                     Conditional expressions select between
                                        two subexpressions under control of a
                                        third subexpression.
* Function Calls::                      A function call is an expression.
* Precedence::                          How various operators nest.
* Locales::                             How the locale affects things.
* Expressions Summary::                 Expressions summary.
* Pattern Overview::                    What goes into a pattern.
* Regexp Patterns::                     Using regexps as patterns.
* Expression Patterns::                 Any expression can be used as a
                                        pattern.
* Ranges::                              Pairs of patterns specify record
                                        ranges.
* BEGIN/END::                           Specifying initialization and cleanup
                                        rules.
* Using BEGIN/END::                     How and why to use BEGIN/END rules.
* I/O And BEGIN/END::                   I/O issues in BEGIN/END rules.
* BEGINFILE/ENDFILE::                   Two special patterns for advanced
                                        control.
* Empty::                               The empty pattern, which matches every
                                        record.
* Using Shell Variables::               How to use shell variables with
                                        @command{awk}.
* Action Overview::                     What goes into an action.
* Statements::                          Describes the various control
                                        statements in detail.
* If Statement::                        Conditionally execute some
                                        @command{awk} statements.
* While Statement::                     Loop until some condition is
                                        satisfied.
* Do Statement::                        Do specified action while looping
                                        until some condition is satisfied.
* For Statement::                       Another looping statement, that
                                        provides initialization and increment
                                        clauses.
* Switch Statement::                    Switch/case evaluation for conditional
                                        execution of statements based on a
                                        value.
* Break Statement::                     Immediately exit the innermost
                                        enclosing loop.
* Continue Statement::                  Skip to the end of the innermost
                                        enclosing loop.
* Next Statement::                      Stop processing the current input
                                        record.
* Nextfile Statement::                  Stop processing the current file.
* Exit Statement::                      Stop execution of @command{awk}.
* Built-in Variables::                  Summarizes the predefined variables.
* User-modified::                       Built-in variables that you change to
                                        control @command{awk}.
* Auto-set::                            Built-in variables where @command{awk}
                                        gives you information.
* ARGC and ARGV::                       Ways to use @code{ARGC} and
                                        @code{ARGV}.
* Pattern Action Summary::              Patterns and Actions summary.
* Array Basics::                        The basics of arrays.
* Array Intro::                         Introduction to Arrays
* Reference to Elements::               How to examine one element of an
                                        array.
* Assigning Elements::                  How to change an element of an array.
* Array Example::                       Basic Example of an Array
* Scanning an Array::                   A variation of the @code{for}
                                        statement. It loops through the
                                        indices of an array's existing
                                        elements.
* Controlling Scanning::                Controlling the order in which arrays
                                        are scanned.
* Numeric Array Subscripts::            How to use numbers as subscripts in
                                        @command{awk}.
* Uninitialized Subscripts::            Using Uninitialized variables as
                                        subscripts.
* Delete::                              The @code{delete} statement removes an
                                        element from an array.
* Multidimensional::                    Emulating multidimensional arrays in
                                        @command{awk}.
* Multiscanning::                       Scanning multidimensional arrays.
* Arrays of Arrays::                    True multidimensional arrays.
* Arrays Summary::                      Summary of arrays.
* Built-in::                            Summarizes the built-in functions.
* Calling Built-in::                    How to call built-in functions.
* Numeric Functions::                   Functions that work with numbers,
                                        including @code{int()}, @code{sin()}
                                        and @code{rand()}.
* String Functions::                    Functions for string manipulation,
                                        such as @code{split()}, @code{match()}
                                        and @code{sprintf()}.
* Gory Details::                        More than you want to know about
                                        @samp{\} and @samp{&} with
                                        @code{sub()}, @code{gsub()}, and
                                        @code{gensub()}.
* I/O Functions::                       Functions for files and shell
                                        commands.
* Time Functions::                      Functions for dealing with timestamps.
* Bitwise Functions::                   Functions for bitwise operations.
* Type Functions::                      Functions for type information.
* I18N Functions::                      Functions for string translation.
* User-defined::                        Describes User-defined functions in
                                        detail.
* Definition Syntax::                   How to write definitions and what they
                                        mean.
* Function Example::                    An example function definition and
                                        what it does.
* Function Calling::                    Calling user-defined functions.
* Calling A Function::                  Don't use spaces.
* Variable Scope::                      Controlling variable scope.
* Pass By Value/Reference::             Passing parameters.
* Function Caveats::                    Other points to know about functions.
* Return Statement::                    Specifying the value a function
                                        returns.
* Dynamic Typing::                      How variable types can change at
                                        runtime.
* Indirect Calls::                      Choosing the function to call at
                                        runtime.
* Functions Summary::                   Summary of functions.
* Library Names::                       How to best name private global
                                        variables in library functions.
* General Functions::                   Functions that are of general use.
* Strtonum Function::                   A replacement for the built-in
                                        @code{strtonum()} function.
* Assert Function::                     A function for assertions in
                                        @command{awk} programs.
* Round Function::                      A function for rounding if
                                        @code{sprintf()} does not do it
                                        correctly.
* Cliff Random Function::               The Cliff Random Number Generator.
* Ordinal Functions::                   Functions for using characters as
                                        numbers and vice versa.
* Join Function::                       A function to join an array into a
                                        string.
* Getlocaltime Function::               A function to get formatted times.
* Readfile Function::                   A function to read an entire file at
                                        once.
* Shell Quoting::                       A function to quote strings for the
                                        shell.
* Data File Management::                Functions for managing command-line
                                        data files.
* Filetrans Function::                  A function for handling data file
                                        transitions.
* Rewind Function::                     A function for rereading the current
                                        file.
* File Checking::                       Checking that data files are readable.
* Empty Files::                         Checking for zero-length files.
* Ignoring Assigns::                    Treating assignments as file names.
* Getopt Function::                     A function for processing command-line
                                        arguments.
* Passwd Functions::                    Functions for getting user
                                        information.
* Group Functions::                     Functions for getting group
                                        information.
* Walking Arrays::                      A function to walk arrays of arrays.
* Library Functions Summary::           Summary of library functions.
* Library Exercises::                   Exercises.
* Running Examples::                    How to run these examples.
* Clones::                              Clones of common utilities.
* Cut Program::                         The @command{cut} utility.
* Egrep Program::                       The @command{egrep} utility.
* Id Program::                          The @command{id} utility.
* Split Program::                       The @command{split} utility.
* Tee Program::                         The @command{tee} utility.
* Uniq Program::                        The @command{uniq} utility.
* Wc Program::                          The @command{wc} utility.
* Miscellaneous Programs::              Some interesting @command{awk}
                                        programs.
* Dupword Program::                     Finding duplicated words in a
                                        document.
* Alarm Program::                       An alarm clock.
* Translate Program::                   A program similar to the @command{tr}
                                        utility.
* Labels Program::                      Printing mailing labels.
* Word Sorting::                        A program to produce a word usage
                                        count.
* History Sorting::                     Eliminating duplicate entries from a
                                        history file.
* Extract Program::                     Pulling out programs from Texinfo
                                        source files.
* Simple Sed::                          A Simple Stream Editor.
* Igawk Program::                       A wrapper for @command{awk} that
                                        includes files.
* Anagram Program::                     Finding anagrams from a dictionary.
* Signature Program::                   People do amazing things with too much
                                        time on their hands.
* Programs Summary::                    Summary of programs.
* Programs Exercises::                  Exercises.
* Nondecimal Data::                     Allowing nondecimal input data.
* Array Sorting::                       Facilities for controlling array
                                        traversal and sorting arrays.
* Controlling Array Traversal::         How to use PROCINFO["sorted_in"].
* Array Sorting Functions::             How to use @code{asort()} and
                                        @code{asorti()}.
* Two-way I/O::                         Two-way communications with another
                                        process.
* TCP/IP Networking::                   Using @command{gawk} for network
                                        programming.
* Profiling::                           Profiling your @command{awk} programs.
* Advanced Features Summary::           Summary of advanced features.
* I18N and L10N::                       Internationalization and Localization.
* Explaining gettext::                  How GNU @command{gettext} works.
* Programmer i18n::                     Features for the programmer.
* Translator i18n::                     Features for the translator.
* String Extraction::                   Extracting marked strings.
* Printf Ordering::                     Rearranging @code{printf} arguments.
* I18N Portability::                    @command{awk}-level portability
                                        issues.
* I18N Example::                        A simple i18n example.
* Gawk I18N::                           @command{gawk} is also
                                        internationalized.
* I18N Summary::                        Summary of I18N stuff.
* Debugging::                           Introduction to @command{gawk}
                                        debugger.
* Debugging Concepts::                  Debugging in General.
* Debugging Terms::                     Additional Debugging Concepts.
* Awk Debugging::                       Awk Debugging.
* Sample Debugging Session::            Sample debugging session.
* Debugger Invocation::                 How to Start the Debugger.
* Finding The Bug::                     Finding the Bug.
* List of Debugger Commands::           Main debugger commands.
* Breakpoint Control::                  Control of Breakpoints.
* Debugger Execution Control::          Control of Execution.
* Viewing And Changing Data::           Viewing and Changing Data.
* Execution Stack::                     Dealing with the Stack.
* Debugger Info::                       Obtaining Information about the
                                        Program and the Debugger State.
* Miscellaneous Debugger Commands::     Miscellaneous Commands.
* Readline Support::                    Readline support.
* Limitations::                         Limitations and future plans.
* Debugging Summary::                   Debugging summary.
* Global Namespace::                    The global namespace in standard
                                        @command{awk}.
* Qualified Names::                     How to qualify names with a namespace.
* Default Namespace::                   The default namespace.
* Changing The Namespace::              How to change the namespace.
* Naming Rules::                        Namespace and Component Naming Rules.
* Internal Name Management::            How names are stored internally.
* Namespace Example::                   An example of code using a namespace.
* Namespace And Features::              Namespaces and other @command{gawk}
                                        features.
* Namespace Summary::                   Summarizing namespaces.
* Computer Arithmetic::                 A quick intro to computer math.
* Math Definitions::                    Defining terms used.
* MPFR features::                       The MPFR features in @command{gawk}.
* FP Math Caution::                     Things to know.
* Inexactness of computations::         Floating point math is not exact.
* Inexact representation::              Numbers are not exactly represented.
* Comparing FP Values::                 How to compare floating point values.
* Errors accumulate::                   Errors get bigger as they go.
* Getting Accuracy::                    Getting more accuracy takes some work.
* Try To Round::                        Add digits and round.
* Setting precision::                   How to set the precision.
* Setting the rounding mode::           How to set the rounding mode.
* Arbitrary Precision Integers::        Arbitrary Precision Integer Arithmetic
                                        with @command{gawk}.
* Checking for MPFR::                   How to check if MPFR is available.
* POSIX Floating Point Problems::       Standards Versus Existing Practice.
* Floating point summary::              Summary of floating point discussion.
* Extension Intro::                     What is an extension.
* Plugin License::                      A note about licensing.
* Extension Mechanism Outline::         An outline of how it works.
* Extension API Description::           A full description of the API.
* Extension API Functions Introduction:: Introduction to the API functions.
* General Data Types::                  The data types.
* Memory Allocation Functions::         Functions for allocating memory.
* Constructor Functions::               Functions for creating values.
* Registration Functions::              Functions to register things with
                                        @command{gawk}.
* Extension Functions::                 Registering extension functions.
* Exit Callback Functions::             Registering an exit callback.
* Extension Version String::            Registering a version string.
* Input Parsers::                       Registering an input parser.
* Output Wrappers::                     Registering an output wrapper.
* Two-way processors::                  Registering a two-way processor.
* Printing Messages::                   Functions for printing messages.
* Updating @code{ERRNO}::               Functions for updating @code{ERRNO}.
* Requesting Values::                   How to get a value.
* Accessing Parameters::                Functions for accessing parameters.
* Symbol Table Access::                 Functions for accessing global
                                        variables.
* Symbol table by name::                Accessing variables by name.
* Symbol table by cookie::              Accessing variables by ``cookie''.
* Cached values::                       Creating and using cached values.
* Array Manipulation::                  Functions for working with arrays.
* Array Data Types::                    Data types for working with arrays.
* Array Functions::                     Functions for working with arrays.
* Flattening Arrays::                   How to flatten arrays.
* Creating Arrays::                     How to create and populate arrays.
* Redirection API::                     How to access and manipulate
                                        redirections.
* Extension API Variables::             Variables provided by the API.
* Extension Versioning::                API Version information.
* Extension GMP/MPFR Versioning::       Version information about GMP and
                                        MPFR.
* Extension API Informational Variables:: Variables providing information about
                                        @command{gawk}'s invocation.
* Extension API Boilerplate::           Boilerplate code for using the API.
* Changes from API V1::                 Changes from V1 of the API.
* Finding Extensions::                  How @command{gawk} finds compiled
                                        extensions.
* Extension Example::                   Example C code for an extension.
* Internal File Description::           What the new functions will do.
* Internal File Ops::                   The code for internal file operations.
* Using Internal File Ops::             How to use an external extension.
* Extension Samples::                   The sample extensions that ship with
                                        @command{gawk}.
* Extension Sample File Functions::     The file functions sample.
* Extension Sample Fnmatch::            An interface to @code{fnmatch()}.
* Extension Sample Fork::               An interface to @code{fork()} and
                                        other process functions.
* Extension Sample Inplace::            Enabling in-place file editing.
* Extension Sample Ord::                Character to value to character
                                        conversions.
* Extension Sample Readdir::            An interface to @code{readdir()}.
* Extension Sample Revout::             Reversing output sample output
                                        wrapper.
* Extension Sample Rev2way::            Reversing data sample two-way
                                        processor.
* Extension Sample Read write array::   Serializing an array to a file.
* Extension Sample Readfile::           Reading an entire file into a string.
* Extension Sample Time::               An interface to @code{gettimeofday()}
                                        and @code{sleep()}.
* Extension Sample API Tests::          Tests for the API.
* gawkextlib::                          The @code{gawkextlib} project.
* Extension summary::                   Extension summary.
* Extension Exercises::                 Exercises.
* V7/SVR3.1::                           The major changes between V7 and
                                        System V Release 3.1.
* SVR4::                                Minor changes between System V
                                        Releases 3.1 and 4.
* POSIX::                               New features from the POSIX standard.
* BTL::                                 New features from Brian Kernighan's
                                        version of @command{awk}.
* POSIX/GNU::                           The extensions in @command{gawk} not
                                        in POSIX @command{awk}.
* Feature History::                     The history of the features in
                                        @command{gawk}.
* Common Extensions::                   Common Extensions Summary.
* Ranges and Locales::                  How locales used to affect regexp
                                        ranges.
* Contributors::                        The major contributors to
                                        @command{gawk}.
* History summary::                     History summary.
* Gawk Distribution::                   What is in the @command{gawk}
                                        distribution.
* Getting::                             How to get the distribution.
* Extracting::                          How to extract the distribution.
* Distribution contents::               What is in the distribution.
* Unix Installation::                   Installing @command{gawk} under
                                        various versions of Unix.
* Quick Installation::                  Compiling @command{gawk} under Unix.
* Shell Startup Files::                 Shell convenience functions.
* Additional Configuration Options::    Other compile-time options.
* Configuration Philosophy::            How it's all supposed to work.
* Non-Unix Installation::               Installation on Other Operating
                                        Systems.
* PC Installation::                     Installing and Compiling
                                        @command{gawk} on Microsoft Windows.
* PC Binary Installation::              Installing a prepared distribution.
* PC Compiling::                        Compiling @command{gawk} for
                                        Windows32.
* PC Using::                            Running @command{gawk} on Windows32.
* Cygwin::                              Building and running @command{gawk}
                                        for Cygwin.
* MSYS::                                Using @command{gawk} In The MSYS
                                        Environment.
* VMS Installation::                    Installing @command{gawk} on VMS.
* VMS Compilation::                     How to compile @command{gawk} under
                                        VMS.
* VMS Dynamic Extensions::              Compiling @command{gawk} dynamic
                                        extensions on VMS.
* VMS Installation Details::            How to install @command{gawk} under
                                        VMS.
* VMS Running::                         How to run @command{gawk} under VMS.
* VMS GNV::                             The VMS GNV Project.
* VMS Old Gawk::                        An old version comes with some VMS
                                        systems.
* Bugs::                                Reporting Problems and Bugs.
* Bug address::                         Where to send reports to.
* Usenet::                              Where not to send reports to.
* Maintainers::                         Maintainers of non-*nix ports.
* Other Versions::                      Other freely available @command{awk}
                                        implementations.
* Installation summary::                Summary of installation.
* Compatibility Mode::                  How to disable certain @command{gawk}
                                        extensions.
* Additions::                           Making Additions To @command{gawk}.
* Accessing The Source::                Accessing the Git repository.
* Adding Code::                         Adding code to the main body of
                                        @command{gawk}.
* New Ports::                           Porting @command{gawk} to a new
                                        operating system.
* Derived Files::                       Why derived files are kept in the Git
                                        repository.
* Future Extensions::                   New features that may be implemented
                                        one day.
* Implementation Limitations::          Some limitations of the
                                        implementation.
* Extension Design::                    Design notes about the extension API.
* Old Extension Problems::              Problems with the old mechanism.
* Extension New Mechanism Goals::       Goals for the new mechanism.
* Extension Other Design Decisions::    Some other design decisions.
* Extension Future Growth::             Some room for future growth.
* Notes summary::                       Summary of implementation notes.
* Basic High Level::                    The high level view.
* Basic Data Typing::                   A very quick intro to data types.
@end detailmenu
@end menu

@c dedication for Info file
@ifinfo
To my parents, for their love, and for the wonderful
example they set for me.
@sp 1
To my wife Miriam, for making me complete.
Thank you for building your life together with me.
@sp 1
To our children Chana, Rivka, Nachum and Malka,
for enrichening our lives in innumerable ways.
@end ifinfo

@summarycontents
@contents

@node Foreword3
@unnumbered Foreword to the Third Edition

@c This bit is post-processed by a script which turns the chapter
@c tag into a preface tag, and moves this stuff to before the title.
@c Bleah.
@docbook
  
    
      Michael
      Brennan
      
      Author of mawk
    
    March 2001
   
@end docbook

Arnold Robbins and I are good friends. We were introduced
@c 11 years ago
in 1990
by circumstances---and our favorite programming language, AWK.
The circumstances started a couple of years
earlier. I was working at a new job and noticed an unplugged
Unix computer sitting in the corner.  No one knew how to use it,
and neither did I.  However,
a couple of days later, it was running, and
I was @code{root} and the one-and-only user.
That day, I began the transition from statistician to Unix programmer.

On one of many trips to the library or bookstore in search of
books on Unix, I found the gray AWK book, a.k.a.@:
Alfred V.@: Aho, Brian W.@: Kernighan, and
Peter J.@: Weinberger's @cite{The AWK Programming Language} (Addison-Wesley,
1988).  @command{awk}'s simple programming paradigm---find a pattern in the
input and then perform an action---often reduced complex or tedious
data manipulations to a few lines of code.  I was excited to try my
hand at programming in AWK.

Alas,  the @command{awk} on my computer was a limited version of the
language described in the gray book.  I discovered that my computer
had ``old @command{awk}'' and the book described
``new @command{awk}.''
I learned that this was typical; the old version refused to step
aside or relinquish its name.  If a system had a new @command{awk}, it was
invariably called @command{nawk}, and few systems had it.
The best way to get a new @command{awk} was to @command{ftp} the source code for
@command{gawk} from @code{prep.ai.mit.edu}.  @command{gawk} was a version of
new @command{awk} written by David Trueman and Arnold, and available under
the GNU General Public License.

(Incidentally,
it's no longer difficult to find a new @command{awk}. @command{gawk} ships with
GNU/Linux, and you can download binaries or source code for almost
any system; my wife uses @command{gawk} on her VMS box.)

My Unix system started out unplugged from the wall; it certainly was not
plugged into a network.  So, oblivious to the existence of @command{gawk}
and the Unix community in general, and desiring a new @command{awk}, I wrote
my own, called @command{mawk}.
Before I was finished, I knew about @command{gawk},
but it was too late to stop, so I eventually posted
to a @code{comp.sources} newsgroup.

A few days after my posting, I got a friendly email
from Arnold introducing
himself.   He suggested we share design and algorithms and
attached a draft of the POSIX standard so
that I could update @command{mawk} to support language extensions added
after publication of @cite{The AWK Programming Language}.

Frankly, if our roles had
been reversed, I would not have been so open and we probably would
have never met.  I'm glad we did meet.
He is an AWK expert's AWK expert and a genuinely nice person.
Arnold contributes significant amounts of his
expertise and time to the Free Software Foundation.

This book is the @command{gawk} reference manual, but at its core it
is a book about AWK programming that
will appeal to a wide audience.
It is a definitive reference to the AWK language as defined by the
1987 Bell Laboratories release and codified in the 1992 POSIX Utilities
standard.

On the other hand, the novice AWK programmer can study
a wealth of practical programs that emphasize
the power of AWK's basic idioms:
data-driven control flow, pattern matching with regular expressions,
and associative arrays.
Those looking for something new can try out @command{gawk}'s
interface to network protocols via special @file{/inet} files.

The programs in this book make clear that an AWK program is
typically much smaller and faster to develop than
a counterpart written in C.
Consequently, there is often a payoff to prototyping an
algorithm or design in AWK to get it running quickly and expose
problems early. Often, the interpreted performance is adequate
and the AWK prototype becomes the product.

The new @command{pgawk} (profiling @command{gawk}), produces
program execution counts.
I recently experimented with an algorithm that for
@ifnotdocbook
@math{n}
@end ifnotdocbook
@ifdocbook
@i{n}
@end ifdocbook
lines of input, exhibited
@tex
$\sim\! Cn^2$
@end tex
@ifnottex
@ifnotdocbook
~ C n^2
@end ifnotdocbook
@end ifnottex
@docbook
∼ Cn2
@end docbook
performance, while
theory predicted
@tex
$\sim\! Cn\log n$
@end tex
@ifnottex
@ifnotdocbook
~ C n log n
@end ifnotdocbook
@end ifnottex
@docbook
∼ Cn log n
@end docbook
behavior. A few minutes poring
over the @file{awkprof.out} profile pinpointed the problem to
a single line of code.  @command{pgawk} is a welcome addition to
my programmer's toolbox.

Arnold has distilled over a decade of experience writing and
using AWK programs, and developing @command{gawk}, into this book.  If you use
AWK or want to learn how, then read this book.

@ifnotdocbook
@cindex Brennan, Michael
@display
Michael Brennan
Author of @command{mawk}
March 2001
@end display
@end ifnotdocbook

@node Foreword4
@unnumbered Foreword to the Fourth Edition

@c This bit is post-processed by a script which turns the chapter
@c tag into a preface tag, and moves this stuff to before the title.
@c Bleah.
@docbook
  
    
      Michael
      Brennan
      
      Author of mawk
    
    October 2014
   
@end docbook

Some things don't change.  Thirteen years ago I wrote:
``If you use AWK or want to learn how, then read this book.''
True then, and still true today.

Learning to use a programming language is about more than mastering the
syntax.  One needs to acquire an understanding of how to use the
features of the language to solve practical programming problems.
A focus of this book is many examples that show how to use AWK.

Some things do change. Our computers are much faster and have more memory.
Consequently, speed and storage inefficiencies of a high-level language
matter less.  Prototyping in AWK and then rewriting in C for performance
reasons happens less, because more often the prototype is fast enough.

Of course, there are computing operations that are best done in C or C++.
With @command{gawk} 4.1 and later, you do not have to choose between writing
your program in AWK or in C/C++.  You can write most of your
program in AWK and the aspects that require C/C++ capabilities can be written
in C/C++, and then the pieces glued together when the @command{gawk} module loads
the C/C++ module as a dynamic plug-in.
@c Chapter 16
@ref{Dynamic Extensions},
has all the
details, and, as expected, many examples to help you learn the ins and outs.

I enjoy programming in AWK and had fun (re)reading this book.
I think you will too.

@ifnotdocbook
@cindex Brennan, Michael
@display
Michael Brennan
Author of @command{mawk}
October 2014
@end display
@end ifnotdocbook

@node Preface
@unnumbered Preface
@c I saw a comment somewhere that the preface should describe the book itself,
@c and the introduction should describe what the book covers.
@c
@c 12/2000: Chuck wants the preface & intro combined.

@c This bit is post-processed by a script which turns the chapter
@c tag into a preface tag, and moves this stuff to before the title.
@c Bleah.
@docbook
  
    
      Arnold
      Robbins
      Nof Ayalon
      Israel
    
    February 2015
   
@end docbook

@cindex @command{awk}
Several kinds of tasks occur repeatedly when working with text files.
You might want to extract certain lines and discard the rest.  Or you
may need to make changes wherever certain patterns appear, but leave the
rest of the file alone.  Such jobs are often easy with @command{awk}.
The @command{awk} utility interprets a special-purpose programming
language that makes it easy to handle simple data-reformatting jobs.

@cindex @command{gawk}
The GNU implementation of @command{awk} is called @command{gawk}; if you
invoke it with the proper options or environment variables,
it is fully compatible with
the POSIX@footnote{The 2018 POSIX standard is accessible online at
@w{@url{https://pubs.opengroup.org/onlinepubs/9699919799/}.}}
specification of the @command{awk} language
and with the Unix version of @command{awk} maintained
by Brian Kernighan.
This means that all
properly written @command{awk} programs should work with @command{gawk}.
So most of the time, we don't distinguish between @command{gawk} and other
@command{awk} implementations.

@cindex @command{awk} @subentry POSIX and @seealso{POSIX @command{awk}}
@cindex @command{awk} @subentry POSIX and
@cindex POSIX @subentry @command{awk} and
@cindex @command{gawk} @subentry @command{awk} and
@cindex @command{awk} @subentry @command{gawk} and
@cindex @command{awk} @subentry uses for
Using @command{awk} you can:

@itemize @value{BULLET}
@item
Manage small, personal databases

@item
Generate reports

@item
Validate data

@item
Produce indexes and perform other document-preparation tasks

@item
Experiment with algorithms that you can adapt later to other computer
languages
@end itemize

@cindex @command{awk} @seealso{@command{gawk}}
@cindex @command{gawk} @seealso{@command{awk}}
@cindex @command{gawk} @subentry uses for
In addition,
@command{gawk}
provides facilities that make it easy to:

@itemize @value{BULLET}
@item
Extract bits and pieces of data for processing

@item
Sort data

@item
Perform simple network communications

@item
Profile and debug @command{awk} programs

@item
Extend the language with functions written in C or C++
@end itemize

This @value{DOCUMENT} teaches you about the @command{awk} language and
how you can use it effectively.  You should already be familiar with basic
system commands, such as @command{cat} and @command{ls},@footnote{These utilities
are available on POSIX-compliant systems, as well as on traditional
Unix-based systems. If you are using some other operating system, you still need to
be familiar with the ideas of I/O redirection and pipes.} as well as basic shell
facilities, such as input/output (I/O) redirection and pipes.

@cindex GNU @command{awk} @seeentry{@command{gawk}}
Implementations of the @command{awk} language are available for many
different computing environments.  This @value{DOCUMENT}, while describing
the @command{awk} language in general, also describes the particular
implementation of @command{awk} called @command{gawk} (which stands for
``GNU @command{awk}'').  @command{gawk} runs on a broad range of Unix systems,
ranging from Intel-architecture PC-based computers
up through large-scale systems.
@command{gawk} has also been ported to Mac OS X,
Microsoft Windows
(all versions),
and OpenVMS.@footnote{Some other, obsolete systems to which @command{gawk}
was once ported are no longer supported and the code for those systems
has been removed.}

@menu
* History::                     The history of @command{gawk} and
                                @command{awk}.
* Names::                       What name to use to find @command{awk}.
* This Manual::                 Using this @value{DOCUMENT}. Includes sample
                                input files that you can use.
* Conventions::                 Typographical Conventions.
* Manual History::              Brief history of the GNU project and this
                                @value{DOCUMENT}.
* How To Contribute::           Helping to save the world.
* Acknowledgments::             Acknowledgments.
@end menu

@node History
@unnumberedsec History of @command{awk} and @command{gawk}
@cindex recipe for a programming language
@cindex programming language, recipe for
@cindex sidebar @subentry Recipe for a Programming Language
@ifdocbook
@docbook
Recipe for a Programming Language
@end docbook


@multitable {2 parts} {1 part  @code{egrep}} {1 part  @code{snobol}}
@item @tab 1 part  @code{egrep} @tab 1 part  @code{snobol}
@item @tab 2 parts @code{ed} @tab 3 parts C
@end multitable

Blend all parts well using @code{lex} and @code{yacc}.
Document minimally and release.

After eight years, add another part @code{egrep} and two
more parts C.  Document very well and release.

@docbook

@end docbook
@end ifdocbook

@ifnotdocbook
@cartouche
@center @b{Recipe for a Programming Language}



@multitable {2 parts} {1 part  @code{egrep}} {1 part  @code{snobol}}
@item @tab 1 part  @code{egrep} @tab 1 part  @code{snobol}
@item @tab 2 parts @code{ed} @tab 3 parts C
@end multitable

Blend all parts well using @code{lex} and @code{yacc}.
Document minimally and release.

After eight years, add another part @code{egrep} and two
more parts C.  Document very well and release.
@end cartouche
@end ifnotdocbook

@cindex Aho, Alfred
@cindex Weinberger, Peter
@cindex Kernighan, Brian
@cindex @command{awk} @subentry history of
The name @command{awk} comes from the initials of its designers: Alfred V.@:
Aho, Peter J.@: Weinberger, and Brian W.@: Kernighan.  The original version of
@command{awk} was written in 1977 at AT&T Bell Laboratories.
In 1985, a new version made the programming
language more powerful, introducing user-defined functions, multiple input
streams, and computed regular expressions.
This new version became widely available with Unix System V
Release 3.1 (1987).
The version in System V Release 4 (1989) added some new features and cleaned
up the behavior in some of the ``dark corners'' of the language.
The specification for @command{awk} in the POSIX Command Language
and Utilities standard further clarified the language.
Both the @command{gawk} designers and the original @command{awk} designers at Bell Laboratories
provided feedback for the POSIX specification.

@cindex Rubin, Paul
@cindex Fenlason, Jay
@cindex Trueman, David
Paul Rubin wrote @command{gawk} in 1986.
Jay Fenlason completed it, with advice from Richard Stallman.  John Woods
contributed parts of the code as well.  In 1988 and 1989, David Trueman, with
help from me, thoroughly reworked @command{gawk} for compatibility
with the newer @command{awk}.
Circa 1994, I became the primary maintainer.
Current development focuses on bug fixes,
performance improvements, standards compliance, and, occasionally, new features.

In May 1997, J@"urgen Kahrs felt the need for network access
from @command{awk}, and with a little help from me, set about adding
features to do this for @command{gawk}.  At that time, he also
wrote the bulk of
@cite{@value{GAWKINETTITLE}}
(a separate document, available as part of the @command{gawk} distribution).
His code finally became part of the main @command{gawk} distribution
with @command{gawk} @value{PVERSION} 3.1.

John Haque rewrote the @command{gawk} internals, in the process providing
an @command{awk}-level debugger. This version became available as
@command{gawk} @value{PVERSION} 4.0 in 2011.

@xref{Contributors}
for a full list of those who have made important contributions to @command{gawk}.

@node Names
@unnumberedsec A Rose by Any Other Name

@cindex @command{awk} @subentry new vs.@: old
The @command{awk} language has evolved over the years. Full details are
provided in @ref{Language History}.
The language described in this @value{DOCUMENT}
is often referred to as ``new @command{awk}.''
By analogy, the original version of @command{awk} is
referred to as ``old @command{awk}.''

On most current systems, when you run the @command{awk} utility
you get some version of new @command{awk}.@footnote{Only
Solaris systems still use an old @command{awk} for the
default @command{awk} utility. A more modern @command{awk} lives in
@file{/usr/xpg6/bin} on these systems.} If your system's standard
@command{awk} is the old one, you will see something like this
if you try the following test program:

@example
@group
$ @kbd{awk 1 /dev/null}
@error{} awk: syntax error near line 1
@error{} awk: bailing out near line 1
@end group
@end example

@noindent
In this case, you should find a version of new @command{awk},
or just install @command{gawk}!

Throughout this @value{DOCUMENT}, whenever we refer to a language feature
that should be available in any complete implementation of POSIX @command{awk},
we simply use the term @command{awk}.  When referring to a feature that is
specific to the GNU implementation, we use the term @command{gawk}.

@node This Manual
@unnumberedsec Using This Book
@cindex @command{awk} @subentry terms describing

The term @command{awk} refers to a particular program as well as to the language you
use to tell this program what to do.  When we need to be careful, we call
the language ``the @command{awk} language,''
and the program ``the @command{awk} utility.''
This @value{DOCUMENT} explains
both how to write programs in the @command{awk} language and how to
run the @command{awk} utility.
The term ``@command{awk} program'' refers to a program written by you in
the @command{awk} programming language.

@cindex @command{gawk} @subentry @command{awk} and
@cindex @command{awk} @subentry @command{gawk} and
@cindex POSIX @command{awk}
Primarily, this @value{DOCUMENT} explains the features of @command{awk}
as defined in the POSIX standard.  It does so in the context of the
@command{gawk} implementation.  While doing so, it also
attempts to describe important differences between @command{gawk}
and other @command{awk}
@ifclear FOR_PRINT
implementations.@footnote{All such differences
appear in the index under the
entry ``differences in @command{awk} and @command{gawk}.''}
@end ifclear
@ifset FOR_PRINT
implementations.
@end ifset
Finally, it notes any @command{gawk} features that are not in
the POSIX standard for @command{awk}.

@ifnotinfo
This @value{DOCUMENT} has the difficult task of being both a tutorial and a reference.
If you are a novice, feel free to skip over details that seem too complex.
You should also ignore the many cross-references; they are for the
expert user and for the Info and
@uref{https://www.gnu.org/software/gawk/manual/, HTML}
versions of the @value{DOCUMENT}.
@end ifnotinfo

There are sidebars
scattered throughout the @value{DOCUMENT}.
They add a more complete explanation of points that are relevant, but not likely
to be of interest on first reading.
@ifclear FOR_PRINT
All appear in the index, under the heading ``sidebar.''
@end ifclear

Most of the time, the examples use complete @command{awk} programs.
Some of the more advanced @value{SECTION}s show only the part of the @command{awk}
program that illustrates the concept being described.

Although this @value{DOCUMENT} is aimed principally at people who have not been
exposed
to @command{awk}, there is a lot of information here that even the @command{awk}
expert should find useful.  In particular, the description of POSIX
@command{awk} and the example programs in
@ref{Library Functions}, and
@ifnotdocbook
in
@end ifnotdocbook
@ref{Sample Programs},
should be of interest.

This @value{DOCUMENT} is split into several parts, as follows:

@c FULLXREF ON

@itemize @value{BULLET}
@item
Part I describes the @command{awk} language and the @command{gawk} program in detail.
It starts with the basics, and continues through all of the features of @command{awk}.
It contains the following chapters:

@c nested
@itemize @value{MINUS}
@item
@ref{Getting Started},
provides the essentials you need to know to begin using @command{awk}.

@item
@ref{Invoking Gawk},
describes how to run @command{gawk}, the meaning of its
command-line options, and how it finds @command{awk}
program source files.

@item
@ref{Regexp},
introduces regular expressions in general, and in particular the flavors
supported by POSIX @command{awk} and @command{gawk}.

@item
@ref{Reading Files},
describes how @command{awk} reads your data.
It introduces the concepts of records and fields, as well
as the @code{getline} command.
I/O redirection is first described here.
Network I/O is also briefly introduced here.

@item
@ref{Printing},
describes how @command{awk} programs can produce output with
@code{print} and @code{printf}.

@item
@ref{Expressions},
describes expressions, which are the basic building blocks
for getting most things done in a program.

@item
@ref{Patterns and Actions},
describes how to write patterns for matching records, actions for
doing something when a record is matched, and the predefined variables
@command{awk} and @command{gawk} use.

@item
@ref{Arrays},
covers @command{awk}'s one-and-only data structure: the associative array.
Deleting array elements and whole arrays is described, as well as
sorting arrays in @command{gawk}.  The @value{CHAPTER} also describes how
@command{gawk} provides arrays of arrays.

@item
@ref{Functions},
describes the built-in functions @command{awk} and @command{gawk} provide,
as well as how to define your own functions.  It also discusses how
@command{gawk} lets you call functions indirectly.
@end itemize

@item
Part II shows how to use @command{awk} and @command{gawk} for problem solving.
There is lots of code here for you to read and learn from.
This part contains the following chapters:

@c nested
@itemize @value{MINUS}
@item
@ref{Library Functions}, provides a number of functions meant to
be used from main @command{awk} programs.

@item
@ref{Sample Programs},
provides many sample @command{awk} programs.
@end itemize

Reading these two chapters allows you to see @command{awk}
solving real problems.

@item
Part III focuses on features specific to @command{gawk}.
It contains the following chapters:

@c nested
@itemize @value{MINUS}
@item
@ref{Advanced Features},
describes a number of advanced features.
Of particular note
are the abilities to control the order of array traversal,
have two-way communications with another process,
perform TCP/IP networking, and
profile your @command{awk} programs.

@item
@ref{Internationalization},
describes special features for translating program
messages into different languages at runtime.

@item
@ref{Debugger}, describes the @command{gawk} debugger.

@item
@ref{Namespaces}, describes how @command{gawk} allows variables and/or
functions of the same name to be in different namespaces.

@item
@ref{Arbitrary Precision Arithmetic},
describes advanced arithmetic facilities.

@item
@ref{Dynamic Extensions}, describes how to add new variables and
functions to @command{gawk} by writing extensions in C or C++.
@end itemize

@item
@ifclear FOR_PRINT
Part IV provides the appendices, the Glossary, and two licenses that cover
the @command{gawk} source code and this @value{DOCUMENT}, respectively.
It contains the following appendices:
@end ifclear
@ifset FOR_PRINT
Part IV provides the following appendices,
including the GNU General Public License:
@end ifset

@itemize @value{MINUS}
@item
@ref{Language History},
describes how the @command{awk} language has evolved since
its first release to the present.  It also describes how @command{gawk}
has acquired features over time.

@item
@ref{Installation},
describes how to get @command{gawk}, how to compile it
on POSIX-compatible systems,
and how to compile and use it on different
non-POSIX systems.  It also describes how to report bugs
in @command{gawk} and where to get other freely
available @command{awk} implementations.

@ifset FOR_PRINT
@item
@ref{Copying},
presents the license that covers the @command{gawk} source code.
@end ifset

@ifclear FOR_PRINT
@item
@ref{Notes},
describes how to disable @command{gawk}'s extensions, as
well as how to contribute new code to @command{gawk},
and some possible future directions for @command{gawk} development.

@item
@ref{Basic Concepts},
provides some very cursory background material for those who
are completely unfamiliar with computer programming.

@item
The @ref{Glossary}, defines most, if not all, of the significant terms used
throughout the @value{DOCUMENT}.  If you find terms that you aren't familiar with,
try looking them up here.

@item
@ref{Copying}, and
@ref{GNU Free Documentation License},
present the licenses that cover the @command{gawk} source code
and this @value{DOCUMENT}, respectively.
@end ifclear
@end itemize
@end itemize

@ifset FOR_PRINT
The version of this @value{DOCUMENT} distributed with @command{gawk}
contains additional appendices and other end material.
To save space, we have omitted them from the
printed edition. You may find them online, as follows:

@itemize @value{BULLET}
@item
@uref{https://www.gnu.org/software/gawk/manual/html_node/Notes.html,
The appendix on implementation notes}
describes how to disable @command{gawk}'s extensions, how to contribute
new code to @command{gawk}, where to find information on some possible
future directions for @command{gawk} development, and the design decisions
behind the extension API.

@item
@uref{https://www.gnu.org/software/gawk/manual/html_node/Basic-Concepts.html,
The appendix on basic concepts}
provides some very cursory background material for those who
are completely unfamiliar with computer programming.

@item
@uref{https://www.gnu.org/software/gawk/manual/html_node/Glossary.html,
The Glossary}
defines most, if not all, of the significant terms used
throughout the @value{DOCUMENT}.  If you find terms that you aren't familiar with,
try looking them up here.

@item
@uref{https://www.gnu.org/software/gawk/manual/html_node/GNU-Free-Documentation-License.html,
The GNU FDL}
is the license that covers this @value{DOCUMENT}.
@end itemize

@c ok not to use CHAPTER / SECTION here
Some of the chapters have exercise sections; these have also been
omitted from the print edition but are available online.
@end ifset

@c FULLXREF OFF

@node Conventions
@unnumberedsec Typographical Conventions

@cindex Texinfo
This @value{DOCUMENT} is written in @uref{https://www.gnu.org/software/texinfo/, Texinfo},
the GNU documentation formatting language.
A single Texinfo source file is used to produce both the printed and online
versions of the documentation.
@ifnotinfo
Because of this, the typographical conventions
are slightly different than in other books you may have read.
@end ifnotinfo
@ifinfo
This @value{SECTION} briefly documents the typographical conventions used in Texinfo.
@end ifinfo

Examples you would type at the command line are preceded by the common
shell primary and secondary prompts, @samp{$} and @samp{>}, respectively.
Input that you type is shown @kbd{like this}.
@c 8/2014: @print{} is stripped from the texi to make docbook.
@ifclear FOR_PRINT
Output from the command is preceded by the glyph ``@print{}''.
This typically represents the command's standard output.
@end ifclear
@ifset FOR_PRINT
Output from the command, usually its standard output, appears
@code{like this}.
@end ifset
Error messages and other output on the command's standard error are preceded
by the glyph ``@error{}''.  For example:

@example
$ @kbd{echo hi on stdout}
@print{} hi on stdout
$ @kbd{echo hello on stderr 1>&2}
@error{} hello on stderr
@end example

@ifnotinfo
In the text, almost anything related to programming, such as
command names,
variable and function names, and string, numeric and regexp constants
appear in @code{this font}. Code fragments
appear in the same font and quoted, @samp{like this}.
Things that are replaced by the user or programmer
appear in @var{this font}.
Options look like this: @option{-f}.
@value{FFN}s are indicated like this: @file{/path/to/ourfile}.
@ifclear FOR_PRINT
Some things are
emphasized @emph{like this}, and if a point needs to be made
strongly, it is done @strong{like this}.
@end ifclear
The first occurrence of
a new term is usually its @dfn{definition} and appears in the same
font as the previous occurrence of ``definition'' in this sentence.
@end ifnotinfo

Characters that you type at the keyboard look @kbd{like this}.  In particular,
there are special characters called ``control characters.''  These are
characters that you type by holding down both the @kbd{CONTROL} key and
another key, at the same time.  For example, a @kbd{Ctrl-d} is typed
by first pressing and holding the @kbd{CONTROL} key, next
pressing the @kbd{d} key, and finally releasing both keys.

For the sake of brevity, throughout this @value{DOCUMENT}, we refer to
Brian Kernighan's version of @command{awk} as ``BWK @command{awk}.''
(@xref{Other Versions} for information on his and other versions.)

@ifset FOR_PRINT
@quotation NOTE
Notes of interest look like this.
@end quotation

@quotation CAUTION
Cautionary or warning notes look like this.
@end quotation
@end ifset

@c fakenode --- for prepinfo
@unnumberedsubsec Dark Corners
@cindex Kernighan, Brian
@quotation
@i{Dark corners are basically fractal---no matter how much
you illuminate, there's always a smaller but darker one.}
@author Brian Kernighan
@end quotation

@cindex d.c. @seeentry{dark corner}
@cindex dark corner
Until the POSIX standard (and @cite{@value{TITLE}}),
many features of @command{awk} were either poorly documented or not
documented at all.  Descriptions of such features
(often called ``dark corners'') are noted in this @value{DOCUMENT} with
@iftex
the picture of a flashlight in the margin, as shown here.
@value{DARKCORNER}
@end iftex
@ifnottex
``(d.c.).''
@end ifnottex
@ifclear FOR_PRINT
They also appear in the index under the heading ``dark corner.''
@end ifclear

But, as noted by the opening quote, any coverage of dark
corners is by definition incomplete.

@cindex c.e. @seeentry{common extensions}
Extensions to the standard @command{awk} language that are supported by
more than one @command{awk} implementation are marked
@ifclear FOR_PRINT
``@value{COMMONEXT},'' and listed in the index under ``common extensions''
and ``extensions, common.''
@end ifclear
@ifset FOR_PRINT
``@value{COMMONEXT}'' for ``common extension.''
@end ifset

@node Manual History
@unnumberedsec The GNU Project and This Book

@cindex FSF (Free Software Foundation)
@cindex Free Software Foundation (FSF)
@cindex Stallman, Richard
The Free Software Foundation (FSF) is a nonprofit organization dedicated
to the production and distribution of freely distributable software.
It was founded by Richard M.@: Stallman, the author of the original
Emacs editor.  GNU Emacs is the most widely used version of Emacs today.

@cindex GNU Project
@cindex GPL (General Public License)
@cindex GNU General Public License @seeentry{GPL}
@cindex General Public License @seeentry{GPL}
@cindex documentation @subentry online
The GNU@footnote{GNU stands for ``GNU's Not Unix.''}
Project is an ongoing effort on the part of the Free Software
Foundation to create a complete, freely distributable, POSIX-compliant
computing environment.
The FSF uses the GNU General Public License (GPL) to ensure that
its software's
source code is always available to the end user.
@ifclear FOR_PRINT
A copy of the GPL is included
@ifnotinfo
in this @value{DOCUMENT}
@end ifnotinfo
for your reference
(@pxref{Copying}).
@end ifclear
The GPL applies to the C language source code for @command{gawk}.
To find out more about the FSF and the GNU Project online,
see @uref{https://www.gnu.org, the GNU Project's home page}.
This @value{DOCUMENT} may also be read from
@uref{https://www.gnu.org/software/gawk/manual/, GNU's website}.

@ifclear FOR_PRINT
A shell, an editor (Emacs), highly portable optimizing C, C++, and
Objective-C compilers, a symbolic debugger and dozens of large and
small utilities (such as @command{gawk}), have all been completed and are
freely available.  The GNU operating
system kernel (the HURD), has been released but remains in an early
stage of development.

@cindex Linux @seeentry{GNU/Linux}
@cindex GNU/Linux
@cindex operating systems @subentry BSD-based
Until the GNU operating system is more fully developed, you should
consider using GNU/Linux, a freely distributable, Unix-like operating
system for Intel,
Power Architecture,
Sun SPARC, IBM S/390, and other
systems.@footnote{The terminology ``GNU/Linux'' is explained
in the @ref{Glossary}.}
Many GNU/Linux distributions are
available for download from the Internet.
@end ifclear

@ifnotinfo
The @value{DOCUMENT} you are reading is actually free---at least, the
information in it is free to anyone.  The machine-readable
source code for the @value{DOCUMENT} comes with @command{gawk}.
@ifclear FOR_PRINT
(Take a moment to check the Free Documentation
License in @ref{GNU Free Documentation License}.)
@end ifclear
@end ifnotinfo

@cindex Close, Diane
The @value{DOCUMENT} itself has gone through multiple previous editions.
Paul Rubin wrote the very first draft of @cite{The GAWK Manual};
it was around 40 pages long.
Diane Close and Richard Stallman improved it, yielding a
version that was
around 90 pages and barely described the original, ``old''
version of @command{awk}.

I started working with that version in the fall of 1988.
As work on it progressed,
the FSF published several preliminary versions (numbered 0.@var{x}).
In 1996, edition 1.0 was released with @command{gawk} 3.0.0.
The FSF published the first two editions under
the title @cite{The GNU Awk User's Guide}.
@ifset FOR_PRINT
SSC published two editions of the @value{DOCUMENT} under the
title @cite{Effective awk Programming}, and O'Reilly published
the third edition in 2001.
@end ifset

This edition maintains the basic structure of the previous editions.
For FSF edition 4.0, the content was thoroughly reviewed and updated. All
references to @command{gawk} versions prior to 4.0 were removed.
Of significant note for that edition was the addition of @ref{Debugger}.

For FSF edition
@ifclear FOR_PRINT
5.0,
@end ifclear
@ifset FOR_PRINT
@value{EDITION}
(the fourth edition as published by O'Reilly),
@end ifset
the content has been reorganized into parts,
and the major new additions are @ref{Arbitrary Precision Arithmetic},
and @ref{Dynamic Extensions}.

This @value{DOCUMENT} will undoubtedly continue to evolve.  If you
find an error in the @value{DOCUMENT}, please report it!  @xref{Bugs}
for information on submitting problem reports electronically.

@ifset FOR_PRINT
@c fakenode --- for prepinfo
@unnumberedsec How to Stay Current

You may have a newer version of @command{gawk} than the
one described here.  To find out what has changed,
you should first look at the @file{NEWS} file in the @command{gawk}
distribution, which provides a high-level summary of the changes in
each release.

You can then look at the @uref{https://www.gnu.org/software/gawk/manual/,
online version} of this @value{DOCUMENT} to read about any new features.
@end ifset

@ifclear FOR_PRINT
@node How To Contribute
@unnumberedsec How to Contribute

As the maintainer of GNU @command{awk}, I once thought that I would be
able to manage a collection of publicly available @command{awk} programs
and I even solicited contributions.  Making things available on the Internet
helps keep the @command{gawk} distribution down to manageable size.

The initial collection of material, such as it is, is still available
at @uref{ftp://ftp.freefriends.org/arnold/Awkstuff}.

In the hopes of doing something more broad, I acquired the
@code{awklang.org} domain.  Late in 2017, a volunteer took on the task
of managing it.

If you have written an interesting @command{awk} program, that
you would like to share with the rest of the world, please see
@uref{http://www.awklang.org} and use the ``Contact'' link.

If you have written a @command{gawk} extension, please see
@ref{gawkextlib}.
@end ifclear

@node Acknowledgments
@unnumberedsec Acknowledgments

The initial draft of @cite{The GAWK Manual} had the following acknowledgments:

@quotation
Many people need to be thanked for their assistance in producing this
manual.  Jay Fenlason contributed many ideas and sample programs.  Richard
Mlynarik and Robert Chassell gave helpful comments on drafts of this
manual.  The paper @cite{A Supplemental Document for AWK} by John W.@:
Pierce of the Chemistry Department at UC San Diego, pinpointed several
issues relevant both to @command{awk} implementation and to this manual, that
would otherwise have escaped us.
@end quotation

@cindex Stallman, Richard
I would like to acknowledge Richard M.@: Stallman, for his vision of a
better world and for his courage in founding the FSF and starting the
GNU Project.

@ifclear FOR_PRINT
Earlier editions of this @value{DOCUMENT} had the following acknowledgements:
@end ifclear
@ifset FOR_PRINT
The previous edition of this @value{DOCUMENT} had
the following acknowledgements:
@end ifset

@quotation
The following people (in alphabetical order)
provided helpful comments on various
versions of this book:
Rick Adams,
Dr.@: Nelson H.F. Beebe,
Karl Berry,
Dr.@: Michael Brennan,
Rich Burridge,
Claire Cloutier,
Diane Close,
Scott Deifik,
Christopher (``Topher'') Eliot,
Jeffrey Friedl,
Dr.@: Darrel Hankerson,
Michal Jaegermann,
Dr.@: Richard J.@: LeBlanc,
Michael Lijewski,
Pat Rankin,
Miriam Robbins,
Mary Sheehan,
and
Chuck Toporek.

@cindex Berry, Karl
@cindex Chassell, Robert J.@:
@c @cindex Texinfo
Robert J.@: Chassell provided much valuable advice on
the use of Texinfo.
He also deserves special thanks for
convincing me @emph{not} to title this @value{DOCUMENT}
@cite{How to Gawk Politely}.
Karl Berry helped significantly with the @TeX{} part of Texinfo.

@cindex Hartholz @subentry Marshall
@cindex Hartholz @subentry Elaine
@cindex Schreiber @subentry Bert
@cindex Schreiber @subentry Rita
I would like to thank Marshall and Elaine Hartholz of Seattle and
Dr.@: Bert and Rita Schreiber of Detroit for large amounts of quiet vacation
time in their homes, which allowed me to make significant progress on
this @value{DOCUMENT} and on @command{gawk} itself.

@cindex Hughes, Phil
Phil Hughes of SSC
contributed in a very important way by loaning me his laptop GNU/Linux
system, not once, but twice, which allowed me to do a lot of work while
away from home.

@cindex Trueman, David
David Trueman deserves special credit; he has done a yeoman job
of evolving @command{gawk} so that it performs well and without bugs.
Although he is no longer involved with @command{gawk},
working with him on this project was a significant pleasure.

@cindex Drepper, Ulrich
@cindex GNITS mailing list
@cindex mailing list, GNITS
The intrepid members of the GNITS mailing list, and most notably Ulrich
Drepper, provided invaluable help and feedback for the design of the
internationalization features.

Chuck Toporek, Mary Sheehan, and Claire Cloutier of O'Reilly & Associates contributed
significant editorial help for this @value{DOCUMENT} for the
3.1 release of @command{gawk}.
@end quotation

@cindex Beebe, Nelson H.F.@:
@cindex Buening, Andreas
@cindex Collado, Manuel
@cindex Colombo, Antonio
@cindex Davies, Stephen
@cindex Deifik, Scott
@cindex Demaille, Akim
@cindex G., Daniel Richard
@cindex Guerrero, Juan Manuel
@cindex Hankerson, Darrel
@cindex Jaegermann, Michal
@cindex Kahrs, J@"urgen
@cindex Kasal, Stepan
@cindex Malmberg, John
@cindex Ramey, Chet
@cindex Rankin, Pat
@cindex Schorr, Andrew
@cindex Vinschen, Corinna
@cindex Zaretskii, Eli

Dr.@: Nelson Beebe,
Andreas Buening,
Dr.@: Manuel Collado,
Antonio Colombo,
Stephen Davies,
Scott Deifik,
Akim Demaille,
Daniel Richard G.,
Juan Manuel Guerrero,
Darrel Hankerson,
Michal Jaegermann,
J@"urgen Kahrs,
Stepan Kasal,
John Malmberg,
Chet Ramey,
Pat Rankin,
Andrew Schorr,
Corinna Vinschen,
and Eli Zaretskii
(in alphabetical order)
make up the current @command{gawk} ``crack portability team.''  Without
their hard work and help, @command{gawk} would not be nearly the robust,
portable program it is today.  It has been and continues to be a pleasure
working with this team of fine people.

Notable code and documentation contributions were made by
a number of people. @xref{Contributors} for the full list.

@ifset FOR_PRINT
@cindex Oram, Andy
Thanks to Andy Oram of O'Reilly Media for initiating
the fourth edition and for his support during the work.
Thanks to Jasmine Kwityn for her copyediting work.
@end ifset

Thanks to Michael Brennan for the Forewords.

@cindex Duman, Patrice
@cindex Berry, Karl
@cindex Smith, Gavin
Thanks to Patrice Dumas for the new @command{makeinfo} program.
Thanks to Karl Berry for his past work on Texinfo, and
to Gavin Smith, who continues to work to improve
the Texinfo markup language.

@cindex Kernighan, Brian
@cindex Brennan, Michael
@cindex Day, Robert P.J.@:
Robert P.J.@: Day, Michael Brennan, and Brian Kernighan kindly acted as
reviewers for the 2015 edition of this @value{DOCUMENT}. Their feedback
helped improve the final work.

I would also like to thank Brian Kernighan for his invaluable assistance during the
testing and debugging of @command{gawk}, and for his ongoing
help and advice in clarifying numerous points about the language.
We could not have done nearly as good a job on either @command{gawk}
or its documentation without his help.

Brian is in a class by himself as a programmer and technical
author.  I have to thank him (yet again) for his ongoing friendship
and for being a role model to me for over 30 years!
Having him as a reviewer is an exciting privilege. It has also
been extremely humbling@enddots{}

@cindex Robbins @subentry Miriam
@cindex Robbins @subentry Jean
@cindex Robbins @subentry Harry
@cindex G-d
I must thank my wonderful wife, Miriam, for her patience through
the many versions of this project, for her proofreading,
and for sharing me with the computer.
I would like to thank my parents for their love, and for the grace with
which they raised and educated me.
Finally, I also must acknowledge my gratitude to G-d, for the many opportunities
He has sent my way, as well as for the gifts He has given me with which to
take advantage of those opportunities.
@ifnotdocbook
@sp 2
@noindent
Arnold Robbins @*
Nof Ayalon @*
Israel @*
March, 2020
@end ifnotdocbook

@ifnotinfo
@part @value{PART1}The @command{awk} Language
@end ifnotinfo

@ifdocbook

Part I describes the @command{awk} language and @command{gawk} program
in detail.  It starts with the basics, and continues through all of
the features of @command{awk}. Included also are many, but not all,
of the features of @command{gawk}.  This part contains the
following chapters:

@itemize @value{BULLET}
@item
@ref{Getting Started}

@item
@ref{Invoking Gawk}

@item
@ref{Regexp}

@item
@ref{Reading Files}

@item
@ref{Printing}

@item
@ref{Expressions}

@item
@ref{Patterns and Actions}

@item
@ref{Arrays}

@item
@ref{Functions}
@end itemize
@end ifdocbook

@node Getting Started
@chapter Getting Started with @command{awk}
@c @cindex script, definition of
@c @cindex rule, definition of
@c @cindex program, definition of
@c @cindex basic function of @command{awk}
@cindex @command{awk} @subentry function of

The basic function of @command{awk} is to search files for lines (or other
units of text) that contain certain patterns.  When a line matches one
of the patterns, @command{awk} performs specified actions on that line.
@command{awk} continues to process input lines in this way until it reaches
the end of the input files.

@cindex @command{awk} @subentry uses for
@cindex programming languages @subentry data-driven vs.@: procedural
@cindex @command{awk} programs
Programs in @command{awk} are different from programs in most other languages,
because @command{awk} programs are @dfn{data driven} (i.e., you describe
the data you want to work with and then what to do when you find it).
Most other languages are @dfn{procedural}; you have to describe, in great
detail, every step the program should take.  When working with procedural
languages, it is usually much
harder to clearly describe the data your program will process.
For this reason, @command{awk} programs are often refreshingly easy to
read and write.

@cindex program, definition of
@cindex rule, definition of
When you run @command{awk}, you specify an @command{awk} @dfn{program} that
tells @command{awk} what to do.  The program consists of a series of
@dfn{rules} (it may also contain @dfn{function definitions},
an advanced feature that we will ignore for now;
@pxref{User-defined}).  Each rule specifies one
pattern to search for and one action to perform
upon finding the pattern.

Syntactically, a rule consists of a @dfn{pattern} followed by an
@dfn{action}.  The action is enclosed in braces to separate it from the
pattern.  Newlines usually separate rules.  Therefore, an @command{awk}
program looks like this:

@example
@var{pattern} @{ @var{action} @}
@var{pattern} @{ @var{action} @}
@dots{}
@end example

@menu
* Running gawk::                How to run @command{gawk} programs; includes
                                command-line syntax.
* Sample Data Files::           Sample data files for use in the @command{awk}
                                programs illustrated in this @value{DOCUMENT}.
* Very Simple::                 A very simple example.
* Two Rules::                   A less simple one-line example using two
                                rules.
* More Complex::                A more complex example.
* Statements/Lines::            Subdividing or combining statements into
                                lines.
* Other Features::              Other Features of @command{awk}.
* When::                        When to use @command{gawk} and when to use
                                other things.
* Intro Summary::               Summary of the introduction.
@end menu

@node Running gawk
@section How to Run @command{awk} Programs

@cindex @command{awk} programs @subentry running
There are several ways to run an @command{awk} program.  If the program is
short, it is easiest to include it in the command that runs @command{awk},
like this:

@example
awk '@var{program}' @var{input-file1} @var{input-file2} @dots{}
@end example

@cindex command line @subentry formats
When the program is long, it is usually more convenient to put it in a file
and run it with a command like this:

@example
awk -f @var{program-file} @var{input-file1} @var{input-file2} @dots{}
@end example

This @value{SECTION} discusses both mechanisms, along with several
variations of each.

@menu
* One-shot::                    Running a short throwaway @command{awk}
                                program.
* Read Terminal::               Using no input files (input from the keyboard
                                instead).
* Long::                        Putting permanent @command{awk} programs in
                                files.
* Executable Scripts::          Making self-contained @command{awk} programs.
* Comments::                    Adding documentation to @command{gawk}
                                programs.
* Quoting::                     More discussion of shell quoting issues.
@end menu

@node One-shot
@subsection One-Shot Throwaway @command{awk} Programs

Once you are familiar with @command{awk}, you will often type in simple
programs the moment you want to use them.  Then you can write the
program as the first argument of the @command{awk} command, like this:

@example
awk '@var{program}' @var{input-file1} @var{input-file2} @dots{}
@end example

@noindent
where @var{program} consists of a series of patterns and
actions, as described earlier.

@cindex single quote (@code{'})
@cindex @code{'} (single quote)
This command format instructs the @dfn{shell}, or command interpreter,
to start @command{awk} and use the @var{program} to process records in the
input file(s).  There are single quotes around @var{program} so
the shell won't interpret any @command{awk} characters as special shell
characters.  The quotes also cause the shell to treat all of @var{program} as
a single argument for @command{awk}, and allow @var{program} to be more
than one line long.

@cindex shells @subentry scripts
@cindex @command{awk} programs @subentry running @subentry from shell scripts
This format is also useful for running short or medium-sized @command{awk}
programs from shell scripts, because it avoids the need for a separate
file for the @command{awk} program.  A self-contained shell script is more
reliable because there are no other files to misplace.

Later in this chapter, in
@ifdocbook
the @value{SECTION}
@end ifdocbook
@ref{Very Simple},
we'll see examples of several short,
self-contained programs.

@node Read Terminal
@subsection Running @command{awk} Without Input Files

@cindex standard input
@cindex input @subentry standard
@cindex input files @subentry running @command{awk} without
You can also run @command{awk} without any input files.  If you type the
following command line:

@example
awk '@var{program}'
@end example

@noindent
@command{awk} applies the @var{program} to the @dfn{standard input},
which usually means whatever you type on the keyboard.  This continues
until you indicate end-of-file by typing @kbd{Ctrl-d}.
(On non-POSIX operating systems, the end-of-file character may be different.)

@cindex files @subentry input @seeentry{input files}
@cindex input files @subentry running @command{awk} without
@cindex @command{awk} programs @subentry running @subentry without input files
As an example, the following program prints a friendly piece of advice
(from Douglas Adams's @cite{The Hitchhiker's Guide to the Galaxy}),
to keep you from worrying about the complexities of computer
programming:

@example
$ @kbd{awk 'BEGIN @{ print "Don\47t Panic!" @}'}
@print{} Don't Panic!
@end example

@command{awk} executes statements associated with @code{BEGIN} before
reading any input.  If there are no other statements in your program,
as is the case here, @command{awk} just stops, instead of trying to read
input it doesn't know how to process.
The @samp{\47} is a magic way (explained later) of getting a single quote into
the program, without having to engage in ugly shell quoting tricks.

@quotation NOTE
If you use Bash as your shell, you should execute the
command @samp{set +H} before running this program interactively, to
disable the C shell-style command history, which treats @samp{!} as a
special character. We recommend putting this command into your personal
startup file.
@end quotation

This next simple @command{awk} program
emulates the @command{cat} utility; it copies whatever you type on the
keyboard to its standard output (why this works is explained shortly):

@example
$ @kbd{awk '@{ print @}'}
@kbd{Now is the time for all good men}
@print{} Now is the time for all good men
@kbd{to come to the aid of their country.}
@print{} to come to the aid of their country.
@kbd{Four score and seven years ago, ...}
@print{} Four score and seven years ago, ...
@kbd{What, me worry?}
@print{} What, me worry?
@kbd{Ctrl-d}
@end example

@node Long
@subsection Running Long Programs

@cindex @command{awk} programs @subentry running
@cindex @command{awk} programs @subentry lengthy
@cindex files @subentry @command{awk} programs in
Sometimes @command{awk} programs are very long.  In these cases, it is
more convenient to put the program into a separate file.  In order to tell
@command{awk} to use that file for its program, you type:

@example
awk -f @var{source-file} @var{input-file1} @var{input-file2} @dots{}
@end example

@cindex @option{-f} option
@cindex command line @subentry option @option{-f}
The @option{-f} instructs the @command{awk} utility to get the
@command{awk} program from the file @var{source-file} (@pxref{Options}).
Any @value{FN} can be used for @var{source-file}.  For example, you
could put the program:

@example
BEGIN @{ print "Don't Panic!" @}
@end example

@noindent
into the file @file{advice}.  Then this command:

@example
awk -f advice
@end example

@noindent
does the same thing as this one:

@example
awk 'BEGIN @{ print "Don\47t Panic!" @}'
@end example

@cindex quoting @subentry in @command{gawk} command lines
@noindent
This was explained earlier
(@pxref{Read Terminal}).
Note that you don't usually need single quotes around the @value{FN} that you
specify with @option{-f}, because most @value{FN}s don't contain any of the shell's
special characters.  Notice that in @file{advice}, the @command{awk}
program did not have single quotes around it.  The quotes are only needed
for programs that are provided on the @command{awk} command line.
(Also, placing the program in a file allows us to use a literal single quote in the program
text, instead of the magic @samp{\47}.)

@cindex single quote (@code{'}) @subentry in @command{gawk} command lines
@cindex @code{'} (single quote) @subentry in @command{gawk} command lines
If you want to clearly identify an @command{awk} program file as such,
you can add the extension @file{.awk} to the @value{FN}.  This doesn't
affect the execution of the @command{awk} program but it does make
``housekeeping'' easier.

@node Executable Scripts
@subsection Executable @command{awk} Programs
@cindex @command{awk} programs
@cindex @code{#} (number sign) @subentry @code{#!} (executable scripts)
@cindex Unix @subentry @command{awk} scripts and
@cindex number sign (@code{#}) @subentry @code{#!} (executable scripts)

Once you have learned @command{awk}, you may want to write self-contained
@command{awk} scripts, using the @samp{#!} script mechanism.  You can do
this on many systems.@footnote{The @samp{#!} mechanism works on
GNU/Linux systems, BSD-based systems, and commercial Unix systems.}
For example, you could update the file @file{advice} to look like this:

@example
#! /bin/awk -f

BEGIN @{ print "Don't Panic!" @}
@end example

@noindent
After making this file executable (with the @command{chmod} utility),
simply type @samp{advice}
at the shell and the system arranges to run @command{awk} as if you had
typed @samp{awk -f advice}:

@example
$ @kbd{chmod +x advice}
$ @kbd{./advice}
@print{} Don't Panic!
@end example

@noindent
Self-contained @command{awk} scripts are useful when you want to write a
program that users can invoke without their having to know that the program is
written in @command{awk}.

@cindex sidebar @subentry Understanding @samp{#!}
@ifdocbook
@docbook
Understanding @samp{#!}
@end docbook

@cindex portability @subentry @code{#!} (executable scripts)

@command{awk} is an @dfn{interpreted} language. This means that the
@command{awk} utility reads your program and then processes your data
according to the instructions in your program. (This is different
from a @dfn{compiled} language such as C, where your program is first
compiled into machine code that is executed directly by your system's
processor.)  The @command{awk} utility is thus termed an @dfn{interpreter}.
Many modern languages are interpreted.

The line beginning with @samp{#!} lists the full @value{FN} of an
interpreter to run and a single optional initial command-line argument
to pass to that interpreter.  The operating system then runs the
interpreter with the given argument and the full argument list of the
executed program.  The first argument in the list is the full @value{FN}
of the @command{awk} program.  The rest of the argument list contains
either options to @command{awk}, or @value{DF}s, or both. (Note that on
many systems @command{awk} is found in @file{/usr/bin} instead of
in @file{/bin}.)

Some systems limit the length of the interpreter name to 32 characters.
Often, this can be dealt with by using a symbolic link.

You should not put more than one argument on the @samp{#!}
line after the path to @command{awk}. It does not work. The operating system
treats the rest of the line as a single argument and passes it to @command{awk}.
Doing this leads to confusing behavior---most likely a usage diagnostic
of some sort from @command{awk}.

@cindex @code{ARGC}/@code{ARGV} variables @subentry portability and
@cindex portability @subentry @code{ARGV} variable
@cindex dark corner @subentry @code{ARGV} variable, value of
Finally, the value of @code{ARGV[0]}
(@pxref{Built-in Variables})
varies depending upon your operating system.
Some systems put @samp{awk} there, some put the full pathname
of @command{awk} (such as @file{/bin/awk}), and some put the name
of your script (@samp{advice}).  @value{DARKCORNER}
Don't rely on the value of @code{ARGV[0]}
to provide your script name.

@docbook

@end docbook
@end ifdocbook

@ifnotdocbook
@cartouche
@center @b{Understanding @samp{#!}}


@cindex portability @subentry @code{#!} (executable scripts)

@command{awk} is an @dfn{interpreted} language. This means that the
@command{awk} utility reads your program and then processes your data
according to the instructions in your program. (This is different
from a @dfn{compiled} language such as C, where your program is first
compiled into machine code that is executed directly by your system's
processor.)  The @command{awk} utility is thus termed an @dfn{interpreter}.
Many modern languages are interpreted.

The line beginning with @samp{#!} lists the full @value{FN} of an
interpreter to run and a single optional initial command-line argument
to pass to that interpreter.  The operating system then runs the
interpreter with the given argument and the full argument list of the
executed program.  The first argument in the list is the full @value{FN}
of the @command{awk} program.  The rest of the argument list contains
either options to @command{awk}, or @value{DF}s, or both. (Note that on
many systems @command{awk} is found in @file{/usr/bin} instead of
in @file{/bin}.)

Some systems limit the length of the interpreter name to 32 characters.
Often, this can be dealt with by using a symbolic link.

You should not put more than one argument on the @samp{#!}
line after the path to @command{awk}. It does not work. The operating system
treats the rest of the line as a single argument and passes it to @command{awk}.
Doing this leads to confusing behavior---most likely a usage diagnostic
of some sort from @command{awk}.

@cindex @code{ARGC}/@code{ARGV} variables @subentry portability and
@cindex portability @subentry @code{ARGV} variable
@cindex dark corner @subentry @code{ARGV} variable, value of
Finally, the value of @code{ARGV[0]}
(@pxref{Built-in Variables})
varies depending upon your operating system.
Some systems put @samp{awk} there, some put the full pathname
of @command{awk} (such as @file{/bin/awk}), and some put the name
of your script (@samp{advice}).  @value{DARKCORNER}
Don't rely on the value of @code{ARGV[0]}
to provide your script name.
@end cartouche
@end ifnotdocbook

@node Comments
@subsection Comments in @command{awk} Programs
@cindex @code{#} (number sign) @subentry commenting
@cindex number sign (@code{#}) @subentry commenting
@cindex commenting
@cindex @command{awk} programs @subentry documenting

A @dfn{comment} is some text that is included in a program for the sake
of human readers; it is not really an executable part of the program.  Comments
can explain what the program does and how it works.  Nearly all
programming languages have provisions for comments, as programs are
typically hard to understand without them.

In the @command{awk} language, a comment starts with the number sign
character (@samp{#}) and continues to the end of the line.
The @samp{#} does not have to be the first character on the line. The
@command{awk} language ignores the rest of a line following a number sign.
For example, we could have put the following into @file{advice}:

@example
# This program prints a nice, friendly message.  It helps
# keep novice users from being afraid of the computer.
BEGIN    @{ print "Don't Panic!" @}
@end example

You can put comment lines into keyboard-composed throwaway @command{awk}
programs, but this usually isn't very useful; the purpose of a
comment is to help you or another person understand the program
when reading it at a later time.

@cindex quoting @subentry for small awk programs
@cindex single quote (@code{'}) @subentry vs.@: apostrophe
@cindex @code{'} (single quote) @subentry vs.@: apostrophe
@quotation CAUTION
As mentioned in
@ref{One-shot},
you can enclose short to medium-sized programs in single quotes,
in order to keep
your shell scripts self-contained.  When doing so, @emph{don't} put
an apostrophe (i.e., a single quote) into a comment (or anywhere else
in your program). The shell interprets the quote as the closing
quote for the entire program. As a result, usually the shell
prints a message about mismatched quotes, and if @command{awk} actually
runs, it will probably print strange messages about syntax errors.
For example, look at the following:

@example
$ @kbd{awk 'BEGIN @{ print "hello" @} # let's be cute'}
>
@end example

The shell sees that the first two quotes match, and that
a new quoted object begins at the end of the command line.
It therefore prompts with the secondary prompt, waiting for more input.
With Unix @command{awk}, closing the quoted string produces this result:

@example
$ @kbd{awk '@{ print "hello" @} # let's be cute'}
> @kbd{'}
@error{} awk: can't open file be
@error{}  source line number 1
@end example

@cindex @code{\} (backslash)
@cindex backslash (@code{\})
Putting a backslash before the single quote in @samp{let's} wouldn't help,
because backslashes are not special inside single quotes.
The next @value{SUBSECTION} describes the shell's quoting rules.
@end quotation

@node Quoting
@subsection Shell Quoting Issues
@cindex shell quoting, rules for

@menu
* DOS Quoting::                 Quoting in Windows Batch Files.
@end menu

For short to medium-length @command{awk} programs, it is most convenient
to enter the program on the @command{awk} command line.
This is best done by enclosing the entire program in single quotes.
This is true whether you are entering the program interactively at
the shell prompt, or writing it as part of a larger shell script:

@example
awk '@var{program text}' @var{input-file1} @var{input-file2} @dots{}
@end example

@cindex shells @subentry quoting @subentry rules for
@cindex Bourne shell, quoting rules for
Once you are working with the shell, it is helpful to have a basic
knowledge of shell quoting rules.  The following rules apply only to
POSIX-compliant, Bourne-style shells (such as Bash, the GNU Bourne-Again
Shell).  If you use the C shell, you're on your own.

Before diving into the rules, we introduce a concept that appears
throughout this @value{DOCUMENT}, which is that of the @dfn{null},
or empty, string.

The null string is character data that has no value.
In other words, it is empty.  It is written in @command{awk} programs
like this: @code{""}. In the shell, it can be written using single
or double quotes: @code{""} or @code{''}. Although the null string has
no characters in it, it does exist. For example, consider this command:

@example
$ @kbd{echo ""}
@end example

@noindent
Here, the @command{echo} utility receives a single argument, even
though that argument has no characters in it. In the rest of this
@value{DOCUMENT}, we use the terms @dfn{null string} and @dfn{empty string}
interchangeably.  Now, on to the quoting rules:

@itemize @value{BULLET}
@item
Quoted items can be concatenated with nonquoted items as well as with other
quoted items.  The shell turns everything into one argument for
the command.

@item
Preceding any single character with a backslash (@samp{\}) quotes
that character.  The shell removes the backslash and passes the quoted
character on to the command.

@item
@cindex @code{\} (backslash) @subentry in shell commands
@cindex backslash (@code{\}) @subentry in shell commands
@cindex single quote (@code{'}) @subentry in shell commands
@cindex @code{'} (single quote) @subentry in shell commands
Single quotes protect everything between the opening and closing quotes.
The shell does no interpretation of the quoted text, passing it on verbatim
to the command.
It is @emph{impossible} to embed a single quote inside single-quoted text.
Refer back to
@ref{Comments}
for an example of what happens if you try.

@item
@cindex double quote (@code{"}) @subentry in shell commands
@cindex @code{"} (double quote) @subentry in shell commands
Double quotes protect most things between the opening and closing quotes.
The shell does at least variable and command substitution on the quoted text.
Different shells may do additional kinds of processing on double-quoted text.

Because certain characters within double-quoted text are processed by the shell,
they must be @dfn{escaped} within the text.  Of note are the characters
@samp{$}, @samp{`}, @samp{\}, and @samp{"}, all of which must be preceded by
a backslash within double-quoted text if they are to be passed on literally
to the program.  (The leading backslash is stripped first.)
Thus, the example seen
@ifnotinfo
previously
@end ifnotinfo
in @ref{Read Terminal}:

@example
awk 'BEGIN @{ print "Don\47t Panic!" @}'
@end example

@noindent
could instead be written this way:

@example
$ @kbd{awk "BEGIN @{ print \"Don't Panic!\" @}"}
@print{} Don't Panic!
@end example

@cindex single quote (@code{'}) @subentry with double quotes
@cindex @code{'} (single quote) @subentry with double quotes
Note that the single quote is not special within double quotes.

@item
Null strings are removed when they occur as part of a non-null
command-line argument, while explicit null objects are kept.
For example, to specify that the field separator @code{FS} should
be set to the null string, use:

@example
awk -F "" '@var{program}' @var{files} # correct
@end example

@noindent
@cindex null strings @subentry in @command{gawk} arguments, quoting and
Don't use this:

@example
awk -F"" '@var{program}' @var{files}  # wrong!
@end example

@noindent
In the second case, @command{awk} attempts to use the text of the program
as the value of @code{FS}, and the first @value{FN} as the text of the program!
This results in syntax errors at best, and confusing behavior at worst.
@end itemize

@cindex quoting @subentry in @command{gawk} command lines @subentry tricks for
Mixing single and double quotes is difficult.  You have to resort
to shell quoting tricks, like this:

@example
$ @kbd{awk 'BEGIN @{ print "Here is a single quote <'"'"'>" @}'}
@print{} Here is a single quote <'>
@end example

@noindent
This program consists of three concatenated quoted strings.  The first and the
third are single-quoted, and the second is double-quoted.

This can be ``simplified'' to:

@example
$ @kbd{awk 'BEGIN @{ print "Here is a single quote <'\''>" @}'}
@print{} Here is a single quote <'>
@end example

@noindent
Judge for yourself which of these two is the more readable.

Another option is to use double quotes, escaping the embedded, @command{awk}-level
double quotes:

@example
$ @kbd{awk "BEGIN @{ print \"Here is a single quote <'>\" @}"}
@print{} Here is a single quote <'>
@end example

@noindent
This option is also painful, because double quotes, backslashes, and dollar signs
are very common in more advanced @command{awk} programs.

A third option is to use the octal escape sequence equivalents
(@pxref{Escape Sequences})
for the
single- and double-quote characters, like so:

@example
@group
$ @kbd{awk 'BEGIN @{ print "Here is a single quote <\47>" @}'}
@print{} Here is a single quote <'>
$ @kbd{awk 'BEGIN @{ print "Here is a double quote <\42>" @}'}
@print{} Here is a double quote <">
@end group
@end example

@noindent
This works nicely, but you should comment clearly what the
escape sequences mean.

A fourth option is to use command-line variable assignment, like this:

@example
$ @kbd{awk -v sq="'" 'BEGIN @{ print "Here is a single quote <" sq ">" @}'}
@print{} Here is a single quote <'>
@end example

(Here, the two string constants and the value of @code{sq} are concatenated
into a single string that is printed by @code{print}.)

If you really need both single and double quotes in your @command{awk}
program, it is probably best to move it into a separate file, where
the shell won't be part of the picture and you can say what you mean.

@node DOS Quoting
@subsubsection Quoting in MS-Windows Batch Files

@ignore
Date: Wed, 21 May 2008 09:58:43 +0200 (CEST)
From: jeroen.brink@inter.NL.net
Subject: (g)awk "contribution"
To: arnold@skeeve.com
Message-id: <42220.193.172.132.34.1211356723.squirrel@webmail.internl.net>

Hello Arnold,

maybe you can help me out. Found your email on the GNU/awk online manual
pages.

I've searched hard to figure out how, on Windows, to print double quotes.
Couldn't find it in the Quotes area, nor on google or elsewhere. Finally i
figured out how to do this myself.

How to print all lines in a file surrounded by double quotes (on Windows):

gawk "{ print \"\042\" $0 \"\042\" }" 

Maybe this is a helpfull tip for other (Windows) gawk users. However, i
don't have a clue as to where to "publish" this tip! Do you?

Kind regards,

Jeroen Brink
@end ignore

Although this @value{DOCUMENT} generally only worries about POSIX systems and the
POSIX shell, the following issue arises often enough for many users that
it is worth addressing.

@cindex Brink, Jeroen
The ``shells'' on Microsoft Windows systems use the double-quote
character for quoting, and make it difficult or impossible to include an
escaped double-quote character in a command-line script.  The following
example, courtesy of Jeroen Brink, shows how to escape the double quotes
from this one liner script that prints all lines in a file surrounded by
double quotes:

@example
@{ print "\"" $0 "\"" @}
@end example

@noindent
In an MS-Windows command-line the one-liner script above may be passed as
follows:

@example
gawk "@{ print \"\042\" $0 \"\042\" @}" @var{file}
@end example

In this example the @samp{\042} is the octal code for a double-quote;
@command{gawk} converts it into a real double-quote for output by
the @code{print} statement.

In MS-Windows escaping double-quotes is a little tricky because you use
backslashes to escape double-quotes, but backslashes themselves are not
escaped in the usual way; indeed they are either duplicated or not,
depending upon whether there is a subsequent double-quote.  The MS-Windows
rule for double-quoting a string is the following:

@enumerate
@item
For each double quote in the original string, let @var{N} be the number
of backslash(es) before it, @var{N} might be zero. Replace these @var{N}
backslash(es) by @math{2@value{TIMES}@var{N}+1} backslash(es)

@item
Let @var{N} be the number of backslash(es) tailing the original string,
@var{N} might be zero. Replace these @var{N} backslash(es) by
@math{2@value{TIMES}@var{N}} backslash(es)

@item
Surround the resulting string by double-quotes.
@end enumerate

So to double-quote the one-liner script @samp{@{ print "\"" $0 "\"" @}}
from the previous example you would do it this way:

@example
gawk "@{ print \"\\\"\" $0 \"\\\"\" @}" @var{file}
@end example

@noindent
However, the use of @samp{\042} instead of @samp{\\\"} is also possible
and easier to read, because backslashes that are not followed by a
double-quote don't need duplication.

@node Sample Data Files
@section @value{DDF}s for the Examples

@cindex input files @subentry examples
@cindex @code{mail-list} file
Many of the examples in this @value{DOCUMENT} take their input from two sample
@value{DF}s.  The first, @file{mail-list}, represents a list of peoples' names
together with their email addresses and information about those people.
The second @value{DF}, called @file{inventory-shipped}, contains
information about monthly shipments.  In both files,
each line is considered to be one @dfn{record}.

In @file{mail-list}, each record contains the name of a person,
his/her phone number, his/her email address, and a code for his/her relationship
with the author of the list.
The columns are aligned using spaces.
An @samp{A} in the last column
means that the person is an acquaintance.  An @samp{F} in the last
column means that the person is a friend.
An @samp{R} means that the person is a relative:

@example
@c system if test ! -d eg      ; then mkdir eg      ; fi
@c system if test ! -d eg/lib  ; then mkdir eg/lib  ; fi
@c system if test ! -d eg/data ; then mkdir eg/data ; fi
@c system if test ! -d eg/prog ; then mkdir eg/prog ; fi
@c system if test ! -d eg/misc ; then mkdir eg/misc ; fi
@c file eg/data/mail-list
Amelia       555-5553     amelia.zodiacusque@@gmail.com    F
Anthony      555-3412     anthony.asserturo@@hotmail.com   A
Becky        555-7685     becky.algebrarum@@gmail.com      A
Bill         555-1675     bill.drowning@@hotmail.com       A
Broderick    555-0542     broderick.aliquotiens@@yahoo.com R
Camilla      555-2912     camilla.infusarum@@skynet.be     R
Fabius       555-1234     fabius.undevicesimus@@ucb.edu    F
Julie        555-6699     julie.perscrutabor@@skeeve.com   F
Martin       555-6480     martin.codicibus@@hotmail.com    A
Samuel       555-3430     samuel.lanceolis@@shu.edu        A
Jean-Paul    555-2127     jeanpaul.campanorum@@nyu.edu     R
@c endfile
@end example

@cindex @code{inventory-shipped} file
The @value{DF} @file{inventory-shipped} represents
information about shipments during the year.
Each record contains the month, the number
of green crates shipped, the number of red boxes shipped, the number of
orange bags shipped, and the number of blue packages shipped,
respectively.  There are 16 entries, covering the 12 months of last year
and the first four months of the current year.
An empty line separates the data for the two years:

@example
@c file eg/data/inventory-shipped
Jan  13  25  15 115
Feb  15  32  24 226
Mar  15  24  34 228
Apr  31  52  63 420
May  16  34  29 208
Jun  31  42  75 492
Jul  24  34  67 436
Aug  15  34  47 316
Sep  13  55  37 277
Oct  29  54  68 525
Nov  20  87  82 577
Dec  17  35  61 401

Jan  21  36  64 620
Feb  26  58  80 652
Mar  24  75  70 495
Apr  21  70  74 514
@c endfile
@end example

The sample files are included in the @command{gawk} distribution,
in the directory @file{awklib/eg/data}.

@node Very Simple
@section Some Simple Examples

The following command runs a simple @command{awk} program that searches the
input file @file{mail-list} for the character string @samp{li} (a
grouping of characters is usually called a @dfn{string};
the term @dfn{string} is based on similar usage in English, such
as ``a string of pearls'' or ``a string of cars in a train''):

@example
awk '/li/ @{ print $0 @}' mail-list
@end example

@noindent
When lines containing @samp{li} are found, they are printed because
@w{@samp{print $0}} means print the current line.  (Just @samp{print} by
itself means the same thing, so we could have written that
instead.)

You will notice that slashes (@samp{/}) surround the string @samp{li}
in the @command{awk} program.  The slashes indicate that @samp{li}
is the pattern to search for.  This type of pattern is called a
@dfn{regular expression}, which is covered in more detail later
(@pxref{Regexp}).
The pattern is allowed to match parts of words.
There are
single quotes around the @command{awk} program so that the shell won't
interpret any of it as special shell characters.

Here is what this program prints:

@example
$ @kbd{awk '/li/ @{ print $0 @}' mail-list}
@print{} Amelia       555-5553     amelia.zodiacusque@@gmail.com    F
@print{} Broderick    555-0542     broderick.aliquotiens@@yahoo.com R
@print{} Julie        555-6699     julie.perscrutabor@@skeeve.com   F
@print{} Samuel       555-3430     samuel.lanceolis@@shu.edu        A
@end example

@cindex actions @subentry default
@cindex patterns @subentry default
In an @command{awk} rule, either the pattern or the action can be omitted,
but not both.  If the pattern is omitted, then the action is performed
for @emph{every} input line.  If the action is omitted, the default
action is to print all lines that match the pattern.

@cindex actions @subentry empty
Thus, we could leave out the action (the @code{print} statement and the
braces) in the previous example and the result would be the same:
@command{awk} prints all lines matching the pattern @samp{li}.  By comparison,
omitting the @code{print} statement but retaining the braces makes an
empty action that does nothing (i.e., no lines are printed).

@cindex @command{awk} programs @subentry one-line examples
Many practical @command{awk} programs are just a line or two long.  Following is a
collection of useful, short programs to get you started.  Some of these
programs contain constructs that haven't been covered yet. (The description
of the program will give you a good idea of what is going on, but you'll
need to read the rest of the @value{DOCUMENT} to become an @command{awk} expert!)
Most of the examples use a @value{DF} named @file{data}.  This is just a
placeholder; if you use these programs yourself, substitute
your own @value{FN}s for @file{data}.
For future reference, note that there is often more than
one way to do things in @command{awk}.  At some point, you may want
to look back at these examples and see if
you can come up with different ways to do the same things shown here:

@itemize @value{BULLET}
@item
Print every line that is longer than 80 characters:

@example
awk 'length($0) > 80' data
@end example

The sole rule has a relational expression as its pattern and has no
action---so it uses the default action, printing the record.

@item
Print the length of the longest input line:

@example
@group
awk '@{ if (length($0) > max) max = length($0) @}
     END @{ print max @}' data
@end group
@end example

The code associated with @code{END} executes after all
input has been read; it's the other side of the coin to @code{BEGIN}.

@cindex @command{expand} utility
@item
Print the length of the longest line in @file{data}:

@example
expand data | awk '@{ if (x < length($0)) x = length($0) @}
                   END @{ print "maximum line length is " x @}'
@end example

This example differs slightly from the previous one:
the input is processed by the @command{expand} utility to change TABs
into spaces, so the widths compared are actually the right-margin columns,
as opposed to the number of input characters on each line.

@item
Print every line that has at least one field:

@example
awk 'NF > 0' data
@end example

This is an easy way to delete blank lines from a file (or rather, to
create a new file similar to the old file but from which the blank lines
have been removed).

@item
Print seven random numbers from 0 to 100, inclusive:

@example
awk 'BEGIN @{ for (i = 1; i <= 7; i++)
                 print int(101 * rand()) @}'
@end example

@item
Print the total number of bytes used by @var{files}:

@example
ls -l @var{files} | awk '@{ x += $5 @}
                   END @{ print "total bytes: " x @}'
@end example

@item
Print the total number of kilobytes used by @var{files}:

@c Don't use \ continuation, not discussed yet
@c Remember that awk does floating point division,
@c no need for (x+1023) / 1024
@example
ls -l @var{files} | awk '@{ x += $5 @}
   END @{ print "total K-bytes:", x / 1024 @}'
@end example

@item
Print a sorted list of the login names of all users:

@example
awk -F: '@{ print $1 @}' /etc/passwd | sort
@end example

@item
Count the lines in a file:

@example
awk 'END @{ print NR @}' data
@end example

@item
Print the even-numbered lines in the @value{DF}:

@example
awk 'NR % 2 == 0' data
@end example

If you used the expression @samp{NR % 2 == 1} instead,
the program would print the odd-numbered lines.
@end itemize

@node Two Rules
@section An Example with Two Rules
@cindex @command{awk} programs

The @command{awk} utility reads the input files one line at a
time.  For each line, @command{awk} tries the patterns of each rule.
If several patterns match, then several actions execute in the order in
which they appear in the @command{awk} program.  If no patterns match, then
no actions run.

After processing all the rules that match the line (and perhaps there are none),
@command{awk} reads the next line.  (However,
@pxref{Next Statement}
@ifdocbook
and @ref{Nextfile Statement}.)
@end ifdocbook
@ifnotdocbook
and also @pxref{Nextfile Statement}.)
@end ifnotdocbook
This continues until the program reaches the end of the file.
For example, the following @command{awk} program contains two rules:

@example
/12/  @{ print $0 @}
/21/  @{ print $0 @}
@end example

@noindent
The first rule has the string @samp{12} as the
pattern and @samp{print $0} as the action.  The second rule has the
string @samp{21} as the pattern and also has @samp{print $0} as the
action.  Each rule's action is enclosed in its own pair of braces.

This program prints every line that contains the string
@samp{12} @emph{or} the string @samp{21}.  If a line contains both
strings, it is printed twice, once by each rule.

This is what happens if we run this program on our two sample @value{DF}s,
@file{mail-list} and @file{inventory-shipped}:

@example
$ @kbd{awk '/12/ @{ print $0 @}}
>      @kbd{/21/ @{ print $0 @}' mail-list inventory-shipped}
@print{} Anthony      555-3412     anthony.asserturo@@hotmail.com   A
@print{} Camilla      555-2912     camilla.infusarum@@skynet.be     R
@print{} Fabius       555-1234     fabius.undevicesimus@@ucb.edu    F
@print{} Jean-Paul    555-2127     jeanpaul.campanorum@@nyu.edu     R
@print{} Jean-Paul    555-2127     jeanpaul.campanorum@@nyu.edu     R
@print{} Jan  21  36  64 620
@print{} Apr  21  70  74 514
@end example

@noindent
Note how the line beginning with @samp{Jean-Paul}
in @file{mail-list} was printed twice, once for each rule.

@node More Complex
@section A More Complex Example

Now that we've mastered some simple tasks, let's look at
what typical @command{awk}
programs do.  This example shows how @command{awk} can be used to
summarize, select, and rearrange the output of another utility.  It uses
features that haven't been covered yet, so don't worry if you don't
understand all the details:

@example
ls -l | awk '$6 == "Nov" @{ sum += $5 @}
             END @{ print sum @}'
@end example

@cindex @command{ls} utility
This command prints the total number of bytes in all the files in the
current directory that were last modified in November (of any year).
The @w{@samp{ls -l}} part of this example is a system command that gives
you a listing of the files in a directory, including each file's size and the date
the file was last modified. Its output looks like this:

@example
-rw-r--r--  1 arnold   user   1933 Nov  7 13:05 Makefile
-rw-r--r--  1 arnold   user  10809 Nov  7 13:03 awk.h
-rw-r--r--  1 arnold   user    983 Apr 13 12:14 awk.tab.h
-rw-r--r--  1 arnold   user  31869 Jun 15 12:20 awkgram.y
-rw-r--r--  1 arnold   user  22414 Nov  7 13:03 awk1.c
-rw-r--r--  1 arnold   user  37455 Nov  7 13:03 awk2.c
-rw-r--r--  1 arnold   user  27511 Dec  9 13:07 awk3.c
-rw-r--r--  1 arnold   user   7989 Nov  7 13:03 awk4.c
@end example

@noindent
@cindex line continuations @subentry with C shell
The first field contains read-write permissions, the second field contains
the number of links to the file, and the third field identifies the file's owner.
The fourth field identifies the file's group.
The fifth field contains the file's size in bytes.  The
sixth, seventh, and eighth fields contain the month, day, and time,
respectively, that the file was last modified.  Finally, the ninth field
contains the @value{FN}.

@c @cindex automatic initialization
@cindex initialization, automatic
The @samp{$6 == "Nov"} in our @command{awk} program is an expression that
tests whether the sixth field of the output from @w{@samp{ls -l}}
matches the string @samp{Nov}.  Each time a line has the string
@samp{Nov} for its sixth field, @command{awk} performs the action
@samp{sum += $5}.  This adds the fifth field (the file's size) to the variable
@code{sum}.  As a result, when @command{awk} has finished reading all the
input lines, @code{sum} is the total of the sizes of the files whose
lines matched the pattern.  (This works because @command{awk} variables
are automatically initialized to zero.)

After the last line of output from @command{ls} has been processed, the
@code{END} rule executes and prints the value of @code{sum}.
In this example, the value of @code{sum} is 80600.

These more advanced @command{awk} techniques are covered in later
@value{SECTION}s
(@pxref{Action Overview}).  Before you can move on to more
advanced @command{awk} programming, you have to know how @command{awk} interprets
your input and displays your output.  By manipulating fields and using
@code{print} statements, you can produce some very useful and
impressive-looking reports.

@node Statements/Lines
@section @command{awk} Statements Versus Lines
@cindex line breaks
@cindex newlines

Most often, each line in an @command{awk} program is a separate statement or
separate rule, like this:

@example
awk '/12/  @{ print $0 @}
     /21/  @{ print $0 @}' mail-list inventory-shipped
@end example

@cindex @command{gawk} @subentry newlines in
However, @command{gawk} ignores newlines after any of the following
symbols and keywords:

@example
,    @{    ?    :    ||    &&    do    else
@end example

@noindent
A newline at any other point is considered the end of the
statement.@footnote{The @samp{?} and @samp{:} referred to here is the
three-operand conditional expression described in
@ref{Conditional Exp}.
Splitting lines after @samp{?} and @samp{:} is a minor @command{gawk}
extension; if @option{--posix} is specified
(@pxref{Options}), then this extension is disabled.}

@cindex @code{\} (backslash) @subentry continuing lines and
@cindex backslash (@code{\}) @subentry continuing lines and
If you would like to split a single statement into two lines at a point
where a newline would terminate it, you can @dfn{continue} it by ending the
first line with a backslash character (@samp{\}).  The backslash must be
the final character on the line in order to be recognized as a continuation
character.  A backslash followed by a newline is allowed anywhere in the statement, even
in the middle of a string or regular expression.  For example:

@example
awk '/This regular expression is too long, so continue it\
 on the next line/ @{ print $1 @}'
@end example

@noindent
@cindex portability @subentry backslash continuation and
We have generally not used backslash continuation in our sample programs.
@command{gawk} places no limit on the
length of a line, so backslash continuation is never strictly necessary;
it just makes programs more readable.  For this same reason, as well as
for clarity, we have kept most statements short in the programs
presented throughout the @value{DOCUMENT}.

Backslash continuation is
most useful when your @command{awk} program is in a separate source file
instead of entered from the command line.  You should also note that
many @command{awk} implementations are more particular about where you
may use backslash continuation. For example, they may not allow you to
split a string constant using backslash continuation.  Thus, for maximum
portability of your @command{awk} programs, it is best not to split your
lines in the middle of a regular expression or a string.
@c 10/2000: gawk, mawk, and current bell labs awk allow it,
@c solaris 2.7 nawk does not. Solaris /usr/xpg4/bin/awk does though!  sigh.

@cindex @command{csh} utility
@cindex backslash (@code{\}) @subentry continuing lines and @subentry in @command{csh}
@cindex @code{\} (backslash) @subentry continuing lines and @subentry in @command{csh}
@quotation CAUTION
@emph{Backslash continuation does not work as described
with the C shell.}  It works for @command{awk} programs in files and
for one-shot programs, @emph{provided} you are using a POSIX-compliant
shell, such as the Unix Bourne shell or Bash.  But the C shell behaves
differently!  There you must use two backslashes in a row, followed by
a newline.  Note also that when using the C shell, @emph{every} newline
in your @command{awk} program must be escaped with a backslash. To illustrate:

@example
% @kbd{awk 'BEGIN @{ \}
? @kbd{  print \\}
? @kbd{      "hello, world" \}
? @kbd{@}'}
@print{} hello, world
@end example

@noindent
Here, the @samp{%} and @samp{?} are the C shell's primary and secondary
prompts, analogous to the standard shell's @samp{$} and @samp{>}.

Compare the previous example to how it is done with a POSIX-compliant shell:

@example
$ @kbd{awk 'BEGIN @{}
>   @kbd{print \}
>       @kbd{"hello, world"}
> @kbd{@}'}
@print{} hello, world
@end example
@end quotation

@command{awk} is a line-oriented language.  Each rule's action has to
begin on the same line as the pattern.  To have the pattern and action
on separate lines, you @emph{must} use backslash continuation; there
is no other option.

@cindex backslash (@code{\}) @subentry continuing lines and @subentry comments and
@cindex @code{\} (backslash) @subentry continuing lines and @subentry comments and
@cindex commenting @subentry backslash continuation and
Another thing to keep in mind is that backslash continuation and
comments do not mix. As soon as @command{awk} sees the @samp{#} that
starts a comment, it ignores @emph{everything} on the rest of the
line. For example:

@example
@group
$ @kbd{gawk 'BEGIN @{ print "dont panic" # a friendly \}
> @kbd{                                   BEGIN rule}
> @kbd{@}'}
@error{} gawk: cmd. line:2:                BEGIN rule
@error{} gawk: cmd. line:2:                ^ syntax error
@end group
@end example

@noindent
In this case, it looks like the backslash would continue the comment onto the
next line. However, the backslash-newline combination is never even
noticed because it is ``hidden'' inside the comment. Thus, the
@code{BEGIN} is noted as a syntax error.

@cindex statements @subentry multiple
@cindex @code{;} (semicolon) @subentry separating statements in actions
@cindex semicolon (@code{;}) @subentry separating statements in actions
@cindex @code{;} (semicolon) @subentry separating rules
@cindex semicolon (@code{;}) @subentry separating rules
When @command{awk} statements within one rule are short, you might want to put
more than one of them on a line.  This is accomplished by separating the statements
with a semicolon (@samp{;}).
This also applies to the rules themselves.
Thus, the program shown at the start of this @value{SECTION}
could also be written this way:

@example
/12/ @{ print $0 @} ; /21/ @{ print $0 @}
@end example

@quotation NOTE
The requirement that states that rules on the same line must be
separated with a semicolon was not in the original @command{awk}
language; it was added for consistency with the treatment of statements
within an action.
@end quotation

@node Other Features
@section Other Features of @command{awk}

@cindex variables
The @command{awk} language provides a number of predefined, or
@dfn{built-in}, variables that your programs can use to get information
from @command{awk}.  There are other variables your program can set
as well to control how @command{awk} processes your data.

In addition, @command{awk} provides a number of built-in functions for doing
common computational and string-related operations.
@command{gawk} provides built-in functions for working with timestamps,
performing bit manipulation, for runtime string translation (internationalization),
determining the type of a variable,
and array sorting.

As we develop our presentation of the @command{awk} language, we will introduce
most of the variables and many of the functions. They are described
systematically in @ref{Built-in Variables} and in
@ref{Built-in}.

@node When
@section When to Use @command{awk}

@cindex @command{awk} @subentry uses for
Now that you've seen some of what @command{awk} can do,
you might wonder how @command{awk} could be useful for you.  By using
utility programs, advanced patterns, field separators, arithmetic
statements, and other selection criteria, you can produce much more
complex output.  The @command{awk} language is very useful for producing
reports from large amounts of raw data, such as summarizing information
from the output of other utility programs like @command{ls}.
(@xref{More Complex}.)

Programs written with @command{awk} are usually much smaller than they would
be in other languages.  This makes @command{awk} programs easy to compose and
use.  Often, @command{awk} programs can be quickly composed at your keyboard,
used once, and thrown away.  Because @command{awk} programs are interpreted, you
can avoid the (usually lengthy) compilation part of the typical
edit-compile-test-debug cycle of software development.

@cindex Brian Kernighan's @command{awk}
Complex programs have been written in @command{awk}, including a complete
retargetable assembler for
@ifclear FOR_PRINT
eight-bit microprocessors (@pxref{Glossary}, for more information),
@end ifclear
@ifset FOR_PRINT
eight-bit microprocessors,
@end ifset
and a microcode assembler for a special-purpose Prolog
computer.
The original @command{awk}'s capabilities were strained by tasks
of such complexity, but modern versions are more capable.

@cindex @command{awk} programs @subentry complex
If you find yourself writing @command{awk} scripts of more than, say,
a few hundred lines, you might consider using a different programming
language.  The shell is good at string and pattern matching; in addition,
it allows powerful use of the system utilities.  Python offers a nice
balance between high-level ease of programming and access to system
facilities.@footnote{Other popular scripting languages include Ruby
and Perl.}

@node Intro Summary
@section Summary

@c FIXME: Review this chapter for summary of builtin functions called.
@itemize @value{BULLET}
@item
Programs in @command{awk} consist of @var{pattern}--@var{action} pairs.

@item
An @var{action} without a @var{pattern} always runs.  The default
@var{action} for a pattern without one is @samp{@{ print $0 @}}.

@item
Use either
@samp{awk '@var{program}' @var{files}}
or
@samp{awk -f @var{program-file} @var{files}}
to run @command{awk}.

@item
You may use the special @samp{#!} header line to create @command{awk}
programs that are directly executable.

@item
Comments in @command{awk} programs start with @samp{#} and continue to
the end of the same line.

@item
Be aware of quoting issues when writing @command{awk} programs as
part of a larger shell script (or MS-Windows batch file).

@item
You may use backslash continuation to continue a source line.
Lines are automatically continued after
a comma, open brace, question mark, colon,
@samp{||}, @samp{&&}, @code{do}, and @code{else}.
@end itemize

@node Invoking Gawk
@chapter Running @command{awk} and @command{gawk}

This @value{CHAPTER} covers how to run @command{awk}, both POSIX-standard
and @command{gawk}-specific command-line options, and what
@command{awk} and
@command{gawk} do with nonoption arguments.
It then proceeds to cover how @command{gawk} searches for source files,
reading standard input along with other files, @command{gawk}'s
environment variables, @command{gawk}'s exit status, using include files,
and obsolete and undocumented options and/or features.

Many of the options and features described here are discussed in
more detail later in the @value{DOCUMENT}; feel free to skip over
things in this @value{CHAPTER} that don't interest you right now.

@menu
* Command Line::                How to run @command{awk}.
* Options::                     Command-line options and their meanings.
* Other Arguments::             Input file names and variable assignments.
* Naming Standard Input::       How to specify standard input with other
                                files.
* Environment Variables::       The environment variables @command{gawk} uses.
* Exit Status::                 @command{gawk}'s exit status.
* Include Files::               Including other files into your program.
* Loading Shared Libraries::    Loading shared libraries into your program.
* Obsolete::                    Obsolete Options and/or features.
* Undocumented::                Undocumented Options and Features.
* Invoking Summary::            Invocation summary.
@end menu

@node Command Line
@section Invoking @command{awk}
@cindex command line @subentry invoking @command{awk} from
@cindex @command{awk} @subentry invoking
@cindex arguments @subentry command-line @subentry invoking @command{awk}
@cindex options @subentry command-line @subentry invoking @command{awk}

There are two ways to run @command{awk}---with an explicit program or with
one or more program files.  Here are templates for both of them; items
enclosed in [@dots{}] in these templates are optional:

@display
@command{awk} [@var{options}] @option{-f} @var{progfile} [@option{--}] @var{file} @dots{}
@command{awk} [@var{options}] [@option{--}] @code{'@var{program}'} @var{file} @dots{}
@end display

@cindex GNU long options
@cindex long options
@cindex options @subentry long
In addition to traditional one-letter POSIX-style options, @command{gawk} also
supports GNU long options.

@cindex dark corner @subentry invoking @command{awk}
@cindex lint checking @subentry empty programs
It is possible to invoke @command{awk} with an empty program:

@example
awk '' datafile1 datafile2
@end example

@cindex @option{--lint} option
@cindex dark corner @subentry empty programs
@noindent
Doing so makes little sense, though; @command{awk} exits
silently when given an empty program.
@value{DARKCORNER}
If @option{--lint} has
been specified on the command line, @command{gawk} issues a
warning that the program is empty.

@node Options
@section Command-Line Options
@cindex options @subentry command-line
@cindex command line @subentry options
@cindex GNU long options
@cindex options @subentry long

Options begin with a dash and consist of a single character.
GNU-style long options consist of two dashes and a keyword.
The keyword can be abbreviated, as long as the abbreviation allows the option
to be uniquely identified.  If the option takes an argument, either the
keyword is immediately followed by an equals sign (@samp{=}) and the
argument's value, or the keyword and the argument's value are separated
by whitespace (spaces or TABs).
If a particular option with a value is given more than once, it is the
last value that counts.

@cindex POSIX @command{awk} @subentry GNU long options and
Each long option for @command{gawk} has a corresponding
POSIX-style short option.
The long and short options are
interchangeable in all contexts.
The following list describes options mandated by the POSIX standard:

@table @code
@item -F @var{fs}
@itemx --field-separator @var{fs}
@cindex @option{-F} option
@cindex @option{--field-separator} option
@cindex @code{FS} variable @subentry @code{--field-separator} option and
Set the @code{FS} variable to @var{fs}
(@pxref{Field Separators}).

@item -f @var{source-file}
@itemx --file @var{source-file}
@cindex @option{-f} option
@cindex @option{--file} option
@cindex @command{awk} programs @subentry location of
Read the @command{awk} program source from @var{source-file}
instead of in the first nonoption argument.
This option may be given multiple times; the @command{awk}
program consists of the concatenation of the contents of
each specified @var{source-file}.

Files named with @option{-f} are treated as if they had @samp{@@namespace "awk"}
at their beginning. @xref{Changing The Namespace}, for more information
on this advanced feature.

@item -v @var{var}=@var{val}
@itemx --assign @var{var}=@var{val}
@cindex @option{-v} option
@cindex @option{--assign} option
@cindex variables @subentry setting
Set the variable @var{var} to the value @var{val} @emph{before}
execution of the program begins.  Such variable values are available
inside the @code{BEGIN} rule
(@pxref{Other Arguments}).

The @option{-v} option can only set one variable, but it can be used
more than once, setting another variable each time, like this:
@samp{awk @w{-v foo=1} @w{-v bar=2} @dots{}}.

@cindex predefined variables @subentry @code{-v} option, setting with
@cindex variables @subentry predefined @subentry @code{-v} option, setting with
@quotation CAUTION
Using @option{-v} to set the values of the built-in
variables may lead to surprising results.  @command{awk} will reset the
values of those variables as it needs to, possibly ignoring any
initial value you may have given.
@end quotation

@item -W @var{gawk-opt}
@cindex @option{-W} option
Provide an implementation-specific option.
This is the POSIX convention for providing implementation-specific options.
These options
also have corresponding GNU-style long options.
Note that the long options may be abbreviated, as long as
the abbreviations remain unique.
The full list of @command{gawk}-specific options is provided next.

@item --
@cindex command line @subentry options @subentry end of
@cindex options @subentry command-line @subentry end of
Signal the end of the command-line options.  The following arguments
are not treated as options even if they begin with @samp{-}.  This
interpretation of @option{--} follows the POSIX argument parsing
conventions.

@cindex @code{-} (hyphen) @subentry file names beginning with
@cindex hyphen (@code{-}) @subentry file names beginning with
This is useful if you have @value{FN}s that start with @samp{-},
or in shell scripts, if you have @value{FN}s that will be specified
by the user that could start with @samp{-}.
It is also useful for passing options on to the @command{awk}
program; see @ref{Getopt Function}.
@end table

The following list describes @command{gawk}-specific options:

@c Have to use @asis here to get docbook to come out right.
@table @asis
@item @option{-b}
@itemx @option{--characters-as-bytes}
@cindex @option{-b} option
@cindex @option{--characters-as-bytes} option
Cause @command{gawk} to treat all input data as single-byte characters.
In addition, all output written with @code{print} or @code{printf}
is treated as single-byte characters.

Normally, @command{gawk} follows the POSIX standard and attempts to process
its input data according to the current locale (@pxref{Locales}). This can often involve
converting multibyte characters into wide characters (internally), and
can lead to problems or confusion if the input data does not contain valid
multibyte characters. This option is an easy way to tell @command{gawk},
``Hands off my data!''

@item @option{-c}
@itemx @option{--traditional}
@cindex @option{-c} option
@cindex @option{--traditional} option
@cindex compatibility mode (@command{gawk}) @subentry specifying
Specify @dfn{compatibility mode}, in which the GNU extensions to
the @command{awk} language are disabled, so that @command{gawk} behaves just
like BWK @command{awk}.
@xref{POSIX/GNU},
which summarizes the extensions.
@ifclear FOR_PRINT
Also see
@ref{Compatibility Mode}.
@end ifclear

@item @option{-C}
@itemx @option{--copyright}
@cindex @option{-C} option
@cindex @option{--copyright} option
@cindex GPL (General Public License) @subentry printing
Print the short version of the General Public License and then exit.

@item @option{-d}[@var{file}]
@itemx @option{--dump-variables}[@code{=}@var{file}]
@cindex @option{-d} option
@cindex @option{--dump-variables} option
@cindex dump all variables of a program
@cindex @file{awkvars.out} file
@cindex files @subentry @file{awkvars.out}
@cindex variables @subentry global @subentry printing list of
Print a sorted list of global variables, their types, and final values
to @var{file}.  If no @var{file} is provided, print this
list to a file named @file{awkvars.out} in the current directory.
No space is allowed between the @option{-d} and @var{file}, if
@var{file} is supplied.

@cindex troubleshooting @subentry typographical errors, global variables
Having a list of all global variables is a good way to look for
typographical errors in your programs.
You would also use this option if you have a large program with a lot of
functions, and you want to be sure that your functions don't
inadvertently use global variables that you meant to be local.
(This is a particularly easy mistake to make with simple variable
names like @code{i}, @code{j}, etc.)

@item @option{-D}[@var{file}]
@itemx @option{--debug}[@code{=}@var{file}]
@cindex @option{-D} option
@cindex @option{--debug} option
@cindex @command{awk} programs @subentry debugging, enabling
Enable debugging of @command{awk} programs
(@pxref{Debugging}).
By default, the debugger reads commands interactively from the keyboard
(standard input).
The optional @var{file} argument allows you to specify a file with a list
of commands for the debugger to execute noninteractively.
No space is allowed between the @option{-D} and @var{file}, if
@var{file} is supplied.

@item @option{-e} @var{program-text}
@itemx @option{--source} @var{program-text}
@cindex @option{-e} option
@cindex @option{--source} option
@cindex source code @subentry mixing
Provide program source code in the @var{program-text}.
This option allows you to mix source code in files with source
code that you enter on the command line.
This is particularly useful
when you have library functions that you want to use from your command-line
programs (@pxref{AWKPATH Variable}).

Note that @command{gawk} treats each string as if it ended with
a newline character (even if it doesn't). This makes building
the total program easier.

@quotation CAUTION
Prior to @value{PVERSION} 5.0, there was
no requirement that each @var{program-text}
be a full syntactic unit. I.e., the following worked:

@example
$ @kbd{gawk -e 'BEGIN @{ a = 5 ;' -e 'print a @}'}
@print{} 5
@end example

@noindent
However, this is no longer true. If you have any scripts that
rely upon this feature, you should revise them.

This is because each @var{program-text} is treated as if it had
@samp{@@namespace "awk"} at its beginning. @xref{Changing The Namespace},
for more information.
@end quotation

@item @option{-E} @var{file}
@itemx @option{--exec} @var{file}
@cindex @option{-E} option
@cindex @option{--exec} option
@cindex @command{awk} programs @subentry location of
@cindex CGI, @command{awk} scripts for
Similar to @option{-f}, read @command{awk} program text from @var{file}.
There are two differences from @option{-f}:

@itemize @value{BULLET}
@item
This option terminates option processing; anything
else on the command line is passed on directly to the @command{awk} program.

@item
Command-line variable assignments of the form
@samp{@var{var}=@var{value}} are disallowed.
@end itemize

This option is particularly necessary for World Wide Web CGI applications
that pass arguments through the URL; using this option prevents a malicious
(or other) user from passing in options, assignments, or @command{awk} source
code (via @option{-e}) to the CGI application.@footnote{For more detail,
please see Section 4.4 of @uref{http://www.ietf.org/rfc/rfc3875,
RFC 3875}. Also see the
@uref{https://lists.gnu.org/archive/html/bug-gawk/2014-11/msg00022.html,
explanatory note sent to the @command{gawk} bug
mailing list}.}
This option should be used
with @samp{#!} scripts (@pxref{Executable Scripts}), like so:

@example
#! /usr/local/bin/gawk -E

@var{awk program here @dots{}}
@end example

@item @option{-g}
@itemx @option{--gen-pot}
@cindex @option{-g} option
@cindex @option{--gen-pot} option
@cindex portable object @subentry files @subentry generating
@cindex files @subentry portable object @subentry generating
Analyze the source program and
generate a GNU @command{gettext} portable object template file on standard
output for all string constants that have been marked for translation.
@xref{Internationalization},
for information about this option.

@item @option{-h}
@itemx @option{--help}
@cindex @option{-h} option
@cindex @option{--help} option
@cindex GNU long options @subentry printing list of
@cindex options @subentry printing list of
@cindex printing @subentry list of options
Print a ``usage'' message summarizing the short- and long-style options
that @command{gawk} accepts and then exit.

@item @option{-i} @var{source-file}
@itemx @option{--include} @var{source-file}
@cindex @option{-i} option
@cindex @option{--include} option
@cindex @command{awk} programs @subentry location of
Read an @command{awk} source library from @var{source-file}.  This option
is completely equivalent to using the @code{@@include} directive inside
your program.  It is very similar to the @option{-f} option,
but there are two important differences.  First, when @option{-i} is
used, the program source is not loaded if it has been previously
loaded, whereas with @option{-f}, @command{gawk} always loads the file.
Second, because this option is intended to be used with code libraries,
@command{gawk} does not recognize such files as constituting main program
input.  Thus, after processing an @option{-i} argument, @command{gawk}
still expects to find the main source code via the @option{-f} option
or on the command line.

Files named with @option{-i} are treated as if they had @samp{@@namespace "awk"}
at their beginning.  @xref{Changing The Namespace}, for more information.

@item @option{-l} @var{ext}
@itemx @option{--load} @var{ext}
@cindex @option{-l} option
@cindex @option{--load} option
@cindex loading extensions
Load a dynamic extension named @var{ext}. Extensions
are stored as system shared libraries.
This option searches for the library using the @env{AWKLIBPATH}
environment variable.  The correct library suffix for your platform will be
supplied by default, so it need not be specified in the extension name.
The extension initialization routine should be named @code{dl_load()}.
An alternative is to use the @code{@@load} keyword inside the program to load
a shared library.  This advanced feature is described in detail in @ref{Dynamic Extensions}.

@item @option{-L}[@var{value}]
@itemx @option{--lint}[@code{=}@var{value}]
@cindex @option{-l} option
@cindex @option{--lint} option
@cindex lint checking @subentry issuing warnings
@cindex warnings, issuing
Warn about constructs that are dubious or nonportable to
other @command{awk} implementations.
No space is allowed between the @option{-L} and @var{value}, if
@var{value} is supplied.
Some warnings are issued when @command{gawk} first reads your program.  Others
are issued at runtime, as your program executes. The optional
argument may be one of the following:

@table @code
@item fatal
Cause lint warnings become fatal errors.
This may be drastic, but its use will certainly encourage the
development of cleaner @command{awk} programs.

@item invalid
Only issue warnings about things
that are actually invalid are issued. (This is not fully implemented yet.)

@item no-ext
Disable warnings about @command{gawk} extensions.
@end table

Some warnings are only printed once, even if the dubious constructs they
warn about occur multiple times in your @command{awk} program.  Thus,
when eliminating problems pointed out by @option{--lint}, you should take
care to search for all occurrences of each inappropriate construct. As
@command{awk} programs are usually short, doing so is not burdensome.

@item @option{-M}
@itemx @option{--bignum}
@cindex @option{-M} option
@cindex @option{--bignum} option
Select arbitrary-precision arithmetic on numbers. This option has no effect
if @command{gawk} is not compiled to use the GNU MPFR and MP libraries
(@pxref{Arbitrary Precision Arithmetic}).

@item @option{-n}
@itemx @option{--non-decimal-data}
@cindex @option{-n} option
@cindex @option{--non-decimal-data} option
@cindex hexadecimal values, enabling interpretation of
@cindex octal values, enabling interpretation of
@cindex troubleshooting @subentry @code{--non-decimal-data} option
Enable automatic interpretation of octal and hexadecimal
values in input data
(@pxref{Nondecimal Data}).

@quotation CAUTION
This option can severely break old programs.  Use with care.  Also note
that this option may disappear in a future version of @command{gawk}.
@end quotation

@item @option{-N}
@itemx @option{--use-lc-numeric}
@cindex @option{-N} option
@cindex @option{--use-lc-numeric} option
Force the use of the locale's decimal point character
when parsing numeric input data (@pxref{Locales}).

@cindex pretty printing
@item @option{-o}[@var{file}]
@itemx @option{--pretty-print}[@code{=}@var{file}]
@cindex @option{-o} option
@cindex @option{--pretty-print} option
Enable pretty-printing of @command{awk} programs.
Implies @option{--no-optimize}.
By default, the output program is created in a file named @file{awkprof.out}
(@pxref{Profiling}).
The optional @var{file} argument allows you to specify a different
@value{FN} for the output.
No space is allowed between the @option{-o} and @var{file}, if
@var{file} is supplied.

@quotation NOTE
In the past, this option would also execute your program.
This is no longer the case.
@end quotation

@item @option{-O}
@itemx @option{--optimize}
@cindex @option{--optimize} option
@cindex @option{-O} option
Enable @command{gawk}'s default optimizations on the internal
representation of the program.  At the moment, this includes just simple
constant folding.

Optimization is enabled by default.
This option remains primarily for backwards compatibility. However, it may
be used to cancel the effect of an earlier @option{-s} option
(see later in this list).

@item @option{-p}[@var{file}]
@itemx @option{--profile}[@code{=}@var{file}]
@cindex @option{-p} option
@cindex @option{--profile} option
@cindex @command{awk} @subentry profiling, enabling
Enable profiling of @command{awk} programs
(@pxref{Profiling}).
Implies @option{--no-optimize}.
By default, profiles are created in a file named @file{awkprof.out}.
The optional @var{file} argument allows you to specify a different
@value{FN} for the profile file.
No space is allowed between the @option{-p} and @var{file}, if
@var{file} is supplied.

The profile contains execution counts for each statement in the program
in the left margin, and function call counts for each function.

@item @option{-P}
@itemx @option{--posix}
@cindex @option{-P} option
@cindex @option{--posix} option
@cindex POSIX mode
@cindex @command{gawk} @subentry extensions, disabling
Operate in strict POSIX mode.  This disables all @command{gawk}
extensions (just like @option{--traditional}) and
disables all extensions not allowed by POSIX.
@xref{Common Extensions} for a summary of the extensions
in @command{gawk} that are disabled by this option.
Also,
the following additional
restrictions apply:

@itemize @value{BULLET}

@cindex newlines
@cindex whitespace @subentry newlines as
@item
Newlines are not allowed after @samp{?} or @samp{:}
(@pxref{Conditional Exp}).


@cindex @code{FS} variable @subentry TAB character as
@item
Specifying @samp{-Ft} on the command line does not set the value
of @code{FS} to be a single TAB character
(@pxref{Field Separators}).

@cindex locale decimal point character
@cindex decimal point character, locale specific
@item
The locale's decimal point character is used for parsing input
data (@pxref{Locales}).
@end itemize

@c @cindex automatic warnings
@c @cindex warnings, automatic
@cindex @option{--traditional} option @subentry @code{--posix} option and
@cindex @option{--posix} option @subentry @code{--traditional} option and
If you supply both @option{--traditional} and @option{--posix} on the
command line, @option{--posix} takes precedence. @command{gawk}
issues a warning if both options are supplied.

@item @option{-r}
@itemx @option{--re-interval}
@cindex @option{-r} option
@cindex @option{--re-interval} option
@cindex regular expressions @subentry interval expressions and
Allow interval expressions
(@pxref{Regexp Operators})
in regexps.
This is now @command{gawk}'s default behavior.
Nevertheless, this option remains (both for backward compatibility
and for use in combination with @option{--traditional}).

@item @option{-s}
@itemx @option{--no-optimize}
@cindex @option{--no-optimize} option
@cindex @option{-s} option
Disable @command{gawk}'s default optimizations on the internal
representation of the program.

@item @option{-S}
@itemx @option{--sandbox}
@cindex @option{-S} option
@cindex @option{--sandbox} option
@cindex sandbox mode
@cindex @code{ARGV} array
Disable the @code{system()} function,
input redirections with @code{getline},
output redirections with @code{print} and @code{printf},
and dynamic extensions.
Also, disallow adding filenames to @code{ARGV} that were
not there when @command{gawk} started running.
This is particularly useful when you want to run @command{awk} scripts
from questionable sources and need to make sure the scripts
can't access your system (other than the specified input @value{DF}s).

@item @option{-t}
@itemx @option{--lint-old}
@cindex @option{-L} option
@cindex @option{--lint-old} option
Warn about constructs that are not available in the original version of
@command{awk} from Version 7 Unix
(@pxref{V7/SVR3.1}).

@item @option{-V}
@itemx @option{--version}
@cindex @option{-V} option
@cindex @option{--version} option
@cindex @command{gawk} @subentry version of @subentry printing information about
Print version information for this particular copy of @command{gawk}.
This allows you to determine if your copy of @command{gawk} is up to date
with respect to whatever the Free Software Foundation is currently
distributing.
It is also useful for bug reports
(@pxref{Bugs}).

@cindex @code{-} (hyphen) @subentry @code{--} end of options marker
@cindex hyphen (@code{-}) @subentry @code{--} end of options marker
@item @code{--}
Mark the end of all options.
Any command-line arguments following @code{--} are placed in @code{ARGV},
even if they start with a minus sign.
@end table

As long as program text has been supplied,
any other options are flagged as invalid with a warning message but
are otherwise ignored.

@cindex @option{-F} option @subentry @option{-Ft} sets @code{FS} to TAB
In compatibility mode, as a special case, if the value of @var{fs} supplied
to the @option{-F} option is @samp{t}, then @code{FS} is set to the TAB
character (@code{"\t"}).  This is true only for @option{--traditional} and not
for @option{--posix}
(@pxref{Field Separators}).

@cindex @option{-f} option @subentry multiple uses
The @option{-f} option may be used more than once on the command line.
If it is, @command{awk} reads its program source from all of the named files, as
if they had been concatenated together into one big file.  This is
useful for creating libraries of @command{awk} functions.  These functions
can be written once and then retrieved from a standard place, instead
of having to be included in each individual program.
The @option{-i} option is similar in this regard.
(As mentioned in
@ref{Definition Syntax},
function names must be unique.)

With standard @command{awk}, library functions can still be used, even
if the program is entered at the keyboard,
by specifying @samp{-f /dev/tty}.  After typing your program,
type @kbd{Ctrl-d} (the end-of-file character) to terminate it.
(You may also use @samp{-f -} to read program source from the standard
input, but then you will not be able to also use the standard input as a
source of data.)

Because it is clumsy using the standard @command{awk} mechanisms to mix
source file and command-line @command{awk} programs, @command{gawk}
provides the @option{-e} option.  This does not require you to
preempt the standard input for your source code, and it allows you to easily
mix command-line and library source code (@pxref{AWKPATH Variable}).
As with @option{-f}, the @option{-e} and @option{-i}
options may also be used multiple times on the command line.

@cindex @option{-e} option
If no @option{-f} option (or @option{-e} option for @command{gawk})
is specified, then @command{awk} uses the first nonoption command-line
argument as the text of the program source code.  Arguments on
the command line that follow the program text are entered into the
@code{ARGV} array; @command{awk} does @emph{not} continue to parse the
command line looking for options.

@cindex @env{POSIXLY_CORRECT} environment variable
@cindex environment variables @subentry @env{POSIXLY_CORRECT}
@cindex lint checking @subentry @env{POSIXLY_CORRECT} environment variable
@cindex POSIX mode
If the environment variable @env{POSIXLY_CORRECT} exists,
then @command{gawk} behaves in strict POSIX mode, exactly as if
you had supplied @option{--posix}.
Many GNU programs look for this environment variable to suppress
extensions that conflict with POSIX, but @command{gawk} behaves
differently: it suppresses all extensions, even those that do not
conflict with POSIX, and behaves in
strict POSIX mode. If @option{--lint} is supplied on the command line
and @command{gawk} turns on POSIX mode because of @env{POSIXLY_CORRECT},
then it issues a warning message indicating that POSIX
mode is in effect.
You would typically set this variable in your shell's startup file.
For a Bourne-compatible shell (such as Bash), you would add these
lines to the @file{.profile} file in your home directory:

@example
POSIXLY_CORRECT=true
export POSIXLY_CORRECT
@end example

@cindex @command{csh} utility @subentry @env{POSIXLY_CORRECT} environment variable
For a C shell-compatible
shell,@footnote{Not recommended.}
you would add this line to the @file{.login} file in your home directory:

@example
setenv POSIXLY_CORRECT true
@end example

@cindex portability @subentry @env{POSIXLY_CORRECT} environment variable
Having @env{POSIXLY_CORRECT} set is not recommended for daily use,
but it is good for testing the portability of your programs to other
environments.

@node Other Arguments
@section Other Command-Line Arguments
@cindex command line @subentry arguments
@cindex arguments @subentry command-line

Any additional arguments on the command line are normally treated as
input files to be processed in the order specified.   However, an
argument that has the form @code{@var{var}=@var{value}}, assigns
the value @var{value} to the variable @var{var}---it does not specify a
file at all.  (See @ref{Assignment Options}.) In the following example,
@var{count=1} is a variable assignment, not a @value{FN}:

@example
awk -f program.awk file1 count=1 file2
@end example

@noindent
As a side point, should you really need to have @command{awk}
process a file named @file{count=1} (or any file whose name looks like
a variable assignment), precede the file name with @samp{./}, like so:

@example
awk -f program.awk file1 ./count=1 file2
@end example

@cindex @command{gawk} @subentry @code{ARGIND} variable in
@cindex @code{ARGIND} variable @subentry command-line arguments
@cindex @code{ARGV} array, indexing into
@cindex @code{ARGC}/@code{ARGV} variables @subentry command-line arguments
@cindex @command{gawk} @subentry @code{PROCINFO} array in
All the command-line arguments are made available to your @command{awk} program in the
@code{ARGV} array (@pxref{Built-in Variables}).  Command-line options
and the program text (if present) are omitted from @code{ARGV}.
All other arguments, including variable assignments, are
included.   As each element of @code{ARGV} is processed, @command{gawk}
sets @code{ARGIND} to the index in @code{ARGV} of the
current element.  (@command{gawk} makes the full command line,
including program text and options, available in @code{PROCINFO["argv"]};
@pxref{Auto-set}.)

@c FIXME: One day, move the ARGC and ARGV node closer to here.
Changing @code{ARGC} and @code{ARGV} in your @command{awk} program lets
you control how @command{awk} processes the input files; this is described
in more detail in @ref{ARGC and ARGV}.

@cindex input files @subentry variable assignments and
@cindex variable assignments and input files
The distinction between @value{FN} arguments and variable-assignment
arguments is made when @command{awk} is about to open the next input file.
At that point in execution, it checks the @value{FN} to see whether
it is really a variable assignment; if so, @command{awk} sets the variable
instead of reading a file.

Therefore, the variables actually receive the given values after all
previously specified files have been read.  In particular, the values of
variables assigned in this fashion are @emph{not} available inside a
@code{BEGIN} rule
(@pxref{BEGIN/END}),
because such rules are run before @command{awk} begins scanning the argument list.

@cindex dark corner @subentry escape sequences
The variable values given on the command line are processed for escape
sequences (@pxref{Escape Sequences}).
@value{DARKCORNER}

In some very early implementations of @command{awk}, when a variable assignment
occurred before any @value{FN}s, the assignment would happen @emph{before}
the @code{BEGIN} rule was executed.  @command{awk}'s behavior was thus
inconsistent; some command-line assignments were available inside the
@code{BEGIN} rule, while others were not.  Unfortunately,
some applications came to depend
upon this ``feature.''  When @command{awk} was changed to be more consistent,
the @option{-v} option was added to accommodate applications that depended
upon the old behavior.

The variable assignment feature is most useful for assigning to variables
such as @code{RS}, @code{OFS}, and @code{ORS}, which control input and
output formats, before scanning the @value{DF}s.  It is also useful for
controlling state if multiple passes are needed over a @value{DF}.  For
example:

@cindex files @subentry multiple passes over
@example
awk 'pass == 1  @{ @var{pass 1 stuff} @}
     pass == 2  @{ @var{pass 2 stuff} @}' pass=1 mydata pass=2 mydata
@end example

Given the variable assignment feature, the @option{-F} option for setting
the value of @code{FS} is not
strictly necessary.  It remains for historical compatibility.

@node Naming Standard Input
@section Naming Standard Input

Often, you may wish to read standard input together with other files.
For example, you may wish to read one file, read standard input coming
from a pipe, and then read another file.

The way to name the standard input, with all versions of @command{awk},
is to use a single, standalone minus sign or dash, @samp{-}.  For example:

@example
@var{some_command} | awk -f myprog.awk file1 - file2
@end example

@noindent
Here, @command{awk} first reads @file{file1}, then it reads
the output of @var{some_command}, and finally it reads
@file{file2}.

You may also use @code{"-"} to name standard input when reading
files with @code{getline} (@pxref{Getline/File}).
And, you can even use @code{"-"} with the @option{-f} option
to read program source code from standard input (@pxref{Options}).

In addition, @command{gawk} allows you to specify the special
@value{FN} @file{/dev/stdin}, both on the command line and
with @code{getline}.
Some other versions of @command{awk} also support this, but it
is not standard.
(Some operating systems provide a @file{/dev/stdin} file
in the filesystem; however, @command{gawk} always processes
this @value{FN} itself.)

@node Environment Variables
@section The Environment Variables @command{gawk} Uses
@cindex environment variables @subentry used by @command{gawk}

A number of environment variables influence how @command{gawk}
behaves.

@menu
* AWKPATH Variable::            Searching directories for @command{awk}
                                programs.
* AWKLIBPATH Variable::         Searching directories for @command{awk} shared
                                libraries.
* Other Environment Variables:: The environment variables.
@end menu

@node AWKPATH Variable
@subsection The @env{AWKPATH} Environment Variable
@cindex @env{AWKPATH} environment variable
@cindex environment variables @subentry @env{AWKPATH}
@cindex directories @subentry searching @subentry for source files
@cindex search paths @subentry for source files
@cindex differences in @command{awk} and @command{gawk} @subentry @env{AWKPATH} environment variable
@ifinfo
The previous @value{SECTION} described how @command{awk} program files can be named
on the command line with the @option{-f} option.
@end ifinfo
In most @command{awk}
implementations, you must supply a precise pathname for each program
file, unless the file is in the current directory.
But with @command{gawk}, if the @value{FN} supplied to the @option{-f}
or @option{-i} options
does not contain a directory separator @samp{/}, then @command{gawk} searches a list of
directories (called the @dfn{search path}) one by one, looking for a
file with the specified name.

The search path is a string consisting of directory names
separated by colons.@footnote{Semicolons on MS-Windows.}
@command{gawk} gets its search path from the
@env{AWKPATH} environment variable.  If that variable does not exist,
or if it has an empty value,
@command{gawk} uses a default path (described shortly).

The search path feature is particularly helpful for building libraries
of useful @command{awk} functions.  The library files can be placed in a
standard directory in the default path and then specified on
the command line with a short @value{FN}.  Otherwise, you would have to
type the full @value{FN} for each file.

By using the @option{-i} or @option{-f} options, your command-line
@command{awk} programs can use facilities in @command{awk} library files
(@pxref{Library Functions}).
Path searching is not done if @command{gawk} is in compatibility mode.
This is true for both @option{--traditional} and @option{--posix}.
@xref{Options}.

If the source code file is not found after the initial search, the path is searched
again after adding the suffix @samp{.awk} to the @value{FN}.

@command{gawk}'s path search mechanism is similar
to the shell's.
(See @uref{https://www.gnu.org/software/bash/manual/,
@cite{The Bourne-Again SHell manual}}.)
It treats a null entry in the path as indicating the current
directory.
(A null entry is indicated by starting or ending the path with a
colon or by placing two colons next to each other [@samp{::}].)

@quotation NOTE
To include the current directory in the path, either place @file{.}
as an entry in the path or write a null entry in the path.

Different past versions of @command{gawk} would also look explicitly in
the current directory, either before or after the path search.  As of
@value{PVERSION} 4.1.2, this no longer happens; if you wish to look
in the current directory, you must include @file{.} either as a separate
entry or as a null entry in the search path.
@end quotation

The default value for @env{AWKPATH} is
@samp{.:/usr/local/share/awk}.@footnote{Your version of @command{gawk}
may use a different directory; it
will depend upon how @command{gawk} was built and installed. The actual
directory is the value of @code{$(pkgdatadir)} generated when
@command{gawk} was configured.
(For more detail, see the @file{INSTALL} file in the source distribution,
and see @ref{Quick Installation}.
You probably don't need to worry about this,
though.)}  Since @file{.} is included at the beginning, @command{gawk}
searches first in the current directory and then in @file{/usr/local/share/awk}.
In practice, this means that you will rarely need to change the
value of @env{AWKPATH}.

@xref{Shell Startup Files}, for information on functions that help to
manipulate the @env{AWKPATH} variable.

@command{gawk} places the value of the search path that it used into
@code{ENVIRON["AWKPATH"]}. This provides access to the actual search
path value from within an @command{awk} program.

Although you can change @code{ENVIRON["AWKPATH"]} within your @command{awk}
program, this has no effect on the running program's behavior.  This makes
sense: the @env{AWKPATH} environment variable is used to find the program
source files.  Once your program is running, all the files have been
found, and @command{gawk} no longer needs to use @env{AWKPATH}.

@node AWKLIBPATH Variable
@subsection The @env{AWKLIBPATH} Environment Variable
@cindex @env{AWKLIBPATH} environment variable
@cindex environment variables @subentry @env{AWKLIBPATH}
@cindex directories @subentry searching @subentry for loadable extensions
@cindex search paths @subentry for loadable extensions
@cindex differences in @command{awk} and @command{gawk} @subentry @code{AWKLIBPATH} environment variable

The @env{AWKLIBPATH} environment variable is similar to the @env{AWKPATH}
variable, but it is used to search for loadable extensions (stored as
system shared libraries) specified with the @option{-l} option rather
than for source files.  If the extension is not found, the path is
searched again after adding the appropriate shared library suffix for
the platform.  For example, on GNU/Linux systems, the suffix @samp{.so}
is used.  The search path specified is also used for extensions loaded
via the @code{@@load} keyword (@pxref{Loading Shared Libraries}).

If @env{AWKLIBPATH} does not exist in the environment, or if it has
an empty value, @command{gawk} uses a default path; this
is typically @samp{/usr/local/lib/gawk}, although it can vary depending
upon how @command{gawk} was built.@footnote{Your version of @command{gawk}
may use a different directory; it
will depend upon how @command{gawk} was built and installed. The actual
directory is the value of @code{$(pkgextensiondir)} generated when
@command{gawk} was configured.
(For more detail, see the @file{INSTALL} file in the source distribution,
and see @ref{Quick Installation}.
You probably don't need to worry about this,
though.)}

@xref{Shell Startup Files}, for information on functions that help to
manipulate the @env{AWKLIBPATH} variable.

@command{gawk} places the value of the search path that it used into
@code{ENVIRON["AWKLIBPATH"]}. This provides access to the actual search
path value from within an @command{awk} program.

Although you can change @code{ENVIRON["AWKLIBPATH"]} within your
@command{awk} program, this has no effect on the running program's
behavior.  This makes sense: the @env{AWKLIBPATH} environment variable
is used to find any requested extensions, and they are loaded before
the program starts to run.  Once your program is running, all the
extensions have been found, and @command{gawk} no longer needs to use
@env{AWKLIBPATH}.

@node Other Environment Variables
@subsection Other Environment Variables

A number of other environment variables affect @command{gawk}'s
behavior, but they are more specialized. Those in the following
list are meant to be used by regular users:

@table @env
@item GAWK_MSEC_SLEEP
Specifies the interval between connection retries,
in milliseconds. On systems that do not support
the @code{usleep()} system call,
the value is rounded up to an integral number of seconds.

@item GAWK_READ_TIMEOUT
Specifies the time, in milliseconds, for @command{gawk} to
wait for input before returning with an error.
@xref{Read Timeout}.

@item GAWK_SOCK_RETRIES
Controls the number of times @command{gawk} attempts to
retry a two-way TCP/IP (socket) connection before giving up.
@xref{TCP/IP Networking}.
Note that when nonfatal I/O is enabled (@pxref{Nonfatal}),
@command{gawk} only tries to open a TCP/IP socket once.

@item POSIXLY_CORRECT
Causes @command{gawk} to switch to POSIX-compatibility
mode, disabling all traditional and GNU extensions.
@xref{Options}.
@end table

The environment variables in the following list are meant
for use by the @command{gawk} developers for testing and tuning.
They are subject to change. The variables are:

@table @env
@item AWKBUFSIZE
This variable only affects @command{gawk} on POSIX-compliant systems.
With a value of @samp{exact}, @command{gawk} uses the size of each input
file as the size of the memory buffer to allocate for I/O. Otherwise,
the value should be a number, and @command{gawk} uses that number as
the size of the buffer to allocate.  (When this variable is not set,
@command{gawk} uses the smaller of the file's size and the ``default''
blocksize, which is usually the filesystem's I/O blocksize.)

@item AWK_HASH
If this variable exists with a value of @samp{gst}, @command{gawk}
switches to using the hash function from GNU Smalltalk for
managing arrays.
This function may be marginally faster than the standard function.

@item AWKREADFUNC
If this variable exists, @command{gawk} switches to reading source
files one line at a time, instead of reading in blocks. This exists
for debugging problems on filesystems on non-POSIX operating systems
where I/O is performed in records, not in blocks.

@item GAWK_MSG_SRC
If this variable exists, @command{gawk} includes the @value{FN}
and line number within the @command{gawk} source code
from which warning and/or fatal messages
are generated.  Its purpose is to help isolate the source of a
message, as there are multiple places that produce the
same warning or error message.

@item GAWK_LOCALE_DIR
Specifies the location of compiled message object files
for @command{gawk} itself. This is passed to the @code{bindtextdomain()}
function when @command{gawk} starts up.

@item GAWK_NO_DFA
If this variable exists, @command{gawk} does not use the DFA regexp matcher
for ``does it match'' kinds of tests. This can cause @command{gawk}
to be slower. Its purpose is to help isolate differences between the
two regexp matchers that @command{gawk} uses internally. (There aren't
supposed to be differences, but occasionally theory and practice don't
coordinate with each other.)

@item GAWK_STACKSIZE
This specifies the amount by which @command{gawk} should grow its
internal evaluation stack, when needed.

@item INT_CHAIN_MAX
This specifies intended maximum number of items @command{gawk} will maintain on a
hash chain for managing arrays indexed by integers.

@item STR_CHAIN_MAX
This specifies intended maximum number of items @command{gawk} will maintain on a
hash chain for managing arrays indexed by strings.

@item TIDYMEM
If this variable exists, @command{gawk} uses the @code{mtrace()} library
calls from the GNU C library to help track down possible memory leaks.
@end table

@node Exit Status
@section @command{gawk}'s Exit Status

@cindex exit status, of @command{gawk}
If the @code{exit} statement is used with a value
(@pxref{Exit Statement}), then @command{gawk} exits with
the numeric value given to it.

Otherwise, if there were no problems during execution,
@command{gawk} exits with the value of the C constant
@code{EXIT_SUCCESS}.  This is usually zero.

If an error occurs, @command{gawk} exits with the value of
the C constant @code{EXIT_FAILURE}.  This is usually one.

If @command{gawk} exits because of a fatal error, the exit
status is two.  On non-POSIX systems, this value may be mapped
to @code{EXIT_FAILURE}.

@node Include Files
@section Including Other Files into Your Program

@c Panos Papadopoulos  contributed the original
@c text for this section.

This @value{SECTION} describes a feature that is specific to @command{gawk}.

@cindex @code{@@} (at-sign) @subentry @code{@@include} directive
@cindex at-sign (@code{@@}) @subentry @code{@@include} directive
@cindex file inclusion, @code{@@include} directive
@cindex including files, @code{@@include} directive
@cindex @code{@@include} directive @sortas{include directive}
The @code{@@include} keyword can be used to read external @command{awk} source
files.  This gives you the ability to split large @command{awk} source files
into smaller, more manageable pieces, and also lets you reuse common @command{awk}
code from various @command{awk} scripts.  In other words, you can group
together @command{awk} functions used to carry out specific tasks
into external files. These files can be used just like function libraries,
using the @code{@@include} keyword in conjunction with the @env{AWKPATH}
environment variable.  Note that source files may also be included
using the @option{-i} option.

Let's see an example.
We'll start with two (trivial) @command{awk} scripts, namely
@file{test1} and @file{test2}. Here is the @file{test1} script:

@example
BEGIN @{
    print "This is script test1."
@}
@end example

@noindent
and here is @file{test2}:

@example
@@include "test1"
BEGIN @{
    print "This is script test2."
@}
@end example

Running @command{gawk} with @file{test2}
produces the following result:

@example
$ @kbd{gawk -f test2}
@print{} This is script test1.
@print{} This is script test2.
@end example

@command{gawk} runs the @file{test2} script, which includes @file{test1}
using the @code{@@include}
keyword.  So, to include external @command{awk} source files, you just
use @code{@@include} followed by the name of the file to be included,
enclosed in double quotes.

@quotation NOTE
Keep in mind that this is a language construct and the @value{FN} cannot
be a string variable, but rather just a literal string constant in double quotes.
@end quotation

The files to be included may be nested; e.g., given a third
script, namely @file{test3}:

@example
@group
@@include "test2"
BEGIN @{
    print "This is script test3."
@}
@end group
@end example

@noindent
Running @command{gawk} with the @file{test3} script produces the
following results:

@example
$ @kbd{gawk -f test3}
@print{} This is script test1.
@print{} This is script test2.
@print{} This is script test3.
@end example

The @value{FN} can, of course, be a pathname. For example:

@example
@@include "../io_funcs"
@end example

@noindent
and:

@example
@@include "/usr/awklib/network"
@end example

@noindent
are both valid. The @env{AWKPATH} environment variable can be of great
value when using @code{@@include}. The same rules for the use
of the @env{AWKPATH} variable in command-line file searches
(@pxref{AWKPATH Variable}) apply to
@code{@@include} also.

This is very helpful in constructing @command{gawk} function libraries.
If you have a large script with useful, general-purpose @command{awk}
functions, you can break it down into library files and put those files
in a special directory.  You can then include those ``libraries,''
either by using the full pathnames of the files, or by setting the @env{AWKPATH}
environment variable accordingly and then using @code{@@include} with
just the file part of the full pathname. Of course,
you can keep library files in more than one directory;
the more complex the working
environment is, the more directories you may need to organize the files
to be included.

Given the ability to specify multiple @option{-f} options, the
@code{@@include} mechanism is not strictly necessary.
However, the @code{@@include} keyword
can help you in constructing self-contained @command{gawk} programs,
thus reducing the need for writing complex and tedious command lines.
In particular, @code{@@include} is very useful for writing CGI scripts
to be run from web pages.

The rules for finding a source file described in @ref{AWKPATH Variable} also
apply to files loaded with @code{@@include}.

Finally, files included with @code{@@include}
are treated as if they had @samp{@@namespace "awk"}
at their beginning.  @xref{Changing The Namespace}, for more information.

@node Loading Shared Libraries
@section Loading Dynamic Extensions into Your Program

This @value{SECTION} describes a feature that is specific to @command{gawk}.

@cindex @code{@@} (at-sign) @subentry @code{@@load} directive
@cindex at-sign (@code{@@}) @subentry @code{@@load} directive
@cindex loading extensions @subentry @code{@@load} directive
@cindex extensions @subentry loadable @subentry loading, @code{@@load} directive
@cindex @code{@@load} directive @sortas{load directive}
The @code{@@load} keyword can be used to read external @command{awk} extensions
(stored as system shared libraries).
This allows you to link in compiled code that may offer superior
performance and/or give you access to extended capabilities not supported
by the @command{awk} language.  The @env{AWKLIBPATH} variable is used to
search for the extension.  Using @code{@@load} is completely equivalent
to using the @option{-l} command-line option.

If the extension is not initially found in @env{AWKLIBPATH}, another
search is conducted after appending the platform's default shared library
suffix to the @value{FN}.  For example, on GNU/Linux systems, the suffix
@samp{.so} is used:

@example
$ @kbd{gawk '@@load "ordchr"; BEGIN @{print chr(65)@}'}
@print{} A
@end example

@noindent
This is equivalent to the following example:

@example
@group
$ @kbd{gawk -lordchr 'BEGIN @{print chr(65)@}'}
@print{} A
@end group
@end example

@noindent
For command-line usage, the @option{-l} option is more convenient,
but @code{@@load} is useful for embedding inside an @command{awk} source file
that requires access to an extension.

@ref{Dynamic Extensions}, describes how to write extensions (in C or C++)
that can be loaded with either @code{@@load} or the @option{-l} option.
It also describes the @code{ordchr} extension.

@node Obsolete
@section Obsolete Options and/or Features

@c update this section for each release!

@cindex options @subentry deprecated
@cindex features @subentry deprecated
@cindex obsolete features
This @value{SECTION} describes features and/or command-line options from
previous releases of @command{gawk} that either are not available in the
current version or are still supported but deprecated (meaning that
they will @emph{not} be in the next release).

The process-related special files @file{/dev/pid}, @file{/dev/ppid},
@file{/dev/pgrpid}, and @file{/dev/user} were deprecated in @command{gawk}
3.1, but still worked.  As of @value{PVERSION} 4.0, they are no longer
interpreted specially by @command{gawk}.  (Use @code{PROCINFO} instead;
see @ref{Auto-set}.)

@ignore
This @value{SECTION}
is thus essentially a place holder,
in case some option becomes obsolete in a future version of @command{gawk}.
@end ignore

@node Undocumented
@section Undocumented Options and Features
@cindex undocumented features
@cindex features @subentry undocumented
@cindex Skywalker, Luke
@cindex Kenobi, Obi-Wan
@cindex jedi knights
@cindex knights, jedi
@quotation
@i{Use the Source, Luke!}
@author Obi-Wan
@end quotation

@cindex shells @subentry sea
This @value{SECTION} intentionally left
blank.

@ignore
@c If these came out in the Info file or TeX document, then they wouldn't
@c be undocumented, would they?

@command{gawk} has one undocumented option:

@table @code
@item -W nostalgia
@itemx --nostalgia
Print the message @samp{awk: bailing out near line 1} and dump core.
This option was inspired by the common behavior of very early versions of
Unix @command{awk} and by a t--shirt.
The message is @emph{not} subject to translation in non-English locales.
@c so there! nyah, nyah.
@end table

Early versions of @command{awk} used to not require any separator (either
a newline or @samp{;}) between the rules in @command{awk} programs.  Thus,
it was common to see one-line programs like:

@example
awk '@{ sum += $1 @} END @{ print sum @}'
@end example

@command{gawk} actually supports this but it is purposely undocumented
because it is bad style.  The correct way to write such a program
is either:

@example
awk '@{ sum += $1 @} ; END @{ print sum @}'
@end example

@noindent
or:

@example
awk '@{ sum += $1 @}
     END @{ print sum @}' data
@end example

@noindent
@xref{Statements/Lines}, for a fuller explanation.

You can insert newlines after the @samp{;} in @code{for} loops.
This seems to have been a long-undocumented feature in Unix @command{awk}.

Similarly, you may use @code{print} or @code{printf} statements in the
@var{init} and @var{increment} parts of a @code{for} loop.  This is another
long-undocumented ``feature'' of Unix @command{awk}.

@command{gawk} lets you use the names of built-in functions that are
@command{gawk} extensions as the names of parameters in user-defined functions.
This is intended to ``future-proof'' old code that happens to use
function names added by @command{gawk} after the code was written.
Standard @command{awk} built-in functions, such as @code{sin()} or
@code{substr()} are @emph{not} shadowed in this way.

You can use a @samp{P} modifier for the @code{printf()} floating-point
format control letters to use the underlying C library's result for
NaN and Infinity values, instead of the special values @command{gawk}
usually produces, as described in @ref{POSIX Floating Point Problems}.
This is mainly useful for the included unit tests.

The @code{typeof()} built-in function
(@pxref{Type Functions})
takes an optional second array argument that, if present, will be cleared
and populated with some information about the internal implementation of
the variable. This can be useful for debugging. At the moment, this
returns a textual version of the flags for scalar variables, and the
array back-end implementation type for arrays. This interface is subject
to change and may not be stable.

When not in POSIX or compatibility mode, if you set @code{LINENO} to a
numeric value using the @option{-v} option, @command{gawk} adds that value
to the real line number for use in error messages.  This is intended for
use within Bash shell scripts, such that the error message will reflect
the line number in the shell script, instead of in the @command{awk}
program. To demonstrate:

@example
$ @kbd{gawk -v LINENO=10 'BEGIN @{ print("hi" @}'}
@error{} gawk: cmd. line:11: BEGIN @{ print("hi" @}
@error{} gawk: cmd. line:11:                    ^ syntax error
@end example

@end ignore

@node Invoking Summary
@section Summary

@itemize @value{BULLET}

@c From Neil R. Ormos
@item
@command{gawk} parses arguments on the command line, left to right, to
determine if they should be treated as options or as non-option arguments.

@item
@command{gawk} recognizes several options which control its operation,
as described in @ref{Options}.  All options begin with @samp{-}.

@item
Any argument that is not recognized as an option is treated as a
non-option argument, even if it begins with @samp{-}.

@itemize @value{MINUS}
@item
However, when an option itself requires an argument, and the option is separated
from that argument on the command line by at least one space, the space 
is ignored, and the argument is considered to be related to the option.  Thus, in
the invocation, @samp{gawk -F x}, the @samp{x} is treated as belonging to the
@option{-F} option, not as a separate non-option argument.
@end itemize

@item
Once @command{gawk} finds a non-option argument, it stops looking for
options. Therefore, all following arguments are also non-option arguments,
even if they resemble recognized options.

@item
If no @option{-e} or @option{-f} options are present, @command{gawk}
expects the program text to be in the first non-option argument.

@item
All non-option arguments, except program text provided in the first
non-option argument, are placed in @code{ARGV} as explained in
@ref{ARGC and ARGV}, and are processed as described in @ref{Other Arguments}.
@c And I wrote:
Adjusting @code{ARGC} and @code{ARGV}
affects how @command{awk} processes input.

@c ----------------------------------------

@item
The three standard options for all versions of @command{awk} are
@option{-f}, @option{-F}, and @option{-v}.  @command{gawk} supplies these
and many others, as well as corresponding GNU-style long options.

@item
Nonoption command-line arguments are usually treated as @value{FN}s,
unless they have the form @samp{@var{var}=@var{value}}, in which case
they are taken as variable assignments to be performed at that point
in processing the input.

@item
You can use a single minus sign (@samp{-}) to refer to standard input
on the command line. @command{gawk} also lets you use the special
@value{FN} @file{/dev/stdin}.

@item
@command{gawk} pays attention to a number of environment variables.
@env{AWKPATH}, @env{AWKLIBPATH}, and @env{POSIXLY_CORRECT} are the
most important ones.

@item
@command{gawk}'s exit status conveys information to the program
that invoked it. Use the @code{exit} statement from within
an @command{awk} program to set the exit status.

@item
@command{gawk} allows you to include other @command{awk} source files into
your program using the @code{@@include} statement and/or the @option{-i}
and @option{-f} command-line options.

@item
@command{gawk} allows you to load additional functions written in C
or C++ using the @code{@@load} statement and/or the @option{-l} option.
(This advanced feature is described later, in @ref{Dynamic Extensions}.)
@end itemize

@node Regexp
@chapter Regular Expressions
@cindex regexp
@cindex regular expressions

A @dfn{regular expression}, or @dfn{regexp}, is a way of describing a
set of strings.
Because regular expressions are such a fundamental part of @command{awk}
programming, their format and use deserve a separate @value{CHAPTER}.

@cindex forward slash (@code{/}) @subentry to enclose regular expressions
@cindex @code{/} (forward slash) @subentry to enclose regular expressions
A regular expression enclosed in slashes (@samp{/})
is an @command{awk} pattern that matches every input record whose text
belongs to that set.
The simplest regular expression is a sequence of letters, numbers, or
both.  Such a regexp matches any string that contains that sequence.
Thus, the regexp @samp{foo} matches any string containing @samp{foo}.
Thus, the pattern @code{/foo/} matches any input record containing
the three adjacent characters @samp{foo} @emph{anywhere} in the record.  Other
kinds of regexps let you specify more complicated classes of strings.

@ifnotinfo
Initially, the examples in this @value{CHAPTER} are simple.
As we explain more about how
regular expressions work, we present more complicated instances.
@end ifnotinfo

@menu
* Regexp Usage::                How to Use Regular Expressions.
* Escape Sequences::            How to write nonprinting characters.
* Regexp Operators::            Regular Expression Operators.
* Bracket Expressions::         What can go between @samp{[...]}.
* Leftmost Longest::            How much text matches.
* Computed Regexps::            Using Dynamic Regexps.
* GNU Regexp Operators::        Operators specific to GNU software.
* Case-sensitivity::            How to do case-insensitive matching.
* Regexp Summary::              Regular expressions summary.
@end menu

@node Regexp Usage
@section How to Use Regular Expressions

@cindex patterns @subentry regexp constants as
@cindex regular expressions @subentry as patterns
A regular expression can be used as a pattern by enclosing it in
slashes.  Then the regular expression is tested against the
entire text of each record.  (Normally, it only needs
to match some part of the text in order to succeed.)  For example, the
following prints the second field of each record where the string
@samp{li} appears anywhere in the record:

@example
$ @kbd{awk '/li/ @{ print $2 @}' mail-list}
@print{} 555-5553
@print{} 555-0542
@print{} 555-6699
@print{} 555-3430
@end example

@cindex regular expressions @subentry operators
@cindex operators @subentry string-matching
@c @cindex operators, @code{~}
@cindex string-matching operators
@cindex @code{~} (tilde), @code{~} operator
@cindex tilde (@code{~}), @code{~} operator
@cindex @code{!} (exclamation point) @subentry @code{!~} operator
@cindex exclamation point (@code{!}) @subentry @code{!~} operator
@c @cindex operators, @code{!~}
@cindex @code{if} statement @subentry use of regexps in
@cindex @code{while} statement @subentry use of regexps in
@cindex @code{do}-@code{while} statement @subentry use of regexps in
@c @cindex statements, @code{if}
@c @cindex statements, @code{while}
@c @cindex statements, @code{do}
Regular expressions can also be used in matching expressions.  These
expressions allow you to specify the string to match against; it need
not be the entire current input record.  The two operators @samp{~}
and @samp{!~} perform regular expression comparisons.  Expressions
using these operators can be used as patterns, or in @code{if},
@code{while}, @code{for}, and @code{do} statements.
(@xref{Statements}.)
For example, the following is true if the expression @var{exp} (taken
as a string) matches @var{regexp}:

@example
@var{exp} ~ /@var{regexp}/
@end example

@noindent
This example matches, or selects, all input records with the uppercase
letter @samp{J} somewhere in the first field:

@example
$ @kbd{awk '$1 ~ /J/' inventory-shipped}
@print{} Jan  13  25  15 115
@print{} Jun  31  42  75 492
@print{} Jul  24  34  67 436
@print{} Jan  21  36  64 620
@end example

So does this:

@example
awk '@{ if ($1 ~ /J/) print @}' inventory-shipped
@end example

This next example is true if the expression @var{exp}
(taken as a character string)
does @emph{not} match @var{regexp}:

@example
@var{exp} !~ /@var{regexp}/
@end example

The following example matches,
or selects, all input records whose first field @emph{does not} contain
the uppercase letter @samp{J}:

@example
$ @kbd{awk '$1 !~ /J/' inventory-shipped}
@print{} Feb  15  32  24 226
@print{} Mar  15  24  34 228
@print{} Apr  31  52  63 420
@print{} May  16  34  29 208
@dots{}
@end example

@cindex regexp constants
@cindex constants @subentry regexp
@cindex regular expressions, constants @seeentry{regexp constants}
When a regexp is enclosed in slashes, such as @code{/foo/}, we call it
a @dfn{regexp constant}, much like @code{5.27} is a numeric constant and
@code{"foo"} is a string constant.

@node Escape Sequences
@section Escape Sequences

@cindex escape sequences
@cindex escape sequences @seealso{backslash}
@cindex backslash (@code{\}) @subentry in escape sequences
@cindex @code{\} (backslash) @subentry in escape sequences
Some characters cannot be included literally in string constants
(@code{"foo"}) or regexp constants (@code{/foo/}).
Instead, they should be represented with @dfn{escape sequences},
which are character sequences beginning with a backslash (@samp{\}).
One use of an escape sequence is to include a double-quote character in
a string constant.  Because a plain double quote ends the string, you
must use @samp{\"} to represent an actual double-quote character as a
part of the string.  For example:

@example
$ @kbd{awk 'BEGIN @{ print "He said \"hi!\" to her." @}'}
@print{} He said "hi!" to her.
@end example

The  backslash character itself is another character that cannot be
included normally; you must write @samp{\\} to put one backslash in the
string or regexp.  Thus, the string whose contents are the two characters
@samp{"} and @samp{\} must be written @code{"\"\\"}.

Other escape sequences represent unprintable characters
such as TAB or newline.  There is nothing to stop you from entering most
unprintable characters directly in a string constant or regexp constant,
but they may look ugly.

The following list presents
all the escape sequences used in @command{awk} and
what they represent. Unless noted otherwise, all these escape
sequences apply to both string constants and regexp constants:

@cindex ASCII
@table @code
@item \\
A literal backslash, @samp{\}.

@c @cindex @command{awk} language, V.4 version
@cindex @code{\} (backslash) @subentry @code{\a} escape sequence
@cindex backslash (@code{\}) @subentry @code{\a} escape sequence
@item \a
The ``alert'' character, @kbd{Ctrl-g}, ASCII code 7 (BEL).
(This often makes some sort of audible noise.)

@cindex @code{\} (backslash) @subentry @code{\b} escape sequence
@cindex backslash (@code{\}) @subentry @code{\b} escape sequence
@item \b
Backspace, @kbd{Ctrl-h}, ASCII code 8 (BS).

@cindex @code{\} (backslash) @subentry @code{\f} escape sequence
@cindex backslash (@code{\}) @subentry @code{\f} escape sequence
@item \f
Formfeed, @kbd{Ctrl-l}, ASCII code 12 (FF).

@cindex @code{\} (backslash) @subentry @code{\n} escape sequence
@cindex backslash (@code{\}) @subentry @code{\n} escape sequence
@item \n
Newline, @kbd{Ctrl-j}, ASCII code 10 (LF).

@cindex @code{\} (backslash) @subentry @code{\r} escape sequence
@cindex backslash (@code{\}) @subentry @code{\r} escape sequence
@item \r
Carriage return, @kbd{Ctrl-m}, ASCII code 13 (CR).

@cindex @code{\} (backslash) @subentry @code{\t} escape sequence
@cindex backslash (@code{\}) @subentry @code{\t} escape sequence
@item \t
Horizontal TAB, @kbd{Ctrl-i}, ASCII code 9 (HT).

@c @cindex @command{awk} language, V.4 version
@cindex @code{\} (backslash) @subentry @code{\v} escape sequence
@cindex backslash (@code{\}) @subentry @code{\v} escape sequence
@item \v
Vertical TAB, @kbd{Ctrl-k}, ASCII code 11 (VT).

@cindex @code{\} (backslash) @subentry @code{\}@var{nnn} escape sequence
@cindex backslash (@code{\}) @subentry @code{\}@var{nnn} escape sequence
@item \@var{nnn}
The octal value @var{nnn}, where @var{nnn} stands for 1 to 3 digits
between @samp{0} and @samp{7}.  For example, the code for the ASCII ESC
(escape) character is @samp{\033}.

@c @cindex @command{awk} language, V.4 version
@c @cindex @command{awk} language, POSIX version
@cindex @code{\} (backslash) @subentry @code{\x} escape sequence
@cindex backslash (@code{\}) @subentry @code{\x} escape sequence
@cindex common extensions @subentry @code{\x} escape sequence
@cindex extensions @subentry common @subentry @code{\x} escape sequence
@item \x@var{hh}@dots{}
The hexadecimal value @var{hh}, where @var{hh} stands for a sequence
of hexadecimal digits (@samp{0}--@samp{9}, and either @samp{A}--@samp{F}
or @samp{a}--@samp{f}).  A maximum of two digts are allowed after
the @samp{\x}. Any further hexadecimal digits are treated as simple
letters or numbers.  @value{COMMONEXT}
(The @samp{\x} escape sequence is not allowed in POSIX awk.)

@quotation CAUTION
In ISO C, the escape sequence continues until the first nonhexadecimal
digit is seen.
For many years, @command{gawk} would continue incorporating
hexadecimal digits into the value until a non-hexadecimal digit
or the end of the string was encountered.
However, using more than two hexadecimal digits produced
undefined results.
As of @value{PVERSION} 4.2, only two digits
are processed.
@end quotation

@cindex @code{\} (backslash) @subentry @code{\/} escape sequence
@cindex backslash (@code{\}) @subentry @code{\/} escape sequence
@item \/
A literal slash (should be used for regexp constants only).
This sequence is used when you want to write a regexp
constant that contains a slash
(such as @code{/.*:\/home\/[[:alnum:]]+:.*/}; the @samp{[[:alnum:]]}
notation is discussed in @ref{Bracket Expressions}).
Because the regexp is delimited by
slashes, you need to escape any slash that is part of the pattern,
in order to tell @command{awk} to keep processing the rest of the regexp.

@cindex @code{\} (backslash) @subentry @code{\"} escape sequence
@cindex backslash (@code{\}) @subentry @code{\"} escape sequence
@item \"
A literal double quote (should be used for string constants only).
This sequence is used when you want to write a string
constant that contains a double quote
(such as @code{"He said \"hi!\" to her."}).
Because the string is delimited by
double quotes, you need to escape any quote that is part of the string,
in order to tell @command{awk} to keep processing the rest of the string.
@end table

In @command{gawk}, a number of additional two-character sequences that begin
with a backslash have special meaning in regexps.
@xref{GNU Regexp Operators}.

In a regexp, a backslash before any character that is not in the previous list
and not listed in
@ref{GNU Regexp Operators}
means that the next character should be taken literally, even if it would
normally be a regexp operator.  For example, @code{/a\+b/} matches the three
characters @samp{a+b}.

@cindex backslash (@code{\}) @subentry in escape sequences
@cindex @code{\} (backslash) @subentry in escape sequences
@cindex portability
For complete portability, do not use a backslash before any character not
shown in the previous list or that is not an operator.

@c 11/2014: Moved so as to not stack sidebars
@cindex sidebar @subentry Backslash Before Regular Characters
@ifdocbook
@docbook
Backslash Before Regular Characters
@end docbook

@cindex portability @subentry backslash in escape sequences
@cindex POSIX @command{awk} @subentry backslashes in string constants
@cindex backslash (@code{\}) @subentry in escape sequences @subentry POSIX and
@cindex @code{\} (backslash) @subentry in escape sequences @subentry POSIX and

@cindex troubleshooting @subentry backslash before nonspecial character
If you place a backslash in a string constant before something that is
not one of the characters previously listed, POSIX @command{awk} purposely
leaves what happens as undefined.  There are two choices:

@c @cindex automatic warnings
@c @cindex warnings, automatic
@cindex Brian Kernighan's @command{awk}
@table @asis
@item Strip the backslash out
This is what BWK @command{awk} and @command{gawk} both do.
For example, @code{"a\qc"} is the same as @code{"aqc"}.
(Because this is such an easy bug both to introduce and to miss,
@command{gawk} warns you about it.)
Consider @samp{FS = @w{"[ \t]+\|[ \t]+"}} to use vertical bars
surrounded by whitespace as the field separator. There should be
two backslashes in the string: @samp{FS = @w{"[ \t]+\\|[ \t]+"}}.)
@c I did this!  This is why I added the warning.

@cindex @command{gawk} @subentry escape sequences
@cindex @command{gawk} @subentry escape sequences @seealso{backslash}
@cindex Unix @command{awk} @subentry backslashes in escape sequences
@cindex @command{mawk} utility
@item Leave the backslash alone
Some other @command{awk} implementations do this.
In such implementations, typing @code{"a\qc"} is the same as typing
@code{"a\\qc"}.
@end table

@docbook

@end docbook
@end ifdocbook

@ifnotdocbook
@cartouche
@center @b{Backslash Before Regular Characters}


@cindex portability @subentry backslash in escape sequences
@cindex POSIX @command{awk} @subentry backslashes in string constants
@cindex backslash (@code{\}) @subentry in escape sequences @subentry POSIX and
@cindex @code{\} (backslash) @subentry in escape sequences @subentry POSIX and

@cindex troubleshooting @subentry backslash before nonspecial character
If you place a backslash in a string constant before something that is
not one of the characters previously listed, POSIX @command{awk} purposely
leaves what happens as undefined.  There are two choices:

@c @cindex automatic warnings
@c @cindex warnings, automatic
@cindex Brian Kernighan's @command{awk}
@table @asis
@item Strip the backslash out
This is what BWK @command{awk} and @command{gawk} both do.
For example, @code{"a\qc"} is the same as @code{"aqc"}.
(Because this is such an easy bug both to introduce and to miss,
@command{gawk} warns you about it.)
Consider @samp{FS = @w{"[ \t]+\|[ \t]+"}} to use vertical bars
surrounded by whitespace as the field separator. There should be
two backslashes in the string: @samp{FS = @w{"[ \t]+\\|[ \t]+"}}.)
@c I did this!  This is why I added the warning.

@cindex @command{gawk} @subentry escape sequences
@cindex @command{gawk} @subentry escape sequences @seealso{backslash}
@cindex Unix @command{awk} @subentry backslashes in escape sequences
@cindex @command{mawk} utility
@item Leave the backslash alone
Some other @command{awk} implementations do this.
In such implementations, typing @code{"a\qc"} is the same as typing
@code{"a\\qc"}.
@end table
@end cartouche
@end ifnotdocbook

To summarize:

@itemize @value{BULLET}
@item
The escape sequences in the preceding list are always processed first,
for both string constants and regexp constants. This happens very early,
as soon as @command{awk} reads your program.

@item
@command{gawk} processes both regexp constants and dynamic regexps
(@pxref{Computed Regexps}),
for the special operators listed in
@ref{GNU Regexp Operators}.

@item
A backslash before any other character means to treat that character
literally.
@end itemize

@cindex sidebar @subentry Escape Sequences for Metacharacters
@ifdocbook
@docbook
Escape Sequences for Metacharacters
@end docbook

@cindex metacharacters @subentry escape sequences for

Suppose you use an octal or hexadecimal
escape to represent a regexp metacharacter.
(See @ref{Regexp Operators}.)
Does @command{awk} treat the character as a literal character or as a regexp
operator?

@cindex dark corner @subentry escape sequences @subentry for metacharacters
Historically, such characters were taken literally.
@value{DARKCORNER}
However, the POSIX standard indicates that they should be treated
as real metacharacters, which is what @command{gawk} does.
In compatibility mode (@pxref{Options}),
@command{gawk} treats the characters represented by octal and hexadecimal
escape sequences literally when used in regexp constants. Thus,
@code{/a\52b/} is equivalent to @code{/a\*b/}.

@docbook

@end docbook
@end ifdocbook

@ifnotdocbook
@cartouche
@center @b{Escape Sequences for Metacharacters}


@cindex metacharacters @subentry escape sequences for

Suppose you use an octal or hexadecimal
escape to represent a regexp metacharacter.
(See @ref{Regexp Operators}.)
Does @command{awk} treat the character as a literal character or as a regexp
operator?

@cindex dark corner @subentry escape sequences @subentry for metacharacters
Historically, such characters were taken literally.
@value{DARKCORNER}
However, the POSIX standard indicates that they should be treated
as real metacharacters, which is what @command{gawk} does.
In compatibility mode (@pxref{Options}),
@command{gawk} treats the characters represented by octal and hexadecimal
escape sequences literally when used in regexp constants. Thus,
@code{/a\52b/} is equivalent to @code{/a\*b/}.
@end cartouche
@end ifnotdocbook

@node Regexp Operators
@section Regular Expression Operators
@cindex regular expressions @subentry operators
@cindex metacharacters @subentry in regular expressions

You can combine regular expressions with special characters,
called @dfn{regular expression operators} or @dfn{metacharacters}, to
increase the power and versatility of regular expressions.

@menu
* Regexp Operator Details::     The actual details.
* Interval Expressions::        Notes on interval expressions.
@end menu

@node Regexp Operator Details
@subsection Regexp Operators in @command{awk}

The escape sequences described
@ifnotinfo
earlier
@end ifnotinfo
in @ref{Escape Sequences}
are valid inside a regexp.  They are introduced by a @samp{\} and
are recognized and converted into corresponding real characters as
the very first step in processing regexps.

Here is a list of metacharacters.  All characters that are not escape
sequences and that are not listed here stand for themselves:

@c Use @asis so the docbook comes out ok. Sigh.
@table @asis
@cindex backslash (@code{\}) @subentry regexp operator
@cindex @code{\} (backslash) @subentry regexp operator
@item @code{\}
This suppresses the special meaning of a character when
matching.  For example, @samp{\$}
matches the character @samp{$}.

@cindex regular expressions @subentry anchors in
@cindex Texinfo @subentry chapter beginnings in files
@cindex @code{^} (caret) @subentry regexp operator
@cindex caret (@code{^}) @subentry regexp operator
@item @code{^}
This matches the beginning of a string.  @samp{^@@chapter}
matches @samp{@@chapter} at the beginning of a string,
for example, and can be used
to identify chapter beginnings in Texinfo source files.
The @samp{^} is known as an @dfn{anchor}, because it anchors the pattern to
match only at the beginning of the string.

It is important to realize that @samp{^} does not match the beginning of
a line (the point right after a @samp{\n} newline character) embedded in a string.
The condition is not true in the following example:

@example
if ("line1\nLINE 2" ~ /^L/) @dots{}
@end example

@cindex @code{$} (dollar sign) @subentry regexp operator
@cindex dollar sign (@code{$}) @subentry regexp operator
@item @code{$}
This is similar to @samp{^}, but it matches only at the end of a string.
For example, @samp{p$}
matches a record that ends with a @samp{p}.  The @samp{$} is an anchor
and does not match the end of a line
(the point right before a @samp{\n} newline character)
embedded in a string.
The condition in the following example is not true:

@example
if ("line1\nLINE 2" ~ /1$/) @dots{}
@end example

@cindex @code{.} (period), regexp operator
@cindex period (@code{.}), regexp operator
@item @code{.} (period)
This matches any single character,
@emph{including} the newline character.  For example, @samp{.P}
matches any single character followed by a @samp{P} in a string.  Using
concatenation, we can make a regular expression such as @samp{U.A}, which
matches any three-character sequence that begins with @samp{U} and ends
with @samp{A}.

@cindex POSIX mode
@cindex POSIX @command{awk} @subentry period (@code{.}), using
In strict POSIX mode (@pxref{Options}),
@samp{.} does not match the @sc{nul}
character, which is a character with all bits equal to zero.
Otherwise, @sc{nul} is just another character. Other versions of @command{awk}
may not be able to match the @sc{nul} character.

@cindex @code{[]} (square brackets), regexp operator
@cindex square brackets (@code{[]}), regexp operator
@cindex bracket expressions
@cindex character sets (in regular expressions) @seeentry{bracket expressions}
@cindex character lists @seeentry{bracket expressions}
@cindex character classes @seeentry{bracket expressions}
@item @code{[}@dots{}@code{]}
This is called a @dfn{bracket expression}.@footnote{In other literature,
you may see a bracket expression referred to as either a
@dfn{character set}, a @dfn{character class}, or a @dfn{character list}.}
It matches any @emph{one} of the characters that are enclosed in
the square brackets.  For example, @samp{[MVX]} matches any one of
the characters @samp{M}, @samp{V}, or @samp{X} in a string.  A full
discussion of what can be inside the square brackets of a bracket expression
is given in
@ref{Bracket Expressions}.

@cindex bracket expressions @subentry complemented
@item @code{[^}@dots{}@code{]}
This is a @dfn{complemented bracket expression}.  The first character after
the @samp{[} @emph{must} be a @samp{^}.  It matches any characters
@emph{except} those in the square brackets.  For example, @samp{[^awk]}
matches any character that is not an @samp{a}, @samp{w},
or @samp{k}.

@cindex @code{|} (vertical bar)
@cindex vertical bar (@code{|})
@item @code{|}
This is the @dfn{alternation operator} and it is used to specify
alternatives.  The @samp{|} has the lowest precedence of all the regular
expression operators.  For example, @samp{^P|[aeiouy]} matches any string
that matches either @samp{^P} or @samp{[aeiouy]}.  This means it matches
any string that starts with @samp{P} or contains (anywhere within it)
a lowercase English vowel.

The alternation applies to the largest possible regexps on either side.

@cindex @code{()} (parentheses) @subentry regexp operator
@cindex parentheses @code{()} @subentry regexp operator
@item @code{(}@dots{}@code{)}
Parentheses are used for grouping in regular expressions, as in
arithmetic.  They can be used to concatenate regular expressions
containing the alternation operator, @samp{|}.  For example,
@samp{@@(samp|code)\@{[^@}]+\@}} matches both @samp{@@code@{foo@}} and
@samp{@@samp@{bar@}}.
(These are Texinfo formatting control sequences. The @samp{+} is
explained further on in this list.)

The left or opening parenthesis is always a metacharacter; to match
one literally, precede it with a backslash. However, the right or
closing parenthesis is only special when paired with a left parenthesis;
an unpaired right parenthesis is (silently) treated as a regular character.

@cindex @code{*} (asterisk) @subentry @code{*} operator @subentry as regexp operator
@cindex asterisk (@code{*}) @subentry @code{*} operator @subentry as regexp operator
@item @code{*}
This symbol means that the preceding regular expression should be
repeated as many times as necessary to find a match.  For example, @samp{ph*}
applies the @samp{*} symbol to the preceding @samp{h} and looks for matches
of one @samp{p} followed by any number of @samp{h}s.  This also matches
just @samp{p} if no @samp{h}s are present.

There are two subtle points to understand about how @samp{*} works.
First, the @samp{*} applies only to the single preceding regular expression
component (e.g., in @samp{ph*}, it applies just to the @samp{h}).
To cause @samp{*} to apply to a larger subexpression, use parentheses:
@samp{(ph)*} matches @samp{ph}, @samp{phph}, @samp{phphph}, and so on.

Second, @samp{*} finds as many repetitions as possible. If the text
to be matched is @samp{phhhhhhhhhhhhhhooey}, @samp{ph*} matches all of
the @samp{h}s.

@cindex @code{+} (plus sign) @subentry regexp operator
@cindex plus sign (@code{+}) @subentry regexp operator
@item @code{+}
This symbol is similar to @samp{*}, except that the preceding expression must be
matched at least once.  This means that @samp{wh+y}
would match @samp{why} and @samp{whhy}, but not @samp{wy}, whereas
@samp{wh*y} would match all three.

@cindex @code{?} (question mark) @subentry regexp operator
@cindex question mark (@code{?}) @subentry regexp operator
@item @code{?}
This symbol is similar to @samp{*}, except that the preceding expression can be
matched either once or not at all.  For example, @samp{fe?d}
matches @samp{fed} and @samp{fd}, but nothing else.

@cindex @code{@{@}} (braces) @subentry regexp operator
@cindex braces (@code{@{@}}) @subentry regexp operator
@cindex interval expressions, regexp operator
@item @code{@{}@var{n}@code{@}}
@itemx @code{@{}@var{n}@code{,@}}
@itemx @code{@{}@var{n}@code{,}@var{m}@code{@}}
One or two numbers inside braces denote an @dfn{interval expression}.
If there is one number in the braces, the preceding regexp is repeated
@var{n} times.
If there are two numbers separated by a comma, the preceding regexp is
repeated @var{n} to @var{m} times.
If there is one number followed by a comma, then the preceding regexp
is repeated at least @var{n} times:

@table @code
@item wh@{3@}y
Matches @samp{whhhy}, but not @samp{why} or @samp{whhhhy}.

@item wh@{3,5@}y
Matches @samp{whhhy}, @samp{whhhhy}, or @samp{whhhhhy} only.

@item wh@{2,@}y
Matches @samp{whhy}, @samp{whhhy}, and so on.
@end table
@end table

@cindex precedence @subentry regexp operators
@cindex regular expressions @subentry operators @subentry precedence of
In regular expressions, the @samp{*}, @samp{+}, and @samp{?} operators,
as well as the braces @samp{@{} and @samp{@}},
have
the highest precedence, followed by concatenation, and finally by @samp{|}.
As in arithmetic, parentheses can change how operators are grouped.

@cindex POSIX @command{awk} @subentry regular expressions and
@cindex @command{gawk} @subentry regular expressions @subentry precedence
In POSIX @command{awk} and @command{gawk}, the @samp{*}, @samp{+}, and
@samp{?} operators stand for themselves when there is nothing in the
regexp that precedes them.  For example, @code{/+/} matches a literal
plus sign.  However, many other versions of @command{awk} treat such a
usage as a syntax error.

@node Interval Expressions
@subsection Some Notes On Interval Expressions

@cindex POSIX @command{awk} @subentry interval expressions in
Interval expressions were not traditionally available in @command{awk}.
They were added as part of the POSIX standard to make @command{awk}
and @command{egrep} consistent with each other.

@cindex @command{gawk} @subentry interval expressions and
Initially, because old programs may use @samp{@{} and @samp{@}} in regexp
constants,
@command{gawk} did @emph{not} match interval expressions
in regexps.

However, beginning with @value{PVERSION} 4.0,
@command{gawk} does match interval expressions by default.
This is because compatibility with POSIX has become more
important to most @command{gawk} users than compatibility with
old programs.

For programs that use @samp{@{} and @samp{@}} in regexp constants,
it is good practice to always escape them with a backslash.  Then the
regexp constants are valid and work the way you want them to, using
any version of @command{awk}.@footnote{Use two backslashes if you're
using a string constant with a regexp operator or function.}

Finally, when @samp{@{} and @samp{@}} appear in regexp constants
in a way that cannot be interpreted as an interval expression
(such as @code{/q@{a@}/}), then they stand for themselves.

As mentioned, interval expressions were not traditionally available
in @command{awk}. In March of 2019, BWK @command{awk} (finally) acquired them.

Nonetheless, because they were not available for
so many decades, @command{gawk} continues to not supply them
when in compatibility mode (@pxref{Options}).

@node Bracket Expressions
@section Using Bracket Expressions
@cindex bracket expressions
@cindex bracket expressions @subentry range expressions
@cindex range expressions (regexps)
@cindex bracket expressions @subentry character lists

As mentioned earlier, a bracket expression matches any character among
those listed between the opening and closing square brackets.

Within a bracket expression, a @dfn{range expression} consists of two
characters separated by a hyphen.  It matches any single character that
sorts between the two characters, based upon the system's native character
set.  For example, @samp{[0-9]} is equivalent to @samp{[0123456789]}.
(See @ref{Ranges and Locales} for an explanation of how the POSIX
standard and @command{gawk} have changed over time.  This is mainly
of historical interest.)

With the increasing popularity of the
@uref{http://www.unicode.org, Unicode character standard},
there is an additional wrinkle to consider. Octal and hexadecimal
escape sequences inside bracket expressions are taken to represent
only single-byte characters (characters whose values fit within
the range 0--256).  To match a range of characters where the endpoints
of the range are larger than 256, enter the multibyte encodings of
the characters directly.

@cindex @code{\} (backslash) @subentry in bracket expressions
@cindex backslash (@code{\}) @subentry in bracket expressions
@cindex @code{^} (caret) @subentry in bracket expressions
@cindex caret (@code{^}) @subentry in bracket expressions
@cindex @code{-} (hyphen) @subentry in bracket expressions
@cindex hyphen (@code{-}) @subentry in bracket expressions
To include one of the characters @samp{\}, @samp{]}, @samp{-}, or @samp{^} in a
bracket expression, put a @samp{\} in front of it.  For example:

@example
[d\]]
@end example

@noindent
matches either @samp{d} or @samp{]}.
Additionally, if you place @samp{]} right after the opening
@samp{[}, the closing bracket is treated as one of the
characters to be matched.

@cindex POSIX @command{awk} @subentry bracket expressions and
@cindex Extended Regular Expressions (EREs)
@cindex EREs (Extended Regular Expressions)
@cindex @command{egrep} utility
The treatment of @samp{\} in bracket expressions
is compatible with other @command{awk}
implementations and is also mandated by POSIX.
The regular expressions in @command{awk} are a superset
of the POSIX specification for Extended Regular Expressions (EREs).
POSIX EREs are based on the regular expressions accepted by the
traditional @command{egrep} utility.

@cindex bracket expressions @subentry character classes
@cindex POSIX @command{awk} @subentry bracket expressions and @subentry character classes
@dfn{Character classes} are a feature introduced in the POSIX standard.
A character class is a special notation for describing
lists of characters that have a specific attribute, but the
actual characters can vary from country to country and/or
from character set to character set.  For example, the notion of what
is an alphabetic character differs between the United States and France.

A character class is only valid in a regexp @emph{inside} the
brackets of a bracket expression.  Character classes consist of @samp{[:},
a keyword denoting the class, and @samp{:]}.
@ref{table-char-classes} lists the character classes defined by the
POSIX standard.

@float Table,table-char-classes
@caption{POSIX character classes}
@multitable @columnfractions .15 .85
@headitem Class @tab Meaning
@item @code{[:alnum:]} @tab Alphanumeric characters
@item @code{[:alpha:]} @tab Alphabetic characters
@item @code{[:blank:]} @tab Space and TAB characters
@item @code{[:cntrl:]} @tab Control characters
@item @code{[:digit:]} @tab Numeric characters
@item @code{[:graph:]} @tab Characters that are both printable and visible
(a space is printable but not visible, whereas an @samp{a} is both)
@item @code{[:lower:]} @tab Lowercase alphabetic characters
@item @code{[:print:]} @tab Printable characters (characters that are not control characters)
@item @code{[:punct:]} @tab Punctuation characters (characters that are not letters, digits,
control characters, or space characters)
@item @code{[:space:]} @tab Space characters (these are: space, TAB, newline, carriage return, formfeed and vertical tab)
@item @code{[:upper:]} @tab Uppercase alphabetic characters
@item @code{[:xdigit:]} @tab Characters that are hexadecimal digits
@end multitable
@end float

For example, before the POSIX standard, you had to write @code{/[A-Za-z0-9]/}
to match alphanumeric characters.  If your
character set had other alphabetic characters in it, this would not
match them.
With the POSIX character classes, you can write
@code{/[[:alnum:]]/} to match the alphabetic
and numeric characters in your character set.

@ignore
From eliz@gnu.org  Fri Feb 15 03:38:41 2019
Date: Fri, 15 Feb 2019 12:38:23 +0200
From: Eli Zaretskii 
To: arnold@skeeve.com
CC: pengyu.ut@gmail.com, bug-gawk@gnu.org
Subject: Re: [bug-gawk] Does gawk character classes follow this?

> From: arnold@skeeve.com
> Date: Fri, 15 Feb 2019 03:01:34 -0700
> Cc: pengyu.ut@gmail.com, bug-gawk@gnu.org
> 
> I get the feeling that there's something really bothering you, but
> I don't understand what.
> 
> Can you clarify, please?

I thought I already did: we cannot be expected to provide a definitive
description of what the named classes stand for, because the answer
depends on various factors out of our control.
@end ignore

@c Thanks to
@c Date: Tue, 01 Jul 2014 07:39:51 +0200
@c From: Hermann Peifer 
@cindex ASCII
Some utilities that match regular expressions provide a nonstandard
@samp{[:ascii:]} character class; @command{awk} does not. However, you
can simulate such a construct using @samp{[\x00-\x7F]}.  This matches
all values numerically between zero and 127, which is the defined
range of the ASCII character set.  Use a complemented character list
(@samp{[^\x00-\x7F]}) to match any single-byte characters that are not
in the ASCII range.

@quotation NOTE
Some older versions of Unix @command{awk}
treat @code{[:blank:]} like @code{[:space:]}, incorrectly matching
more characters than they should.  Caveat Emptor.
@end quotation

@cindex bracket expressions @subentry collating elements
@cindex bracket expressions @subentry non-ASCII
@cindex collating elements
Two additional special sequences can appear in bracket expressions.
These apply to non-ASCII character sets, which can have single symbols
(called @dfn{collating elements}) that are represented with more than one
character. They can also have several characters that are equivalent for
@dfn{collating}, or sorting, purposes.  (For example, in French, a plain ``e''
and a grave-accented ``@`e'' are equivalent.)
These sequences are:

@table @asis
@cindex bracket expressions @subentry collating symbols
@cindex collating symbols
@item Collating symbols
Multicharacter collating elements enclosed between
@samp{[.} and @samp{.]}.  For example, if @samp{ch} is a collating element,
then @samp{[[.ch.]]} is a regexp that matches this collating element, whereas
@samp{[ch]} is a regexp that matches either @samp{c} or @samp{h}.

@cindex bracket expressions @subentry equivalence classes
@item Equivalence classes
Locale-specific names for a list of
characters that are equal. The name is enclosed between
@samp{[=} and @samp{=]}.
For example, the name @samp{e} might be used to represent all of
``e,'' ``@^e,'' ``@`e,'' and ``@'e.'' In this case, @samp{[[=e=]]} is a regexp
that matches any of @samp{e}, @samp{@^e}, @samp{@'e}, or @samp{@`e}.
@end table

These features are very valuable in non-English-speaking locales.

@cindex internationalization @subentry localization @subentry character classes
@cindex @command{gawk} @subentry character classes and
@cindex POSIX @command{awk} @subentry bracket expressions and @subentry character classes
@quotation CAUTION
The library functions that @command{gawk} uses for regular
expression matching currently recognize only POSIX character classes;
they do not recognize collating symbols or equivalence classes.
@end quotation
@c maybe one day ...

Inside a bracket expression, an opening bracket (@samp{[}) that does
not start a character class, collating element or equivalence class is
taken literally. This is also true of @samp{.} and @samp{*}.

@node Leftmost Longest
@section How Much Text Matches?

@cindex regular expressions @subentry leftmost longest match
@c @cindex matching, leftmost longest
Consider the following:

@example
echo aaaabcd | awk '@{ sub(/a+/, ""); print @}'
@end example

This example uses the @code{sub()} function to make a change to the input
record.  (@code{sub()} replaces the first instance of any text matched
by the first argument with the string provided as the second argument;
@pxref{String Functions}.)  Here, the regexp @code{/a+/} indicates ``one
or more @samp{a} characters,'' and the replacement text is @samp{}.

The input contains four @samp{a} characters.
@command{awk} (and POSIX) regular expressions always match
the leftmost, @emph{longest} sequence of input characters that can
match.  Thus, all four @samp{a} characters are
replaced with @samp{} in this example:

@example
$ @kbd{echo aaaabcd | awk '@{ sub(/a+/, ""); print @}'}
@print{} bcd
@end example

For simple match/no-match tests, this is not so important. But when doing
text matching and substitutions with the @code{match()}, @code{sub()}, @code{gsub()},
and @code{gensub()} functions, it is very important.
@ifinfo
@xref{String Functions},
for more information on these functions.
@end ifinfo
Understanding this principle is also important for regexp-based record
and field splitting (@pxref{Records},
and also @pxref{Field Separators}).

@node Computed Regexps
@section Using Dynamic Regexps

@cindex regular expressions @subentry computed
@cindex regular expressions @subentry dynamic
@cindex @code{~} (tilde), @code{~} operator
@cindex tilde (@code{~}), @code{~} operator
@cindex @code{!} (exclamation point) @subentry @code{!~} operator
@cindex exclamation point (@code{!}) @subentry @code{!~} operator
@c @cindex operators, @code{~}
@c @cindex operators, @code{!~}
The righthand side of a @samp{~} or @samp{!~} operator need not be a
regexp constant (i.e., a string of characters between slashes).  It may
be any expression.  The expression is evaluated and converted to a string
if necessary; the contents of the string are then used as the
regexp.  A regexp computed in this way is called a @dfn{dynamic
regexp} or a @dfn{computed regexp}:

@example
BEGIN @{ digits_regexp = "[[:digit:]]+" @}
$0 ~ digits_regexp    @{ print @}
@end example

@noindent
This sets @code{digits_regexp} to a regexp that describes one or more digits,
and tests whether the input record matches this regexp.

@quotation NOTE
When using the @samp{~} and @samp{!~}
operators, be aware that there is a difference between a regexp constant
enclosed in slashes and a string constant enclosed in double quotes.
If you are going to use a string constant, you have to understand that
the string is, in essence, scanned @emph{twice}: the first time when
@command{awk} reads your program, and the second time when it goes to
match the string on the lefthand side of the operator with the pattern
on the right.  This is true of any string-valued expression (such as
@code{digits_regexp}, shown in the previous example), not just string constants.
@end quotation

@cindex regexp constants @subentry slashes vs.@: quotes
@cindex @code{\} (backslash) @subentry in regexp constants
@cindex backslash (@code{\}) @subentry in regexp constants
@cindex @code{"} (double quote) @subentry in regexp constants
@cindex double quote (@code{"}) @subentry in regexp constants
What difference does it make if the string is
scanned twice? The answer has to do with escape sequences, and particularly
with backslashes.  To get a backslash into a regular expression inside a
string, you have to type two backslashes.

For example, @code{/\*/} is a regexp constant for a literal @samp{*}.
Only one backslash is needed.  To do the same thing with a string,
you have to type @code{"\\*"}.  The first backslash escapes the
second one so that the string actually contains the
two characters @samp{\} and @samp{*}.

@cindex troubleshooting @subentry regexp constants vs.@: string constants
@cindex regexp constants @subentry vs.@: string constants
@cindex string @subentry constants @subentry vs.@: regexp constants
Given that you can use both regexp and string constants to describe
regular expressions, which should you use?  The answer is ``regexp
constants,'' for several reasons:

@itemize @value{BULLET}
@item
String constants are more complicated to write and
more difficult to read. Using regexp constants makes your programs
less error-prone.  Not understanding the difference between the two
kinds of constants is a common source of errors.

@item
It is more efficient to use regexp constants. @command{awk} can note
that you have supplied a regexp and store it internally in a form that
makes pattern matching more efficient.  When using a string constant,
@command{awk} must first convert the string into this internal form and
then perform the pattern matching.

@item
Using regexp constants is better form; it shows clearly that you
intend a regexp match.
@end itemize

@cindex sidebar @subentry Using @code{\n} in Bracket Expressions of Dynamic Regexps
@ifdocbook
@docbook
Using @code{\n} in Bracket Expressions of Dynamic Regexps
@end docbook

@cindex regular expressions @subentry dynamic @subentry with embedded newlines
@cindex newlines @subentry in dynamic regexps

Some older versions of @command{awk} do not allow the newline
character to be used inside a bracket expression for a dynamic regexp:

@example
$ @kbd{awk '$0 ~ "[ \t\n]"'}
@error{} awk: newline in character class [
@error{} ]...
@error{}  source line number 1
@error{}  context is
@error{}        $0 ~ "[ >>>  \t\n]" <<<
@end example

@cindex newlines @subentry in regexp constants
But a newline in a regexp constant works with no problem:

@example
$ @kbd{awk '$0 ~ /[ \t\n]/'}
@kbd{here is a sample line}
@print{} here is a sample line
@kbd{Ctrl-d}
@end example

@command{gawk} does not have this problem, and it isn't likely to
occur often in practice, but it's worth noting for future reference.

@docbook

@end docbook
@end ifdocbook

@ifnotdocbook
@cartouche
@center @b{Using @code{\n} in Bracket Expressions of Dynamic Regexps}


@cindex regular expressions @subentry dynamic @subentry with embedded newlines
@cindex newlines @subentry in dynamic regexps

Some older versions of @command{awk} do not allow the newline
character to be used inside a bracket expression for a dynamic regexp:

@example
$ @kbd{awk '$0 ~ "[ \t\n]"'}
@error{} awk: newline in character class [
@error{} ]...
@error{}  source line number 1
@error{}  context is
@error{}        $0 ~ "[ >>>  \t\n]" <<<
@end example

@cindex newlines @subentry in regexp constants
But a newline in a regexp constant works with no problem:

@example
$ @kbd{awk '$0 ~ /[ \t\n]/'}
@kbd{here is a sample line}
@print{} here is a sample line
@kbd{Ctrl-d}
@end example

@command{gawk} does not have this problem, and it isn't likely to
occur often in practice, but it's worth noting for future reference.
@end cartouche
@end ifnotdocbook

@node GNU Regexp Operators
@section @command{gawk}-Specific Regexp Operators

@c This section adapted (long ago) from the regex-0.12 manual

@cindex regular expressions @subentry operators @subentry @command{gawk}
@cindex @command{gawk} @subentry regular expressions @subentry operators
@cindex operators @subentry GNU-specific
@cindex regular expressions @subentry operators @subentry for words
@cindex word, regexp definition of
GNU software that deals with regular expressions provides a number of
additional regexp operators.  These operators are described in this
@value{SECTION} and are specific to @command{gawk};
they are not available in other @command{awk} implementations.
Most of the additional operators deal with word matching.
For our purposes, a @dfn{word} is a sequence of one or more letters, digits,
or underscores (@samp{_}):

@table @code
@c @cindex operators, @code{\s} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\s} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\s} operator (@command{gawk})
@item \s
Matches any space character as defined by the current locale.
Think of it as shorthand for
@w{@samp{[[:space:]]}}.

@c @cindex operators, @code{\S} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\S} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\S} operator (@command{gawk})
@item \S
Matches any character that is not a space, as defined by the current locale.
Think of it as shorthand for
@w{@samp{[^[:space:]]}}.

@c @cindex operators, @code{\w} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\w} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\w} operator (@command{gawk})
@item \w
Matches any word-constituent character---that is, it matches any
letter, digit, or underscore. Think of it as shorthand for
@w{@samp{[[:alnum:]_]}}.

@c @cindex operators, @code{\W} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\W} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\W} operator (@command{gawk})
@item \W
Matches any character that is not word-constituent.
Think of it as shorthand for
@w{@samp{[^[:alnum:]_]}}.

@c @cindex operators, @code{\<} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\<} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\<} operator (@command{gawk})
@item \<
Matches the empty string at the beginning of a word.
For example, @code{/\} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\>} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\>} operator (@command{gawk})
@item \>
Matches the empty string at the end of a word.
For example, @code{/stow\>/} matches @samp{stow} but not @samp{stowaway}.

@c @cindex operators, @code{\y} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\y} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\y} operator (@command{gawk})
@cindex word boundaries, matching
@item \y
Matches the empty string at either the beginning or the
end of a word (i.e., the word boundar@strong{y}).  For example, @samp{\yballs?\y}
matches either @samp{ball} or @samp{balls}, as a separate word.

@c @cindex operators, @code{\B} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\B} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\B} operator (@command{gawk})
@item \B
Matches the empty string that occurs between two
word-constituent characters. For example,
@code{/\Brat\B/} matches @samp{crate}, but it does not match @samp{dirty rat}.
@samp{\B} is essentially the opposite of @samp{\y}.
@end table

@cindex buffers @subentry operators for
@cindex regular expressions @subentry operators @subentry for buffers
@cindex operators @subentry string-matching @subentry for buffers
There are two other operators that work on buffers.  In Emacs, a
@dfn{buffer} is, naturally, an Emacs buffer.
Other GNU programs, including @command{gawk},
consider the entire string to match as the buffer.
The operators are:

@table @code
@item \`
@c @cindex operators, @code{\`} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\`} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\`} operator (@command{gawk})
Matches the empty string at the
beginning of a buffer (string)

@c @cindex operators, @code{\'} (@command{gawk})
@cindex backslash (@code{\}) @subentry @code{\'} operator (@command{gawk})
@cindex @code{\} (backslash) @subentry @code{\'} operator (@command{gawk})
@item \'
Matches the empty string at the
end of a buffer (string)
@end table

@cindex @code{^} (caret) @subentry regexp operator
@cindex caret (@code{^}) @subentry regexp operator
@cindex @code{?} (question mark) @subentry regexp operator
@cindex question mark (@code{?}) @subentry regexp operator
Because @samp{^} and @samp{$} always work in terms of the beginning
and end of strings, these operators don't add any new capabilities
for @command{awk}.  They are provided for compatibility with other
GNU software.

@cindex @command{gawk} @subentry word-boundary operator
@cindex word-boundary operator (@command{gawk})
@cindex operators @subentry word-boundary (@command{gawk})
In other GNU software, the word-boundary operator is @samp{\b}. However,
that conflicts with the @command{awk} language's definition of @samp{\b}
as backspace, so @command{gawk} uses a different letter.
An alternative method would have been to require two backslashes in the
GNU operators, but this was deemed too confusing. The current
method of using @samp{\y} for the GNU @samp{\b} appears to be the
lesser of two evils.

@cindex regular expressions @subentry @command{gawk}, command-line options
@cindex @command{gawk} @subentry command-line options, regular expressions and
The various command-line options
(@pxref{Options})
control how @command{gawk} interprets characters in regexps:

@table @asis
@item No options
In the default case, @command{gawk} provides all the facilities of
POSIX regexps and the
@ifnotinfo
previously described
GNU regexp operators.
@end ifnotinfo
@ifnottex
@ifnotdocbook
GNU regexp operators described
in @ref{Regexp Operators}.
@end ifnotdocbook
@end ifnottex

@item @code{--posix}
Match only POSIX regexps; the GNU operators are not special
(e.g., @samp{\w} matches a literal @samp{w}).  Interval expressions
are allowed.

@cindex Brian Kernighan's @command{awk}
@item @code{--traditional}
Match traditional Unix @command{awk} regexps. The GNU operators
are not special, and interval expressions are not available.
Because BWK @command{awk} supports them,
the POSIX character classes (@samp{[[:alnum:]]}, etc.) are available.
Characters described by octal and hexadecimal escape sequences are
treated literally, even if they represent regexp metacharacters.

@item @code{--re-interval}
Allow interval expressions in regexps, if @option{--traditional}
has been provided.
Otherwise, interval expressions are available by default.
@end table

@node Case-sensitivity
@section Case Sensitivity in Matching

@cindex regular expressions @subentry case sensitivity
@cindex case sensitivity @subentry regexps and
Case is normally significant in regular expressions, both when matching
ordinary characters (i.e., not metacharacters) and inside bracket
expressions.  Thus, a @samp{w} in a regular expression matches only a lowercase
@samp{w} and not an uppercase @samp{W}.

The simplest way to do a case-independent match is to use a bracket
expression---for example, @samp{[Ww]}.  However, this can be cumbersome if
you need to use it often, and it can make the regular expressions harder
to read.  There are two alternatives that you might prefer.

One way to perform a case-insensitive match at a particular point in the
program is to convert the data to a single case, using the
@code{tolower()} or @code{toupper()} built-in string functions (which we
haven't discussed yet;
@pxref{String Functions}).
For example:

@example
tolower($1) ~ /foo/  @{ @dots{} @}
@end example

@noindent
converts the first field to lowercase before matching against it.
This works in any POSIX-compliant @command{awk}.

@cindex @command{gawk} @subentry regular expressions @subentry case sensitivity
@cindex case sensitivity @subentry @command{gawk}
@cindex differences in @command{awk} and @command{gawk} @subentry regular expressions
@cindex @code{~} (tilde), @code{~} operator
@cindex tilde (@code{~}), @code{~} operator
@cindex @code{!} (exclamation point) @subentry @code{!~} operator
@cindex exclamation point (@code{!}) @subentry @code{!~} operator
@cindex @code{IGNORECASE} variable @subentry with @code{~} and @code{!~} operators
@cindex @command{gawk} @subentry @code{IGNORECASE} variable in
@c @cindex variables, @code{IGNORECASE}
Another method, specific to @command{gawk}, is to set the variable
@code{IGNORECASE} to a nonzero value (@pxref{Built-in Variables}).
When @code{IGNORECASE} is not zero, @emph{all} regexp and string
operations ignore case.

Changing the value of @code{IGNORECASE} dynamically controls the
case sensitivity of the program as it runs.  Case is significant by
default because @code{IGNORECASE} (like most variables) is initialized
to zero:

@example
x = "aB"
if (x ~ /ab/) @dots{}   # this test will fail

IGNORECASE = 1
if (x ~ /ab/) @dots{}   # now it will succeed
@end example

In general, you cannot use @code{IGNORECASE} to make certain rules
case insensitive and other rules case sensitive, as there is no
straightforward way
to set @code{IGNORECASE} just for the pattern of
a particular rule.@footnote{Experienced C and C++ programmers will note
that it is possible, using something like
@samp{IGNORECASE = 1 && /foObAr/ @{ @dots{} @}}
and
@samp{IGNORECASE = 0 || /foobar/ @{ @dots{} @}}.
However, this is somewhat obscure and we don't recommend it.}
To do this, use either bracket expressions or @code{tolower()}.  However, one
thing you can do with @code{IGNORECASE} only is dynamically turn
case sensitivity on or off for all the rules at once.

@code{IGNORECASE} can be set on the command line or in a @code{BEGIN} rule
(@pxref{Other Arguments}; also
@pxref{Using BEGIN/END}).
Setting @code{IGNORECASE} from the command line is a way to make
a program case insensitive without having to edit it.

@c @cindex ISO 8859-1
@c @cindex ISO Latin-1
In multibyte locales, the equivalences between upper- and lowercase
characters are tested based on the wide-character values of the locale's
character set.  Prior to @value{PVERSION} 5.0, single-byte characters were
tested based on the ISO-8859-1 (ISO Latin-1) character set.  However, as
of @value{PVERSION} 5.0, single-byte characters are also tested based on
the values of the locale's character set.@footnote{If you don't understand
this, don't worry about it; it just means that @command{gawk} does the
right thing.}

The value of @code{IGNORECASE} has no effect if @command{gawk} is in
compatibility mode (@pxref{Options}).
Case is always significant in compatibility mode.

@node Regexp Summary
@section Summary

@itemize @value{BULLET}
@item
Regular expressions describe sets of strings to be matched.
In @command{awk}, regular expression constants are written enclosed
between slashes: @code{/}@dots{}@code{/}.

@item
Regexp constants may be used standalone in patterns and
in conditional expressions, or as part of matching expressions
using the @samp{~} and @samp{!~} operators.

@item
Escape sequences let you represent nonprintable characters and
also let you represent regexp metacharacters as literal characters
to be matched.

@item
Regexp operators provide grouping, alternation, and repetition.

@item
Bracket expressions give you a shorthand for specifying sets
of characters that can match at a particular point in a regexp.
Within bracket expressions, POSIX character classes let you specify
certain groups of characters in a locale-independent fashion.

@item
Regular expressions match the leftmost longest text in the string being
matched.  This matters for cases where you need to know the extent of
the match, such as for text substitution and when the record separator
is a regexp.

@item
Matching expressions may use dynamic regexps (i.e., string values
treated as regular expressions).

@item
@command{gawk}'s @code{IGNORECASE} variable lets you control the
case sensitivity of regexp matching.  In other @command{awk}
versions, use @code{tolower()} or @code{toupper()}.

@end itemize


@node Reading Files
@chapter Reading Input Files

@cindex reading input files
@cindex input files @subentry reading
@cindex input files
@cindex @code{FILENAME} variable
In the typical @command{awk} program,
@command{awk} reads all input either from the
standard input (by default, this is the keyboard, but often it is a pipe from another
command) or from files whose names you specify on the @command{awk}
command line.  If you specify input files, @command{awk} reads them
in order, processing all the data from one before going on to the next.
The name of the current input file can be found in the predefined variable
@code{FILENAME}
(@pxref{Built-in Variables}).

@cindex records
@cindex fields
The input is read in units called @dfn{records}, and is processed by the
rules of your program one record at a time.
By default, each record is one line.  Each
record is automatically split into chunks called @dfn{fields}.
This makes it more convenient for programs to work on the parts of a record.

@cindex @code{getline} command
On rare occasions, you may need to use the @code{getline} command.
The  @code{getline} command is valuable both because it
can do explicit input from any number of files, and because the files
used with it do not have to be named on the @command{awk} command line
(@pxref{Getline}).

@menu
* Records::                     Controlling how data is split into records.
* Fields::                      An introduction to fields.
* Nonconstant Fields::          Nonconstant Field Numbers.
* Changing Fields::             Changing the Contents of a Field.
* Field Separators::            The field separator and how to change it.
* Constant Size::               Reading constant width data.
* Splitting By Content::        Defining Fields By Content
* Testing field creation::      Checking how @command{gawk} is splitting
                                records.
* Multiple Line::               Reading multiline records.
* Getline::                     Reading files under explicit program control
                                using the @code{getline} function.
* Read Timeout::                Reading input with a timeout.
* Retrying Input::              Retrying input after certain errors.
* Command-line directories::    What happens if you put a directory on the
                                command line.
* Input Summary::               Input summary.
* Input Exercises::             Exercises.
@end menu

@node Records
@section How Input Is Split into Records

@cindex input @subentry splitting into records
@cindex records @subentry splitting input into
@cindex @code{NR} variable
@cindex @code{FNR} variable
@command{awk} divides the input for your program into records and fields.
It keeps track of the number of records that have been read so far from
the current input file.  This value is stored in a predefined variable
called @code{FNR}, which is reset to zero every time a new file is started.
Another predefined variable, @code{NR}, records the total number of input
records read so far from all @value{DF}s.  It starts at zero, but is
never automatically reset to zero.

Normally, records are separated by newline characters.  You can control how
records are separated by assigning values to the built-in variable @code{RS}.
If @code{RS} is any single character, that character separates records.
Otherwise (in @command{gawk}), @code{RS} is treated as a regular expression.
This mechanism is explained in greater detail shortly.

@menu
* awk split records::           How standard @command{awk} splits records.
* gawk split records::          How @command{gawk} splits records.
@end menu

@node awk split records
@subsection Record Splitting with Standard @command{awk}

@cindex separators @subentry for records
@cindex record separators
Records are separated by a character called the @dfn{record separator}.
By default, the record separator is the newline character.
This is why records are, by default, single lines.
To use a different character for the record separator,
simply assign that character to the predefined variable @code{RS}.

@cindex record separators @subentry newlines as
@cindex newlines @subentry as record separators
@cindex @code{RS} variable
Like any other variable,
the value of @code{RS} can be changed in the @command{awk} program
with the assignment operator, @samp{=}
(@pxref{Assignment Ops}).
The new record-separator character should be enclosed in quotation marks,
which indicate a string constant.  Often, the right time to do this is
at the beginning of execution, before any input is processed,
so that the very first record is read with the proper separator.
To do this, use the special @code{BEGIN} pattern
(@pxref{BEGIN/END}).
For example:

@example
awk 'BEGIN @{ RS = "u" @}
     @{ print $0 @}' mail-list
@end example

@noindent
changes the value of @code{RS} to @samp{u}, before reading any input.
The new value is a string whose first character is the letter ``u''; as a result, records
are separated by the letter ``u''.  Then the input file is read, and the second
rule in the @command{awk} program (the action with no pattern) prints each
record.  Because each @code{print} statement adds a newline at the end of
its output, this @command{awk} program copies the input
with each @samp{u} changed to a newline.  Here are the results of running
the program on @file{mail-list}:

@example
@group
$ @kbd{awk 'BEGIN @{ RS = "u" @}}
>      @kbd{@{ print $0 @}' mail-list}
@end group
@print{} Amelia       555-5553     amelia.zodiac
@print{} sq
@print{} e@@gmail.com    F
@print{} Anthony      555-3412     anthony.assert
@print{} ro@@hotmail.com   A
@print{} Becky        555-7685     becky.algebrar
@print{} m@@gmail.com      A
@print{} Bill         555-1675     bill.drowning@@hotmail.com       A
@print{} Broderick    555-0542     broderick.aliq
@print{} otiens@@yahoo.com R
@print{} Camilla      555-2912     camilla.inf
@print{} sar
@print{} m@@skynet.be     R
@print{} Fabi
@print{} s       555-1234     fabi
@print{} s.
@print{} ndevicesim
@print{} s@@
@print{} cb.ed
@print{}     F
@print{} J
@print{} lie        555-6699     j
@print{} lie.perscr
@print{} tabor@@skeeve.com   F
@print{} Martin       555-6480     martin.codicib
@print{} s@@hotmail.com    A
@print{} Sam
@print{} el       555-3430     sam
@print{} el.lanceolis@@sh
@print{} .ed
@print{}         A
@print{} Jean-Pa
@print{} l    555-2127     jeanpa
@print{} l.campanor
@print{} m@@ny
@print{} .ed
@print{}      R
@print{}
@end example

@noindent
Note that the entry for the name @samp{Bill} is not split.
In the original @value{DF}
(@pxref{Sample Data Files}),
the line looks like this:

@example
Bill         555-1675     bill.drowning@@hotmail.com       A
@end example

@noindent
It contains no @samp{u}, so there is no reason to split the record,
unlike the others, which each have one or more occurrences of the @samp{u}.
In fact, this record is treated as part of the previous record;
the newline separating them in the output
is the original newline in the @value{DF}, not the one added by
@command{awk} when it printed the record!

@cindex record separators @subentry changing
@cindex separators @subentry for records
Another way to change the record separator is on the command line,
using the variable-assignment feature
(@pxref{Other Arguments}):

@example
awk '@{ print $0 @}' RS="u" mail-list
@end example

@noindent
This sets @code{RS} to @samp{u} before processing @file{mail-list}.

Using an alphabetic character such as @samp{u} for the record separator
is highly likely to produce strange results.
Using an unusual character such as @samp{/} is more likely to
produce correct behavior in the majority of cases, but there
are no guarantees. The moral is: Know Your Data.

@command{gawk} allows @code{RS} to be a full regular expression
(discussed shortly; @pxref{gawk split records}).  Even so, using
a regular expression metacharacter, such as @samp{.} as the single
character in the value of @code{RS} has no special effect: it is
treated literally. This is required for backwards compatibility with
both Unix @command{awk} and with POSIX.

When using regular characters as the record separator,
there is one unusual case that occurs when @command{gawk} is
being fully POSIX-compliant (@pxref{Options}).
Then, the following (extreme) pipeline prints a surprising @samp{1}:

@example
$ @kbd{echo | gawk --posix 'BEGIN @{ RS = "a" @} ; @{ print NF @}'}
@print{} 1
@end example

There is one field, consisting of a newline.  The value of the built-in
variable @code{NF} is the number of fields in the current record.
(In the normal case, @command{gawk} treats the newline as whitespace,
printing @samp{0} as the result. Most other versions of @command{awk}
also act this way.)

@cindex dark corner @subentry input files
Reaching the end of an input file terminates the current input record,
even if the last character in the file is not the character in @code{RS}.
@value{DARKCORNER}

@cindex empty strings @seeentry{null strings}
@cindex null strings
@cindex strings @subentry empty @seeentry{null strings}
The empty string @code{""} (a string without any characters)
has a special meaning
as the value of @code{RS}. It means that records are separated
by one or more blank lines and nothing else.
@xref{Multiple Line} for more details.

If you change the value of @code{RS} in the middle of an @command{awk} run,
the new value is used to delimit subsequent records, but the record
currently being processed, as well as records already processed, are not
affected.

@cindex @command{gawk} @subentry @code{RT} variable in
@cindex @code{RT} variable
@cindex records @subentry terminating
@cindex terminating records
@cindex differences in @command{awk} and @command{gawk} @subentry record separators
@cindex differences in @command{awk} and @command{gawk} @subentry @code{RS}/@code{RT} variables
@cindex regular expressions @subentry as record separators
@cindex record separators @subentry regular expressions as
@cindex separators @subentry for records @subentry regular expressions as
After the end of the record has been determined, @command{gawk}
sets the variable @code{RT} to the text in the input that matched
@code{RS}.

@node gawk split records
@subsection Record Splitting with @command{gawk}

@cindex common extensions @subentry @code{RS} as a regexp
@cindex extensions @subentry common @subentry @code{RS} as a regexp
When using @command{gawk}, the value of @code{RS} is not limited to a
one-character string.  If it contains more than one character, it is
treated as a regular expression
(@pxref{Regexp}). @value{COMMONEXT}
In general, each record
ends at the next string that matches the regular expression; the next
record starts at the end of the matching string.  This general rule is
actually at work in the usual case, where @code{RS} contains just a
newline: a record ends at the beginning of the next matching string (the
next newline in the input), and the following record starts just after
the end of this string (at the first character of the following line).
The newline, because it matches @code{RS}, is not part of either record.

When @code{RS} is a single character, @code{RT}
contains the same single character. However, when @code{RS} is a
regular expression, @code{RT} contains
the actual input text that matched the regular expression.

If the input file ends without any text matching @code{RS},
@command{gawk} sets @code{RT} to the null string.

The following example illustrates both of these features.
It sets @code{RS} equal to a regular expression that
matches either a newline or a series of one or more uppercase letters
with optional leading and/or trailing whitespace:

@example
@group
$ @kbd{echo record 1 AAAA record 2 BBBB record 3 |}
> @kbd{gawk 'BEGIN @{ RS = "\n|( *[[:upper:]]+ *)" @}}
>             @kbd{@{ print "Record =", $0,"and RT = [" RT "]" @}'}
@end group
@print{} Record = record 1 and RT = [ AAAA ]
@print{} Record = record 2 and RT = [ BBBB ]
@print{} Record = record 3 and RT = [
@print{} ]
@end example

@noindent
The square brackets delineate the contents of @code{RT}, letting you
see the leading and trailing whitespace. The final value of
@code{RT} is a newline.
@xref{Simple Sed} for a more useful example
of @code{RS} as a regexp and @code{RT}.

If you set @code{RS} to a regular expression that allows optional
trailing text, such as @samp{RS = "abc(XYZ)?"}, it is possible, due
to implementation constraints, that @command{gawk} may match the leading
part of the regular expression, but not the trailing part, particularly
if the input text that could match the trailing part is fairly long.
@command{gawk} attempts to avoid this problem, but currently, there's
no guarantee that this will never happen.

@quotation NOTE
Remember that in @command{awk}, the @samp{^} and @samp{$} anchor
metacharacters match the beginning and end of a @emph{string}, and not
the beginning and end of a @emph{line}.  As a result, something like
@samp{RS = "^[[:upper:]]"} can only match at the beginning of a file.
This is because @command{gawk} views the input file as one long string
that happens to contain newline characters.
It is thus best to avoid anchor metacharacters in the value of @code{RS}.
@end quotation

@cindex @command{gawk} @subentry @code{RT} variable in
@cindex @code{RT} variable
@cindex differences in @command{awk} and @command{gawk} @subentry @code{RS}/@code{RT} variables
The use of @code{RS} as a regular expression and the @code{RT}
variable are @command{gawk} extensions; they are not available in
compatibility mode
(@pxref{Options}).
In compatibility mode, only the first character of the value of
@code{RS} determines the end of the record.

@cindex Brian Kernighan's @command{awk}
@command{mawk} has allowed @code{RS} to be a regexp for decades.
As of October, 2019, BWK @command{awk} also supports it.  Neither
version supplies @code{RT}, however.

@cindex sidebar @subentry @code{RS = "\0"} Is Not Portable
@ifdocbook
@docbook
@code{RS = "\0"} Is Not Portable
@end docbook

@cindex portability @subentry data files as single record
There are times when you might want to treat an entire @value{DF} as a
single record.  The only way to make this happen is to give @code{RS}
a value that you know doesn't occur in the input file.  This is hard
to do in a general way, such that a program always works for arbitrary
input files.

You might think that for text files, the @sc{nul} character, which
consists of a character with all bits equal to zero, is a good
value to use for @code{RS} in this case:

@example
BEGIN @{ RS = "\0" @}  # whole file becomes one record?
@end example

@cindex differences in @command{awk} and @command{gawk} @subentry strings @subentry storing
@command{gawk} in fact accepts this, and uses the @sc{nul}
character for the record separator.
This works for certain special files, such as @file{/proc/environ} on
GNU/Linux systems, where the @sc{nul} character is in fact the record separator.
However, this usage is @emph{not} portable
to most other @command{awk} implementations.

@cindex dark corner @subentry strings, storing
Almost all other @command{awk} implementations@footnote{At least that we know
about.} store strings internally as C-style strings.  C strings use the
@sc{nul} character as the string terminator.  In effect, this means that
@samp{RS = "\0"} is the same as @samp{RS = ""}.
@value{DARKCORNER}

It happens that recent versions of @command{mawk} can use the @sc{nul}
character as a record separator. However, this is a special case:
@command{mawk} does not allow embedded @sc{nul} characters in strings.
(This may change in a future version of @command{mawk}.)

@cindex records @subentry treating files as
@cindex treating files, as single records
@cindex single records, treating files as
@xref{Readfile Function} for an interesting way to read
whole files.  If you are using @command{gawk}, see @ref{Extension Sample
Readfile} for another option.

@docbook

@end docbook
@end ifdocbook

@ifnotdocbook
@cartouche
@center @b{@code{RS = "\0"} Is Not Portable}


@cindex portability @subentry data files as single record
There are times when you might want to treat an entire @value{DF} as a
single record.  The only way to make this happen is to give @code{RS}
a value that you know doesn't occur in the input file.  This is hard
to do in a general way, such that a program always works for arbitrary
input files.

You might think that for text files, the @sc{nul} character, which
consists of a character with all bits equal to zero, is a good
value to use for @code{RS} in this case:

@example
BEGIN @{ RS = "\0" @}  # whole file becomes one record?
@end example

@cindex differences in @command{awk} and @command{gawk} @subentry strings @subentry storing
@command{gawk} in fact accepts this, and uses the @sc{nul}
character for the record separator.
This works for certain special files, such as @file{/proc/environ} on
GNU/Linux systems, where the @sc{nul} character is in fact the record separator.
However, this usage is @emph{not} portable
to most other @command{awk} implementations.

@cindex dark corner @subentry strings, storing
Almost all other @command{awk} implementations@footnote{At least that we know
about.} store strings internally as C-style strings.  C strings use the
@sc{nul} character as the string terminator.  In effect, this means that
@samp{RS = "\0"} is the same as @samp{RS = ""}.
@value{DARKCORNER}

It happens that recent versions of @command{mawk} can use the @sc{nul}
character as a record separator. However, this is a special case:
@command{mawk} does not allow embedded @sc{nul} characters in strings.
(This may change in a future version of @command{mawk}.)

@cindex records @subentry treating files as
@cindex treating files, as single records
@cindex single records, treating files as
@xref{Readfile Function} for an interesting way to read
whole files.  If you are using @command{gawk}, see @ref{Extension Sample
Readfile} for another option.
@end cartouche
@end ifnotdocbook

@node Fields
@section Examining Fields

@cindex examining fields
@cindex fields
@cindex accessing fields
@cindex fields @subentry examining
@cindex whitespace @subentry definition of
When @command{awk} reads an input record, the record is
automatically @dfn{parsed} or separated by the @command{awk} utility into chunks
called @dfn{fields}.  By default, fields are separated by @dfn{whitespace},
like words in a line.
Whitespace in @command{awk} means any string of one or more spaces,
TABs, or newlines; other characters
that are considered whitespace by other languages
(such as formfeed, vertical tab, etc.) are @emph{not} considered
whitespace by @command{awk}.

The purpose of fields is to make it more convenient for you to refer to
these pieces of the record.  You don't have to use them---you can
operate on the whole record if you want---but fields are what make
simple @command{awk} programs so powerful.

@cindex field operator @code{$}
@cindex @code{$} (dollar sign) @subentry @code{$} field operator
@cindex dollar sign (@code{$}) @subentry @code{$} field operator
@cindex field operators, dollar sign as
You use a dollar sign (@samp{$})
to refer to a field in an @command{awk} program,
followed by the number of the field you want.  Thus, @code{$1}
refers to the first field, @code{$2} to the second, and so on.
(Unlike in the Unix shells, the field numbers are not limited to single digits.
@code{$127} is the 127th field in the record.)
For example, suppose the following is a line of input:

@example
This seems like a pretty nice example.
@end example

@noindent
Here the first field, or @code{$1}, is @samp{This}, the second field, or
@code{$2}, is @samp{seems}, and so on.  Note that the last field,
@code{$7}, is @samp{example.}.  Because there is no space between the
@samp{e} and the @samp{.}, the period is considered part of the seventh
field.

@cindex @code{NF} variable
@cindex fields @subentry number of
@code{NF} is a predefined variable whose value is the number of fields
in the current record.  @command{awk} automatically updates the value
of @code{NF} each time it reads a record.  No matter how many fields
there are, the last field in a record can be represented by @code{$NF}.
So, @code{$NF} is the same as @code{$7}, which is @samp{example.}.
If you try to reference a field beyond the last
one (such as @code{$8} when the record has only seven fields), you get
the empty string.  (If used in a numeric operation, you get zero.)

The use of @code{$0}, which looks like a reference to the ``zeroth'' field, is
a special case: it represents the whole input record. Use it
when you are not interested in specific fields.
Here are some more examples:

@example
$ @kbd{awk '$1 ~ /li/ @{ print $0 @}' mail-list}
@print{} Amelia       555-5553     amelia.zodiacusque@@gmail.com    F
@print{} Julie        555-6699     julie.perscrutabor@@skeeve.com   F
@end example

@noindent
This example prints each record in the file @file{mail-list} whose first
field contains the string @samp{li}.

By contrast, the following example looks for @samp{li} in @emph{the
entire record} and prints the first and last fields for each matching
input record:

@example
$ @kbd{awk '/li/ @{ print $1, $NF @}' mail-list}
@print{} Amelia F
@print{} Broderick R
@print{} Julie F
@print{} Samuel A
@end example

@node Nonconstant Fields
@section Nonconstant Field Numbers
@cindex fields @subentry numbers
@cindex field numbers

A field number need not be a constant.  Any expression in
the @command{awk} language can be used after a @samp{$} to refer to a
field.  The value of the expression specifies the field number.  If the
value is a string, rather than a number, it is converted to a number.
Consider this example:

@example
awk '@{ print $NR @}'
@end example

@noindent
Recall that @code{NR} is the number of records read so far: one in the
first record, two in the second, and so on.  So this example prints the first
field of the first record, the second field of the second record, and so
on.  For the twentieth record, field number 20 is printed; most likely,
the record has fewer than 20 fields, so this prints a blank line.
Here is another example of using expressions as field numbers:

@example
awk '@{ print $(2*2) @}' mail-list
@end example

@command{awk} evaluates the expression @samp{(2*2)} and uses
its value as the number of the field to print.  The @samp{*}
represents multiplication, so the expression @samp{2*2} evaluates to four.
The parentheses are used so that the multiplication is done before the
@samp{$} operation; they are necessary whenever there is a binary
operator@footnote{A @dfn{binary operator}, such as @samp{*} for
multiplication, is one that takes two operands. The distinction
is required because @command{awk} also has unary (one-operand)
and ternary (three-operand) operators.}
in the field-number expression.  This example, then, prints the
type of relationship (the fourth field) for every line of the file
@file{mail-list}.  (All of the @command{awk} operators are listed, in
order of decreasing precedence, in
@ref{Precedence}.)

If the field number you compute is zero, you get the entire record.
Thus, @samp{$(2-2)} has the same value as @code{$0}.  Negative field
numbers are not allowed; trying to reference one usually terminates
the program.  (The POSIX standard does not define
what happens when you reference a negative field number.  @command{gawk}
notices this and terminates your program.  Other @command{awk}
implementations may behave differently.)

As mentioned in @ref{Fields},
@command{awk} stores the current record's number of fields in the built-in
variable @code{NF} (also @pxref{Built-in Variables}).  Thus, the expression
@code{$NF} is not a special feature---it is the direct consequence of
evaluating @code{NF} and using its value as a field number.

@node Changing Fields
@section Changing the Contents of a Field

@cindex fields @subentry changing contents of
The contents of a field, as seen by @command{awk}, can be changed within an
@command{awk} program; this changes what @command{awk} perceives as the
current input record.  (The actual input is untouched; @command{awk} @emph{never}
modifies the input file.)
Consider the following example and its output:

@example
$ @kbd{awk '@{ nboxes = $3 ; $3 = $3 - 10}
>        @kbd{print nboxes, $3 @}' inventory-shipped}
@print{} 25 15
@print{} 32 22
@print{} 24 14
@dots{}
@end example

@noindent
The program first saves the original value of field three in the variable
@code{nboxes}.
The @samp{-} sign represents subtraction, so this program reassigns
field three, @code{$3}, as the original value of field three minus ten:
@samp{$3 - 10}.  (@xref{Arithmetic Ops}.)
Then it prints the original and new values for field three.
(Someone in the warehouse made a consistent mistake while inventorying
the red boxes.)

For this to work, the text in @code{$3} must make sense
as a number; the string of characters must be converted to a number
for the computer to do arithmetic on it.  The number resulting
from the subtraction is converted back to a string of characters that
then becomes field three.
@xref{Conversion}.

When the value of a field is changed (as perceived by @command{awk}), the
text of the input record is recalculated to contain the new field where
the old one was.  In other words, @code{$0} changes to reflect the altered
field.  Thus, this program
prints a copy of the input file, with 10 subtracted from the second
field of each line:

@example
$ @kbd{awk '@{ $2 = $2 - 10; print $0 @}' inventory-shipped}
@print{} Jan 3 25 15 115
@print{} Feb 5 32 24 226
@print{} Mar 5 24 34 228
@dots{}
@end example

It is also possible to assign contents to fields that are out
of range.  For example:

@example
$ @kbd{awk '@{ $6 = ($5 + $4 + $3 + $2)}
> @kbd{       print $6 @}' inventory-shipped}
@print{} 168
@print{} 297
@print{} 301
@dots{}
@end example

@cindex adding @subentry fields
@cindex fields @subentry adding
@noindent
We've just created @code{$6}, whose value is the sum of fields
@code{$2}, @code{$3}, @code{$4}, and @code{$5}.  The @samp{+} sign
represents addition.  For the file @file{inventory-shipped}, @code{$6}
represents the total number of parcels shipped for a particular month.

Creating a new field changes @command{awk}'s internal copy of the current
input record, which is the value of @code{$0}.  Thus, if you do @samp{print $0}
after adding a field, the record printed includes the new field, with
the appropriate number of field separators between it and the previously
existing fields.

@cindex @code{OFS} variable
@cindex output field separator @seeentry{@code{OFS} variable}
@cindex field separator @seealso{@code{OFS}}
This recomputation affects and is affected by
@code{NF} (the number of fields; @pxref{Fields}).
For example, the value of @code{NF} is set to the number of the highest
field you create.
The exact format of @code{$0} is also affected by a feature that has not been discussed yet:
the @dfn{output field separator}, @code{OFS},
used to separate the fields (@pxref{Output Separators}).

Note, however, that merely @emph{referencing} an out-of-range field
does @emph{not} change the value of either @code{$0} or @code{NF}.
Referencing an out-of-range field only produces an empty string.  For
example:

@example
if ($(NF+1) != "")
    print "can't happen"
else
    print "everything is normal"
@end example

@noindent
should print @samp{everything is normal}, because @code{NF+1} is certain
to be out of range.  (@xref{If Statement}
for more information about @command{awk}'s @code{if-else} statements.
@xref{Typing and Comparison}
for more information about the @samp{!=} operator.)

It is important to note that making an assignment to an existing field
changes the
value of @code{$0} but does not change the value of @code{NF},
even when you assign the empty string to a field.  For example:

@example
$ @kbd{echo a b c d | awk '@{ OFS = ":"; $2 = ""}
>                       @kbd{print $0; print NF @}'}
@print{} a::c:d
@print{} 4
@end example

@noindent
The field is still there; it just has an empty value, delimited by
the two colons between @samp{a} and @samp{c}.
This example shows what happens if you create a new field:

@example
$ @kbd{echo a b c d | awk '@{ OFS = ":"; $2 = ""; $6 = "new"}
>                       @kbd{print $0; print NF @}'}
@print{} a::c:d::new
@print{} 6
@end example

@noindent
The intervening field, @code{$5}, is created with an empty value
(indicated by the second pair of adjacent colons),
and @code{NF} is updated with the value six.

@cindex dark corner @subentry @code{NF} variable, decrementing
@cindex @code{NF} variable @subentry decrementing
Decrementing @code{NF} throws away the values of the fields
after the new value of @code{NF} and recomputes @code{$0}.
@value{DARKCORNER}
Here is an example:

@example
$ @kbd{echo a b c d e f | awk '@{ print "NF =", NF;}
> @kbd{                          NF = 3; print $0 @}'}
@print{} NF = 6
@print{} a b c
@end example

@cindex portability @subentry @code{NF} variable, decrementing
@quotation CAUTION
Some versions of @command{awk} don't
rebuild @code{$0} when @code{NF} is decremented.
Until August, 2018, this included BWK @command{awk}; fortunately
his version now handles this correctly.
@end quotation

Finally, there are times when it is convenient to force
@command{awk} to rebuild the entire record, using the current
values of the fields and @code{OFS}.  To do this, use the
seemingly innocuous assignment:

@example
@group
$1 = $1   # force record to be reconstituted
print $0  # or whatever else with $0
@end group
@end example

@noindent
This forces @command{awk} to rebuild the record.  It does help
to add a comment, as we've shown here.

There is a flip side to the relationship between @code{$0} and
the fields.  Any assignment to @code{$0} causes the record to be
reparsed into fields using the @emph{current} value of @code{FS}.
This also applies to any built-in function that updates @code{$0},
such as @code{sub()} and @code{gsub()}
(@pxref{String Functions}).

@cindex sidebar @subentry Understanding @code{$0}
@ifdocbook
@docbook
Understanding @code{$0}
@end docbook


It is important to remember that @code{$0} is the @emph{full}
record, exactly as it was read from the input.  This includes
any leading or trailing whitespace, and the exact whitespace (or other
characters) that separates the fields.

It is a common error to try to change the field separators
in a record simply by setting @code{FS} and @code{OFS}, and then
expecting a plain @samp{print} or @samp{print $0} to print the
modified record.

But this does not work, because nothing was done to change the record
itself.  Instead, you must force the record to be rebuilt, typically
with a statement such as @samp{$1 = $1}, as described earlier.

@docbook

@end docbook
@end ifdocbook

@ifnotdocbook
@cartouche
@center @b{Understanding @code{$0}}



It is important to remember that @code{$0} is the @emph{full}
record, exactly as it was read from the input.  This includes
any leading or trailing whitespace, and the exact whitespace (or other
characters) that separates the fields.

It is a common error to try to change the field separators
in a record simply by setting @code{FS} and @code{OFS}, and then
expecting a plain @samp{print} or @samp{print $0} to print the
modified record.

But this does not work, because nothing was done to change the record
itself.  Instead, you must force the record to be rebuilt, typically
with a statement such as @samp{$1 = $1}, as described earlier.
@end cartouche
@end ifnotdocbook


@node Field Separators
@section Specifying How Fields Are Separated

@menu
* Default Field Splitting::      How fields are normally separated.
* Regexp Field Splitting::       Using regexps as the field separator.
* Single Character Fields::      Making each character a separate field.
* Command Line Field Separator:: Setting @code{FS} from the command line.
* Full Line Fields::             Making the full line be a single field.
* Field Splitting Summary::      Some final points and a summary table.
@end menu

@cindex @code{FS} variable
@cindex fields @subentry separating
@cindex field separator
@cindex fields @subentry separating
The @dfn{field separator}, which is either a single character or a regular
expression, controls the way @command{awk} splits an input record into fields.
@command{awk} scans the input record for character sequences that
match the separator; the fields themselves are the text between the matches.

In the examples that follow, we use the bullet symbol (@bullet{}) to
represent spaces in the output.
If the field separator is @samp{oo}, then the following line:

@example
moo goo gai pan
@end example

@noindent
is split into three fields: @samp{m}, @samp{@bullet{}g}, and
@samp{@bullet{}gai@bullet{}pan}.
Note the leading spaces in the values of the second and third fields.

@cindex troubleshooting @subentry @command{awk} uses @code{FS} not @code{IFS}
The field separator is represented by the predefined variable @code{FS}.
Shell programmers take note:  @command{awk} does @emph{not} use the
name @code{IFS} that is used by the POSIX-compliant shells (such as
the Unix Bourne shell, @command{sh}, or Bash).

@cindex @code{FS} variable @subentry changing value of
The value of @code{FS} can be changed in the @command{awk} program with the
assignment operator, @samp{=} (@pxref{Assignment Ops}).
Often, the right time to do this is at the beginning of execution
before any input has been processed, so that the very first record
is read with the proper separator.  To do this, use the special
@code{BEGIN} pattern
(@pxref{BEGIN/END}).
For example, here we set the value of @code{FS} to the string
@code{","}:

@example
awk 'BEGIN @{ FS = "," @} ; @{ print $2 @}'
@end example

@cindex @code{BEGIN} pattern
@noindent
Given the input line:

@example
John Q. Smith, 29 Oak St., Walamazoo, MI 42139
@end example

@noindent
this @command{awk} program extracts and prints the string
@samp{@bullet{}29@bullet{}Oak@bullet{}St.}.

@cindex field separator @subentry choice of
@cindex regular expressions @subentry as field separators
@cindex field separator @subentry regular expression as
Sometimes the input data contains separator characters that don't
separate fields the way you thought they would.  For instance, the
person's name in the example we just used might have a title or
suffix attached, such as:

@example
John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139
@end example

@noindent
The same program would extract @samp{@bullet{}LXIX} instead of
@samp{@bullet{}29@bullet{}Oak@bullet{}St.}.
If you were expecting the program to print the
address, you would be surprised.  The moral is to choose your data layout and
separator characters carefully to prevent such problems.
(If the data is not in a form that is easy to process, perhaps you
can massage it first with a separate @command{awk} program.)


@node Default Field Splitting
@subsection Whitespace Normally Separates Fields

@cindex field separator @subentry whitespace as
@cindex whitespace @subentry as field separators
@cindex field separator @subentry @code{FS} variable and
@cindex separators @subentry field @subentry @code{FS} variable and
Fields are normally separated by whitespace sequences
(spaces, TABs, and newlines), not by single spaces.  Two spaces in a row do not
delimit an empty field.  The default value of the field separator @code{FS}
is a string containing a single space, @w{@code{" "}}.  If @command{awk}
interpreted this value in the usual way, each space character would separate
fields, so two spaces in a row would make an empty field between them.
The reason this does not happen is that a single space as the value of
@code{FS} is a special case---it is taken to specify the default manner
of delimiting fields.

If @code{FS} is any other single character, such as @code{","}, then
each occurrence of that character separates two fields.  Two consecutive
occurrences delimit an empty field.  If the character occurs at the
beginning or the end of the line, that too delimits an empty field.  The
space character is the only single character that does not follow these
rules.

@node Regexp Field Splitting
@subsection Using Regular Expressions to Separate Fields

@cindex regular expressions @subentry as field separators
@cindex field separator @subentry regular expression as
The previous @value{SUBSECTION}
discussed the use of single characters or simple strings as the
value of @code{FS}.
More generally, the value of @code{FS} may be a string containing any
regular expression.  In this case, each match in the record for the regular
expression separates fields.  For example, the assignment:

@example
FS = ", \t"
@end example

@noindent
makes every area of an input line that consists of a comma followed by a
space and a TAB into a field separator.
@ifinfo
(@samp{\t}
is an @dfn{escape sequence} that stands for a TAB;
@pxref{Escape Sequences},
for the complete list of similar escape sequences.)
@end ifinfo

For a less trivial example of a regular expression, try using
single spaces to separate fields the way single commas are used.
@code{FS} can be set to @w{@code{"[@ ]"}} (left bracket, space, right
bracket).  This regular expression matches a single space and nothing else
(@pxref{Regexp}).

There is an important difference between the two cases of @samp{FS = @w{" "}}
(a single space) and @samp{FS = @w{"[ \t\n]+"}}
(a regular expression matching one or more spaces, TABs, or newlines).
For both values of @code{FS}, fields are separated by @dfn{runs}
(multiple adjacent occurrences) of spaces, TABs,
and/or newlines.  However, when the value of @code{FS} is @w{@code{" "}},
@command{awk} first strips leading and trailing whitespace from
the record and then decides where the fields are.
For example, the following pipeline prints @samp{b}:

@example
$ @kbd{echo ' a b c d ' | awk '@{ print $2 @}'}
@print{} b
@end example

@noindent
However, this pipeline prints @samp{a} (note the extra spaces around
each letter):

@example
$ @kbd{echo ' a  b  c  d ' | awk 'BEGIN @{ FS = "[ \t\n]+" @}}
>                                  @kbd{@{ print $2 @}'}
@print{} a
@end example

@noindent
@cindex null strings
@cindex strings @subentry null
In this case, the first field is null, or empty.

The stripping of leading and trailing whitespace also comes into
play whenever @code{$0} is recomputed.  For instance, study this pipeline:

@example
$ @kbd{echo '   a b c d' | awk '@{ print; $2 = $2; print @}'}
@print{}    a b c d
@print{} a b c d
@end example

@noindent
The first @code{print} statement prints the record as it was read,
with leading whitespace intact.  The assignment to @code{$2} rebuilds
@code{$0} by concatenating @code{$1} through @code{$NF} together,
separated by the value of @code{OFS} (which is a space by default).
Because the leading whitespace was ignored when finding @code{$1},
it is not part of the new @code{$0}.  Finally, the last @code{print}
statement prints the new @code{$0}.

@cindex @code{FS} variable @subentry containing @code{^}
@cindex @code{^} (caret) @subentry in @code{FS}
@cindex dark corner @subentry @code{^}, in @code{FS}
There is an additional subtlety to be aware of when using regular expressions
for field splitting.
It is not well specified in the POSIX standard, or anywhere else, what @samp{^}
means when splitting fields.  Does the @samp{^}  match only at the beginning of
the entire record? Or is each field separator a new string?  It turns out that
different @command{awk} versions answer this question differently, and you
should not rely on any specific behavior in your programs.
@value{DARKCORNER}

@cindex Brian Kernighan's @command{awk}
As a point of information, BWK @command{awk} allows @samp{^}
to match only at the beginning of the record. @command{gawk}
also works this way. For example:

@example
$ @kbd{echo 'xxAA  xxBxx  C' |}
> @kbd{gawk -F '(^x+)|( +)' '@{ for (i = 1; i <= NF; i++)}
> @kbd{                            printf "-->%s<--\n", $i @}'}
@print{} --><--
@print{} -->AA<--
@print{} -->xxBxx<--
@print{} -->C<--
@end example

@node Single Character Fields
@subsection Making Each Character a Separate Field

@cindex common extensions @subentry single character fields
@cindex extensions @subentry common @subentry single character fields
@cindex differences in @command{awk} and @command{gawk} @subentry single-character fields
@cindex single-character fields
@cindex fields @subentry single-character
There are times when you may want to examine each character
of a record separately.  This can be done in @command{gawk} by
simply assigning the null string (@code{""}) to @code{FS}. @value{COMMONEXT}
In this case,
each individual character in the record becomes a separate field.
For example:

@example
$ @kbd{echo a b | gawk 'BEGIN @{ FS = "" @}}
>                  @kbd{@{}
>                      @kbd{for (i = 1; i <= NF; i = i + 1)}
>                          @kbd{print "Field", i, "is", $i}
>                  @kbd{@}'}
@print{} Field 1 is a
@print{} Field 2 is
@print{} Field 3 is b
@end example

@cindex dark corner @subentry @code{FS} as null string
@cindex @code{FS} variable @subentry null string as
Traditionally, the behavior of @code{FS} equal to @code{""} was not defined.
In this case, most versions of Unix @command{awk} simply treat the entire record
as only having one field.
@value{DARKCORNER}
In compatibility mode
(@pxref{Options}),
if @code{FS} is the null string, then @command{gawk} also
behaves this way.

@node Command Line Field Separator
@subsection Setting @code{FS} from the Command Line
@cindex @option{-F} option @subentry command-line
@cindex field separator @subentry on command line
@cindex command line @subentry @code{FS} on, setting
@cindex @code{FS} variable @subentry setting from command line

@code{FS} can be set on the command line.  Use the @option{-F} option to
do so.  For example:

@example
awk -F, '@var{program}' @var{input-files}
@end example

@noindent
sets @code{FS} to the @samp{,} character.  Notice that the option uses
an uppercase @samp{F} instead of a lowercase @samp{f}. The latter
option (@option{-f}) specifies a file containing an @command{awk} program.

The value used for the argument to @option{-F} is processed in exactly the
same way as assignments to the predefined variable @code{FS}.
Any special characters in the field separator must be escaped
appropriately.  For example, to use a @samp{\} as the field separator
on the command line, you would have to type:

@example
# same as FS = "\\"
awk -F\\\\ '@dots{}' files @dots{}
@end example

@noindent
@cindex field separator @subentry backslash (@code{\}) as
@cindex @code{\} (backslash) @subentry as field separator
@cindex backslash (@code{\}) @subentry as field separator
Because @samp{\} is used for quoting in the shell, @command{awk} sees
@samp{-F\\}.  Then @command{awk} processes the @samp{\\} for escape
characters (@pxref{Escape Sequences}), finally yielding
a single @samp{\} to use for the field separator.

@c @cindex historical features
As a special case, in compatibility mode
(@pxref{Options}),
if the argument to @option{-F} is @samp{t}, then @code{FS} is set to
the TAB character.  If you type @samp{-F\t} at the
shell, without any quotes, the @samp{\} gets deleted, so @command{awk}
figures that you really want your fields to be separated with TABs and
not @samp{t}s.  Use @samp{-v FS="t"} or @samp{-F"[t]"} on the command line
if you really do want to separate your fields with @samp{t}s.
Use @samp{-F '\t'} when not in compatibility mode to specify that TABs
separate fields.

As an example, let's use an @command{awk} program file called @file{edu.awk}
that contains the pattern @code{/edu/} and the action @samp{print $1}:

@example
/edu/   @{ print $1 @}
@end example

Let's also set @code{FS} to be the @samp{-} character and run the
program on the file @file{mail-list}.  The following command prints a
list of the names of the people that work at or attend a university, and
the first three digits of their phone numbers:

@example
$ @kbd{awk -F- -f edu.awk mail-list}
@print{} Fabius       555
@print{} Samuel       555
@print{} Jean
@end example

@noindent
Note the third line of output.  The third line
in the original file looked like this:

@example
Jean-Paul    555-2127     jeanpaul.campanorum@@nyu.edu     R
@end example

The @samp{-} as part of the person's name was used as the field
separator, instead of the @samp{-} in the phone number that was
originally intended.  This demonstrates why you have to be careful in
choosing your field and record separators.

@cindex Unix @command{awk} @subentry password files, field separators and
Perhaps the most common use of a single character as the field separator
occurs when processing the Unix system password file.  On many Unix
systems, each user has a separate entry in the system password file, with one
line per user.  The information in these lines is separated by colons.
The first field is the user's login name and the second is the user's
encrypted or shadow password.  (A shadow password is indicated by the
presence of a single @samp{x} in the second field.)  A password file
entry might look like this:

@cindex Robbins @subentry Arnold
@example
arnold:x:2076:10:Arnold Robbins:/home/arnold:/bin/bash
@end example

The following program searches the system password file and prints
the entries for users whose full name is not indicated:

@example
awk -F: '$5 == ""' /etc/passwd
@end example

@node Full Line Fields
@subsection Making the Full Line Be a Single Field

Occasionally, it's useful to treat the whole input line as a
single field.  This can be done easily and portably simply by
setting @code{FS} to @code{"\n"} (a newline):@footnote{Thanks to
Andrew Schorr for this tip.}

@example
awk -F'\n' '@var{program}' @var{files @dots{}}
@end example

@noindent
When you do this, @code{$1} is the same as @code{$0}.

@cindex sidebar @subentry Changing @code{FS} Does Not Affect the Fields
@ifdocbook
@docbook
Changing @code{FS} Does Not Affect the Fields
@end docbook


@cindex POSIX @command{awk} @subentry field separators and
@cindex field separator @subentry POSIX and
According to the POSIX standard, @command{awk} is supposed to behave
as if each record is split into fields at the time it is read.
In particular, this means that if you change the value of @code{FS}
after a record is read, the values of the fields (i.e., how they were split)
should reflect the old value of @code{FS}, not the new one.

@cindex dark corner @subentry field separators
@cindex @command{sed} utility
@cindex stream editors
However, many older implementations of @command{awk} do not work this way.  Instead,
they defer splitting the fields until a field is actually
referenced.  The fields are split
using the @emph{current} value of @code{FS}!
@value{DARKCORNER}
This behavior can be difficult
to diagnose. The following example illustrates the difference
between the two methods:

@example
sed 1q /etc/passwd | awk '@{ FS = ":" ; print $1 @}'
@end example

@noindent
which usually prints:

@example
root
@end example

@noindent
on an incorrect implementation of @command{awk}, while @command{gawk}
prints the full first line of the file, something like:

@example
root:x:0:0:Root:/:
@end example

(The @command{sed}@footnote{The @command{sed} utility is a ``stream editor.''
Its behavior is also defined by the POSIX standard.}
command prints just the first line of @file{/etc/passwd}.)

@docbook

@end docbook
@end ifdocbook

@ifnotdocbook
@cartouche
@center @b{Changing @code{FS} Does Not Affect the Fields}



@cindex POSIX @command{awk} @subentry field separators and
@cindex field separator @subentry POSIX and
According to the POSIX standard, @command{awk} is supposed to behave
as if each record is split into fields at the time it is read.
In particular, this means that if you change the value of @code{FS}
after a record is read, the values of the fields (i.e., how they were split)
should reflect the old value of @code{FS}, not the new one.

@cindex dark corner @subentry field separators
@cindex @command{sed} utility
@cindex stream editors
However, many older implementations of @command{awk} do not work this way.  Instead,
they defer splitting the fields until a field is actually
referenced.  The fields are split
using the @emph{current} value of @code{FS}!
@value{DARKCORNER}
This behavior can be difficult
to diagnose. The following example illustrates the difference
between the two methods:

@example
sed 1q /etc/passwd | awk '@{ FS = ":" ; print $1 @}'
@end example

@noindent
which usually prints:

@example
root
@end example

@noindent
on an incorrect implementation of @command{awk}, while @command{gawk}
prints the full first line of the file, something like:

@example
root:x:0:0:Root:/:
@end example

(The @command{sed}@footnote{The @command{sed} utility is a ``stream editor.''
Its behavior is also defined by the POSIX standard.}
command prints just the first line of @file{/etc/passwd}.)
@end cartouche
@end ifnotdocbook

@node Field Splitting Summary
@subsection Field-Splitting Summary

It is important to remember that when you assign a string constant
as the value of @code{FS}, it undergoes normal @command{awk} string
processing.  For example, with Unix @command{awk} and @command{gawk},
the assignment @samp{FS = "\.."} assigns the character string @code{".."}
to @code{FS} (the backslash is stripped).  This creates a regexp meaning
``fields are separated by occurrences of any two characters.''
If instead you want fields to be separated by a literal period followed
by any single character, use @samp{FS = "\\.."}.

The following list summarizes how fields are split, based on the value
of @code{FS} (@samp{==} means ``is equal to''):

@table @code
@item FS == " "
Fields are separated by runs of whitespace.  Leading and trailing
whitespace are ignored.  This is the default.

@item FS == @var{any other single character}
Fields are separated by each occurrence of the character.  Multiple
successive occurrences delimit empty fields, as do leading and
trailing occurrences.
The character can even be a regexp metacharacter; it does not need
to be escaped.

@item FS == @var{regexp}
Fields are separated by occurrences of characters that match @var{regexp}.
Leading and trailing matches of @var{regexp} delimit empty fields.

@item FS == ""
Each individual character in the record becomes a separate field.
(This is a common extension; it is not specified by the POSIX standard.)
@end table

@cindex sidebar @subentry @code{FS} and @code{IGNORECASE}
@ifdocbook
@docbook
@code{FS} and @code{IGNORECASE}
@end docbook


The @code{IGNORECASE} variable
(@pxref{User-modified})
affects field splitting @emph{only} when the value of @code{FS} is a regexp.
It has no effect when @code{FS} is a single character, even if
that character is a letter.  Thus, in the following code:

@example
FS = "c"
IGNORECASE = 1
$0 = "aCa"
print $1
@end example

@noindent
The output is @samp{aCa}.  If you really want to split fields on an
alphabetic character while ignoring case, use a regexp that will
do it for you (e.g., @samp{FS = "[c]"}).  In this case, @code{IGNORECASE}
will take effect.

@docbook

@end docbook
@end ifdocbook

@ifnotdocbook
@cartouche
@center @b{@code{FS} and @code{IGNORECASE}}



The @code{IGNORECASE} variable
(@pxref{User-modified})
affects field splitting @emph{only} when the value of @code{FS} is a regexp.
It has no effect when @code{FS} is a single character, even if
that character is a letter.  Thus, in the following code:

@example
FS = "c"
IGNORECASE = 1
$0 = "aCa"
print $1
@end example

@noindent
The output is @samp{aCa}.  If you really want to split fields on an
alphabetic character while ignoring case, use a regexp that will
do it for you (e.g., @samp{FS = "[c]"}).  In this case, @code{IGNORECASE}
will take effect.
@end cartouche
@end ifnotdocbook


@node Constant Size
@section Reading Fixed-Width Data

@cindex data, fixed-width
@cindex fixed-width data
@cindex advanced features @subentry fixed-width data

@c O'Reilly doesn't like it as a note the first thing in the section.
This @value{SECTION} discusses an advanced
feature of @command{gawk}.  If you are a novice @command{awk} user,
you might want to skip it on the first reading.

@command{gawk} provides a facility for dealing with fixed-width fields
with no distinctive field separator. We discuss this feature in
the following @value{SUBSECTION}s.

@menu
* Fixed width data::            Processing fixed-width data.
* Skipping intervening::        Skipping intervening fields.
* Allowing trailing data::      Capturing optional trailing data.
* Fields with fixed data::      Field values with fixed-width data.
@end menu

@node Fixed width data
@subsection Processing Fixed-Width Data

An example of fixed-width data would be the input for old Fortran programs
where numbers are run together, or the output of programs that did not
anticipate the use of their output as input for other programs.

An example of the latter is a table where all the columns are lined up
by the use of a variable number of spaces and @emph{empty fields are
just spaces}.  Clearly, @command{awk}'s normal field splitting based
on @code{FS} does not work well in this case.  Although a portable
@command{awk} program can use a series of @code{substr()} calls on
@code{$0} (@pxref{String Functions}), this is awkward and inefficient
for a large number of fields.

@cindex troubleshooting @subentry fatal errors @subentry field widths, specifying
@cindex @command{w} utility
@cindex @code{FIELDWIDTHS} variable
@cindex @command{gawk} @subentry @code{FIELDWIDTHS} variable in
The splitting of an input record into fixed-width fields is specified by
assigning a string containing space-separated numbers to the built-in
variable @code{FIELDWIDTHS}.  Each number specifies the width of the
field, @emph{including} columns between fields.  If you want to ignore
the columns between fields, you can specify the width as a separate
field that is subsequently ignored.  It is a fatal error to supply a
field width that has a negative value.

The following data is the output of the Unix @command{w} utility.  It is useful
to illustrate the use of @code{FIELDWIDTHS}:

@example
@group
 10:06pm  up 21 days, 14:04,  23 users
User     tty       login@  idle   JCPU   PCPU  what
hzuo     ttyV0     8:58pm            9      5  vi p24.tex
hzang    ttyV3     6:37pm    50                -csh
eklye    ttyV5     9:53pm            7      1  em thes.tex
dportein ttyV6     8:17pm  1:47                -csh
gierd    ttyD3    10:00pm     1                elm
dave     ttyD4     9:47pm            4      4  w
brent    ttyp0    26Jun91  4:46  26:46   4:41  bash
dave     ttyq4    26Jun9115days     46     46  wnewmail
@end group
@end example

The following program takes this input, converts the idle time to
number of seconds, and prints out the first two fields and the calculated
idle time:

@example
BEGIN  @{ FIELDWIDTHS = "9 6 10 6 7 7 35" @}
NR > 2 @{
    idle = $4
    sub(/^ +/, "", idle)   # strip leading spaces
    if (idle == "")
        idle = 0
    if (idle ~ /:/) @{      # hh:mm
        split(idle, t, ":")
        idle = t[1] * 60 + t[2]
    @}
    if (idle ~ /days/)
        idle *= 24 * 60 * 60

    print $1, $2, idle
@}
@end example

@quotation NOTE
The preceding program uses a number of @command{awk} features that
haven't been introduced yet.
@end quotation

Running the program on the data produces the following results:

@example
hzuo      ttyV0  0
hzang     ttyV3  50
eklye     ttyV5  0
dportein  ttyV6  107
gierd     ttyD3  1
dave      ttyD4  0
brent     ttyp0  286
dave      ttyq4  1296000
@end example

Another (possibly more practical) example of fixed-width input data
is the input from a deck of balloting cards.  In some parts of
the United States, voters mark their choices by punching holes in computer
cards.  These cards are then processed to count the votes for any particular
candidate or on any particular issue.  Because a voter may choose not to
vote on some issue, any column on the card may be empty.  An @command{awk}
program for processing such data could use the @code{FIELDWIDTHS} feature
to simplify reading the data.  (Of course, getting @command{gawk} to run on
a system with card readers is another story!)

@node Skipping intervening
@subsection Skipping Intervening Fields

Starting in @value{PVERSION} 4.2, each field width may optionally be
preceded by a colon-separated value specifying the number of characters
to skip before the field starts.  Thus, the preceding program could be
rewritten to specify @code{FIELDWIDTHS} like so:

@example
BEGIN  @{ FIELDWIDTHS = "8 1:5 4:7 6 1:6 1:6 2:33" @}
@end example

This strips away some of the white space separating the fields. With such
a change, the program produces the following results:

@example
hzang    ttyV3 50
eklye    ttyV5 0
dportein ttyV6 107
gierd    ttyD3 1
dave     ttyD4 0
brent    ttyp0 286
dave     ttyq4 1296000
@end example

@node Allowing trailing data
@subsection Capturing Optional Trailing Data

There are times when fixed-width data may be followed by additional data
that has no fixed length.  Such data may or may not be present, but if
it is, it should be possible to get at it from an @command{awk} program.

Starting with @value{PVERSION} 4.2, in order to provide a way to say ``anything
else in the record after the defined fields,'' @command{gawk}
allows you to add a final @samp{*} character to the value of
@code{FIELDWIDTHS}. There can only be one such character, and it must
be the final non-whitespace character in @code{FIELDWIDTHS}.
For example:

@example
$ @kbd{cat fw.awk}                         @ii{Show the program}
@print{} BEGIN @{ FIELDWIDTHS = "2 2 *" @}
@print{} @{ print NF, $1, $2, $3 @}
$ @kbd{cat fw.in}                          @ii{Show sample input}
@print{} 1234abcdefghi
$ @kbd{gawk -f fw.awk fw.in}               @ii{Run the program}
@print{} 3 12 34 abcdefghi
@end example

@node Fields with fixed data
@subsection Field Values With Fixed-Width Data

So far, so good.  But what happens if there isn't as much data as there
should be based on the contents of @code{FIELDWIDTHS}? Or, what happens
if there is more data than expected?

For many years, what happens in these cases was not well defined. Starting
with @value{PVERSION} 4.2, the rules are as follows:

@table @asis
@item Enough data for some fields
For example, if @code{FIELDWIDTHS} is set to @code{"2 3 4"} and the
input record is @samp{aabbb}.  In this case, @code{NF} is set to two.

@item Not enough data for a field
For example, if @code{FIELDWIDTHS} is set to @code{"2 3 4"} and the
input record is @samp{aab}.  In this case, @code{NF} is set to two and
@code{$2} has the value @code{"b"}. The idea is that even though there
aren't as many characters as were expected, there are some, so the data
should be made available to the program.

@item Too much data
For example, if @code{FIELDWIDTHS} is set to @code{"2 3 4"} and the
input record is @samp{aabbbccccddd}.  In this case, @code{NF} is set to
three and the extra characters (@samp{ddd}) are ignored.  If you want
@command{gawk} to capture the extra characters, supply a final @samp{*}
in the value of @code{FIELDWIDTHS}.

@item Too much data, but with @samp{*} supplied
For example, if @code{FIELDWIDTHS} is set to @code{"2 3 4 *"} and the
input record is @samp{aabbbccccddd}.  In this case, @code{NF} is set to
four, and @code{$4} has the value @code{"ddd"}.

@end table

@node Splitting By Content
@section Defining Fields by Content

@menu
* More CSV::                    More on CSV files.
@end menu

@c O'Reilly doesn't like it as a note the first thing in the section.
This @value{SECTION} discusses an advanced
feature of @command{gawk}.  If you are a novice @command{awk} user,
you might want to skip it on the first reading.

@cindex advanced features @subentry specifying field content
Normally, when using @code{FS}, @command{gawk} defines the fields as the
parts of the record that occur in between each field separator. In other
words, @code{FS} defines what a field @emph{is not}, instead of what a field
@emph{is}.
However, there are times when you really want to define the fields by
what they are, and not by what they are not.

The most notorious such case
is so-called @dfn{comma-separated values} (CSV) data. Many spreadsheet programs,
for example, can export their data into text files, where each record is
terminated with a newline, and fields are separated by commas. If
commas only separated the data, there wouldn't be an issue. The problem comes when
one of the fields contains an @emph{embedded} comma.
In such cases, most programs embed the field in double quotes.@footnote{The
CSV format lacked a formal standard definition for many years.
@uref{http://www.ietf.org/rfc/rfc4180.txt, RFC 4180}
standardizes the most common practices.}
So, we might have data like this:

@example
@c file eg/misc/addresses.csv
Robbins,Arnold,"1234 A Pretty Street, NE",MyTown,MyState,12345-6789,USA
@c endfile
@end example

@cindex @command{gawk} @subentry @code{FPAT} variable in
@cindex @code{FPAT} variable
The @code{FPAT} variable offers a solution for cases like this.
The value of @code{FPAT} should be a string that provides a regular expression.
This regular expression describes the contents of each field.

In the case of CSV data as presented here, each field is either ``anything that
is not a comma,'' or ``a double quote, anything that is not a double quote, and a
closing double quote.''  (There are more complicated definitions of CSV data,
treated shortly.)
If written as a regular expression constant
(@pxref{Regexp}),
we would have @code{/([^,]+)|("[^"]+")/}.
Writing this as a string requires us to escape the double quotes, leading to:

@example
FPAT = "([^,]+)|(\"[^\"]+\")"
@end example

Putting this to use, here is a simple program to parse the data:

@example
@c file eg/misc/simple-csv.awk
@group
BEGIN @{
    FPAT = "([^,]+)|(\"[^\"]+\")"
@}
@end group

@group
@{
    print "NF = ", NF
    for (i = 1; i <= NF; i++) @{
        printf("$%d = <%s>\n", i, $i)
    @}
@}
@end group
@c endfile
@end example

When run, we get the following:

@example
$ @kbd{gawk -f simple-csv.awk addresses.csv}
NF =  7
$1 = 
$2 = 
$3 = <"1234 A Pretty Street, NE">
$4 = 
$5 = 
$6 = <12345-6789>
$7 = 
@end example

Note the embedded comma in the value of @code{$3}.

A straightforward improvement when processing CSV data of this sort
would be to remove the quotes when they occur, with something like this:

@example
if (substr($i, 1, 1) == "\"") @{
    len = length($i)
    $i = substr($i, 2, len - 2)    # Get text within the two quotes
@}
@end example

@quotation NOTE
Some programs export CSV data that contains embedded newlines between
the double quotes.  @command{gawk} provides no way to deal with this.
Even though a formal specification for CSV data exists, there isn't much
more to be done;
the @code{FPAT} mechanism provides an elegant solution for the majority
of cases, and the @command{gawk} developers are satisfied with that.
@end quotation

As written, the regexp used for @code{FPAT} requires that each field
contain at least one character.  A straightforward modification
(changing the first @samp{+} to @samp{*}) allows fields to be empty:

@example
FPAT = "([^,]*)|(\"[^\"]+\")"
@end example

@c FIXME: 4/2015
@c Consider use of FPAT = "([^,]*)|(\"[^\"]*\")"
@c (star in latter part of value) to allow quoted strings to be empty.
@c Per email from Ed Morton 

As with @code{FS}, the @code{IGNORECASE} variable (@pxref{User-modified})
affects field splitting with @code{FPAT}.

Assigning a value to @code{FPAT} overrides field splitting
with @code{FS} and with @code{FIELDWIDTHS}.

Finally, the @code{patsplit()} function makes the same functionality
available for splitting regular strings (@pxref{String Functions}).

@node More CSV
@subsection More on CSV Files

@cindex Collado, Manuel
Manuel Collado notes that in addition to commas, a CSV field can also
contains quotes, that have to be escaped by doubling them. The previously
described regexps fail to accept quoted fields with both commas and
quotes inside. He suggests that the simplest @code{FPAT} expression that
recognizes this kind of fields is @code{/([^,]*)|("([^"]|"")+")/}. He
provides the following input data to test these variants:

@example
@c file eg/misc/sample.csv
p,"q,r",s
p,"q""r",s
p,"q,""r",s
p,"",s
p,,s
@c endfile
@end example

@noindent
And here is his test program:

@example
@c file eg/misc/test-csv.awk
@group
BEGIN @{
     fp[0] = "([^,]+)|(\"[^\"]+\")"
     fp[1] = "([^,]*)|(\"[^\"]+\")"
     fp[2] = "([^,]*)|(\"([^\"]|\"\")+\")"
     FPAT = fp[fpat+0]
@}
@end group

@group
@{
     print "<" $0 ">"
     printf("NF = %s ", NF)
     for (i = 1; i <= NF; i++) @{
         printf("<%s>", $i)
     @}
     print ""
@}
@end group
@c endfile
@end example

When run on the third variant, it produces:

@example
$ @kbd{gawk -v fpat=2 -f test-csv.awk sample.csv}
@print{} 
@print{} NF = 3 

<"q,r"> @print{} @print{} NF = 3

<"q""r"> @print{} @print{} NF = 3

<"q,""r"> @print{} @print{} NF = 3

<""> @print{} @print{} NF = 3

<> @end example @node Testing field creation @section Checking How @command{gawk} Is Splitting Records @cindex @command{gawk} @subentry splitting fields and As we've seen, @command{gawk} provides three independent methods to split input records into fields. The mechanism used is based on which of the three variables---@code{FS}, @code{FIELDWIDTHS}, or @code{FPAT}---was last assigned to. In addition, an API input parser may choose to override the record parsing mechanism; please refer to @ref{Input Parsers} for further information about this feature. To restore normal field splitting after using @code{FIELDWIDTHS} and/or @code{FPAT}, simply assign a value to @code{FS}. You can use @samp{FS = FS} to do this, without having to know the current value of @code{FS}. In order to tell which kind of field splitting is in effect, use @code{PROCINFO["FS"]} (@pxref{Auto-set}). The value is @code{"FS"} if regular field splitting is being used, @code{"FIELDWIDTHS"} if fixed-width field splitting is being used, or @code{"FPAT"} if content-based field splitting is being used: @example if (PROCINFO["FS"] == "FS") @var{regular field splitting} @dots{} else if (PROCINFO["FS"] == "FIELDWIDTHS") @var{fixed-width field splitting} @dots{} else if (PROCINFO["FS"] == "FPAT") @var{content-based field splitting} @dots{} else @var{API input parser field splitting} @dots{} @ii{(advanced feature)} @end example This information is useful when writing a function that needs to temporarily change @code{FS} or @code{FIELDWIDTHS}, read some records, and then restore the original settings (@pxref{Passwd Functions} for an example of such a function). @node Multiple Line @section Multiple-Line Records @cindex multiple-line records @cindex records @subentry multiline @cindex input @subentry multiline records @cindex files @subentry reading @subentry multiline records @cindex input, files @seeentry{input files} In some databases, a single line cannot conveniently hold all the information in one entry. In such cases, you can use multiline records. The first step in doing this is to choose your data format. @cindex record separators @subentry with multiline records One technique is to use an unusual character or string to separate records. For example, you could use the formfeed character (written @samp{\f} in @command{awk}, as in C) to separate them, making each record a page of the file. To do this, just set the variable @code{RS} to @code{"\f"} (a string containing the formfeed character). Any other character could equally well be used, as long as it won't be part of the data in a record. @cindex @code{RS} variable @subentry multiline records and Another technique is to have blank lines separate records. By a special dispensation, an empty string as the value of @code{RS} indicates that records are separated by one or more blank lines. When @code{RS} is set to the empty string, each record always ends at the first blank line encountered. The next record doesn't start until the first nonblank line that follows. No matter how many blank lines appear in a row, they all act as one record separator. (Blank lines must be completely empty; lines that contain only whitespace do not count.) @cindex leftmost longest match @cindex matching @subentry leftmost longest You can achieve the same effect as @samp{RS = ""} by assigning the string @code{"\n\n+"} to @code{RS}. This regexp matches the newline at the end of the record and one or more blank lines after the record. In addition, a regular expression always matches the longest possible sequence when there is a choice (@pxref{Leftmost Longest}). So, the next record doesn't start until the first nonblank line that follows---no matter how many blank lines appear in a row, they are considered one record separator. @cindex dark corner @subentry multiline records However, there is an important difference between @samp{RS = ""} and @samp{RS = "\n\n+"}. In the first case, leading newlines in the input @value{DF} are ignored, and if a file ends without extra blank lines after the last record, the final newline is removed from the record. In the second case, this special processing is not done. @value{DARKCORNER} @cindex field separator @subentry in multiline records @cindex @code{FS} variable @subentry in multiline records Now that the input is separated into records, the second step is to separate the fields in the records. One way to do this is to divide each of the lines into fields in the normal manner. This happens by default as the result of a special feature. When @code{RS} is set to the empty string @emph{and} @code{FS} is set to a single character, the newline character @emph{always} acts as a field separator. This is in addition to whatever field separations result from @code{FS}. @quotation NOTE When @code{FS} is the null string (@code{""}) or a regexp, this special feature of @code{RS} does not apply. It does apply to the default field separator of a single space: @samp{FS = @w{" "}}. Note that language in the POSIX specification implies that this special feature should apply when @code{FS} is a regexp. However, Unix @command{awk} has never behaved that way, nor has @command{gawk}. This is essentially a bug in POSIX. @c Noted as of 4/2019; working to get the standard fixed. @end quotation The original motivation for this special exception was probably to provide useful behavior in the default case (i.e., @code{FS} is equal to @w{@code{" "}}). This feature can be a problem if you really don't want the newline character to separate fields, because there is no way to prevent it. However, you can work around this by using the @code{split()} function to break up the record manually (@pxref{String Functions}). If you have a single-character field separator, you can work around the special feature in a different way, by making @code{FS} into a regexp for that single character. For example, if the field separator is a percent character, instead of @samp{FS = "%"}, use @samp{FS = "[%]"}. Another way to separate fields is to put each field on a separate line: to do this, just set the variable @code{FS} to the string @code{"\n"}. (This single-character separator matches a single newline.) A practical example of a @value{DF} organized this way might be a mailing list, where blank lines separate the entries. Consider a mailing list in a file named @file{addresses}, which looks like this: @example Jane Doe 123 Main Street Anywhere, SE 12345-6789 John Smith 456 Tree-lined Avenue Smallville, MW 98765-4321 @dots{} @end example @noindent A simple program to process this file is as follows: @example # addrs.awk --- simple mailing list program # Records are separated by blank lines. # Each line is one field. BEGIN @{ RS = "" ; FS = "\n" @} @{ print "Name is:", $1 print "Address is:", $2 print "City and State are:", $3 print "" @} @end example Running the program produces the following output: @example $ @kbd{awk -f addrs.awk addresses} @print{} Name is: Jane Doe @print{} Address is: 123 Main Street @print{} City and State are: Anywhere, SE 12345-6789 @print{} @print{} Name is: John Smith @print{} Address is: 456 Tree-lined Avenue @print{} City and State are: Smallville, MW 98765-4321 @print{} @dots{} @end example @xref{Labels Program} for a more realistic program dealing with address lists. The following list summarizes how records are split, based on the value of @ifinfo @code{RS}. (@samp{==} means ``is equal to.'') @end ifinfo @ifnotinfo @code{RS}: @end ifnotinfo @table @code @item RS == "\n" Records are separated by the newline character (@samp{\n}). In effect, every line in the @value{DF} is a separate record, including blank lines. This is the default. @item RS == @var{any single character} Records are separated by each occurrence of the character. Multiple successive occurrences delimit empty records. @item RS == "" Records are separated by runs of blank lines. When @code{FS} is a single character, then the newline character always serves as a field separator, in addition to whatever value @code{FS} may have. Leading and trailing newlines in a file are ignored. @item RS == @var{regexp} Records are separated by occurrences of characters that match @var{regexp}. Leading and trailing matches of @var{regexp} delimit empty records. (This is a @command{gawk} extension; it is not specified by the POSIX standard.) @end table @cindex @command{gawk} @subentry @code{RT} variable in @cindex @code{RT} variable @cindex differences in @command{awk} and @command{gawk} @subentry @code{RS}/@code{RT} variables If not in compatibility mode (@pxref{Options}), @command{gawk} sets @code{RT} to the input text that matched the value specified by @code{RS}. But if the input file ended without any text that matches @code{RS}, then @command{gawk} sets @code{RT} to the null string. @node Getline @section Explicit Input with @code{getline} @cindex @code{getline} command @subentry explicit input with @cindex input @subentry explicit So far we have been getting our input data from @command{awk}'s main input stream---either the standard input (usually your keyboard, sometimes the output from another program) or the files specified on the command line. The @command{awk} language has a special built-in command called @code{getline} that can be used to read input under your explicit control. The @code{getline} command is used in several different ways and should @emph{not} be used by beginners. The examples that follow the explanation of the @code{getline} command include material that has not been covered yet. Therefore, come back and study the @code{getline} command @emph{after} you have reviewed the rest of @ifinfo this @value{DOCUMENT} @end ifinfo @ifhtml this @value{DOCUMENT} @end ifhtml @ifnotinfo @ifnothtml Parts I and II @end ifnothtml @end ifnotinfo and have a good knowledge of how @command{awk} works. @cindex @command{gawk} @subentry @code{ERRNO} variable in @cindex @code{ERRNO} variable @subentry with @command{getline} command @cindex differences in @command{awk} and @command{gawk} @subentry @code{getline} command @cindex @code{getline} command @subentry return values @cindex @option{--sandbox} option @subentry input redirection with @code{getline} The @code{getline} command returns 1 if it finds a record and 0 if it encounters the end of the file. If there is some error in getting a record, such as a file that cannot be opened, then @code{getline} returns @minus{}1. In this case, @command{gawk} sets the variable @code{ERRNO} to a string describing the error that occurred. If @code{ERRNO} indicates that the I/O operation may be retried, and @code{PROCINFO["@var{input}", "RETRY"]} is set, then @code{getline} returns @minus{}2 instead of @minus{}1, and further calls to @code{getline} may be attempted. @xref{Retrying Input} for further information about this feature. In the following examples, @var{command} stands for a string value that represents a shell command. @quotation NOTE When @option{--sandbox} is specified (@pxref{Options}), reading lines from files, pipes, and coprocesses is disabled. @end quotation @menu * Plain Getline:: Using @code{getline} with no arguments. * Getline/Variable:: Using @code{getline} into a variable. * Getline/File:: Using @code{getline} from a file. * Getline/Variable/File:: Using @code{getline} into a variable from a file. * Getline/Pipe:: Using @code{getline} from a pipe. * Getline/Variable/Pipe:: Using @code{getline} into a variable from a pipe. * Getline/Coprocess:: Using @code{getline} from a coprocess. * Getline/Variable/Coprocess:: Using @code{getline} into a variable from a coprocess. * Getline Notes:: Important things to know about @code{getline}. * Getline Summary:: Summary of @code{getline} Variants. @end menu @node Plain Getline @subsection Using @code{getline} with No Arguments The @code{getline} command can be used without arguments to read input from the current input file. All it does in this case is read the next input record and split it up into fields. This is useful if you've finished processing the current record, but want to do some special processing on the next record @emph{right now}. For example: @c 6/2019: Thanks to Mark Krauze for suggested @c improvements (the inner while loop). @example # Remove text between /* and */, inclusive @{ while ((start = index($0, "/*")) != 0) @{ out = substr($0, 1, start - 1) # leading part of the string rest = substr($0, start + 2) # ... */ ... while ((end = index(rest, "*/")) == 0) @{ # is */ in trailing part? # get more text if (getline <= 0) @{ print("unexpected EOF or error:", ERRNO) > "/dev/stderr" exit @} # build up the line using string concatenation rest = rest $0 @} rest = substr(rest, end + 2) # remove comment # build up the output line using string concatenation $0 = out rest @} print $0 @} @end example This @command{awk} program deletes C-style comments (@samp{/* @dots{} */}) from the input. It uses a number of features we haven't covered yet, including string concatenation (@pxref{Concatenation}) and the @code{index()} and @code{substr()} built-in functions (@pxref{String Functions}). By replacing the @samp{print $0} with other statements, you could perform more complicated processing on the decommented input, such as searching for matches of a regular expression. Here is some sample input: @example mon/*comment*/key rab/*commen t*/bit horse /*comment*/more text part 1 /*comment*/part 2 /*comment*/part 3 no comment @end example When run, the output is: @example $ @kbd{awk -f strip_comments.awk example_text} @print{} monkey @print{} rabbit @print{} horse more text @print{} part 1 part 2 part 3 @print{} no comment @end example This form of the @code{getline} command sets @code{NF}, @code{NR}, @code{FNR}, @code{RT}, and the value of @code{$0}. @quotation NOTE The new value of @code{$0} is used to test the patterns of any subsequent rules. The original value of @code{$0} that triggered the rule that executed @code{getline} is lost. By contrast, the @code{next} statement reads a new record but immediately begins processing it normally, starting with the first rule in the program. @xref{Next Statement}. @end quotation @node Getline/Variable @subsection Using @code{getline} into a Variable @cindex @code{getline} command @subentry into a variable @cindex variables @subentry @code{getline} command into, using You can use @samp{getline @var{var}} to read the next record from @command{awk}'s input into the variable @var{var}. No other processing is done. For example, suppose the next line is a comment or a special string, and you want to read it without triggering any rules. This form of @code{getline} allows you to read that line and store it in a variable so that the main read-a-line-and-check-each-rule loop of @command{awk} never sees it. The following example swaps every two lines of input: @example @group @{ if ((getline tmp) > 0) @{ print tmp print $0 @} else print $0 @} @end group @end example @noindent It takes the following list: @example wan tew free phore @end example @noindent and produces these results: @example tew wan phore free @end example The @code{getline} command used in this way sets only the variables @code{NR}, @code{FNR}, and @code{RT} (and, of course, @var{var}). The record is not split into fields, so the values of the fields (including @code{$0}) and the value of @code{NF} do not change. @node Getline/File @subsection Using @code{getline} from a File @cindex @code{getline} command @subentry from a file @cindex input redirection @cindex redirection @subentry of input @cindex @code{<} (left angle bracket) @subentry @code{<} operator (I/O) @cindex left angle bracket (@code{<}) @subentry @code{<} operator (I/O) @cindex operators @subentry input/output Use @samp{getline < @var{file}} to read the next record from @var{file}. Here, @var{file} is a string-valued expression that specifies the @value{FN}. @samp{< @var{file}} is called a @dfn{redirection} because it directs input to come from a different place. For example, the following program reads its input record from the file @file{secondary.input} when it encounters a first field with a value equal to 10 in the current input file: @example @{ if ($1 == 10) @{ getline < "secondary.input" print @} else print @} @end example Because the main input stream is not used, the values of @code{NR} and @code{FNR} are not changed. However, the record it reads is split into fields in the normal manner, so the values of @code{$0} and the other fields are changed, resulting in a new value of @code{NF}. @code{RT} is also set. @cindex POSIX @command{awk} @subentry @code{<} operator and @c Thanks to Paul Eggert for initial wording here According to POSIX, @samp{getline < @var{expression}} is ambiguous if @var{expression} contains unparenthesized operators other than @samp{$}; for example, @samp{getline < dir "/" file} is ambiguous because the concatenation operator (not discussed yet; @pxref{Concatenation}) is not parenthesized. You should write it as @samp{getline < (dir "/" file)} if you want your program to be portable to all @command{awk} implementations. @node Getline/Variable/File @subsection Using @code{getline} into a Variable from a File @cindex variables @subentry @code{getline} command into, using Use @samp{getline @var{var} < @var{file}} to read input from the file @var{file}, and put it in the variable @var{var}. As earlier, @var{file} is a string-valued expression that specifies the file from which to read. In this version of @code{getline}, none of the predefined variables are changed and the record is not split into fields. The only variable changed is @var{var}.@footnote{This is not quite true. @code{RT} could be changed if @code{RS} is a regular expression.} For example, the following program copies all the input files to the output, except for records that say @w{@samp{@@include @var{filename}}}. Such a record is replaced by the contents of the file @var{filename}: @example @{ if (NF == 2 && $1 == "@@include") @{ while ((getline line < $2) > 0) print line close($2) @} else print @} @end example Note here how the name of the extra input file is not built into the program; it is taken directly from the data, specifically from the second field on the @code{@@include} line. The @code{close()} function is called to ensure that if two identical @code{@@include} lines appear in the input, the entire specified file is included twice. @xref{Close Files And Pipes}. One deficiency of this program is that it does not process nested @code{@@include} statements (i.e., @code{@@include} statements in included files) the way a true macro preprocessor would. @xref{Igawk Program} for a program that does handle nested @code{@@include} statements. @node Getline/Pipe @subsection Using @code{getline} from a Pipe @c From private email, dated October 2, 1988. Used by permission, March 2013. @cindex Kernighan, Brian @quotation @i{Omniscience has much to recommend it. Failing that, attention to details would be useful.} @author Brian Kernighan @end quotation @cindex @code{|} (vertical bar) @subentry @code{|} operator (I/O) @cindex vertical bar (@code{|}) @subentry @code{|} operator (I/O) @cindex input pipeline @cindex pipe @subentry input @cindex operators @subentry input/output The output of a command can also be piped into @code{getline}, using @samp{@var{command} | getline}. In this case, the string @var{command} is run as a shell command and its output is piped into @command{awk} to be used as input. This form of @code{getline} reads one record at a time from the pipe. For example, the following program copies its input to its output, except for lines that begin with @samp{@@execute}, which are replaced by the output produced by running the rest of the line as a shell command: @example @group @{ if ($1 == "@@execute") @{ tmp = substr($0, 10) # Remove "@@execute" while ((tmp | getline) > 0) print close(tmp) @} else print @} @end group @end example @noindent The @code{close()} function is called to ensure that if two identical @samp{@@execute} lines appear in the input, the command is run for each one. @ifnottex @ifnotdocbook @xref{Close Files And Pipes}. @end ifnotdocbook @end ifnottex @c This example is unrealistic, since you could just use system Given the input: @example foo bar baz @@execute who bletch @end example @noindent the program might produce: @cindex Robbins @subentry Bill @cindex Robbins @subentry Miriam @cindex Robbins @subentry Arnold @example foo bar baz arnold ttyv0 Jul 13 14:22 miriam ttyp0 Jul 13 14:23 (murphy:0) bill ttyp1 Jul 13 14:23 (murphy:0) bletch @end example @noindent Notice that this program ran the command @command{who} and printed the result. (If you try this program yourself, you will of course get different results, depending upon who is logged in on your system.) This variation of @code{getline} splits the record into fields, sets the value of @code{NF}, and recomputes the value of @code{$0}. The values of @code{NR} and @code{FNR} are not changed. @code{RT} is set. @cindex POSIX @command{awk} @subentry @code{|} I/O operator and @c Thanks to Paul Eggert for initial wording here According to POSIX, @samp{@var{expression} | getline} is ambiguous if @var{expression} contains unparenthesized operators other than @samp{$}---for example, @samp{@w{"echo "} "date" | getline} is ambiguous because the concatenation operator is not parenthesized. You should write it as @samp{(@w{"echo "} "date") | getline} if you want your program to be portable to all @command{awk} implementations. @cindex Brian Kernighan's @command{awk} @cindex @command{mawk} utility @quotation NOTE Unfortunately, @command{gawk} has not been consistent in its treatment of a construct like @samp{@w{"echo "} "date" | getline}. Most versions, including the current version, treat it as @samp{@w{("echo "} "date") | getline}. (This is also how BWK @command{awk} behaves.) Some versions instead treat it as @samp{@w{"echo "} ("date" | getline)}. (This is how @command{mawk} behaves.) In short, @emph{always} use explicit parentheses, and then you won't have to worry. @end quotation @node Getline/Variable/Pipe @subsection Using @code{getline} into a Variable from a Pipe @cindex variables @subentry @code{getline} command into, using When you use @samp{@var{command} | getline @var{var}}, the output of @var{command} is sent through a pipe to @code{getline} and into the variable @var{var}. For example, the following program reads the current date and time into the variable @code{current_time}, using the @command{date} utility, and then prints it: @example BEGIN @{ "date" | getline current_time close("date") print "Report printed on " current_time @} @end example In this version of @code{getline}, none of the predefined variables are changed and the record is not split into fields. However, @code{RT} is set. @ifinfo @c Thanks to Paul Eggert for initial wording here According to POSIX, @samp{@var{expression} | getline @var{var}} is ambiguous if @var{expression} contains unparenthesized operators other than @samp{$}; for example, @samp{@w{"echo "} "date" | getline @var{var}} is ambiguous because the concatenation operator is not parenthesized. You should write it as @samp{(@w{"echo "} "date") | getline @var{var}} if you want your program to be portable to other @command{awk} implementations. @end ifinfo @node Getline/Coprocess @subsection Using @code{getline} from a Coprocess @cindex coprocesses @subentry @code{getline} from @cindex @code{getline} command @subentry coprocesses, using from @cindex @code{|} (vertical bar) @subentry @code{|&} operator (I/O) @cindex vertical bar (@code{|}) @subentry @code{|&} operator (I/O) @cindex operators @subentry input/output @cindex differences in @command{awk} and @command{gawk} @subentry input/output operators Reading input into @code{getline} from a pipe is a one-way operation. The command that is started with @samp{@var{command} | getline} only sends data @emph{to} your @command{awk} program. On occasion, you might want to send data to another program for processing and then read the results back. @command{gawk} allows you to start a @dfn{coprocess}, with which two-way communications are possible. This is done with the @samp{|&} operator. Typically, you write data to the coprocess first and then read the results back, as shown in the following: @example print "@var{some query}" |& "db_server" "db_server" |& getline @end example @noindent which sends a query to @command{db_server} and then reads the results. The values of @code{NR} and @code{FNR} are not changed, because the main input stream is not used. However, the record is split into fields in the normal manner, thus changing the values of @code{$0}, of the other fields, and of @code{NF} and @code{RT}. Coprocesses are an advanced feature. They are discussed here only because this is the @value{SECTION} on @code{getline}. @xref{Two-way I/O}, where coprocesses are discussed in more detail. @node Getline/Variable/Coprocess @subsection Using @code{getline} into a Variable from a Coprocess @cindex variables @subentry @code{getline} command into, using When you use @samp{@var{command} |& getline @var{var}}, the output from the coprocess @var{command} is sent through a two-way pipe to @code{getline} and into the variable @var{var}. In this version of @code{getline}, none of the predefined variables are changed and the record is not split into fields. The only variable changed is @var{var}. However, @code{RT} is set. @ifinfo Coprocesses are an advanced feature. They are discussed here only because this is the @value{SECTION} on @code{getline}. @xref{Two-way I/O}, where coprocesses are discussed in more detail. @end ifinfo @node Getline Notes @subsection Points to Remember About @code{getline} Here are some miscellaneous points about @code{getline} that you should bear in mind: @itemize @value{BULLET} @item When @code{getline} changes the value of @code{$0} and @code{NF}, @command{awk} does @emph{not} automatically jump to the start of the program and start testing the new record against every pattern. However, the new record is tested against any subsequent rules. @cindex differences in @command{awk} and @command{gawk} @subentry implementation limitations @cindex implementation issues, @command{gawk} @subentry limits @cindex @command{awk} @subentry implementations @subentry limits @cindex @command{gawk} @subentry implementation issues @subentry limits @item Some very old @command{awk} implementations limit the number of pipelines that an @command{awk} program may have open to just one. In @command{gawk}, there is no such limit. You can open as many pipelines (and coprocesses) as the underlying operating system permits. @cindex side effects @subentry @code{FILENAME} variable @cindex @code{FILENAME} variable @subentry @code{getline}, setting with @cindex dark corner @subentry @code{FILENAME} variable @cindex @code{getline} command @subentry @code{FILENAME} variable and @cindex @code{BEGIN} pattern @subentry @code{getline} and @item An interesting side effect occurs if you use @code{getline} without a redirection inside a @code{BEGIN} rule. Because an unredirected @code{getline} reads from the command-line @value{DF}s, the first @code{getline} command causes @command{awk} to set the value of @code{FILENAME}. Normally, @code{FILENAME} does not have a value inside @code{BEGIN} rules, because you have not yet started to process the command-line @value{DF}s. @value{DARKCORNER} (See @ref{BEGIN/END}; also @pxref{Auto-set}.) @item Using @code{FILENAME} with @code{getline} (@samp{getline < FILENAME}) is likely to be a source of confusion. @command{awk} opens a separate input stream from the current input file. However, by not using a variable, @code{$0} and @code{NF} are still updated. If you're doing this, it's probably by accident, and you should reconsider what it is you're trying to accomplish. @item @ifdocbook The next @value{SECTION} @end ifdocbook @ifnotdocbook @ref{Getline Summary}, @end ifnotdocbook presents a table summarizing the @code{getline} variants and which variables they can affect. It is worth noting that those variants that do not use redirection can cause @code{FILENAME} to be updated if they cause @command{awk} to start reading a new input file. @item @cindex Moore, Duncan If the variable being assigned is an expression with side effects, different versions of @command{awk} behave differently upon encountering end-of-file. Some versions don't evaluate the expression; many versions (including @command{gawk}) do. Here is an example, courtesy of Duncan Moore: @ignore Date: Sun, 01 Apr 2012 11:49:33 +0100 From: Duncan Moore @end ignore @example BEGIN @{ system("echo 1 > f") while ((getline a[++c] < "f") > 0) @{ @} print c @} @end example @noindent Here, the side effect is the @samp{++c}. Is @code{c} incremented if end-of-file is encountered before the element in @code{a} is assigned? @command{gawk} treats @code{getline} like a function call, and evaluates the expression @samp{a[++c]} before attempting to read from @file{f}. However, some versions of @command{awk} only evaluate the expression once they know that there is a string value to be assigned. @end itemize @node Getline Summary @subsection Summary of @code{getline} Variants @cindex @code{getline} command @subentry variants @ref{table-getline-variants} summarizes the eight variants of @code{getline}, listing which predefined variables are set by each one, and whether the variant is standard or a @command{gawk} extension. Note: for each variant, @command{gawk} sets the @code{RT} predefined variable. @float Table,table-getline-variants @caption{@code{getline} variants and what they set} @multitable @columnfractions .33 .38 .27 @headitem Variant @tab Effect @tab @command{awk} / @command{gawk} @item @code{getline} @tab Sets @code{$0}, @code{NF}, @code{FNR}, @code{NR}, and @code{RT} @tab @command{awk} @item @code{getline} @var{var} @tab Sets @var{var}, @code{FNR}, @code{NR}, and @code{RT} @tab @command{awk} @item @code{getline <} @var{file} @tab Sets @code{$0}, @code{NF}, and @code{RT} @tab @command{awk} @item @code{getline @var{var} < @var{file}} @tab Sets @var{var} and @code{RT} @tab @command{awk} @item @var{command} @code{| getline} @tab Sets @code{$0}, @code{NF}, and @code{RT} @tab @command{awk} @item @var{command} @code{| getline} @var{var} @tab Sets @var{var} and @code{RT} @tab @command{awk} @item @var{command} @code{|& getline} @tab Sets @code{$0}, @code{NF}, and @code{RT} @tab @command{gawk} @item @var{command} @code{|& getline} @var{var} @tab Sets @var{var} and @code{RT} @tab @command{gawk} @end multitable @end float @node Read Timeout @section Reading Input with a Timeout @cindex timeout, reading input @cindex differences in @command{awk} and @command{gawk} @subentry read timeouts This @value{SECTION} describes a feature that is specific to @command{gawk}. You may specify a timeout in milliseconds for reading input from the keyboard, a pipe, or two-way communication, including TCP/IP sockets. This can be done on a per-input, per-command, or per-connection basis, by setting a special element in the @code{PROCINFO} array (@pxref{Auto-set}): @example PROCINFO["input_name", "READ_TIMEOUT"] = @var{timeout in milliseconds} @end example When set, this causes @command{gawk} to time out and return failure if no data is available to read within the specified timeout period. For example, a TCP client can decide to give up on receiving any response from the server after a certain amount of time: @example @group Service = "/inet/tcp/0/localhost/daytime" PROCINFO[Service, "READ_TIMEOUT"] = 100 if ((Service |& getline) > 0) print $0 else if (ERRNO != "") print ERRNO @end group @end example Here is how to read interactively from the user@footnote{This assumes that standard input is the keyboard.} without waiting for more than five seconds: @example PROCINFO["/dev/stdin", "READ_TIMEOUT"] = 5000 while ((getline < "/dev/stdin") > 0) print $0 @end example @command{gawk} terminates the read operation if input does not arrive after waiting for the timeout period, returns failure, and sets @code{ERRNO} to an appropriate string value. A negative or zero value for the timeout is the same as specifying no timeout at all. A timeout can also be set for reading from the keyboard in the implicit loop that reads input records and matches them against patterns, like so: @example $ @kbd{gawk 'BEGIN @{ PROCINFO["-", "READ_TIMEOUT"] = 5000 @}} > @kbd{@{ print "You entered: " $0 @}'} @kbd{gawk} @print{} You entered: gawk @end example In this case, failure to respond within five seconds results in the following error message: @example @error{} gawk: cmd. line:2: (FILENAME=- FNR=1) fatal: error reading input file `-': Connection timed out @end example The timeout can be set or changed at any time, and will take effect on the next attempt to read from the input device. In the following example, we start with a timeout value of one second, and progressively reduce it by one-tenth of a second until we wait indefinitely for the input to arrive: @example PROCINFO[Service, "READ_TIMEOUT"] = 1000 while ((Service |& getline) > 0) @{ print $0 PROCINFO[Service, "READ_TIMEOUT"] -= 100 @} @end example @quotation NOTE You should not assume that the read operation will block exactly after the tenth record has been printed. It is possible that @command{gawk} will read and buffer more than one record's worth of data the first time. Because of this, changing the value of timeout like in the preceding example is not very useful. @end quotation @cindex @env{GAWK_READ_TIMEOUT} environment variable @cindex environment variables @subentry @env{GAWK_READ_TIMEOUT} If the @code{PROCINFO} element is not present and the @env{GAWK_READ_TIMEOUT} environment variable exists, @command{gawk} uses its value to initialize the timeout value. The exclusive use of the environment variable to specify timeout has the disadvantage of not being able to control it on a per-command or per-connection basis. @command{gawk} considers a timeout event to be an error even though the attempt to read from the underlying device may succeed in a later attempt. This is a limitation, and it also means that you cannot use this to multiplex input from two or more sources. @xref{Retrying Input} for a way to enable later I/O attempts to succeed. Assigning a timeout value prevents read operations from blocking indefinitely. But bear in mind that there are other ways @command{gawk} can stall waiting for an input device to be ready. A network client can sometimes take a long time to establish a connection before it can start reading any data, or the attempt to open a FIFO special file for reading can block indefinitely until some other process opens it for writing. @node Retrying Input @section Retrying Reads After Certain Input Errors @cindex retrying input @cindex differences in @command{awk} and @command{gawk} @subentry retrying input This @value{SECTION} describes a feature that is specific to @command{gawk}. When @command{gawk} encounters an error while reading input, by default @code{getline} returns @minus{}1, and subsequent attempts to read from that file result in an end-of-file indication. However, you may optionally instruct @command{gawk} to allow I/O to be retried when certain errors are encountered by setting a special element in the @code{PROCINFO} array (@pxref{Auto-set}): @example PROCINFO["@var{input_name}", "RETRY"] = 1 @end example When this element exists, @command{gawk} checks the value of the system (C language) @code{errno} variable when an I/O error occurs. If @code{errno} indicates a subsequent I/O attempt may succeed, @code{getline} instead returns @minus{}2 and further calls to @code{getline} may succeed. This applies to the @code{errno} values @code{EAGAIN}, @code{EWOULDBLOCK}, @code{EINTR}, or @code{ETIMEDOUT}. This feature is useful in conjunction with @code{PROCINFO["@var{input_name}", "READ_TIMEOUT"]} or situations where a file descriptor has been configured to behave in a non-blocking fashion. @node Command-line directories @section Directories on the Command Line @cindex differences in @command{awk} and @command{gawk} @subentry command-line directories @cindex directories @subentry command-line @cindex command line @subentry directories on According to the POSIX standard, files named on the @command{awk} command line must be text files; it is a fatal error if they are not. Most versions of @command{awk} treat a directory on the command line as a fatal error. By default, @command{gawk} produces a warning for a directory on the command line, but otherwise ignores it. This makes it easier to use shell wildcards with your @command{awk} program: @example $ @kbd{gawk -f whizprog.awk *} @ii{Directories could kill this program} @end example If either of the @option{--posix} or @option{--traditional} options is given, then @command{gawk} reverts to treating a directory on the command line as a fatal error. @xref{Extension Sample Readdir} for a way to treat directories as usable data from an @command{awk} program. @node Input Summary @section Summary @itemize @value{BULLET} @item Input is split into records based on the value of @code{RS}. The possibilities are as follows: @multitable @columnfractions .25 .35 .40 @headitem Value of @code{RS} @tab Records are split on @dots{} @tab @command{awk} / @command{gawk} @item Any single character @tab That character @tab @command{awk} @item The empty string (@code{""}) @tab Runs of two or more newlines @tab @command{awk} @item A regexp @tab Text that matches the regexp @tab @command{gawk} @end multitable @item @code{FNR} indicates how many records have been read from the current input file; @code{NR} indicates how many records have been read in total. @item @command{gawk} sets @code{RT} to the text matched by @code{RS}. @item After splitting the input into records, @command{awk} further splits the records into individual fields, named @code{$1}, @code{$2}, and so on. @code{$0} is the whole record, and @code{NF} indicates how many fields there are. The default way to split fields is between whitespace characters. @item Fields may be referenced using a variable, as in @code{$NF}. Fields may also be assigned values, which causes the value of @code{$0} to be recomputed when it is later referenced. Assigning to a field with a number greater than @code{NF} creates the field and rebuilds the record, using @code{OFS} to separate the fields. Incrementing @code{NF} does the same thing. Decrementing @code{NF} throws away fields and rebuilds the record. @item Field splitting is more complicated than record splitting: @multitable @columnfractions .40 .40 .20 @headitem Field separator value @tab Fields are split @dots{} @tab @command{awk} / @command{gawk} @item @code{FS == " "} @tab On runs of whitespace @tab @command{awk} @item @code{FS == @var{any single character}} @tab On that character @tab @command{awk} @item @code{FS == @var{regexp}} @tab On text matching the regexp @tab @command{awk} @item @code{FS == ""} @tab Such that each individual character is a separate field @tab @command{gawk} @item @code{FIELDWIDTHS == @var{list of columns}} @tab Based on character position @tab @command{gawk} @item @code{FPAT == @var{regexp}} @tab On the text surrounding text matching the regexp @tab @command{gawk} @end multitable @item Using @samp{FS = "\n"} causes the entire record to be a single field (assuming that newlines separate records). @item @code{FS} may be set from the command line using the @option{-F} option. This can also be done using command-line variable assignment. @item Use @code{PROCINFO["FS"]} to see how fields are being split. @item Use @code{getline} in its various forms to read additional records from the default input stream, from a file, or from a pipe or coprocess. @item Use @code{PROCINFO[@var{file}, "READ_TIMEOUT"]} to cause reads to time out for @var{file}. @cindex POSIX mode @item Directories on the command line are fatal for standard @command{awk}; @command{gawk} ignores them if not in POSIX mode. @end itemize @c EXCLUDE START @node Input Exercises @section Exercises @enumerate @item Using the @code{FIELDWIDTHS} variable (@pxref{Constant Size}), write a program to read election data, where each record represents one voter's votes. Come up with a way to define which columns are associated with each ballot item, and print the total votes, including abstentions, for each item. @end enumerate @c EXCLUDE END @node Printing @chapter Printing Output @cindex printing @cindex output, printing @seeentry{printing} One of the most common programming actions is to @dfn{print}, or output, some or all of the input. Use the @code{print} statement for simple output, and the @code{printf} statement for fancier formatting. The @code{print} statement is not limited when computing @emph{which} values to print. However, with two exceptions, you cannot specify @emph{how} to print them---how many columns, whether to use exponential notation or not, and so on. (For the exceptions, @pxref{Output Separators} and @ref{OFMT}.) For printing with specifications, you need the @code{printf} statement (@pxref{Printf}). @cindex @code{print} statement @cindex @code{printf} statement Besides basic and formatted printing, this @value{CHAPTER} also covers I/O redirections to files and pipes, introduces the special @value{FN}s that @command{gawk} processes internally, and discusses the @code{close()} built-in function. @menu * Print:: The @code{print} statement. * Print Examples:: Simple examples of @code{print} statements. * Output Separators:: The output separators and how to change them. * OFMT:: Controlling Numeric Output With @code{print}. * Printf:: The @code{printf} statement. * Redirection:: How to redirect output to multiple files and pipes. * Special FD:: Special files for I/O. * Special Files:: File name interpretation in @command{gawk}. @command{gawk} allows access to inherited file descriptors. * Close Files And Pipes:: Closing Input and Output Files and Pipes. * Nonfatal:: Enabling Nonfatal Output. * Output Summary:: Output summary. * Output Exercises:: Exercises. @end menu @node Print @section The @code{print} Statement Use the @code{print} statement to produce output with simple, standardized formatting. You specify only the strings or numbers to print, in a list separated by commas. They are output, separated by single spaces, followed by a newline. The statement looks like this: @example print @var{item1}, @var{item2}, @dots{} @end example @noindent The entire list of items may be optionally enclosed in parentheses. The parentheses are necessary if any of the item expressions uses the @samp{>} relational operator; otherwise it could be confused with an output redirection (@pxref{Redirection}). The items to print can be constant strings or numbers, fields of the current record (such as @code{$1}), variables, or any @command{awk} expression. Numeric values are converted to strings and then printed. @cindex records @subentry printing @cindex lines @subentry blank, printing @cindex text, printing The simple statement @samp{print} with no items is equivalent to @samp{print $0}: it prints the entire current record. To print a blank line, use @samp{print ""}. To print a fixed piece of text, use a string constant, such as @w{@code{"Don't Panic"}}, as one item. If you forget to use the double-quote characters, your text is taken as an @command{awk} expression, and you will probably get an error. Keep in mind that a space is printed between any two items. Note that the @code{print} statement is a statement and not an expression---you can't use it in the pattern part of a pattern--action statement, for example. @node Print Examples @section @code{print} Statement Examples Each @code{print} statement makes at least one line of output. However, it isn't limited to only one line. If an item value is a string containing a newline, the newline is output along with the rest of the string. A single @code{print} statement can make any number of lines this way. @cindex newlines @subentry printing The following is an example of printing a string that contains embedded @ifinfo newlines (the @samp{\n} is an escape sequence, used to represent the newline character; @pxref{Escape Sequences}): @end ifinfo @ifhtml newlines (the @samp{\n} is an escape sequence, used to represent the newline character; @pxref{Escape Sequences}): @end ifhtml @ifnotinfo @ifnothtml newlines: @end ifnothtml @end ifnotinfo @example @group $ @kbd{awk 'BEGIN @{ print "line one\nline two\nline three" @}'} @print{} line one @print{} line two @print{} line three @end group @end example @cindex fields @subentry printing The next example, which is run on the @file{inventory-shipped} file, prints the first two fields of each input record, with a space between them: @example $ @kbd{awk '@{ print $1, $2 @}' inventory-shipped} @print{} Jan 13 @print{} Feb 15 @print{} Mar 15 @dots{} @end example @cindex @code{print} statement @subentry commas, omitting @cindex troubleshooting @subentry @code{print} statement, omitting commas A common mistake in using the @code{print} statement is to omit the comma between two items. This often has the effect of making the items run together in the output, with no space. The reason for this is that juxtaposing two string expressions in @command{awk} means to concatenate them. Here is the same program, without the comma: @example $ @kbd{awk '@{ print $1 $2 @}' inventory-shipped} @print{} Jan13 @print{} Feb15 @print{} Mar15 @dots{} @end example @cindex @code{BEGIN} pattern @subentry headings, adding To someone unfamiliar with the @file{inventory-shipped} file, neither example's output makes much sense. A heading line at the beginning would make it clearer. Let's add some headings to our table of months (@code{$1}) and green crates shipped (@code{$2}). We do this using a @code{BEGIN} rule (@pxref{BEGIN/END}) so that the headings are only printed once: @example awk 'BEGIN @{ print "Month Crates" print "----- ------" @} @{ print $1, $2 @}' inventory-shipped @end example @noindent When run, the program prints the following: @example Month Crates ----- ------ Jan 13 Feb 15 Mar 15 @dots{} @end example @noindent The only problem, however, is that the headings and the table data don't line up! We can fix this by printing some spaces between the two fields: @example @group awk 'BEGIN @{ print "Month Crates" print "----- ------" @} @{ print $1, " ", $2 @}' inventory-shipped @end group @end example @cindex @code{printf} statement @subentry columns, aligning @cindex columns @subentry aligning Lining up columns this way can get pretty complicated when there are many columns to fix. Counting spaces for two or three columns is simple, but any more than this can take up a lot of time. This is why the @code{printf} statement was created (@pxref{Printf}); one of its specialties is lining up columns of data. @cindex line continuations @subentry in @code{print} statement @cindex @code{print} statement @subentry line continuations and @quotation NOTE You can continue either a @code{print} or @code{printf} statement simply by putting a newline after any comma (@pxref{Statements/Lines}). @end quotation @node Output Separators @section Output Separators @cindex @code{OFS} variable As mentioned previously, a @code{print} statement contains a list of items separated by commas. In the output, the items are normally separated by single spaces. However, this doesn't need to be the case; a single space is simply the default. Any string of characters may be used as the @dfn{output field separator} by setting the predefined variable @code{OFS}. The initial value of this variable is the string @w{@code{" "}} (i.e., a single space). The output from an entire @code{print} statement is called an @dfn{output record}. Each @code{print} statement outputs one output record, and then outputs a string called the @dfn{output record separator} (or @code{ORS}). The initial value of @code{ORS} is the string @code{"\n"} (i.e., a newline character). Thus, each @code{print} statement normally makes a separate line. @cindex output @subentry records @cindex output record separator @seeentry{@code{ORS} variable} @cindex @code{ORS} variable @cindex @code{BEGIN} pattern @subentry @code{OFS}/@code{ORS} variables, assigning values to In order to change how output fields and records are separated, assign new values to the variables @code{OFS} and @code{ORS}. The usual place to do this is in the @code{BEGIN} rule (@pxref{BEGIN/END}), so that it happens before any input is processed. It can also be done with assignments on the command line, before the names of the input files, or using the @option{-v} command-line option (@pxref{Options}). The following example prints the first and second fields of each input record, separated by a semicolon, with a blank line added after each newline: @example $ @kbd{awk 'BEGIN @{ OFS = ";"; ORS = "\n\n" @}} > @kbd{@{ print $1, $2 @}' mail-list} @print{} Amelia;555-5553 @print{} @print{} Anthony;555-3412 @print{} @print{} Becky;555-7685 @print{} @print{} Bill;555-1675 @print{} @print{} Broderick;555-0542 @print{} @print{} Camilla;555-2912 @print{} @print{} Fabius;555-1234 @print{} @print{} Julie;555-6699 @print{} @print{} Martin;555-6480 @print{} @print{} Samuel;555-3430 @print{} @print{} Jean-Paul;555-2127 @print{} @end example If the value of @code{ORS} does not contain a newline, the program's output runs together on a single line. @node OFMT @section Controlling Numeric Output with @code{print} @cindex numeric @subentry output format @cindex formats, numeric output When printing numeric values with the @code{print} statement, @command{awk} internally converts each number to a string of characters and prints that string. @command{awk} uses the @code{sprintf()} function to do this conversion (@pxref{String Functions}). For now, it suffices to say that the @code{sprintf()} function accepts a @dfn{format specification} that tells it how to format numbers (or strings), and that there are a number of different ways in which numbers can be formatted. The different format specifications are discussed more fully in @ref{Control Letters}. @cindexawkfunc{sprintf} @cindex @code{OFMT} variable @cindex output @subentry format specifier, @code{OFMT} The predefined variable @code{OFMT} contains the format specification that @code{print} uses with @code{sprintf()} when it wants to convert a number to a string for printing. The default value of @code{OFMT} is @code{"%.6g"}. The way @code{print} prints numbers can be changed by supplying a different format specification for the value of @code{OFMT}, as shown in the following example: @example $ @kbd{awk 'BEGIN @{} > @kbd{OFMT = "%.0f" # print numbers as integers (rounds)} > @kbd{print 17.23, 17.54 @}'} @print{} 17 18 @end example @noindent @cindex dark corner @subentry @code{OFMT} variable @cindex POSIX @command{awk} @subentry @code{OFMT} variable and @cindex @code{OFMT} variable @subentry POSIX @command{awk} and According to the POSIX standard, @command{awk}'s behavior is undefined if @code{OFMT} contains anything but a floating-point conversion specification. @value{DARKCORNER} @node Printf @section Using @code{printf} Statements for Fancier Printing @cindex @code{printf} statement @cindex output @subentry formatted @cindex formatting @subentry output For more precise control over the output format than what is provided by @code{print}, use @code{printf}. With @code{printf} you can specify the width to use for each item, as well as various formatting choices for numbers (such as what output base to use, whether to print an exponent, whether to print a sign, and how many digits to print after the decimal point). @menu * Basic Printf:: Syntax of the @code{printf} statement. * Control Letters:: Format-control letters. * Format Modifiers:: Format-specification modifiers. * Printf Examples:: Several examples. @end menu @node Basic Printf @subsection Introduction to the @code{printf} Statement @cindex @code{printf} statement @subentry syntax of A simple @code{printf} statement looks like this: @example printf @var{format}, @var{item1}, @var{item2}, @dots{} @end example @noindent As for @code{print}, the entire list of arguments may optionally be enclosed in parentheses. Here too, the parentheses are necessary if any of the item expressions uses the @samp{>} relational operator; otherwise, it can be confused with an output redirection (@pxref{Redirection}). @cindex format specifiers The difference between @code{printf} and @code{print} is the @var{format} argument. This is an expression whose value is taken as a string; it specifies how to output each of the other arguments. It is called the @dfn{format string}. The format string is very similar to that in the ISO C library function @code{printf()}. Most of @var{format} is text to output verbatim. Scattered among this text are @dfn{format specifiers}---one per item. Each format specifier says to output the next item in the argument list at that place in the format. The @code{printf} statement does not automatically append a newline to its output. It outputs only what the format string specifies. So if a newline is needed, you must include one in the format string. The output separator variables @code{OFS} and @code{ORS} have no effect on @code{printf} statements. For example: @example @group $ @kbd{awk 'BEGIN @{} > @kbd{ORS = "\nOUCH!\n"; OFS = "+"} > @kbd{msg = "Don\47t Panic!"} > @kbd{printf "%s\n", msg} > @kbd{@}'} @print{} Don't Panic! @end group @end example @noindent Here, neither the @samp{+} nor the @samp{OUCH!} appears in the output message. @node Control Letters @subsection Format-Control Letters @cindex @code{printf} statement @subentry format-control characters @cindex format specifiers @subentry @code{printf} statement A format specifier starts with the character @samp{%} and ends with a @dfn{format-control letter}---it tells the @code{printf} statement how to output one item. The format-control letter specifies what @emph{kind} of value to print. The rest of the format specifier is made up of optional @dfn{modifiers} that control @emph{how} to print the value, such as the field width. Here is a list of the format-control letters: @c @asis for docbook to come out right @table @asis @item @code{%a}, @code{%A} A floating point number of the form [@code{-}]@code{0x@var{h}.@var{hhhh}p+-@var{dd}} (C99 hexadecimal floating point format). For @code{%A}, uppercase letters are used instead of lowercase ones. @quotation NOTE The current POSIX standard requires support for @code{%a} and @code{%A} in @command{awk}. As far as we know, besides @command{gawk}, the only other version of @command{awk} that actually implements it is BWK @command{awk}. It's use is thus highly nonportable! Furthermore, these formats are not available on any system where the underlying C library @code{printf()} function does not support them. As of this writing, among current systems, only OpenVMS is known to not support them. @end quotation @item @code{%c} Print a number as a character; thus, @samp{printf "%c", 65} outputs the letter @samp{A}. The output for a string value is the first character of the string. @cindex dark corner @subentry format-control characters @cindex @command{gawk} @subentry format-control characters @quotation NOTE The POSIX standard says the first character of a string is printed. In locales with multibyte characters, @command{gawk} attempts to convert the leading bytes of the string into a valid wide character and then to print the multibyte encoding of that character. Similarly, when printing a numeric value, @command{gawk} allows the value to be within the numeric range of values that can be held in a wide character. If the conversion to multibyte encoding fails, @command{gawk} uses the low eight bits of the value as the character to print. Other @command{awk} versions generally restrict themselves to printing the first byte of a string or to numeric values within the range of a single byte (0--255). @value{DARKCORNER} @end quotation @item @code{%d}, @code{%i} Print a decimal integer. The two control letters are equivalent. (The @samp{%i} specification is for compatibility with ISO C.) @item @code{%e}, @code{%E} Print a number in scientific (exponential) notation. For example: @example printf "%4.3e\n", 1950 @end example @noindent prints @samp{1.950e+03}, with a total of four significant figures, three of which follow the decimal point. (The @samp{4.3} represents two modifiers, discussed in the next @value{SUBSECTION}.) @samp{%E} uses @samp{E} instead of @samp{e} in the output. @item @code{%f} Print a number in floating-point notation. For example: @example printf "%4.3f", 1950 @end example @noindent prints @samp{1950.000}, with a minimum of four significant figures, three of which follow the decimal point. (The @samp{4.3} represents two modifiers, discussed in the next @value{SUBSECTION}.) On systems supporting IEEE 754 floating-point format, values representing negative infinity are formatted as @samp{-inf} or @samp{-infinity}, and positive infinity as @samp{inf} or @samp{infinity}. The special ``not a number'' value formats as @samp{-nan} or @samp{nan} (@pxref{Math Definitions}). @item @code{%F} Like @samp{%f}, but the infinity and ``not a number'' values are spelled using uppercase letters. The @samp{%F} format is a POSIX extension to ISO C; not all systems support it. On those that don't, @command{gawk} uses @samp{%f} instead. @item @code{%g}, @code{%G} Print a number in either scientific notation or in floating-point notation, whichever uses fewer characters; if the result is printed in scientific notation, @samp{%G} uses @samp{E} instead of @samp{e}. @item @code{%o} Print an unsigned octal integer (@pxref{Nondecimal-numbers}). @item @code{%s} Print a string. @item @code{%u} Print an unsigned decimal integer. (This format is of marginal use, because all numbers in @command{awk} are floating point; it is provided primarily for compatibility with C.) @item @code{%x}, @code{%X} Print an unsigned hexadecimal integer; @samp{%X} uses the letters @samp{A} through @samp{F} instead of @samp{a} through @samp{f} (@pxref{Nondecimal-numbers}). @item @code{%%} Print a single @samp{%}. This does not consume an argument and it ignores any modifiers. @end table @cindex dark corner @subentry format-control characters @cindex @command{gawk} @subentry format-control characters @quotation NOTE When using the integer format-control letters for values that are outside the range of the widest C integer type, @command{gawk} switches to the @samp{%g} format specifier. If @option{--lint} is provided on the command line (@pxref{Options}), @command{gawk} warns about this. Other versions of @command{awk} may print invalid values or do something else entirely. @value{DARKCORNER} @end quotation @quotation NOTE The IEEE 754 standard for floating-point arithmetic allows for special values that represent ``infinity'' (positive and negative) and values that are ``not a number'' (NaN). Input and output of these values occurs as text strings. This is somewhat problematic for the @command{awk} language, which predates the IEEE standard. Further details are provided in @ref{POSIX Floating Point Problems}; please see there. @end quotation @node Format Modifiers @subsection Modifiers for @code{printf} Formats @cindex @code{printf} statement @subentry modifiers @cindex modifiers, in format specifiers A format specification can also include @dfn{modifiers} that can control how much of the item's value is printed, as well as how much space it gets. The modifiers come between the @samp{%} and the format-control letter. We use the bullet symbol ``@bullet{}'' in the following examples to represent spaces in the output. Here are the possible modifiers, in the order in which they may appear: @table @asis @cindex differences in @command{awk} and @command{gawk} @subentry @code{print}/@code{printf} statements @cindex @code{printf} statement @subentry positional specifiers @c the code{} does NOT start a secondary @cindex positional specifiers, @code{printf} statement @item @code{@var{N}$} An integer constant followed by a @samp{$} is a @dfn{positional specifier}. Normally, format specifications are applied to arguments in the order given in the format string. With a positional specifier, the format specification is applied to a specific argument, instead of what would be the next argument in the list. Positional specifiers begin counting with one. Thus: @example printf "%s %s\n", "don't", "panic" printf "%2$s %1$s\n", "panic", "don't" @end example @noindent prints the famous friendly message twice. At first glance, this feature doesn't seem to be of much use. It is in fact a @command{gawk} extension, intended for use in translating messages at runtime. @xref{Printf Ordering}, which describes how and why to use positional specifiers. For now, we ignore them. @item @code{-} (Minus) The minus sign, used before the width modifier (see later on in this list), says to left-justify the argument within its specified width. Normally, the argument is printed right-justified in the specified width. Thus: @example printf "%-4s", "foo" @end example @noindent prints @samp{foo@bullet{}}. @item @var{space} For numeric conversions, prefix positive values with a space and negative values with a minus sign. @item @code{+} The plus sign, used before the width modifier (see later on in this list), says to always supply a sign for numeric conversions, even if the data to format is positive. The @samp{+} overrides the space modifier. @item @code{#} Use an ``alternative form'' for certain control letters. For @samp{%o}, supply a leading zero. For @samp{%x} and @samp{%X}, supply a leading @samp{0x} or @samp{0X} for a nonzero result. For @samp{%e}, @samp{%E}, @samp{%f}, and @samp{%F}, the result always contains a decimal point. For @samp{%g} and @samp{%G}, trailing zeros are not removed from the result. @item @code{0} A leading @samp{0} (zero) acts as a flag indicating that output should be padded with zeros instead of spaces. This applies only to the numeric output formats. This flag only has an effect when the field width is wider than the value to print. @item @code{'} A single quote or apostrophe character is a POSIX extension to ISO C. It indicates that the integer part of a floating-point value, or the entire part of an integer decimal value, should have a thousands-separator character in it. This only works in locales that support such characters. For example: @example $ @kbd{cat thousands.awk} @ii{Show source program} @print{} BEGIN @{ printf "%'d\n", 1234567 @} $ @kbd{LC_ALL=C gawk -f thousands.awk} @print{} 1234567 @ii{Results in} "C" @ii{locale} $ @kbd{LC_ALL=en_US.UTF-8 gawk -f thousands.awk} @print{} 1,234,567 @ii{Results in US English UTF locale} @end example @noindent For more information about locales and internationalization issues, see @ref{Locales}. @quotation NOTE The @samp{'} flag is a nice feature, but its use complicates things: it becomes difficult to use it in command-line programs. For information on appropriate quoting tricks, see @ref{Quoting}. @end quotation @item @var{width} This is a number specifying the desired minimum width of a field. Inserting any number between the @samp{%} sign and the format-control character forces the field to expand to this width. The default way to do this is to pad with spaces on the left. For example: @example printf "%4s", "foo" @end example @noindent prints @samp{@bullet{}foo}. The value of @var{width} is a minimum width, not a maximum. If the item value requires more than @var{width} characters, it can be as wide as necessary. Thus, the following: @example printf "%4s", "foobar" @end example @noindent prints @samp{foobar}. Preceding the @var{width} with a minus sign causes the output to be padded with spaces on the right, instead of on the left. @item @code{.@var{prec}} A period followed by an integer constant specifies the precision to use when printing. The meaning of the precision varies by control letter: @table @asis @item @code{%d}, @code{%i}, @code{%o}, @code{%u}, @code{%x}, @code{%X} Minimum number of digits to print. @item @code{%e}, @code{%E}, @code{%f}, @code{%F} Number of digits to the right of the decimal point. @item @code{%g}, @code{%G} Maximum number of significant digits. @item @code{%s} Maximum number of characters from the string that should print. @end table Thus, the following: @example printf "%.4s", "foobar" @end example @noindent prints @samp{foob}. @end table The C library @code{printf}'s dynamic @var{width} and @var{prec} capability (e.g., @code{"%*.*s"}) is supported. Instead of supplying explicit @var{width} and/or @var{prec} values in the format string, they are passed in the argument list. For example: @example w = 5 p = 3 s = "abcdefg" printf "%*.*s\n", w, p, s @end example @noindent is exactly equivalent to: @example s = "abcdefg" printf "%5.3s\n", s @end example @noindent Both programs output @samp{@w{@bullet{}@bullet{}abc}}. Earlier versions of @command{awk} did not support this capability. If you must use such a version, you may simulate this feature by using concatenation to build up the format string, like so: @example w = 5 p = 3 s = "abcdefg" printf "%" w "." p "s\n", s @end example @noindent This is not particularly easy to read, but it does work. @c @cindex lint checks @cindex troubleshooting @subentry fatal errors @subentry @code{printf} format strings @cindex POSIX @command{awk} @subentry @code{printf} format strings and C programmers may be used to supplying additional modifiers (@samp{h}, @samp{j}, @samp{l}, @samp{L}, @samp{t}, and @samp{z}) in @code{printf} format strings. These are not valid in @command{awk}. Most @command{awk} implementations silently ignore them. If @option{--lint} is provided on the command line (@pxref{Options}), @command{gawk} warns about their use. If @option{--posix} is supplied, their use is a fatal error. @node Printf Examples @subsection Examples Using @code{printf} The following simple example shows how to use @code{printf} to make an aligned table: @example awk '@{ printf "%-10s %s\n", $1, $2 @}' mail-list @end example @noindent This command prints the names of the people (@code{$1}) in the file @file{mail-list} as a string of 10 characters that are left-justified. It also prints the phone numbers (@code{$2}) next on the line. This produces an aligned two-column table of names and phone numbers, as shown here: @example $ @kbd{awk '@{ printf "%-10s %s\n", $1, $2 @}' mail-list} @print{} Amelia 555-5553 @print{} Anthony 555-3412 @print{} Becky 555-7685 @print{} Bill 555-1675 @print{} Broderick 555-0542 @print{} Camilla 555-2912 @print{} Fabius 555-1234 @print{} Julie 555-6699 @print{} Martin 555-6480 @print{} Samuel 555-3430 @print{} Jean-Paul 555-2127 @end example In this case, the phone numbers had to be printed as strings because the numbers are separated by dashes. Printing the phone numbers as numbers would have produced just the first three digits: @samp{555}. This would have been pretty confusing. It wasn't necessary to specify a width for the phone numbers because they are last on their lines. They don't need to have spaces after them. The table could be made to look even nicer by adding headings to the tops of the columns. This is done using a @code{BEGIN} rule (@pxref{BEGIN/END}) so that the headers are only printed once, at the beginning of the @command{awk} program: @example awk 'BEGIN @{ print "Name Number" print "---- ------" @} @{ printf "%-10s %s\n", $1, $2 @}' mail-list @end example The preceding example mixes @code{print} and @code{printf} statements in the same program. Using just @code{printf} statements can produce the same results: @example awk 'BEGIN @{ printf "%-10s %s\n", "Name", "Number" printf "%-10s %s\n", "----", "------" @} @{ printf "%-10s %s\n", $1, $2 @}' mail-list @end example @noindent Printing each column heading with the same format specification used for the column elements ensures that the headings are aligned just like the columns. The fact that the same format specification is used three times can be emphasized by storing it in a variable, like this: @example awk 'BEGIN @{ format = "%-10s %s\n" printf format, "Name", "Number" printf format, "----", "------" @} @{ printf format, $1, $2 @}' mail-list @end example @node Redirection @section Redirecting Output of @code{print} and @code{printf} @cindex output redirection @cindex redirection @subentry of output @cindex @option{--sandbox} option @subentry output redirection with @code{print} @subentry @code{printf} So far, the output from @code{print} and @code{printf} has gone to the standard output, usually the screen. Both @code{print} and @code{printf} can also send their output to other places. This is called @dfn{redirection}. @quotation NOTE When @option{--sandbox} is specified (@pxref{Options}), redirecting output to files, pipes, and coprocesses is disabled. @end quotation A redirection appears after the @code{print} or @code{printf} statement. Redirections in @command{awk} are written just like redirections in shell commands, except that they are written inside the @command{awk} program. @c the commas here are part of the see also @cindex @code{print} statement @seealso{redirection of output} @cindex @code{printf} statement @seealso{redirection of output} There are four forms of output redirection: output to a file, output appended to a file, output through a pipe to another command, and output to a coprocess. We show them all for the @code{print} statement, but they work identically for @code{printf}: @table @code @cindex @code{>} (right angle bracket) @subentry @code{>} operator (I/O) @cindex right angle bracket (@code{>}) @subentry @code{>} operator (I/O) @cindex operators @subentry input/output @item print @var{items} > @var{output-file} This redirection prints the items into the output file named @var{output-file}. The @value{FN} @var{output-file} can be any expression. Its value is changed to a string and then used as a @value{FN} (@pxref{Expressions}). When this type of redirection is used, the @var{output-file} is erased before the first output is written to it. Subsequent writes to the same @var{output-file} do not erase @var{output-file}, but append to it. (This is different from how you use redirections in shell scripts.) If @var{output-file} does not exist, it is created. For example, here is how an @command{awk} program can write a list of peoples' names to one file named @file{name-list}, and a list of phone numbers to another file named @file{phone-list}: @example $ @kbd{awk '@{ print $2 > "phone-list"} > @kbd{print $1 > "name-list" @}' mail-list} $ @kbd{cat phone-list} @print{} 555-5553 @print{} 555-3412 @dots{} $ @kbd{cat name-list} @print{} Amelia @print{} Anthony @dots{} @end example @noindent Each output file contains one name or number per line. @cindex @code{>} (right angle bracket) @subentry @code{>>} operator (I/O) @cindex right angle bracket (@code{>}) @subentry @code{>>} operator (I/O) @item print @var{items} >> @var{output-file} This redirection prints the items into the preexisting output file named @var{output-file}. The difference between this and the single-@samp{>} redirection is that the old contents (if any) of @var{output-file} are not erased. Instead, the @command{awk} output is appended to the file. If @var{output-file} does not exist, then it is created. @cindex @code{|} (vertical bar) @subentry @code{|} operator (I/O) @cindex pipe @subentry output @cindex output @subentry pipes @item print @var{items} | @var{command} It is possible to send output to another program through a pipe instead of into a file. This redirection opens a pipe to @var{command}, and writes the values of @var{items} through this pipe to another process created to execute @var{command}. The redirection argument @var{command} is actually an @command{awk} expression. Its value is converted to a string whose contents give the shell command to be run. For example, the following produces two files, one unsorted list of peoples' names, and one list sorted in reverse alphabetical order: @ignore 10/2000: This isn't the best style, since COMMAND is assigned for each record. It's done to avoid overfull hboxes in TeX. Leave it alone for now and let's hope no-one notices. @end ignore @example @group awk '@{ print $1 > "names.unsorted" command = "sort -r > names.sorted" print $1 | command @}' mail-list @end group @end example The unsorted list is written with an ordinary redirection, while the sorted list is written by piping through the @command{sort} utility. The next example uses redirection to mail a message to the mailing list @code{bug-system}. This might be useful when trouble is encountered in an @command{awk} script run periodically for system maintenance: @example report = "mail bug-system" print("Awk script failed:", $0) | report print("at record number", FNR, "of", FILENAME) | report close(report) @end example The @code{close()} function is called here because it's a good idea to close the pipe as soon as all the intended output has been sent to it. @xref{Close Files And Pipes} for more information. This example also illustrates the use of a variable to represent a @var{file} or @var{command}---it is not necessary to always use a string constant. Using a variable is generally a good idea, because (if you mean to refer to that same file or command) @command{awk} requires that the string value be written identically every time. @cindex coprocesses @cindex @code{|} (vertical bar) @subentry @code{|&} operator (I/O) @cindex operators @subentry input/output @cindex differences in @command{awk} and @command{gawk} @subentry input/output operators @item print @var{items} |& @var{command} This redirection prints the items to the input of @var{command}. The difference between this and the single-@samp{|} redirection is that the output from @var{command} can be read with @code{getline}. Thus, @var{command} is a @dfn{coprocess}, which works together with but is subsidiary to the @command{awk} program. This feature is a @command{gawk} extension, and is not available in POSIX @command{awk}. @ifnotdocbook @xref{Getline/Coprocess}, for a brief discussion. @xref{Two-way I/O}, for a more complete discussion. @end ifnotdocbook @ifdocbook @xref{Getline/Coprocess} for a brief discussion and @ref{Two-way I/O} for a more complete discussion. @end ifdocbook @end table Redirecting output using @samp{>}, @samp{>>}, @samp{|}, or @samp{|&} asks the system to open a file, pipe, or coprocess only if the particular @var{file} or @var{command} you specify has not already been written to by your program or if it has been closed since it was last written to. @cindex troubleshooting @subentry printing It is a common error to use @samp{>} redirection for the first @code{print} to a file, and then to use @samp{>>} for subsequent output: @example # clear the file print "Don't panic" > "guide.txt" @dots{} # append print "Avoid improbability generators" >> "guide.txt" @end example @noindent This is indeed how redirections must be used from the shell. But in @command{awk}, it isn't necessary. In this kind of case, a program should use @samp{>} for all the @code{print} statements, because the output file is only opened once. (It happens that if you mix @samp{>} and @samp{>>} output is produced in the expected order. However, mixing the operators for the same file is definitely poor style, and is confusing to readers of your program.) @cindex differences in @command{awk} and @command{gawk} @subentry implementation limitations @cindex implementation issues, @command{gawk} @subentry limits @cindex @command{awk} @subentry implementation issues @subentry pipes @cindex @command{gawk} @subentry implementation issues @subentry pipes @ifnotinfo As mentioned earlier (@pxref{Getline Notes}), many @end ifnotinfo @ifnottex @ifnotdocbook Many @end ifnotdocbook @end ifnottex older @command{awk} implementations limit the number of pipelines that an @command{awk} program may have open to just one! In @command{gawk}, there is no such limit. @command{gawk} allows a program to open as many pipelines as the underlying operating system permits. @cindex sidebar @subentry Piping into @command{sh} @ifdocbook @docbook Piping into @command{sh} @end docbook @cindex shells @subentry piping commands into A particularly powerful way to use redirection is to build command lines and pipe them into the shell, @command{sh}. For example, suppose you have a list of files brought over from a system where all the @value{FN}s are stored in uppercase, and you wish to rename them to have names in all lowercase. The following program is both simple and efficient: @c @cindex @command{mv} utility @example @{ printf("mv %s %s\n", $0, tolower($0)) | "sh" @} END @{ close("sh") @} @end example The @code{tolower()} function returns its argument string with all uppercase characters converted to lowercase (@pxref{String Functions}). The program builds up a list of command lines, using the @command{mv} utility to rename the files. It then sends the list to the shell for execution. @xref{Shell Quoting} for a function that can help in generating command lines to be fed to the shell. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Piping into @command{sh}} @cindex shells @subentry piping commands into A particularly powerful way to use redirection is to build command lines and pipe them into the shell, @command{sh}. For example, suppose you have a list of files brought over from a system where all the @value{FN}s are stored in uppercase, and you wish to rename them to have names in all lowercase. The following program is both simple and efficient: @c @cindex @command{mv} utility @example @{ printf("mv %s %s\n", $0, tolower($0)) | "sh" @} END @{ close("sh") @} @end example The @code{tolower()} function returns its argument string with all uppercase characters converted to lowercase (@pxref{String Functions}). The program builds up a list of command lines, using the @command{mv} utility to rename the files. It then sends the list to the shell for execution. @xref{Shell Quoting} for a function that can help in generating command lines to be fed to the shell. @end cartouche @end ifnotdocbook @node Special FD @section Special Files for Standard Preopened Data Streams @cindex standard input @cindex input @subentry standard @cindex standard output @cindex output @subentry standard @cindex error output @cindex standard error @cindex file descriptors @cindex files @subentry descriptors @seeentry{file descriptors} Running programs conventionally have three input and output streams already available to them for reading and writing. These are known as the @dfn{standard input}, @dfn{standard output}, and @dfn{standard error output}. These open streams (and any other open files or pipes) are often referred to by the technical term @dfn{file descriptors}. These streams are, by default, connected to your keyboard and screen, but they are often redirected with the shell, via the @samp{<}, @samp{<<}, @samp{>}, @samp{>>}, @samp{>&}, and @samp{|} operators. Standard error is typically used for writing error messages; the reason there are two separate streams, standard output and standard error, is so that they can be redirected separately. @cindex differences in @command{awk} and @command{gawk} @subentry error messages @cindex error handling In traditional implementations of @command{awk}, the only way to write an error message to standard error in an @command{awk} program is as follows: @example print "Serious error detected!" | "cat 1>&2" @end example @noindent This works by opening a pipeline to a shell command that can access the standard error stream that it inherits from the @command{awk} process. @c 8/2014: Mike Brennan says not to cite this as inefficient. So, fixed. This is far from elegant, and it also requires a separate process. So people writing @command{awk} programs often don't do this. Instead, they send the error messages to the screen, like this: @example print "Serious error detected!" > "/dev/tty" @end example @noindent (@file{/dev/tty} is a special file supplied by the operating system that is connected to your keyboard and screen. It represents the ``terminal,''@footnote{The ``tty'' in @file{/dev/tty} stands for ``Teletype,'' a serial terminal.} which on modern systems is a keyboard and screen, not a serial console.) This generally has the same effect, but not always: although the standard error stream is usually the screen, it can be redirected; when that happens, writing to the screen is not correct. In fact, if @command{awk} is run from a background job, it may not have a terminal at all. Then opening @file{/dev/tty} fails. @command{gawk}, BWK @command{awk}, and @command{mawk} provide special @value{FN}s for accessing the three standard streams. If the @value{FN} matches one of these special names when @command{gawk} (or one of the others) redirects input or output, then it directly uses the descriptor that the @value{FN} stands for. These special @value{FN}s work for all operating systems that @command{gawk} has been ported to, not just those that are POSIX-compliant: @cindex common extensions @subentry @code{/dev/stdin} special file @cindex common extensions @subentry @code{/dev/stdout} special file @cindex common extensions @subentry @code{/dev/stderr} special file @cindex extensions @subentry common @subentry @code{/dev/stdin} special file @cindex extensions @subentry common @subentry @code{/dev/stdout} special file @cindex extensions @subentry common @subentry @code{/dev/stderr} special file @cindex file names @subentry standard streams in @command{gawk} @cindex @code{/dev/@dots{}} special files @cindex files @subentry @code{/dev/@dots{}} special files @cindex @code{/dev/fd/@var{N}} special files (@command{gawk}) @table @file @item /dev/stdin The standard input (file descriptor 0). @item /dev/stdout The standard output (file descriptor 1). @item /dev/stderr The standard error output (file descriptor 2). @end table With these facilities, the proper way to write an error message then becomes: @example print "Serious error detected!" > "/dev/stderr" @end example @cindex troubleshooting @subentry quotes with file names Note the use of quotes around the @value{FN}. Like with any other redirection, the value must be a string. It is a common error to omit the quotes, which leads to confusing results. @command{gawk} does not treat these @value{FN}s as special when in POSIX-compatibility mode. However, because BWK @command{awk} supports them, @command{gawk} does support them even when invoked with the @option{--traditional} option (@pxref{Options}). @node Special Files @section Special @value{FFN}s in @command{gawk} @cindex @command{gawk} @subentry file names in Besides access to standard input, standard output, and standard error, @command{gawk} provides access to any open file descriptor. Additionally, there are special @value{FN}s reserved for TCP/IP networking. @menu * Other Inherited Files:: Accessing other open files with @command{gawk}. * Special Network:: Special files for network communications. * Special Caveats:: Things to watch out for. @end menu @node Other Inherited Files @subsection Accessing Other Open Files with @command{gawk} Besides the @code{/dev/stdin}, @code{/dev/stdout}, and @code{/dev/stderr} special @value{FN}s mentioned earlier, @command{gawk} provides syntax for accessing any other inherited open file: @table @file @item /dev/fd/@var{N} The file associated with file descriptor @var{N}. Such a file must be opened by the program initiating the @command{awk} execution (typically the shell). Unless special pains are taken in the shell from which @command{gawk} is invoked, only descriptors 0, 1, and 2 are available. @end table The @value{FN}s @file{/dev/stdin}, @file{/dev/stdout}, and @file{/dev/stderr} are essentially aliases for @file{/dev/fd/0}, @file{/dev/fd/1}, and @file{/dev/fd/2}, respectively. However, those names are more self-explanatory. Note that using @code{close()} on a @value{FN} of the form @code{"/dev/fd/@var{N}"}, for file descriptor numbers above two, does actually close the given file descriptor. @node Special Network @subsection Special Files for Network Communications @cindex networks @subentry support for @cindex TCP/IP @subentry support for @command{gawk} programs can open a two-way TCP/IP connection, acting as either a client or a server. This is done using a special @value{FN} of the form: @example @file{/@var{net-type}/@var{protocol}/@var{local-port}/@var{remote-host}/@var{remote-port}} @end example The @var{net-type} is one of @samp{inet}, @samp{inet4}, or @samp{inet6}. The @var{protocol} is one of @samp{tcp} or @samp{udp}, and the other fields represent the other essential pieces of information for making a networking connection. These @value{FN}s are used with the @samp{|&} operator for communicating with @w{a coprocess} (@pxref{Two-way I/O}). This is an advanced feature, mentioned here only for completeness. Full discussion is delayed until @ref{TCP/IP Networking}. @node Special Caveats @subsection Special @value{FFN} Caveats Here are some things to bear in mind when using the special @value{FN}s that @command{gawk} provides: @itemize @value{BULLET} @cindex compatibility mode (@command{gawk}) @subentry file names @cindex file names @subentry in compatibility mode @cindex POSIX mode @item Recognition of the @value{FN}s for the three standard preopened files is disabled only in POSIX mode. @item Recognition of the other special @value{FN}s is disabled if @command{gawk} is in compatibility mode (either @option{--traditional} or @option{--posix}; @pxref{Options}). @item @command{gawk} @emph{always} interprets these special @value{FN}s. For example, using @samp{/dev/fd/4} for output actually writes on file descriptor 4, and not on a new file descriptor that is @code{dup()}ed from file descriptor 4. Most of the time this does not matter; however, it is important to @emph{not} close any of the files related to file descriptors 0, 1, and 2. Doing so results in unpredictable behavior. @end itemize @node Close Files And Pipes @section Closing Input and Output Redirections @cindex files @subentry output @seeentry{output files} @cindex input files @subentry closing @cindex output @subentry files, closing @cindex pipe @subentry closing @cindex coprocesses @subentry closing @cindex @code{getline} command @subentry coprocesses, using from If the same @value{FN} or the same shell command is used with @code{getline} more than once during the execution of an @command{awk} program (@pxref{Getline}), the file is opened (or the command is executed) the first time only. At that time, the first record of input is read from that file or command. The next time the same file or command is used with @code{getline}, another record is read from it, and so on. Similarly, when a file or pipe is opened for output, @command{awk} remembers the @value{FN} or command associated with it, and subsequent writes to the same file or command are appended to the previous writes. The file or pipe stays open until @command{awk} exits. @cindexawkfunc{close} This implies that special steps are necessary in order to read the same file again from the beginning, or to rerun a shell command (rather than reading more output from the same command). The @code{close()} function makes these things possible: @example close(@var{filename}) @end example @noindent or: @example close(@var{command}) @end example The argument @var{filename} or @var{command} can be any expression. Its value must @emph{exactly} match the string that was used to open the file or start the command (spaces and other ``irrelevant'' characters included). For example, if you open a pipe with this: @example "sort -r names" | getline foo @end example @noindent then you must close it with this: @example close("sort -r names") @end example Once this function call is executed, the next @code{getline} from that file or command, or the next @code{print} or @code{printf} to that file or command, reopens the file or reruns the command. Because the expression that you use to close a file or pipeline must exactly match the expression used to open the file or run the command, it is good practice to use a variable to store the @value{FN} or command. The previous example becomes the following: @example @group sortcom = "sort -r names" sortcom | getline foo @end group @group @dots{} close(sortcom) @end group @end example @noindent This helps avoid hard-to-find typographical errors in your @command{awk} programs. Here are some of the reasons for closing an output file: @itemize @value{BULLET} @item To write a file and read it back later on in the same @command{awk} program. Close the file after writing it, then begin reading it with @code{getline}. @item To write numerous files, successively, in the same @command{awk} program. If the files aren't closed, eventually @command{awk} may exceed a system limit on the number of open files in one process. It is best to close each one when the program has finished writing it. @item To make a command finish. When output is redirected through a pipe, the command reading the pipe normally continues to try to read input as long as the pipe is open. Often this means the command cannot really do its work until the pipe is closed. For example, if output is redirected to the @command{mail} program, the message is not actually sent until the pipe is closed. @item To run the same program a second time, with the same arguments. This is not the same thing as giving more input to the first run! For example, suppose a program pipes output to the @command{mail} program. If it outputs several lines redirected to this pipe without closing it, they make a single message of several lines. By contrast, if the program closes the pipe after each line of output, then each line makes a separate message. @end itemize @cindex differences in @command{awk} and @command{gawk} @subentry @code{close()} function @cindex portability @subentry @code{close()} function and @cindex @code{close()} function @subentry portability If you use more files than the system allows you to have open, @command{gawk} attempts to multiplex the available open files among your @value{DF}s. @command{gawk}'s ability to do this depends upon the facilities of your operating system, so it may not always work. It is therefore both good practice and good portability advice to always use @code{close()} on your files when you are done with them. In fact, if you are using a lot of pipes, it is essential that you close commands when done. For example, consider something like this: @example @{ @dots{} command = ("grep " $1 " /some/file | my_prog -q " $3) while ((command | getline) > 0) @{ @var{process output of} command @} # need close(command) here @} @end example This example creates a new pipeline based on data in @emph{each} record. Without the call to @code{close()} indicated in the comment, @command{awk} creates child processes to run the commands, until it eventually runs out of file descriptors for more pipelines. Even though each command has finished (as indicated by the end-of-file return status from @code{getline}), the child process is not terminated;@footnote{The technical terminology is rather morbid. The finished child is called a ``zombie,'' and cleaning up after it is referred to as ``reaping.''} @c Good old UNIX: give the marketing guys fits, that's the ticket more importantly, the file descriptor for the pipe is not closed and released until @code{close()} is called or @command{awk} exits. @code{close()} silently does nothing if given an argument that does not represent a file, pipe, or coprocess that was opened with a redirection. In such a case, it returns a negative value, indicating an error. In addition, @command{gawk} sets @code{ERRNO} to a string indicating the error. Note also that @samp{close(FILENAME)} has no ``magic'' effects on the implicit loop that reads through the files named on the command line. It is, more likely, a close of a file that was never opened with a redirection, so @command{awk} silently does nothing, except return a negative value. @cindex @code{|} (vertical bar) @subentry @code{|&} operator (I/O) @subentry pipes, closing When using the @samp{|&} operator to communicate with a coprocess, it is occasionally useful to be able to close one end of the two-way pipe without closing the other. This is done by supplying a second argument to @code{close()}. As in any other call to @code{close()}, the first argument is the name of the command or special file used to start the coprocess. The second argument should be a string, with either of the values @code{"to"} or @code{"from"}. Case does not matter. As this is an advanced feature, discussion is delayed until @ref{Two-way I/O}, which describes it in more detail and gives an example. @cindex sidebar @subentry Using @code{close()}'s Return Value @ifdocbook @docbook Using @code{close()}'s Return Value @end docbook @cindex dark corner @subentry @code{close()} function @cindex @code{close()} function @subentry return value @cindex return value, @code{close()} function @cindex differences in @command{awk} and @command{gawk} @subentry @code{close()} function @cindex Unix @command{awk} @subentry @code{close()} function and In many older versions of Unix @command{awk}, the @code{close()} function is actually a statement. @value{DARKCORNER} It is a syntax error to try and use the return value from @code{close()}: @example command = "@dots{}" command | getline info retval = close(command) # syntax error in many Unix awks @end example @cindex @command{gawk} @subentry @code{ERRNO} variable in @cindex @code{ERRNO} variable @subentry with @command{close()} function @command{gawk} treats @code{close()} as a function. The return value is @minus{}1 if the argument names something that was never opened with a redirection, or if there is a system problem closing the file or process. In these cases, @command{gawk} sets the predefined variable @code{ERRNO} to a string describing the problem. In @command{gawk}, starting with @value{PVERSION} 4.2, when closing a pipe or coprocess (input or output), the return value is the exit status of the command, as described in @ref{table-close-pipe-return-values}.@footnote{Prior to @value{PVERSION} 4.2, the return value from closing a pipe or co-process was the full 16-bit exit value as defined by the @code{wait()} system call.} Otherwise, it is the return value from the system's @code{close()} or @code{fclose()} C functions when closing input or output files, respectively. This value is zero if the close succeeds, or @minus{}1 if it fails. @float Table,table-close-pipe-return-values @caption{Return values from @code{close()} of a pipe} @multitable @columnfractions .50 .50 @headitem Situation @tab Return value from @code{close()} @item Normal exit of command @tab Command's exit status @item Death by signal of command @tab 256 + number of murderous signal @item Death by signal of command with core dump @tab 512 + number of murderous signal @item Some kind of error @tab @minus{}1 @end multitable @end float @cindex POSIX mode The POSIX standard is very vague; it says that @code{close()} returns zero on success and a nonzero value otherwise. In general, different implementations vary in what they report when closing pipes; thus, the return value cannot be used portably. @value{DARKCORNER} In POSIX mode (@pxref{Options}), @command{gawk} just returns zero when closing a pipe. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Using @code{close()}'s Return Value} @cindex dark corner @subentry @code{close()} function @cindex @code{close()} function @subentry return value @cindex return value, @code{close()} function @cindex differences in @command{awk} and @command{gawk} @subentry @code{close()} function @cindex Unix @command{awk} @subentry @code{close()} function and In many older versions of Unix @command{awk}, the @code{close()} function is actually a statement. @value{DARKCORNER} It is a syntax error to try and use the return value from @code{close()}: @example command = "@dots{}" command | getline info retval = close(command) # syntax error in many Unix awks @end example @cindex @command{gawk} @subentry @code{ERRNO} variable in @cindex @code{ERRNO} variable @subentry with @command{close()} function @command{gawk} treats @code{close()} as a function. The return value is @minus{}1 if the argument names something that was never opened with a redirection, or if there is a system problem closing the file or process. In these cases, @command{gawk} sets the predefined variable @code{ERRNO} to a string describing the problem. In @command{gawk}, starting with @value{PVERSION} 4.2, when closing a pipe or coprocess (input or output), the return value is the exit status of the command, as described in @ref{table-close-pipe-return-values}.@footnote{Prior to @value{PVERSION} 4.2, the return value from closing a pipe or co-process was the full 16-bit exit value as defined by the @code{wait()} system call.} Otherwise, it is the return value from the system's @code{close()} or @code{fclose()} C functions when closing input or output files, respectively. This value is zero if the close succeeds, or @minus{}1 if it fails. @float Table,table-close-pipe-return-values @caption{Return values from @code{close()} of a pipe} @multitable @columnfractions .50 .50 @headitem Situation @tab Return value from @code{close()} @item Normal exit of command @tab Command's exit status @item Death by signal of command @tab 256 + number of murderous signal @item Death by signal of command with core dump @tab 512 + number of murderous signal @item Some kind of error @tab @minus{}1 @end multitable @end float @cindex POSIX mode The POSIX standard is very vague; it says that @code{close()} returns zero on success and a nonzero value otherwise. In general, different implementations vary in what they report when closing pipes; thus, the return value cannot be used portably. @value{DARKCORNER} In POSIX mode (@pxref{Options}), @command{gawk} just returns zero when closing a pipe. @end cartouche @end ifnotdocbook @node Nonfatal @section Enabling Nonfatal Output This @value{SECTION} describes a @command{gawk}-specific feature. In standard @command{awk}, output with @code{print} or @code{printf} to a nonexistent file, or some other I/O error (such as filling up the disk) is a fatal error. @example $ @kbd{gawk 'BEGIN @{ print "hi" > "/no/such/file" @}'} @error{} gawk: cmd. line:1: fatal: can't redirect to `/no/such/file' (No @error{} such file or directory) @end example @command{gawk} makes it possible to detect that an error has occurred, allowing you to possibly recover from the error, or at least print an error message of your choosing before exiting. You can do this in one of two ways: @itemize @bullet @item For all output files, by assigning any value to @code{PROCINFO["NONFATAL"]}. @item On a per-file basis, by assigning any value to @code{PROCINFO[@var{filename}, "NONFATAL"]}. Here, @var{filename} is the name of the file to which you wish output to be nonfatal. @end itemize Once you have enabled nonfatal output, you must check @code{ERRNO} after every relevant @code{print} or @code{printf} statement to see if something went wrong. It is also a good idea to initialize @code{ERRNO} to zero before attempting the output. For example: @example $ @kbd{gawk '} > @kbd{BEGIN @{} > @kbd{ PROCINFO["NONFATAL"] = 1} > @kbd{ ERRNO = 0} > @kbd{ print "hi" > "/no/such/file"} > @kbd{ if (ERRNO) @{} > @kbd{ print("Output failed:", ERRNO) > "/dev/stderr"} > @kbd{ exit 1} > @kbd{ @}} > @kbd{@}'} @error{} Output failed: No such file or directory @end example Here, @command{gawk} did not produce a fatal error; instead it let the @command{awk} program code detect the problem and handle it. This mechanism works also for standard output and standard error. For standard output, you may use @code{PROCINFO["-", "NONFATAL"]} or @code{PROCINFO["/dev/stdout", "NONFATAL"]}. For standard error, use @code{PROCINFO["/dev/stderr", "NONFATAL"]}. @cindex @env{GAWK_SOCK_RETRIES} environment variable @cindex environment variables @subentry @env{GAWK_SOCK_RETRIES} When attempting to open a TCP/IP socket (@pxref{TCP/IP Networking}), @command{gawk} tries multiple times. The @env{GAWK_SOCK_RETRIES} environment variable (@pxref{Other Environment Variables}) allows you to override @command{gawk}'s builtin default number of attempts. However, once nonfatal I/O is enabled for a given socket, @command{gawk} only retries once, relying on @command{awk}-level code to notice that there was a problem. @node Output Summary @section Summary @itemize @value{BULLET} @item The @code{print} statement prints comma-separated expressions. Each expression is separated by the value of @code{OFS} and terminated by the value of @code{ORS}. @code{OFMT} provides the conversion format for numeric values for the @code{print} statement. @item The @code{printf} statement provides finer-grained control over output, with format-control letters for different data types and various flags that modify the behavior of the format-control letters. @item Output from both @code{print} and @code{printf} may be redirected to files, pipes, and coprocesses. @item @command{gawk} provides special @value{FN}s for access to standard input, output, and error, and for network communications. @item Use @code{close()} to close open file, pipe, and coprocess redirections. For coprocesses, it is possible to close only one direction of the communications. @item Normally errors with @code{print} or @code{printf} are fatal. @command{gawk} lets you make output errors be nonfatal either for all files or on a per-file basis. You must then check for errors after every relevant output statement. @end itemize @c EXCLUDE START @node Output Exercises @section Exercises @enumerate @item Rewrite the program: @example awk 'BEGIN @{ print "Month Crates" print "----- ------" @} @{ print $1, " ", $2 @}' inventory-shipped @end example @noindent from @ref{Output Separators}, by using a new value of @code{OFS}. @item Use the @code{printf} statement to line up the headings and table data for the @file{inventory-shipped} example that was covered in @ref{Print}. @item What happens if you forget the double quotes when redirecting output, as follows: @example BEGIN @{ print "Serious error detected!" > /dev/stderr @} @end example @end enumerate @c EXCLUDE END @node Expressions @chapter Expressions @cindex expressions Expressions are the basic building blocks of @command{awk} patterns and actions. An expression evaluates to a value that you can print, test, or pass to a function. Additionally, an expression can assign a new value to a variable or a field by using an assignment operator. An expression can serve as a pattern or action statement on its own. Most other kinds of statements contain one or more expressions that specify the data on which to operate. As in other languages, expressions in @command{awk} can include variables, array references, constants, and function calls, as well as combinations of these with various operators. @menu * Values:: Constants, Variables, and Regular Expressions. * All Operators:: @command{gawk}'s operators. * Truth Values and Conditions:: Testing for true and false. * Function Calls:: A function call is an expression. * Precedence:: How various operators nest. * Locales:: How the locale affects things. * Expressions Summary:: Expressions summary. @end menu @node Values @section Constants, Variables, and Conversions Expressions are built up from values and the operations performed upon them. This @value{SECTION} describes the elementary objects that provide the values used in expressions. @menu * Constants:: String, numeric and regexp constants. * Using Constant Regexps:: When and how to use a regexp constant. * Variables:: Variables give names to values for later use. * Conversion:: The conversion of strings to numbers and vice versa. @end menu @node Constants @subsection Constant Expressions @cindex constants @subentry types of The simplest type of expression is the @dfn{constant}, which always has the same value. There are three types of constants: numeric, string, and regular expression. Each is used in the appropriate context when you need a data value that isn't going to change. Numeric constants can have different forms, but are internally stored in an identical manner. @menu * Scalar Constants:: Numeric and string constants. * Nondecimal-numbers:: What are octal and hex numbers. * Regexp Constants:: Regular Expression constants. @end menu @node Scalar Constants @subsubsection Numeric and String Constants @cindex constants @subentry numeric @cindex numeric @subentry constants A @dfn{numeric constant} stands for a number. This number can be an integer, a decimal fraction, or a number in scientific (exponential) notation.@footnote{The internal representation of all numbers, including integers, uses double-precision floating-point numbers. On most modern systems, these are in IEEE 754 standard format. @xref{Arbitrary Precision Arithmetic}, for much more information.} Here are some examples of numeric constants that all have the same value: @example 105 1.05e+2 1050e-1 @end example @cindex string @subentry constants @cindex constants @subentry string A @dfn{string constant} consists of a sequence of characters enclosed in double quotation marks. For example: @example "parrot" @end example @noindent @cindex differences in @command{awk} and @command{gawk} @subentry strings @cindex strings @subentry length limitations @cindex ASCII represents the string whose contents are @samp{parrot}. Strings in @command{gawk} can be of any length, and they can contain any of the possible eight-bit ASCII characters, including ASCII @sc{nul} (character code zero). Other @command{awk} implementations may have difficulty with some character codes. Some languages allow you to continue long strings across multiple lines by ending the line with a backslash. For example in C: @example #include int main() @{ printf("hello, \ world\n"); return 0; @} @end example @noindent In such a case, the C compiler removes both the backslash and the newline, producing a string as if it had been typed @samp{"hello, world\n"}. This is useful when a single string needs to contain a large amount of text. The POSIX standard says explicitly that newlines are not allowed inside string constants. And indeed, all @command{awk} implementations report an error if you try to do so. For example: @example $ @kbd{gawk 'BEGIN @{ print "hello, } > @kbd{world" @}'} @print{} gawk: cmd. line:1: BEGIN @{ print "hello, @print{} gawk: cmd. line:1: ^ unterminated string @print{} gawk: cmd. line:1: BEGIN @{ print "hello, @print{} gawk: cmd. line:1: ^ syntax error @end example @cindex dark corner @subentry string continuation @cindex strings @subentry continuation across lines @cindex differences in @command{awk} and @command{gawk} @subentry strings Although POSIX doesn't define what happens if you use an escaped newline, as in the previous C example, all known versions of @command{awk} allow you to do so. Unfortunately, what each one does with such a string varies. @value{DARKCORNER} @command{gawk}, @command{mawk}, and the OpenSolaris POSIX @command{awk} (@pxref{Other Versions}) elide the backslash and newline, as in C: @example $ @kbd{gawk 'BEGIN @{ print "hello, \} > @kbd{world" @}'} @print{} hello, world @end example @cindex POSIX mode In POSIX mode (@pxref{Options}), @command{gawk} does not allow escaped newlines. Otherwise, it behaves as just described. Brian Kernighan's @command{awk} and BusyBox @command{awk} remove the backslash but leave the newline intact, as part of the string: @example $ @kbd{nawk 'BEGIN @{ print "hello, \} > @kbd{world" @}'} @print{} hello, @print{} world @end example @node Nondecimal-numbers @subsubsection Octal and Hexadecimal Numbers @cindex octal numbers @cindex hexadecimal numbers @cindex numbers @subentry octal @cindex numbers @subentry hexadecimal In @command{awk}, all numbers are in decimal (i.e., base 10). Many other programming languages allow you to specify numbers in other bases, often octal (base 8) and hexadecimal (base 16). In octal, the numbers go 0, 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, and so on. Just as @samp{11} in decimal is 1 times 10 plus 1, so @samp{11} in octal is 1 times 8 plus 1. This equals 9 in decimal. In hexadecimal, there are 16 digits. Because the everyday decimal number system only has ten digits (@samp{0}--@samp{9}), the letters @samp{a} through @samp{f} represent the rest. (Case in the letters is usually irrelevant; hexadecimal @samp{a} and @samp{A} have the same value.) Thus, @samp{11} in hexadecimal is 1 times 16 plus 1, which equals 17 in decimal. Just by looking at plain @samp{11}, you can't tell what base it's in. So, in C, C++, and other languages derived from C, @c such as PERL, but we won't mention that.... there is a special notation to signify the base. Octal numbers start with a leading @samp{0}, and hexadecimal numbers start with a leading @samp{0x} or @samp{0X}: @table @code @item 11 Decimal value 11 @item 011 Octal 11, decimal value 9 @item 0x11 Hexadecimal 11, decimal value 17 @end table This example shows the difference: @example $ @kbd{gawk 'BEGIN @{ printf "%d, %d, %d\n", 011, 11, 0x11 @}'} @print{} 9, 11, 17 @end example Being able to use octal and hexadecimal constants in your programs is most useful when working with data that cannot be represented conveniently as characters or as regular numbers, such as binary data of various sorts. @cindex @command{gawk} @subentry octal numbers and @cindex @command{gawk} @subentry hexadecimal numbers and @command{gawk} allows the use of octal and hexadecimal constants in your program text. However, such numbers in the input data are not treated differently; doing so by default would break old programs. (If you really need to do this, use the @option{--non-decimal-data} command-line option; @pxref{Nondecimal Data}.) If you have octal or hexadecimal data, you can use the @code{strtonum()} function (@pxref{String Functions}) to convert the data into a number. Most of the time, you will want to use octal or hexadecimal constants when working with the built-in bit-manipulation functions; see @ref{Bitwise Functions} for more information. Unlike in some early C implementations, @samp{8} and @samp{9} are not valid in octal constants. For example, @command{gawk} treats @samp{018} as decimal 18: @example $ @kbd{gawk 'BEGIN @{ print "021 is", 021 ; print 018 @}'} @print{} 021 is 17 @print{} 18 @end example @cindex compatibility mode (@command{gawk}) @subentry octal numbers @cindex compatibility mode (@command{gawk}) @subentry hexadecimal numbers Octal and hexadecimal source code constants are a @command{gawk} extension. If @command{gawk} is in compatibility mode (@pxref{Options}), they are not available. @cindex sidebar @subentry A Constant's Base Does Not Affect Its Value @ifdocbook @docbook A Constant's Base Does Not Affect Its Value @end docbook Once a numeric constant has been converted internally into a number, @command{gawk} no longer remembers what the original form of the constant was; the internal value is always used. This has particular consequences for conversion of numbers to strings: @example $ @kbd{gawk 'BEGIN @{ printf "0x11 is <%s>\n", 0x11 @}'} @print{} 0x11 is <17> @end example @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{A Constant's Base Does Not Affect Its Value} Once a numeric constant has been converted internally into a number, @command{gawk} no longer remembers what the original form of the constant was; the internal value is always used. This has particular consequences for conversion of numbers to strings: @example $ @kbd{gawk 'BEGIN @{ printf "0x11 is <%s>\n", 0x11 @}'} @print{} 0x11 is <17> @end example @end cartouche @end ifnotdocbook @node Regexp Constants @subsubsection Regular Expression Constants @cindex regexp constants @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point) @subentry @code{!~} operator @cindex exclamation point (@code{!}) @subentry @code{!~} operator A @dfn{regexp constant} is a regular expression description enclosed in slashes, such as @code{@w{/^beginning and end$/}}. Most regexps used in @command{awk} programs are constant, but the @samp{~} and @samp{!~} matching operators can also match computed or dynamic regexps (which are typically just ordinary strings or variables that contain a regexp, but could be more complex expressions). @node Using Constant Regexps @subsection Using Regular Expression Constants Regular expression constants consist of text describing a regular expression enclosed in slashes (such as @code{/the +answer/}). This @value{SECTION} describes how such constants work in POSIX @command{awk} and @command{gawk}, and then goes on to describe @dfn{strongly typed regexp constants}, which are a @command{gawk} extension. @menu * Standard Regexp Constants:: Regexp constants in standard @command{awk}. * Strong Regexp Constants:: Strongly typed regexp constants. @end menu @node Standard Regexp Constants @subsubsection Standard Regular Expression Constants @cindex dark corner @subentry regexp constants When used on the righthand side of the @samp{~} or @samp{!~} operators, a regexp constant merely stands for the regexp that is to be matched. However, regexp constants (such as @code{/foo/}) may be used like simple expressions. When a regexp constant appears by itself, it has the same meaning as if it appeared in a pattern (i.e., @samp{($0 ~ /foo/)}). @value{DARKCORNER} @xref{Expression Patterns}. This means that the following two code segments: @example if ($0 ~ /barfly/ || $0 ~ /camelot/) print "found" @end example @noindent and: @example if (/barfly/ || /camelot/) print "found" @end example @noindent are exactly equivalent. One rather bizarre consequence of this rule is that the following Boolean expression is valid, but does not do what its author probably intended: @example # Note that /foo/ is on the left of the ~ if (/foo/ ~ $1) print "found foo" @end example @c @cindex automatic warnings @c @cindex warnings, automatic @cindex @command{gawk} @subentry regexp constants and @cindex regexp constants @subentry in @command{gawk} @noindent This code is ``obviously'' testing @code{$1} for a match against the regexp @code{/foo/}. But in fact, the expression @samp{/foo/ ~ $1} really means @samp{($0 ~ /foo/) ~ $1}. In other words, first match the input record against the regexp @code{/foo/}. The result is either zero or one, depending upon the success or failure of the match. That result is then matched against the first field in the record. Because it is unlikely that you would ever really want to make this kind of test, @command{gawk} issues a warning when it sees this construct in a program. Another consequence of this rule is that the assignment statement: @example matches = /foo/ @end example @noindent assigns either zero or one to the variable @code{matches}, depending upon the contents of the current input record. @cindex differences in @command{awk} and @command{gawk} @subentry regexp constants @cindex dark corner @subentry regexp constants @subentry as arguments to user-defined functions @cindexgawkfunc{gensub} @cindexawkfunc{sub} @cindexawkfunc{gsub} Constant regular expressions are also used as the first argument for the @code{gensub()}, @code{sub()}, and @code{gsub()} functions, as the second argument of the @code{match()} function, and as the third argument of the @code{split()} and @code{patsplit()} functions (@pxref{String Functions}). Modern implementations of @command{awk}, including @command{gawk}, allow the third argument of @code{split()} to be a regexp constant, but some older implementations do not. @value{DARKCORNER} Because some built-in functions accept regexp constants as arguments, confusion can arise when attempting to use regexp constants as arguments to user-defined functions (@pxref{User-defined}). For example: @example @group function mysub(pat, repl, str, global) @{ if (global) gsub(pat, repl, str) else sub(pat, repl, str) return str @} @end group @group @{ @dots{} text = "hi! hi yourself!" mysub(/hi/, "howdy", text, 1) @dots{} @} @end group @end example @c @cindex automatic warnings @c @cindex warnings, automatic In this example, the programmer wants to pass a regexp constant to the user-defined function @code{mysub()}, which in turn passes it on to either @code{sub()} or @code{gsub()}. However, what really happens is that the @code{pat} parameter is assigned a value of either one or zero, depending upon whether or not @code{$0} matches @code{/hi/}. @command{gawk} issues a warning when it sees a regexp constant used as a parameter to a user-defined function, because passing a truth value in this way is probably not what was intended. @node Strong Regexp Constants @subsubsection Strongly Typed Regexp Constants This @value{SECTION} describes a @command{gawk}-specific feature. As we saw in the previous @value{SECTION}, regexp constants (@code{/@dots{}/}) hold a strange position in the @command{awk} language. In most contexts, they act like an expression: @samp{$0 ~ /@dots{}/}. In other contexts, they denote only a regexp to be matched. In no case are they really a ``first class citizen'' of the language. That is, you cannot define a scalar variable whose type is ``regexp'' in the same sense that you can define a variable to be a number or a string: @example num = 42 @ii{Numeric variable} str = "hi" @ii{String variable} re = /foo/ @ii{Wrong!} re @ii{is the result of} $0 ~ /foo/ @end example For a number of more advanced use cases, it would be nice to have regexp constants that are @dfn{strongly typed}; in other words, that denote a regexp useful for matching, and not an expression. @cindex values @subentry regexp @command{gawk} provides this feature. A strongly typed regexp constant looks almost like a regular regexp constant, except that it is preceded by an @samp{@@} sign: @example re = @@/foo/ @ii{Regexp variable} @end example Strongly typed regexp constants @emph{cannot} be used everywhere that a regular regexp constant can, because this would make the language even more confusing. Instead, you may use them only in certain contexts: @itemize @bullet @item On the righthand side of the @samp{~} and @samp{!~} operators: @samp{some_var ~ @@/foo/} (@pxref{Regexp Usage}). @item In the @code{case} part of a @code{switch} statement (@pxref{Switch Statement}). @item As an argument to one of the built-in functions that accept regexp constants: @code{gensub()}, @code{gsub()}, @code{match()}, @code{patsplit()}, @code{split()}, and @code{sub()} (@pxref{String Functions}). @item As a parameter in a call to a user-defined function (@pxref{User-defined}). @item On the righthand side of an assignment to a variable: @samp{some_var = @@/foo/}. In this case, the type of @code{some_var} is regexp. Additionally, @code{some_var} can be used with @samp{~} and @samp{!~}, passed to one of the built-in functions listed above, or passed as a parameter to a user-defined function. @end itemize You may use the @code{typeof()} built-in function (@pxref{Type Functions}) to determine if a variable or function parameter is a regexp variable. The true power of this feature comes from the ability to create variables that have regexp type. Such variables can be passed on to user-defined functions, without the confusing aspects of computed regular expressions created from strings or string constants. They may also be passed through indirect function calls (@pxref{Indirect Calls}) and on to the built-in functions that accept regexp constants. When used in numeric conversions, strongly typed regexp variables convert to zero. When used in string conversions, they convert to the string value of the original regexp text. @node Variables @subsection Variables @cindex variables @subentry user-defined @cindex user-defined @subentry variables @dfn{Variables} are ways of storing values at one point in your program for use later in another part of your program. They can be manipulated entirely within the program text, and they can also be assigned values on the @command{awk} command line. @menu * Using Variables:: Using variables in your programs. * Assignment Options:: Setting variables on the command line and a summary of command-line syntax. This is an advanced method of input. @end menu @node Using Variables @subsubsection Using Variables in a Program Variables let you give names to values and refer to them later. Variables have already been used in many of the examples. The name of a variable must be a sequence of letters, digits, or underscores, and it may not begin with a digit. Here, a @dfn{letter} is any one of the 52 upper- and lowercase English letters. Other characters that may be defined as letters in non-English locales are not valid in variable names. Case is significant in variable names; @code{a} and @code{A} are distinct variables. A variable name is a valid expression by itself; it represents the variable's current value. Variables are given new values with @dfn{assignment operators}, @dfn{increment operators}, and @dfn{decrement operators} (@pxref{Assignment Ops}). In addition, the @code{sub()} and @code{gsub()} functions can change a variable's value, and the @code{match()}, @code{split()}, and @code{patsplit()} functions can change the contents of their array parameters (@pxref{String Functions}). @cindex variables @subentry built-in @cindex variables @subentry initializing A few variables have special built-in meanings, such as @code{FS} (the field separator) and @code{NF} (the number of fields in the current input record). @xref{Built-in Variables} for a list of the predefined variables. These predefined variables can be used and assigned just like all other variables, but their values are also used or changed automatically by @command{awk}. All predefined variables' names are entirely uppercase. Variables in @command{awk} can be assigned either numeric or string values. The kind of value a variable holds can change over the life of a program. By default, variables are initialized to the empty string, which is zero if converted to a number. There is no need to explicitly initialize a variable in @command{awk}, which is what you would do in C and in most other traditional languages. @node Assignment Options @subsubsection Assigning Variables on the Command Line @cindex variables @subentry assigning on command line @cindex command line @subentry variables, assigning on Any @command{awk} variable can be set by including a @dfn{variable assignment} among the arguments on the command line when @command{awk} is invoked (@pxref{Other Arguments}). Such an assignment has the following form: @example @var{variable}=@var{text} @end example @cindex @option{-v} option @noindent With it, a variable is set either at the beginning of the @command{awk} run or in between input files. When the assignment is preceded with the @option{-v} option, as in the following: @example -v @var{variable}=@var{text} @end example @noindent the variable is set at the very beginning, even before the @code{BEGIN} rules execute. The @option{-v} option and its assignment must precede all the @value{FN} arguments, as well as the program text. (@xref{Options} for more information about the @option{-v} option.) Otherwise, the variable assignment is performed at a time determined by its position among the input file arguments---after the processing of the preceding input file argument. For example: @example awk '@{ print $n @}' n=4 inventory-shipped n=2 mail-list @end example @noindent prints the value of field number @code{n} for all input records. Before the first file is read, the command line sets the variable @code{n} equal to four. This causes the fourth field to be printed in lines from @file{inventory-shipped}. After the first file has finished, but before the second file is started, @code{n} is set to two, so that the second field is printed in lines from @file{mail-list}: @example $ @kbd{awk '@{ print $n @}' n=4 inventory-shipped n=2 mail-list} @print{} 15 @print{} 24 @dots{} @print{} 555-5553 @print{} 555-3412 @dots{} @end example @cindex dark corner @subentry command-line arguments Command-line arguments are made available for explicit examination by the @command{awk} program in the @code{ARGV} array (@pxref{ARGC and ARGV}). @command{awk} processes the values of command-line assignments for escape sequences (@pxref{Escape Sequences}). @value{DARKCORNER} Normally, variables assigned on the command line (with or without the @option{-v} option) are treated as strings. When such variables are used as numbers, @command{awk}'s normal automatic conversion of strings to numbers takes place, and everything ``just works.'' However, @command{gawk} supports variables whose types are ``regexp''. You can assign variables of this type using the following syntax: @example gawk -v 're1=@@/foo|bar/' '@dots{}' /path/to/file1 're2=@@/baz|quux/' /path/to/file2 @end example @noindent Strongly typed regexps are an advanced feature (@pxref{Strong Regexp Constants}). We mention them here only for completeness. @node Conversion @subsection Conversion of Strings and Numbers Number-to-string and string-to-number conversion are generally straightforward. There can be subtleties to be aware of; this @value{SECTION} discusses this important facet of @command{awk}. @menu * Strings And Numbers:: How @command{awk} Converts Between Strings And Numbers. * Locale influences conversions:: How the locale may affect conversions. @end menu @node Strings And Numbers @subsubsection How @command{awk} Converts Between Strings and Numbers @cindex converting @subentry string to numbers @cindex strings @subentry converting @cindex numbers @subentry converting @cindex converting @subentry numbers to strings Strings are converted to numbers and numbers are converted to strings, if the context of the @command{awk} program demands it. For example, if the value of either @code{foo} or @code{bar} in the expression @samp{foo + bar} happens to be a string, it is converted to a number before the addition is performed. If numeric values appear in string concatenation, they are converted to strings. Consider the following: @example @group two = 2; three = 3 print (two three) + 4 @end group @end example @noindent This prints the (numeric) value 27. The numeric values of the variables @code{two} and @code{three} are converted to strings and concatenated together. The resulting string is converted back to the number 23, to which 4 is then added. @cindex null strings @subentry converting numbers to strings @cindex type @subentry conversion If, for some reason, you need to force a number to be converted to a string, concatenate that number with the empty string, @code{""}. To force a string to be converted to a number, add zero to that string. A string is converted to a number by interpreting any numeric prefix of the string as numerals: @code{"2.5"} converts to 2.5, @code{"1e3"} converts to 1,000, and @code{"25fix"} has a numeric value of 25. Strings that can't be interpreted as valid numbers convert to zero. @cindex @code{CONVFMT} variable The exact manner in which numbers are converted into strings is controlled by the @command{awk} predefined variable @code{CONVFMT} (@pxref{Built-in Variables}). Numbers are converted using the @code{sprintf()} function with @code{CONVFMT} as the format specifier (@pxref{String Functions}). @code{CONVFMT}'s default value is @code{"%.6g"}, which creates a value with at most six significant digits. For some applications, you might want to change it to specify more precision. On most modern machines, 17 digits is usually enough to capture a floating-point number's value exactly.@footnote{Pathological cases can require up to 752 digits (!), but we doubt that you need to worry about this.} @cindex dark corner @subentry @code{CONVFMT} variable Strange results can occur if you set @code{CONVFMT} to a string that doesn't tell @code{sprintf()} how to format floating-point numbers in a useful way. For example, if you forget the @samp{%} in the format, @command{awk} converts all numbers to the same constant string. As a special case, if a number is an integer, then the result of converting it to a string is @emph{always} an integer, no matter what the value of @code{CONVFMT} may be. Given the following code fragment: @example CONVFMT = "%2.2f" a = 12 b = a "" @end example @noindent @code{b} has the value @code{"12"}, not @code{"12.00"}. @value{DARKCORNER} @cindex sidebar @subentry Pre-POSIX @command{awk} Used @code{OFMT} for String Conversion @ifdocbook @docbook Pre-POSIX @command{awk} Used @code{OFMT} for String Conversion @end docbook @cindex POSIX @command{awk} @subentry @code{OFMT} variable and @cindex @code{OFMT} variable @cindex portability @subentry new @command{awk} vs.@: old @command{awk} @cindex @command{awk} @subentry new vs.@: old @subentry @code{OFMT} variable Prior to the POSIX standard, @command{awk} used the value of @code{OFMT} for converting numbers to strings. @code{OFMT} specifies the output format to use when printing numbers with @code{print}. @code{CONVFMT} was introduced in order to separate the semantics of conversion from the semantics of printing. Both @code{CONVFMT} and @code{OFMT} have the same default value: @code{"%.6g"}. In the vast majority of cases, old @command{awk} programs do not change their behavior. @xref{Print} for more information on the @code{print} statement. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Pre-POSIX @command{awk} Used @code{OFMT} for String Conversion} @cindex POSIX @command{awk} @subentry @code{OFMT} variable and @cindex @code{OFMT} variable @cindex portability @subentry new @command{awk} vs.@: old @command{awk} @cindex @command{awk} @subentry new vs.@: old @subentry @code{OFMT} variable Prior to the POSIX standard, @command{awk} used the value of @code{OFMT} for converting numbers to strings. @code{OFMT} specifies the output format to use when printing numbers with @code{print}. @code{CONVFMT} was introduced in order to separate the semantics of conversion from the semantics of printing. Both @code{CONVFMT} and @code{OFMT} have the same default value: @code{"%.6g"}. In the vast majority of cases, old @command{awk} programs do not change their behavior. @xref{Print} for more information on the @code{print} statement. @end cartouche @end ifnotdocbook @node Locale influences conversions @subsubsection Locales Can Influence Conversion Where you are can matter when it comes to converting between numbers and strings. The local character set and language---the @dfn{locale}---can affect numeric formats. In particular, for @command{awk} programs, it affects the decimal point character and the thousands-separator character. The @code{"C"} locale, and most English-language locales, use the period character (@samp{.}) as the decimal point and don't have a thousands separator. However, many (if not most) European and non-English locales use the comma (@samp{,}) as the decimal point character. European locales often use either a space or a period as the thousands separator, if they have one. @cindex dark corner @subentry locale's decimal point character The POSIX standard says that @command{awk} always uses the period as the decimal point when reading the @command{awk} program source code, and for command-line variable assignments (@pxref{Other Arguments}). However, when interpreting input data, for @code{print} and @code{printf} output, and for number-to-string conversion, the local decimal point character is used. @value{DARKCORNER} In all cases, numbers in source code and in input data cannot have a thousands separator. Here are some examples indicating the difference in behavior, on a GNU/Linux system: @example $ @kbd{export POSIXLY_CORRECT=1} @ii{Force POSIX behavior} $ @kbd{gawk 'BEGIN @{ printf "%g\n", 3.1415927 @}'} @print{} 3.14159 $ @kbd{LC_ALL=en_DK.utf-8 gawk 'BEGIN @{ printf "%g\n", 3.1415927 @}'} @print{} 3,14159 $ @kbd{echo 4,321 | gawk '@{ print $1 + 1 @}'} @print{} 5 $ @kbd{echo 4,321 | LC_ALL=en_DK.utf-8 gawk '@{ print $1 + 1 @}'} @print{} 5,321 @end example @noindent The @code{en_DK.utf-8} locale is for English in Denmark, where the comma acts as the decimal point separator. In the normal @code{"C"} locale, @command{gawk} treats @samp{4,321} as 4, while in the Danish locale, it's treated as the full number including the fractional part, 4.321. @cindex POSIX mode Some earlier versions of @command{gawk} fully complied with this aspect of the standard. However, many users in non-English locales complained about this behavior, because their data used a period as the decimal point, so the default behavior was restored to use a period as the decimal point character. You can use the @option{--use-lc-numeric} option (@pxref{Options}) to force @command{gawk} to use the locale's decimal point character. (@command{gawk} also uses the locale's decimal point character when in POSIX mode, either via @option{--posix} or the @env{POSIXLY_CORRECT} environment variable, as shown previously.) @ref{table-locale-affects} describes the cases in which the locale's decimal point character is used and when a period is used. Some of these features have not been described yet. @float Table,table-locale-affects @caption{Locale decimal point versus a period} @multitable @columnfractions .15 .20 .45 @headitem Feature @tab Default @tab @option{--posix} or @option{--use-lc-numeric} @item @code{%'g} @tab Use locale @tab Use locale @item @code{%g} @tab Use period @tab Use locale @item Input @tab Use period @tab Use locale @item @code{strtonum()} @tab Use period @tab Use locale @end multitable @end float Finally, modern-day formal standards and the IEEE standard floating-point representation can have an unusual but important effect on the way @command{gawk} converts some special string values to numbers. The details are presented in @ref{POSIX Floating Point Problems}. @node All Operators @section Operators: Doing Something with Values This @value{SECTION} introduces the @dfn{operators} that make use of the values provided by constants and variables. @menu * Arithmetic Ops:: Arithmetic operations (@samp{+}, @samp{-}, etc.) * Concatenation:: Concatenating strings. * Assignment Ops:: Changing the value of a variable or a field. * Increment Ops:: Incrementing the numeric value of a variable. @end menu @node Arithmetic Ops @subsection Arithmetic Operators @cindex arithmetic operators @cindex operators @subentry arithmetic @c @cindex addition @c @cindex subtraction @c @cindex multiplication @c @cindex division @c @cindex remainder @c @cindex quotient @c @cindex exponentiation The @command{awk} language uses the common arithmetic operators when evaluating expressions. All of these arithmetic operators follow normal precedence rules and work as you would expect them to. The following example uses a file named @file{grades}, which contains a list of student names as well as three test scores per student (it's a small class): @example Pat 100 97 58 Sandy 84 72 93 Chris 72 92 89 @end example @noindent This program takes the file @file{grades} and prints the average of the scores: @example $ @kbd{awk '@{ sum = $2 + $3 + $4 ; avg = sum / 3} > @kbd{print $1, avg @}' grades} @print{} Pat 85 @print{} Sandy 83 @print{} Chris 84.3333 @end example The following list provides the arithmetic operators in @command{awk}, in order from the highest precedence to the lowest: @table @code @cindex common extensions @subentry @code{**} operator @cindex extensions @subentry common @subentry @code{**} operator @cindex POSIX @command{awk} @subentry arithmetic operators and @item @var{x} ^ @var{y} @itemx @var{x} ** @var{y} Exponentiation; @var{x} raised to the @var{y} power. @samp{2 ^ 3} has the value eight; the character sequence @samp{**} is equivalent to @samp{^}. @value{COMMONEXT} @item - @var{x} Negation. @item + @var{x} Unary plus; the expression is converted to a number. @item @var{x} * @var{y} Multiplication. @cindex troubleshooting @subentry division @cindex division @item @var{x} / @var{y} Division; because all numbers in @command{awk} are floating-point numbers, the result is @emph{not} rounded to an integer---@samp{3 / 4} has the value 0.75. (It is a common mistake, especially for C programmers, to forget that @emph{all} numbers in @command{awk} are floating point, and that division of integer-looking constants produces a real number, not an integer.) @item @var{x} % @var{y} Remainder; further discussion is provided in the text, just after this list. @item @var{x} + @var{y} Addition. @item @var{x} - @var{y} Subtraction. @end table Unary plus and minus have the same precedence, the multiplication operators all have the same precedence, and addition and subtraction have the same precedence. @cindex differences in @command{awk} and @command{gawk} @subentry trunc-mod operation @cindex trunc-mod operation When computing the remainder of @samp{@var{x} % @var{y}}, the quotient is rounded toward zero to an integer and multiplied by @var{y}. This result is subtracted from @var{x}; this operation is sometimes known as ``trunc-mod.'' The following relation always holds: @example b * int(a / b) + (a % b) == a @end example One possibly undesirable effect of this definition of remainder is that @samp{@var{x} % @var{y}} is negative if @var{x} is negative. Thus: @example -17 % 8 = -1 @end example In other @command{awk} implementations, the signedness of the remainder may be machine-dependent. @c FIXME !!! what does posix say? @cindex portability @subentry @code{**} operator and @cindex @code{*} (asterisk) @subentry @code{**} operator @cindex asterisk (@code{*}) @subentry @code{**} operator @quotation NOTE The POSIX standard only specifies the use of @samp{^} for exponentiation. For maximum portability, do not use the @samp{**} operator. @end quotation @node Concatenation @subsection String Concatenation @cindex Kernighan, Brian @quotation @i{It seemed like a good idea at the time.} @author Brian Kernighan @end quotation @cindex string @subentry operators @cindex operators @subentry string @cindex concatenating There is only one string operation: concatenation. It does not have a specific operator to represent it. Instead, concatenation is performed by writing expressions next to one another, with no operator. For example: @example $ @kbd{awk '@{ print "Field number one: " $1 @}' mail-list} @print{} Field number one: Amelia @print{} Field number one: Anthony @dots{} @end example Without the space in the string constant after the @samp{:}, the line runs together. For example: @example $ @kbd{awk '@{ print "Field number one:" $1 @}' mail-list} @print{} Field number one:Amelia @print{} Field number one:Anthony @dots{} @end example @cindex troubleshooting @subentry string concatenation Because string concatenation does not have an explicit operator, it is often necessary to ensure that it happens at the right time by using parentheses to enclose the items to concatenate. For example, you might expect that the following code fragment concatenates @code{file} and @code{name}: @example file = "file" name = "name" print "something meaningful" > file name @end example @cindex Brian Kernighan's @command{awk} @cindex @command{mawk} utility @noindent This produces a syntax error with some versions of Unix @command{awk}.@footnote{It happens that BWK @command{awk}, @command{gawk}, and @command{mawk} all ``get it right,'' but you should not rely on this.} It is necessary to use the following: @example print "something meaningful" > (file name) @end example @cindex order of evaluation, concatenation @cindex evaluation order @subentry concatenation @cindex side effects Parentheses should be used around concatenation in all but the most common contexts, such as on the righthand side of @samp{=}. Be careful about the kinds of expressions used in string concatenation. In particular, the order of evaluation of expressions used for concatenation is undefined in the @command{awk} language. Consider this example: @example BEGIN @{ a = "don't" print (a " " (a = "panic")) @} @end example @noindent It is not defined whether the second assignment to @code{a} happens before or after the value of @code{a} is retrieved for producing the concatenated value. The result could be either @samp{don't panic}, or @samp{panic panic}. @c see test/nasty.awk for a worse example The precedence of concatenation, when mixed with other operators, is often counter-intuitive. Consider this example: @ignore > To: bug-gnu-utils@@gnu.org > CC: arnold@@gnu.org > Subject: gawk 3.0.4 bug with {print -12 " " -24} > From: Russell Schulz > Date: Tue, 8 Feb 2000 19:56:08 -0700 > > gawk 3.0.4 on NT gives me: > > prompt> cat bad.awk > BEGIN { print -12 " " -24; } > > prompt> gawk -f bad.awk > -12-24 > > when I would expect > > -12 -24 > > I have not investigated the source, or other implementations. The > bug is there on my NT and DOS versions 2.15.6 . @end ignore @example $ @kbd{awk 'BEGIN @{ print -12 " " -24 @}'} @print{} -12-24 @end example This ``obviously'' is concatenating @minus{}12, a space, and @minus{}24. But where did the space disappear to? The answer lies in the combination of operator precedences and @command{awk}'s automatic conversion rules. To get the desired result, write the program this way: @example $ @kbd{awk 'BEGIN @{ print -12 " " (-24) @}'} @print{} -12 -24 @end example This forces @command{awk} to treat the @samp{-} on the @samp{-24} as unary. Otherwise, it's parsed as follows: @display @minus{}12 (@code{"@ "} @minus{} 24) @result{} @minus{}12 (0 @minus{} 24) @result{} @minus{}12 (@minus{}24) @result{} @minus{}12@minus{}24 @end display As mentioned earlier, when mixing concatenation with other operators, @emph{parenthesize}. Otherwise, you're never quite sure what you'll get. @node Assignment Ops @subsection Assignment Expressions @cindex assignment operators @cindex operators @subentry assignment @cindex expressions @subentry assignment @cindex @code{=} (equals sign) @subentry @code{=} operator @cindex equals sign (@code{=}) @subentry @code{=} operator An @dfn{assignment} is an expression that stores a (usually different) value into a variable. For example, let's assign the value one to the variable @code{z}: @example z = 1 @end example After this expression is executed, the variable @code{z} has the value one. Whatever old value @code{z} had before the assignment is forgotten. Assignments can also store string values. For example, the following stores the value @code{"this food is good"} in the variable @code{message}: @example thing = "food" predicate = "good" message = "this " thing " is " predicate @end example @noindent @cindex side effects @subentry assignment expressions This also illustrates string concatenation. The @samp{=} sign is called an @dfn{assignment operator}. It is the simplest assignment operator because the value of the righthand operand is stored unchanged. Most operators (addition, concatenation, and so on) have no effect except to compute a value. If the value isn't used, there's no reason to use the operator. An assignment operator is different; it does produce a value, but even if you ignore it, the assignment still makes itself felt through the alteration of the variable. We call this a @dfn{side effect}. @cindex lvalues/rvalues @cindex rvalues/lvalues @cindex assignment operators @subentry lvalues/rvalues @cindex operators @subentry assignment The lefthand operand of an assignment need not be a variable (@pxref{Variables}); it can also be a field (@pxref{Changing Fields}) or an array element (@pxref{Arrays}). These are all called @dfn{lvalues}, which means they can appear on the lefthand side of an assignment operator. The righthand operand may be any expression; it produces the new value that the assignment stores in the specified variable, field, or array element. (Such values are called @dfn{rvalues}.) @cindex variables @subentry types of It is important to note that variables do @emph{not} have permanent types. A variable's type is simply the type of whatever value was last assigned to it. In the following program fragment, the variable @code{foo} has a numeric value at first, and a string value later on: @example @group foo = 1 print foo @end group @group foo = "bar" print foo @end group @end example @noindent When the second assignment gives @code{foo} a string value, the fact that it previously had a numeric value is forgotten. String values that do not begin with a digit have a numeric value of zero. After executing the following code, the value of @code{foo} is five: @example foo = "a string" foo = foo + 5 @end example @quotation NOTE Using a variable as a number and then later as a string can be confusing and is poor programming style. The previous two examples illustrate how @command{awk} works, @emph{not} how you should write your programs! @end quotation An assignment is an expression, so it has a value---the same value that is assigned. Thus, @samp{z = 1} is an expression with the value one. One consequence of this is that you can write multiple assignments together, such as: @example x = y = z = 5 @end example @noindent This example stores the value five in all three variables (@code{x}, @code{y}, and @code{z}). It does so because the value of @samp{z = 5}, which is five, is stored into @code{y} and then the value of @samp{y = z = 5}, which is five, is stored into @code{x}. Assignments may be used anywhere an expression is called for. For example, it is valid to write @samp{x != (y = 1)} to set @code{y} to one, and then test whether @code{x} equals one. But this style tends to make programs hard to read; such nesting of assignments should be avoided, except perhaps in a one-shot program. @cindex @code{+} (plus sign) @subentry @code{+=} operator @cindex plus sign (@code{+}) @subentry @code{+=} operator Aside from @samp{=}, there are several other assignment operators that do arithmetic with the old value of the variable. For example, the operator @samp{+=} computes a new value by adding the righthand value to the old value of the variable. Thus, the following assignment adds five to the value of @code{foo}: @example foo += 5 @end example @noindent This is equivalent to the following: @example foo = foo + 5 @end example @noindent Use whichever makes the meaning of your program clearer. There are situations where using @samp{+=} (or any assignment operator) is @emph{not} the same as simply repeating the lefthand operand in the righthand expression. For example: @cindex Rankin, Pat @example @group # Thanks to Pat Rankin for this example BEGIN @{ foo[rand()] += 5 for (x in foo) print x, foo[x] @end group @group bar[rand()] = bar[rand()] + 5 for (x in bar) print x, bar[x] @} @end group @end example @cindex operators @subentry assignment @subentry evaluation order @cindex assignment operators @subentry evaluation order @noindent The indices of @code{bar} are practically guaranteed to be different, because @code{rand()} returns different values each time it is called. (Arrays and the @code{rand()} function haven't been covered yet. @xref{Arrays}, and @ifnotdocbook @pxref{Numeric Functions} @end ifnotdocbook @ifdocbook @ref{Numeric Functions} @end ifdocbook for more information.) This example illustrates an important fact about assignment operators: the lefthand expression is only evaluated @emph{once}. It is up to the implementation as to which expression is evaluated first, the lefthand or the righthand. Consider this example: @example i = 1 a[i += 2] = i + 1 @end example @noindent The value of @code{a[3]} could be either two or four. @ref{table-assign-ops} lists the arithmetic assignment operators. In each case, the righthand operand is an expression whose value is converted to a number. @cindex @code{-} (hyphen) @subentry @code{-=} operator @cindex hyphen (@code{-}) @subentry @code{-=} operator @cindex @code{*} (asterisk) @subentry @code{*=} operator @cindex asterisk (@code{*}) @subentry @code{*=} operator @cindex @code{/} (forward slash) @subentry @code{/=} operator @cindex forward slash (@code{/}) @subentry @code{/=} operator @cindex @code{%} (percent sign) @subentry @code{%=} operator @cindex percent sign (@code{%}) @subentry @code{%=} operator @cindex @code{^} (caret) @subentry @code{^=} operator @cindex caret (@code{^}) @subentry @code{^=} operator @cindex @code{*} (asterisk) @subentry @code{**=} operator @cindex asterisk (@code{*}) @subentry @code{**=} operator @float Table,table-assign-ops @caption{Arithmetic assignment operators} @multitable @columnfractions .30 .70 @headitem Operator @tab Effect @item @var{lvalue} @code{+=} @var{increment} @tab Add @var{increment} to the value of @var{lvalue}. @item @var{lvalue} @code{-=} @var{decrement} @tab Subtract @var{decrement} from the value of @var{lvalue}. @item @var{lvalue} @code{*=} @var{coefficient} @tab Multiply the value of @var{lvalue} by @var{coefficient}. @item @var{lvalue} @code{/=} @var{divisor} @tab Divide the value of @var{lvalue} by @var{divisor}. @item @var{lvalue} @code{%=} @var{modulus} @tab Set @var{lvalue} to its remainder by @var{modulus}. @cindex common extensions @subentry @code{**=} operator @cindex extensions @subentry common @subentry @code{**=} operator @cindex @command{awk} @subentry language, POSIX version @cindex POSIX @command{awk} @item @var{lvalue} @code{^=} @var{power} @tab Raise @var{lvalue} to the power @var{power}. @item @var{lvalue} @code{**=} @var{power} @tab Raise @var{lvalue} to the power @var{power}. @value{COMMONEXT} @end multitable @end float @cindex POSIX @command{awk} @subentry @code{**=} operator and @cindex portability @subentry @code{**=} operator and @quotation NOTE Only the @samp{^=} operator is specified by POSIX. For maximum portability, do not use the @samp{**=} operator. @end quotation @cindex sidebar @subentry Syntactic Ambiguities Between @samp{/=} and Regular Expressions @ifdocbook @docbook Syntactic Ambiguities Between @samp{/=} and Regular Expressions @end docbook @cindex dark corner @subentry regexp constants @subentry @code{/=} operator and @cindex @code{/} (forward slash) @subentry @code{/=} operator @subentry vs. @code{/=@dots{}/} regexp constant @cindex forward slash (@code{/}) @subentry @code{/=} operator @subentry vs. @code{/=@dots{}/} regexp constant @cindex regexp constants @subentry @code{/=@dots{}/} @subentry @code{/=} operator and @c derived from email from "Nelson H. F. Beebe" @c Date: Mon, 1 Sep 1997 13:38:35 -0600 (MDT) @cindex dark corner @subentry @code{/=} operator vs. @code{/=@dots{}/} regexp constant @cindex ambiguity, syntactic: @code{/=} operator vs. @code{/=@dots{}/} regexp constant @cindex syntactic ambiguity: @code{/=} operator vs. @code{/=@dots{}/} regexp constant @cindex @code{/=} operator vs. @code{/=@dots{}/} regexp constant There is a syntactic ambiguity between the @code{/=} assignment operator and regexp constants whose first character is an @samp{=}. @value{DARKCORNER} This is most notable in some commercial @command{awk} versions. For example: @example $ @kbd{awk /==/ /dev/null} @error{} awk: syntax error at source line 1 @error{} context is @error{} >>> /= <<< @error{} awk: bailing out at source line 1 @end example @noindent A workaround is: @example awk '/[=]=/' /dev/null @end example @command{gawk} does not have this problem; BWK @command{awk} and @command{mawk} also do not. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Syntactic Ambiguities Between @samp{/=} and Regular Expressions} @cindex dark corner @subentry regexp constants @subentry @code{/=} operator and @cindex @code{/} (forward slash) @subentry @code{/=} operator @subentry vs. @code{/=@dots{}/} regexp constant @cindex forward slash (@code{/}) @subentry @code{/=} operator @subentry vs. @code{/=@dots{}/} regexp constant @cindex regexp constants @subentry @code{/=@dots{}/} @subentry @code{/=} operator and @c derived from email from "Nelson H. F. Beebe" @c Date: Mon, 1 Sep 1997 13:38:35 -0600 (MDT) @cindex dark corner @subentry @code{/=} operator vs. @code{/=@dots{}/} regexp constant @cindex ambiguity, syntactic: @code{/=} operator vs. @code{/=@dots{}/} regexp constant @cindex syntactic ambiguity: @code{/=} operator vs. @code{/=@dots{}/} regexp constant @cindex @code{/=} operator vs. @code{/=@dots{}/} regexp constant There is a syntactic ambiguity between the @code{/=} assignment operator and regexp constants whose first character is an @samp{=}. @value{DARKCORNER} This is most notable in some commercial @command{awk} versions. For example: @example $ @kbd{awk /==/ /dev/null} @error{} awk: syntax error at source line 1 @error{} context is @error{} >>> /= <<< @error{} awk: bailing out at source line 1 @end example @noindent A workaround is: @example awk '/[=]=/' /dev/null @end example @command{gawk} does not have this problem; BWK @command{awk} and @command{mawk} also do not. @end cartouche @end ifnotdocbook @node Increment Ops @subsection Increment and Decrement Operators @cindex increment operators @cindex operators @subentry decrement/increment @dfn{Increment} and @dfn{decrement operators} increase or decrease the value of a variable by one. An assignment operator can do the same thing, so the increment operators add no power to the @command{awk} language; however, they are convenient abbreviations for very common operations. @cindex side effects @cindex @code{+} (plus sign) @subentry @code{++} operator @cindex plus sign (@code{+}) @subentry @code{++} operator @cindex side effects @subentry decrement/increment operators The operator used for adding one is written @samp{++}. It can be used to increment a variable either before or after taking its value. To @dfn{pre-increment} a variable @code{v}, write @samp{++v}. This adds one to the value of @code{v}---that new value is also the value of the expression. (The assignment expression @samp{v += 1} is completely equivalent.) Writing the @samp{++} after the variable specifies @dfn{post-increment}. This increments the variable value just the same; the difference is that the value of the increment expression itself is the variable's @emph{old} value. Thus, if @code{foo} has the value four, then the expression @samp{foo++} has the value four, but it changes the value of @code{foo} to five. In other words, the operator returns the old value of the variable, but with the side effect of incrementing it. The post-increment @samp{foo++} is nearly the same as writing @samp{(foo += 1) - 1}. It is not perfectly equivalent because all numbers in @command{awk} are floating point---in floating point, @samp{foo + 1 - 1} does not necessarily equal @code{foo}. But the difference is minute as long as you stick to numbers that are fairly small (less than @iftex @math{10^{12}}). @end iftex @ifinfo 10e12). @end ifinfo @ifnottex @ifnotinfo 10@sup{12}). @end ifnotinfo @end ifnottex @cindex @code{$} (dollar sign) @subentry incrementing fields and arrays @cindex dollar sign (@code{$}) @subentry incrementing fields and arrays Fields and array elements are incremented just like variables. (Use @samp{$(i++)} when you want to do a field reference and a variable increment at the same time. The parentheses are necessary because of the precedence of the field reference operator @samp{$}.) @cindex decrement operators The decrement operator @samp{--} works just like @samp{++}, except that it subtracts one instead of adding it. As with @samp{++}, it can be used before the lvalue to pre-decrement or after it to post-decrement. Following is a summary of increment and decrement expressions: @table @code @cindex @code{+} (plus sign) @subentry @code{++} operator @cindex plus sign (@code{+}) @subentry @code{++} operator @item ++@var{lvalue} Increment @var{lvalue}, returning the new value as the value of the expression. @item @var{lvalue}++ Increment @var{lvalue}, returning the @emph{old} value of @var{lvalue} as the value of the expression. @cindex @code{-} (hyphen) @subentry @code{--} operator @cindex hyphen (@code{-}) @subentry @code{--} operator @item --@var{lvalue} Decrement @var{lvalue}, returning the new value as the value of the expression. (This expression is like @samp{++@var{lvalue}}, but instead of adding, it subtracts.) @item @var{lvalue}-- Decrement @var{lvalue}, returning the @emph{old} value of @var{lvalue} as the value of the expression. (This expression is like @samp{@var{lvalue}++}, but instead of adding, it subtracts.) @end table @cindex sidebar @subentry Operator Evaluation Order @ifdocbook @docbook Operator Evaluation Order @end docbook @cindex precedence @cindex operators @subentry precedence of @cindex portability @subentry operators @cindex evaluation order @cindex Marx, Groucho @quotation @i{Doctor, it hurts when I do this!@* Then don't do that!} @author Groucho Marx @end quotation @noindent What happens for something like the following? @example b = 6 print b += b++ @end example @noindent Or something even stranger? @example b = 6 b += ++b + b++ print b @end example @cindex side effects In other words, when do the various side effects prescribed by the postfix operators (@samp{b++}) take effect? When side effects happen is @dfn{implementation-defined}. In other words, it is up to the particular version of @command{awk}. The result for the first example may be 12 or 13, and for the second, it may be 22 or 23. In short, doing things like this is not recommended and definitely not anything that you can rely upon for portability. You should avoid such things in your own programs. @c You'll sleep better at night and be able to look at yourself @c in the mirror in the morning. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Operator Evaluation Order} @cindex precedence @cindex operators @subentry precedence of @cindex portability @subentry operators @cindex evaluation order @cindex Marx, Groucho @quotation @i{Doctor, it hurts when I do this!@* Then don't do that!} @author Groucho Marx @end quotation @noindent What happens for something like the following? @example b = 6 print b += b++ @end example @noindent Or something even stranger? @example b = 6 b += ++b + b++ print b @end example @cindex side effects In other words, when do the various side effects prescribed by the postfix operators (@samp{b++}) take effect? When side effects happen is @dfn{implementation-defined}. In other words, it is up to the particular version of @command{awk}. The result for the first example may be 12 or 13, and for the second, it may be 22 or 23. In short, doing things like this is not recommended and definitely not anything that you can rely upon for portability. You should avoid such things in your own programs. @c You'll sleep better at night and be able to look at yourself @c in the mirror in the morning. @end cartouche @end ifnotdocbook @node Truth Values and Conditions @section Truth Values and Conditions In certain contexts, expression values also serve as ``truth values''; i.e., they determine what should happen next as the program runs. This @value{SECTION} describes how @command{awk} defines ``true'' and ``false'' and how values are compared. @menu * Truth Values:: What is ``true'' and what is ``false''. * Typing and Comparison:: How variables acquire types and how this affects comparison of numbers and strings with @samp{<}, etc. * Boolean Ops:: Combining comparison expressions using boolean operators @samp{||} (``or''), @samp{&&} (``and'') and @samp{!} (``not''). * Conditional Exp:: Conditional expressions select between two subexpressions under control of a third subexpression. @end menu @node Truth Values @subsection True and False in @command{awk} @cindex truth values @cindex logical false/true @cindex false, logical @cindex true, logical @cindex null strings Many programming languages have a special representation for the concepts of ``true'' and ``false.'' Such languages usually use the special constants @code{true} and @code{false}, or perhaps their uppercase equivalents. However, @command{awk} is different. It borrows a very simple concept of true and false from C. In @command{awk}, any nonzero numeric value @emph{or} any nonempty string value is true. Any other value (zero or the null string, @code{""}) is false. The following program prints @samp{A strange truth value} three times: @example BEGIN @{ if (3.1415927) print "A strange truth value" if ("Four Score And Seven Years Ago") print "A strange truth value" if (j = 57) print "A strange truth value" @} @end example @cindex dark corner @subentry @code{"0"} is actually true There is a surprising consequence of the ``nonzero or non-null'' rule: the string constant @code{"0"} is actually true, because it is non-null. @value{DARKCORNER} @node Typing and Comparison @subsection Variable Typing and Comparison Expressions @quotation @i{The Guide is definitive. Reality is frequently inaccurate.} @author Douglas Adams, @cite{The Hitchhiker's Guide to the Galaxy} @end quotation @c 2/2015: Antonio Colombo points out that this is really from @c The Restaurant at the End of the Universe. But I'm going to @c leave it alone. @cindex comparison expressions @cindex expressions @subentry comparison @cindex expressions, matching @seeentry{comparison expressions} @cindex matching @subentry expressions @seeentry{comparison expressions} @cindex relational operators @seeentry{comparison operators} @cindex operators, relational @seeentry{operators, comparison} @cindex variables @subentry types of @subentry comparison expressions and Unlike in other programming languages, in @command{awk} variables do not have a fixed type. Instead, they can be either a number or a string, depending upon the value that is assigned to them. We look now at how variables are typed, and how @command{awk} compares variables. @menu * Variable Typing:: String type versus numeric type. * Comparison Operators:: The comparison operators. * POSIX String Comparison:: String comparison with POSIX rules. @end menu @node Variable Typing @subsubsection String Type versus Numeric Type Scalar objects in @command{awk} (variables, array elements, and fields) are @emph{dynamically} typed. This means their type can change as the program runs, from @dfn{untyped} before any use,@footnote{@command{gawk} calls this @dfn{unassigned}, as the following example shows.} to string or number, and then from string to number or number to string, as the program progresses. (@command{gawk} also provides regexp-typed scalars, but let's ignore that for now; @pxref{Strong Regexp Constants}.) You can't do much with untyped variables, other than tell that they are untyped. The following program tests @code{a} against @code{""} and @code{0}; the test succeeds when @code{a} has never been assigned a value. It also uses the built-in @code{typeof()} function (not presented yet; @pxref{Type Functions}) to show @code{a}'s type: @example $ @kbd{gawk 'BEGIN @{ print (a == "" && a == 0 ?} > @kbd{"a is untyped" : "a has a type!") ; print typeof(a) @}'} @print{} a is untyped @print{} unassigned @end example A scalar has numeric type when assigned a numeric value, such as from a numeric constant, or from another scalar with numeric type: @example $ @kbd{gawk 'BEGIN @{ a = 42 ; print typeof(a)} > @kbd{b = a ; print typeof(b) @}'} number number @end example Similarly, a scalar has string type when assigned a string value, such as from a string constant, or from another scalar with string type: @example $ @kbd{gawk 'BEGIN @{ a = "forty two" ; print typeof(a)} > @kbd{b = a ; print typeof(b) @}'} string string @end example So far, this is all simple and straightforward. What happens, though, when @command{awk} has to process data from a user? Let's start with field data. What should the following command produce as output? @example echo hello | awk '@{ printf("%s %s < 42\n", $1, ($1 < 42 ? "is" : "is not")) @}' @end example @noindent Since @samp{hello} is alphabetic data, @command{awk} can only do a string comparison. Internally, it converts @code{42} into @code{"42"} and compares the two string values @code{"hello"} and @code{"42"}. Here's the result: @example $ @kbd{echo hello | awk '@{ printf("%s %s < 42\n", $1,} > @kbd{ ($1 < 42 ? "is" : "is not")) @}'} @print{} hello is not < 42 @end example However, what happens when data from a user @emph{looks like} a number? On the one hand, in reality, the input data consists of characters, not binary numeric values. But, on the other hand, the data looks numeric, and @command{awk} really ought to treat it as such. And indeed, it does: @example $ @kbd{echo 37 | awk '@{ printf("%s %s < 42\n", $1,} > @kbd{ ($1 < 42 ? "is" : "is not")) @}'} @print{} 37 is < 42 @end example Here are the rules for when @command{awk} treats data as a number, and for when it treats data as a string. @cindex numeric @subentry strings @cindex strings @subentry numeric @cindex POSIX @command{awk} @subentry numeric strings and The POSIX standard uses the term @dfn{numeric string} for input data that looks numeric. The @samp{37} in the previous example is a numeric string. So what is the type of a numeric string? Answer: numeric. The type of a variable is important because the types of two variables determine how they are compared. Variable typing follows these definitions and rules: @itemize @value{BULLET} @item A numeric constant or the result of a numeric operation has the @dfn{numeric} attribute. @item A string constant or the result of a string operation has the @dfn{string} attribute. @item Fields, @code{getline} input, @code{FILENAME}, @code{ARGV} elements, @code{ENVIRON} elements, and the elements of an array created by @code{match()}, @code{split()}, and @code{patsplit()} that are numeric strings have the @dfn{strnum} attribute.@footnote{Thus, a POSIX numeric string and @command{gawk}'s strnum are the same thing.} Otherwise, they have the @dfn{string} attribute. Uninitialized variables also have the @dfn{strnum} attribute. @item Attributes propagate across assignments but are not changed by any use. @c (Although a use may cause the entity to acquire an additional @c value such that it has both a numeric and string value, this leaves the @c attribute unchanged.) @c This is important but not relevant @end itemize The last rule is particularly important. In the following program, @code{a} has numeric type, even though it is later used in a string operation: @example BEGIN @{ a = 12.345 b = a " is a cute number" print b @} @end example When two operands are compared, either string comparison or numeric comparison may be used. This depends upon the attributes of the operands, according to the following symmetric matrix: @c thanks to Karl Berry, kb@cs.umb.edu, for major help with TeX tables @tex \centerline{ \vbox{\bigskip % space above the table (about 1 linespace) % Because we have vertical rules, we can't let TeX insert interline space % in its usual way. \offinterlineskip % % Define the table template. & separates columns, and \cr ends the % template (and each row). # is replaced by the text of that entry on % each row. The template for the first column breaks down like this: % \strut -- a way to make each line have the height and depth % of a normal line of type, since we turned off interline spacing. % \hfil -- infinite glue; has the effect of right-justifying in this case. % # -- replaced by the text (for instance, `STRNUM', in the last row). % \quad -- about the width of an `M'. Just separates the columns. % % The second column (\vrule#) is what generates the vertical rule that % spans table rows. % % The doubled && before the next entry means `repeat the following % template as many times as necessary on each line' -- in our case, twice. % % The template itself, \quad#\hfil, left-justifies with a little space before. % \halign{\strut\hfil#\quad&\vrule#&&\quad#\hfil\cr &&STRING &NUMERIC &STRNUM\cr % The \omit tells TeX to skip inserting the template for this column on % this particular row. In this case, we only want a little extra space % to separate the heading row from the rule below it. the depth 2pt -- % `\vrule depth 2pt' is that little space. \omit &depth 2pt\cr % This is the horizontal rule below the heading. Since it has nothing to % do with the columns of the table, we use \noalign to get it in there. \noalign{\hrule} % Like above, this time a little more space. \omit &depth 4pt\cr % The remaining rows have nothing special about them. STRING &&string &string &string\cr NUMERIC &&string &numeric &numeric\cr STRNUM &&string &numeric &numeric\cr }}} @end tex @ifnottex @ifnotdocbook @verbatim +---------------------------------------------- | STRING NUMERIC STRNUM --------+---------------------------------------------- | STRING | string string string | NUMERIC | string numeric numeric | STRNUM | string numeric numeric --------+---------------------------------------------- @end verbatim @end ifnotdocbook @end ifnottex @docbook STRING NUMERIC STRNUM STRING string string string NUMERIC string numeric numeric STRNUM string numeric numeric @end docbook The basic idea is that user input that looks numeric---and @emph{only} user input---should be treated as numeric, even though it is actually made of characters and is therefore also a string. Thus, for example, the string constant @w{@code{" +3.14"}}, when it appears in program source code, is a string---even though it looks numeric---and is @emph{never} treated as a number for comparison purposes. In short, when one operand is a ``pure'' string, such as a string constant, then a string comparison is performed. Otherwise, a numeric comparison is performed. (The primary difference between a number and a strnum is that for strnums @command{gawk} preserves the original string value that the scalar had when it came in.) This point bears additional emphasis: Input that looks numeric @emph{is} numeric. All other input is treated as strings. Thus, the six-character input string @w{@samp{ +3.14}} receives the strnum attribute. In contrast, the eight characters @w{@code{" +3.14"}} appearing in program text comprise a string constant. The following examples print @samp{1} when the comparison between the two different constants is true, and @samp{0} otherwise: @c 22.9.2014: Tested with mawk and BWK awk, got same results. @example $ @kbd{echo ' +3.14' | awk '@{ print($0 == " +3.14") @}'} @ii{True} @print{} 1 $ @kbd{echo ' +3.14' | awk '@{ print($0 == "+3.14") @}'} @ii{False} @print{} 0 $ @kbd{echo ' +3.14' | awk '@{ print($0 == "3.14") @}'} @ii{False} @print{} 0 $ @kbd{echo ' +3.14' | awk '@{ print($0 == 3.14) @}'} @ii{True} @print{} 1 $ @kbd{echo ' +3.14' | awk '@{ print($1 == " +3.14") @}'} @ii{False} @print{} 0 $ @kbd{echo ' +3.14' | awk '@{ print($1 == "+3.14") @}'} @ii{True} @print{} 1 $ @kbd{echo ' +3.14' | awk '@{ print($1 == "3.14") @}'} @ii{False} @print{} 0 $ @kbd{echo ' +3.14' | awk '@{ print($1 == 3.14) @}'} @ii{True} @print{} 1 @end example You can see the type of an input field (or other user input) using @code{typeof()}: @example $ @kbd{echo hello 37 | gawk '@{ print typeof($1), typeof($2) @}'} @print{} string strnum @end example @node Comparison Operators @subsubsection Comparison Operators @cindex operators @subentry comparison @dfn{Comparison expressions} compare strings or numbers for relationships such as equality. They are written using @dfn{relational operators}, which are a superset of those in C. @ref{table-relational-ops} describes them. @cindex @code{<} (left angle bracket) @subentry @code{<} operator @cindex left angle bracket (@code{<}) @subentry @code{<} operator @cindex @code{<} (left angle bracket) @subentry @code{<=} operator @cindex left angle bracket (@code{<}) @subentry @code{<=} operator @cindex @code{>} (right angle bracket) @subentry @code{>=} operator @cindex right angle bracket (@code{>}) @subentry @code{>=} operator @cindex @code{>} (right angle bracket) @subentry @code{>} operator @cindex right angle bracket (@code{>}) @subentry @code{>} operator @cindex @code{=} (equals sign) @subentry @code{==} operator @cindex equals sign (@code{=}) @subentry @code{==} operator @cindex @code{!} (exclamation point) @subentry @code{!=} operator @cindex exclamation point (@code{!}) @subentry @code{!=} operator @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point) @subentry @code{!~} operator @cindex exclamation point (@code{!}) @subentry @code{!~} operator @cindex @code{in} operator @float Table,table-relational-ops @caption{Relational operators} @multitable @columnfractions .25 .75 @headitem Expression @tab Result @item @var{x} @code{<} @var{y} @tab True if @var{x} is less than @var{y} @item @var{x} @code{<=} @var{y} @tab True if @var{x} is less than or equal to @var{y} @item @var{x} @code{>} @var{y} @tab True if @var{x} is greater than @var{y} @item @var{x} @code{>=} @var{y} @tab True if @var{x} is greater than or equal to @var{y} @item @var{x} @code{==} @var{y} @tab True if @var{x} is equal to @var{y} @item @var{x} @code{!=} @var{y} @tab True if @var{x} is not equal to @var{y} @item @var{x} @code{~} @var{y} @tab True if the string @var{x} matches the regexp denoted by @var{y} @item @var{x} @code{!~} @var{y} @tab True if the string @var{x} does not match the regexp denoted by @var{y} @item @var{subscript} @code{in} @var{array} @tab True if the array @var{array} has an element with the subscript @var{subscript} @end multitable @end float Comparison expressions have the value one if true and zero if false. When comparing operands of mixed types, numeric operands are converted to strings using the value of @code{CONVFMT} (@pxref{Conversion}). Strings are compared by comparing the first character of each, then the second character of each, and so on. Thus, @code{"10"} is less than @code{"9"}. If there are two strings where one is a prefix of the other, the shorter string is less than the longer one. Thus, @code{"abc"} is less than @code{"abcd"}. @cindex troubleshooting @subentry @code{==} operator It is very easy to accidentally mistype the @samp{==} operator and leave off one of the @samp{=} characters. The result is still valid @command{awk} code, but the program does not do what is intended: @example @group if (a = b) # oops! should be a == b @dots{} else @dots{} @end group @end example @noindent Unless @code{b} happens to be zero or the null string, the @code{if} part of the test always succeeds. Because the operators are so similar, this kind of error is very difficult to spot when scanning the source code. The following list of expressions illustrates the kinds of comparisons @command{awk} performs, as well as what the result of each comparison is: @table @code @item 1.5 <= 2.0 Numeric comparison (true) @item "abc" >= "xyz" String comparison (false) @item 1.5 != " +2" String comparison (true) @item "1e2" < "3" String comparison (true) @item a = 2; b = "2" @itemx a == b String comparison (true) @item a = 2; b = " +2" @itemx a == b String comparison (false) @end table In this example: @example $ @kbd{echo 1e2 3 | awk '@{ print ($1 < $2) ? "true" : "false" @}'} @print{} false @end example @cindex comparison expressions @subentry string vs.@: regexp @c @cindex string comparison vs.@: regexp comparison @c @cindex regexp comparison vs.@: string comparison @noindent the result is @samp{false} because both @code{$1} and @code{$2} are user input. They are numeric strings---therefore both have the strnum attribute, dictating a numeric comparison. The purpose of the comparison rules and the use of numeric strings is to attempt to produce the behavior that is ``least surprising,'' while still ``doing the right thing.'' String comparisons and regular expression comparisons are very different. For example: @example x == "foo" @end example @noindent has the value one, or is true if the variable @code{x} is precisely @samp{foo}. By contrast: @example x ~ /foo/ @end example @noindent has the value one if @code{x} contains @samp{foo}, such as @code{"Oh, what a fool am I!"}. @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point) @subentry @code{!~} operator @cindex exclamation point (@code{!}) @subentry @code{!~} operator The righthand operand of the @samp{~} and @samp{!~} operators may be either a regexp constant (@code{/}@dots{}@code{/}) or an ordinary expression. In the latter case, the value of the expression as a string is used as a dynamic regexp (@pxref{Regexp Usage}; also @pxref{Computed Regexps}). @cindex @command{awk} @subentry regexp constants and @cindex regexp constants A constant regular expression in slashes by itself is also an expression. @code{/@var{regexp}/} is an abbreviation for the following comparison expression: @example $0 ~ /@var{regexp}/ @end example One special place where @code{/foo/} is @emph{not} an abbreviation for @samp{$0 ~ /foo/} is when it is the righthand operand of @samp{~} or @samp{!~}. @xref{Using Constant Regexps}, where this is discussed in more detail. @node POSIX String Comparison @subsubsection String Comparison Based on Locale Collating Order The POSIX standard used to say that all string comparisons are performed based on the locale's @dfn{collating order}. This is the order in which characters sort, as defined by the locale (for more discussion, @pxref{Locales}). This order is usually very different from the results obtained when doing straight byte-by-byte comparison.@footnote{Technically, string comparison is supposed to behave the same way as if the strings were compared with the C @code{strcoll()} function.} @cindex POSIX mode Because this behavior differs considerably from existing practice, @command{gawk} only implemented it when in POSIX mode (@pxref{Options}). Here is an example to illustrate the difference, in an @code{en_US.UTF-8} locale: @example $ @kbd{gawk 'BEGIN @{ printf("ABC < abc = %s\n",} > @kbd{("ABC" < "abc" ? "TRUE" : "FALSE")) @}'} @print{} ABC < abc = TRUE $ @kbd{gawk --posix 'BEGIN @{ printf("ABC < abc = %s\n",} > @kbd{("ABC" < "abc" ? "TRUE" : "FALSE")) @}'} @print{} ABC < abc = FALSE @end example Fortunately, as of August 2016, comparison based on locale collating order is no longer required for the @code{==} and @code{!=} operators.@footnote{See @uref{http://austingroupbugs.net/view.php?id=1070, the Austin Group website}.} However, comparison based on locales is still required for @code{<}, @code{<=}, @code{>}, and @code{>=}. POSIX thus recommends as follows: @quotation Since the @code{==} operator checks whether strings are identical, not whether they collate equally, applications needing to check whether strings collate equally can use: @example a <= b && a >= b @end example @end quotation @cindex POSIX mode As of @value{PVERSION} 4.2, @command{gawk} continues to use locale collating order for @code{<}, @code{<=}, @code{>}, and @code{>=} only in POSIX mode. @ignore References: http://austingroupbugs.net/view.php?id=963 and http://austingroupbugs.net/view.php?id=1070. @end ignore @node Boolean Ops @subsection Boolean Expressions @cindex and Boolean-logic operator @cindex or Boolean-logic operator @cindex not Boolean-logic operator @cindex expressions @subentry Boolean @cindex Boolean expressions @cindex operators, Boolean @seeentry{Boolean expressions} @cindex Boolean operators @seeentry{Boolean expressions} @cindex logical operators @seeentry{Boolean expressions} @cindex operators, logical @seeentry{Boolean expressions} A @dfn{Boolean expression} is a combination of comparison expressions or matching expressions, using the Boolean operators ``or'' (@samp{||}), ``and'' (@samp{&&}), and ``not'' (@samp{!}), along with parentheses to control nesting. The truth value of the Boolean expression is computed by combining the truth values of the component expressions. Boolean expressions are also referred to as @dfn{logical expressions}. The terms are equivalent. Boolean expressions can be used wherever comparison and matching expressions can be used. They can be used in @code{if}, @code{while}, @code{do}, and @code{for} statements (@pxref{Statements}). They have numeric values (one if true, zero if false) that come into play if the result of the Boolean expression is stored in a variable or used in arithmetic. In addition, every Boolean expression is also a valid pattern, so you can use one as a pattern to control the execution of rules. The Boolean operators are: @table @code @item @var{boolean1} && @var{boolean2} True if both @var{boolean1} and @var{boolean2} are true. For example, the following statement prints the current input record if it contains both @samp{edu} and @samp{li}: @example if ($0 ~ /edu/ && $0 ~ /li/) print @end example @cindex side effects @subentry Boolean operators The subexpression @var{boolean2} is evaluated only if @var{boolean1} is true. This can make a difference when @var{boolean2} contains expressions that have side effects. In the case of @samp{$0 ~ /foo/ && ($2 == bar++)}, the variable @code{bar} is not incremented if there is no substring @samp{foo} in the record. @item @var{boolean1} || @var{boolean2} True if at least one of @var{boolean1} or @var{boolean2} is true. For example, the following statement prints all records in the input that contain @emph{either} @samp{edu} or @samp{li}: @example if ($0 ~ /edu/ || $0 ~ /li/) print @end example The subexpression @var{boolean2} is evaluated only if @var{boolean1} is false. This can make a difference when @var{boolean2} contains expressions that have side effects. (Thus, this test never really distinguishes records that contain both @samp{edu} and @samp{li}---as soon as @samp{edu} is matched, the full test succeeds.) @item ! @var{boolean} True if @var{boolean} is false. For example, the following program prints @samp{no home!} in the unusual event that the @env{HOME} environment variable is not defined: @example BEGIN @{ if (! ("HOME" in ENVIRON)) print "no home!" @} @end example (The @code{in} operator is described in @ref{Reference to Elements}.) @end table @cindex short-circuit operators @cindex operators @subentry short-circuit @cindex @code{&} (ampersand) @subentry @code{&&} operator @cindex ampersand (@code{&}) @subentry @code{&&} operator @cindex @code{|} (vertical bar) @subentry @code{||} operator @cindex vertical bar (@code{|}) @subentry @code{||} operator The @samp{&&} and @samp{||} operators are called @dfn{short-circuit} operators because of the way they work. Evaluation of the full expression is ``short-circuited'' if the result can be determined partway through its evaluation. @cindex line continuations Statements that end with @samp{&&} or @samp{||} can be continued simply by putting a newline after them. But you cannot put a newline in front of either of these operators without using backslash continuation (@pxref{Statements/Lines}). @cindex @code{!} (exclamation point) @subentry @code{!} operator @cindex exclamation point (@code{!}) @subentry @code{!} operator @cindex newlines @cindex variables @subentry flag @cindex flag variables The actual value of an expression using the @samp{!} operator is either one or zero, depending upon the truth value of the expression it is applied to. The @samp{!} operator is often useful for changing the sense of a flag variable from false to true and back again. For example, the following program is one way to print lines in between special bracketing lines: @example $1 == "START" @{ interested = ! interested; next @} interested @{ print @} $1 == "END" @{ interested = ! interested; next @} @end example @noindent The variable @code{interested}, as with all @command{awk} variables, starts out initialized to zero, which is also false. When a line is seen whose first field is @samp{START}, the value of @code{interested} is toggled to true, using @samp{!}. The next rule prints lines as long as @code{interested} is true. When a line is seen whose first field is @samp{END}, @code{interested} is toggled back to false.@footnote{This program has a bug; it prints lines starting with @samp{END}. How would you fix it?} @ignore Scott Deifik points out that this program isn't robust against bogus input data, but the point is to illustrate the use of `!', so we'll leave well enough alone. @end ignore Most commonly, the @samp{!} operator is used in the conditions of @code{if} and @code{while} statements, where it often makes more sense to phrase the logic in the negative: @example if (! @var{some condition} || @var{some other condition}) @{ @var{@dots{} do whatever processing @dots{}} @} @end example @cindex @code{next} statement @quotation NOTE The @code{next} statement is discussed in @ref{Next Statement}. @code{next} tells @command{awk} to skip the rest of the rules, get the next record, and start processing the rules over again at the top. The reason it's there is to avoid printing the bracketing @samp{START} and @samp{END} lines. @end quotation @node Conditional Exp @subsection Conditional Expressions @cindex conditional expressions @cindex expressions @subentry conditional @cindex expressions @subentry selecting A @dfn{conditional expression} is a special kind of expression that has three operands. It allows you to use one expression's value to select one of two other expressions. The conditional expression in @command{awk} is the same as in the C language, as shown here: @example @var{selector} ? @var{if-true-exp} : @var{if-false-exp} @end example @noindent There are three subexpressions. The first, @var{selector}, is always computed first. If it is ``true'' (not zero or not null), then @var{if-true-exp} is computed next, and its value becomes the value of the whole expression. Otherwise, @var{if-false-exp} is computed next, and its value becomes the value of the whole expression. For example, the following expression produces the absolute value of @code{x}: @example x >= 0 ? x : -x @end example @cindex side effects @subentry conditional expressions Each time the conditional expression is computed, only one of @var{if-true-exp} and @var{if-false-exp} is used; the other is ignored. This is important when the expressions have side effects. For example, this conditional expression examines element @code{i} of either array @code{a} or array @code{b}, and increments @code{i}: @example x == y ? a[i++] : b[i++] @end example @noindent This is guaranteed to increment @code{i} exactly once, because each time only one of the two increment expressions is executed and the other is not. @xref{Arrays}, for more information about arrays. @cindex differences in @command{awk} and @command{gawk} @subentry line continuations @cindex line continuations @subentry @command{gawk} @cindex @command{gawk} @subentry line continuation in As a minor @command{gawk} extension, a statement that uses @samp{?:} can be continued simply by putting a newline after either character. However, putting a newline in front of either character does not work without using backslash continuation (@pxref{Statements/Lines}). If @option{--posix} is specified (@pxref{Options}), this extension is disabled. @node Function Calls @section Function Calls @cindex function calls A @dfn{function} is a name for a particular calculation. This enables you to ask for it by name at any point in the program. For example, the function @code{sqrt()} computes the square root of a number. @cindex functions @subentry built-in A fixed set of functions are @dfn{built in}, which means they are available in every @command{awk} program. The @code{sqrt()} function is one of these. @xref{Built-in} for a list of built-in functions and their descriptions. In addition, you can define functions for use in your program. @xref{User-defined} for instructions on how to do this. Finally, @command{gawk} lets you write functions in C or C++ that may be called from your program (@pxref{Dynamic Extensions}). @cindex arguments @subentry in function calls The way to use a function is with a @dfn{function call} expression, which consists of the function name followed immediately by a list of @dfn{arguments} in parentheses. The arguments are expressions that provide the raw materials for the function's calculations. When there is more than one argument, they are separated by commas. If there are no arguments, just write @samp{()} after the function name. The following examples show function calls with and without arguments: @example sqrt(x^2 + y^2) @ii{one argument} atan2(y, x) @ii{two arguments} rand() @ii{no arguments} @end example @cindex troubleshooting @subentry function call syntax @quotation CAUTION Do not put any space between the function name and the opening parenthesis! A user-defined function name looks just like the name of a variable---a space would make the expression look like concatenation of a variable with an expression inside parentheses. With built-in functions, space before the parenthesis is harmless, but it is best not to get into the habit of using space to avoid mistakes with user-defined functions. @end quotation Each function expects a particular number of arguments. For example, the @code{sqrt()} function must be called with a single argument, the number of which to take the square root: @example sqrt(@var{argument}) @end example Some of the built-in functions have one or more optional arguments. If those arguments are not supplied, the functions use a reasonable default value. @xref{Built-in} for full details. If arguments are omitted in calls to user-defined functions, then those arguments are treated as local variables. Such local variables act like the empty string if referenced where a string value is required, and like zero if referenced where a numeric value is required (@pxref{User-defined}). As an advanced feature, @command{gawk} provides indirect function calls, which is a way to choose the function to call at runtime, instead of when you write the source code to your program. We defer discussion of this feature until later; see @ref{Indirect Calls}. @cindex side effects @subentry function calls Like every other expression, the function call has a value, often called the @dfn{return value}, which is computed by the function based on the arguments you give it. In this example, the return value of @samp{sqrt(@var{argument})} is the square root of @var{argument}. The following program reads numbers, one number per line, and prints the square root of each one: @example $ @kbd{awk '@{ print "The square root of", $1, "is", sqrt($1) @}'} @kbd{1} @print{} The square root of 1 is 1 @kbd{3} @print{} The square root of 3 is 1.73205 @kbd{5} @print{} The square root of 5 is 2.23607 @kbd{Ctrl-d} @end example A function can also have side effects, such as assigning values to certain variables or doing I/O. This program shows how the @code{match()} function (@pxref{String Functions}) changes the variables @code{RSTART} and @code{RLENGTH}: @example @{ if (match($1, $2)) print RSTART, RLENGTH else print "no match" @} @end example @noindent Here is a sample run: @example $ @kbd{awk -f matchit.awk} @kbd{aaccdd c+} @print{} 3 2 @kbd{foo bar} @print{} no match @kbd{abcdefg e} @print{} 5 1 @end example @node Precedence @section Operator Precedence (How Operators Nest) @cindex precedence @cindex operators @subentry precedence of @dfn{Operator precedence} determines how operators are grouped when different operators appear close by in one expression. For example, @samp{*} has higher precedence than @samp{+}; thus, @samp{a + b * c} means to multiply @code{b} and @code{c}, and then add @code{a} to the product (i.e., @samp{a + (b * c)}). The normal precedence of the operators can be overruled by using parentheses. Think of the precedence rules as saying where the parentheses are assumed to be. In fact, it is wise to always use parentheses whenever there is an unusual combination of operators, because other people who read the program may not remember what the precedence is in this case. Even experienced programmers occasionally forget the exact rules, which leads to mistakes. Explicit parentheses help prevent any such mistakes. When operators of equal precedence are used together, the leftmost operator groups first, except for the assignment, conditional, and exponentiation operators, which group in the opposite order. Thus, @samp{a - b + c} groups as @samp{(a - b) + c} and @samp{a = b = c} groups as @samp{a = (b = c)}. Normally the precedence of prefix unary operators does not matter, because there is only one way to interpret them: innermost first. Thus, @samp{$++i} means @samp{$(++i)} and @samp{++$x} means @samp{++($x)}. However, when another operator follows the operand, then the precedence of the unary operators can matter. @samp{$x^2} means @samp{($x)^2}, but @samp{-x^2} means @samp{-(x^2)}, because @samp{-} has lower precedence than @samp{^}, whereas @samp{$} has higher precedence. Also, operators cannot be combined in a way that violates the precedence rules; for example, @samp{$$0++--} is not a valid expression because the first @samp{$} has higher precedence than the @samp{++}; to avoid the problem the expression can be rewritten as @samp{$($0++)--}. This list presents @command{awk}'s operators, in order of highest to lowest precedence: @c @asis for docbook to come out right @table @asis @item @code{(}@dots{}@code{)} Grouping. @cindex @code{$} (dollar sign) @subentry @code{$} field operator @cindex dollar sign (@code{$}) @subentry @code{$} field operator @item @code{$} Field reference. @cindex @code{+} (plus sign) @subentry @code{++} operator @cindex plus sign (@code{+}) @subentry @code{++} operator @cindex @code{-} (hyphen) @subentry @code{--} operator @cindex hyphen (@code{-}) @subentry @code{--} operator @item @code{++ --} Increment, decrement. @cindex @code{^} (caret) @subentry @code{^} operator @cindex caret (@code{^}) @subentry @code{^} operator @cindex @code{*} (asterisk) @subentry @code{**} operator @cindex asterisk (@code{*}) @subentry @code{**} operator @item @code{^ **} Exponentiation. These operators group right to left. @cindex @code{+} (plus sign) @subentry @code{+} operator @cindex plus sign (@code{+}) @subentry @code{+} operator @cindex @code{-} (hyphen) @subentry @code{-} operator @cindex hyphen (@code{-}) @subentry @code{-} operator @cindex @code{!} (exclamation point) @subentry @code{!} operator @cindex exclamation point (@code{!}) @subentry @code{!} operator @item @code{+ - !} Unary plus, minus, logical ``not.'' @cindex @code{*} (asterisk) @subentry @code{*} operator @subentry as multiplication operator @cindex asterisk (@code{*}) @subentry @code{*} operator @subentry as multiplication operator @cindex @code{/} (forward slash) @subentry @code{/} operator @cindex forward slash (@code{/}) @subentry @code{/} operator @cindex @code{%} (percent sign) @subentry @code{%} operator @cindex percent sign (@code{%}) @subentry @code{%} operator @item @code{* / %} Multiplication, division, remainder. @cindex @code{+} (plus sign) @subentry @code{+} operator @cindex plus sign (@code{+}) @subentry @code{+} operator @cindex @code{-} (hyphen) @subentry @code{-} operator @cindex hyphen (@code{-}) @subentry @code{-} operator @item @code{+ -} Addition, subtraction. @item String concatenation There is no special symbol for concatenation. The operands are simply written side by side (@pxref{Concatenation}). @cindex @code{<} (left angle bracket) @subentry @code{<} operator @cindex left angle bracket (@code{<}) @subentry @code{<} operator @cindex @code{<} (left angle bracket) @subentry @code{<=} operator @cindex left angle bracket (@code{<}) @subentry @code{<=} operator @cindex @code{>} (right angle bracket) @subentry @code{>=} operator @cindex right angle bracket (@code{>}) @subentry @code{>=} operator @cindex @code{>} (right angle bracket) @subentry @code{>} operator @cindex right angle bracket (@code{>}) @subentry @code{>} operator @cindex @code{=} (equals sign) @subentry @code{==} operator @cindex equals sign (@code{=}) @subentry @code{==} operator @cindex @code{!} (exclamation point) @subentry @code{!=} operator @cindex exclamation point (@code{!}) @subentry @code{!=} operator @cindex @code{>} (right angle bracket) @subentry @code{>>} operator (I/O) @cindex right angle bracket (@code{>}) @subentry @code{>>} operator (I/O) @cindex operators @subentry input/output @cindex @code{|} (vertical bar) @subentry @code{|} operator (I/O) @cindex vertical bar (@code{|}) @subentry @code{|} operator (I/O) @cindex operators @subentry input/output @cindex @code{|} (vertical bar) @subentry @code{|&} operator (I/O) @cindex vertical bar (@code{|}) @subentry @code{|&} operator (I/O) @cindex operators @subentry input/output @item @code{< <= == != > >= >> | |&} Relational and redirection. The relational operators and the redirections have the same precedence level. Characters such as @samp{>} serve both as relationals and as redirections; the context distinguishes between the two meanings. @cindex @code{print} statement @subentry I/O operators in @cindex @code{printf} statement @subentry I/O operators in Note that the I/O redirection operators in @code{print} and @code{printf} statements belong to the statement level, not to expressions. The redirection does not produce an expression that could be the operand of another operator. As a result, it does not make sense to use a redirection operator near another operator of lower precedence without parentheses. Such combinations (e.g., @samp{print foo > a ? b : c}) result in syntax errors. The correct way to write this statement is @samp{print foo > (a ? b : c)}. @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point) @subentry @code{!~} operator @cindex exclamation point (@code{!}) @subentry @code{!~} operator @item @code{~ !~} Matching, nonmatching. @cindex @code{in} operator @item @code{in} Array membership. @cindex @code{&} (ampersand) @subentry @code{&&} operator @cindex ampersand (@code{&}) @subentry @code{&&} operator @item @code{&&} Logical ``and.'' @cindex @code{|} (vertical bar) @subentry @code{||} operator @cindex vertical bar (@code{|}) @subentry @code{||} operator @item @code{||} Logical ``or.'' @cindex @code{?} (question mark) @subentry @code{?:} operator @cindex question mark (@code{?}) @subentry @code{?:} operator @cindex @code{:} (colon) @subentry @code{?:} operator @cindex colon (@code{:}) @subentry @code{?:} operator @item @code{?:} Conditional. This operator groups right to left. @cindex @code{+} (plus sign) @subentry @code{+=} operator @cindex plus sign (@code{+}) @subentry @code{+=} operator @cindex @code{-} (hyphen) @subentry @code{-=} operator @cindex hyphen (@code{-}) @subentry @code{-=} operator @cindex @code{*} (asterisk) @subentry @code{*=} operator @cindex asterisk (@code{*}) @subentry @code{*=} operator @cindex @code{*} (asterisk) @subentry @code{**=} operator @cindex asterisk (@code{*}) @subentry @code{**=} operator @cindex @code{/} (forward slash) @subentry @code{/=} operator @cindex forward slash (@code{/}) @subentry @code{/=} operator @cindex @code{%} (percent sign) @subentry @code{%=} operator @cindex percent sign (@code{%}) @subentry @code{%=} operator @cindex @code{^} (caret) @subentry @code{^=} operator @cindex caret (@code{^}) @subentry @code{^=} operator @item @code{= += -= *= /= %= ^= **=} Assignment. These operators group right to left. @end table @cindex POSIX @command{awk} @subentry @code{**} operator and @cindex portability @subentry operators @subentry not in POSIX @command{awk} @quotation NOTE The @samp{|&}, @samp{**}, and @samp{**=} operators are not specified by POSIX. For maximum portability, do not use them. @end quotation @node Locales @section Where You Are Makes a Difference @cindex locale, definition of Modern systems support the notion of @dfn{locales}: a way to tell the system about the local character set and language. The ISO C standard defines a default @code{"C"} locale, which is an environment that is typical of what many C programmers are used to. Once upon a time, the locale setting used to affect regexp matching, but this is no longer true (@pxref{Ranges and Locales}). Locales can affect record splitting. For the normal case of @samp{RS = "\n"}, the locale is largely irrelevant. For other single-character record separators, setting @samp{LC_ALL=C} in the environment will give you much better performance when reading records. Otherwise, @command{gawk} has to make several function calls, @emph{per input character}, to find the record terminator. Locales can affect how dates and times are formatted (@pxref{Time Functions}). For example, a common way to abbreviate the date September 4, 2015, in the United States is ``9/4/15.'' In many countries in Europe, however, it is abbreviated ``4.9.15.'' Thus, the @samp{%x} specification in a @code{"US"} locale might produce @samp{9/4/15}, while in a @code{"EUROPE"} locale, it might produce @samp{4.9.15}. According to POSIX, string comparison is also affected by locales (similar to regular expressions). The details are presented in @ref{POSIX String Comparison}. Finally, the locale affects the value of the decimal point character used when @command{gawk} parses input data. This is discussed in detail in @ref{Conversion}. @node Expressions Summary @section Summary @itemize @value{BULLET} @item Expressions are the basic elements of computation in programs. They are built from constants, variables, function calls, and combinations of the various kinds of values with operators. @item @command{awk} supplies three kinds of constants: numeric, string, and regexp. @command{gawk} lets you specify numeric constants in octal and hexadecimal (bases 8 and 16) as well as decimal (base 10). In certain contexts, a standalone regexp constant such as @code{/foo/} has the same meaning as @samp{$0 ~ /foo/}. @item Variables hold values between uses in computations. A number of built-in variables provide information to your @command{awk} program, and a number of others let you control how @command{awk} behaves. @item Numbers are automatically converted to strings, and strings to numbers, as needed by @command{awk}. Numeric values are converted as if they were formatted with @code{sprintf()} using the format in @code{CONVFMT}. Locales can influence the conversions. @item @command{awk} provides the usual arithmetic operators (addition, subtraction, multiplication, division, modulus), and unary plus and minus. It also provides comparison operators, Boolean operators, an array membership testing operator, and regexp matching operators. String concatenation is accomplished by placing two expressions next to each other; there is no explicit operator. The three-operand @samp{?:} operator provides an ``if-else'' test within expressions. @item Assignment operators provide convenient shorthands for common arithmetic operations. @item In @command{awk}, a value is considered to be true if it is nonzero @emph{or} non-null. Otherwise, the value is false. @item A variable's type is set upon each assignment and may change over its lifetime. The type determines how it behaves in comparisons (string or numeric). @item Function calls return a value that may be used as part of a larger expression. Expressions used to pass parameter values are fully evaluated before the function is called. @command{awk} provides built-in and user-defined functions; this is described in @ref{Functions}. @item Operator precedence specifies the order in which operations are performed, unless explicitly overridden by parentheses. @command{awk}'s operator precedence is compatible with that of C. @item Locales can affect the format of data as output by an @command{awk} program, and occasionally the format for data read as input. @end itemize @node Patterns and Actions @chapter Patterns, Actions, and Variables @cindex patterns As you have already seen, each @command{awk} statement consists of a pattern with an associated action. This @value{CHAPTER} describes how you build patterns and actions, what kinds of things you can do within actions, and @command{awk}'s predefined variables. The pattern--action rules and the statements available for use within actions form the core of @command{awk} programming. In a sense, everything covered up to here has been the foundation that programs are built on top of. Now it's time to start building something useful. @menu * Pattern Overview:: What goes into a pattern. * Using Shell Variables:: How to use shell variables with @command{awk}. * Action Overview:: What goes into an action. * Statements:: Describes the various control statements in detail. * Built-in Variables:: Summarizes the predefined variables. * Pattern Action Summary:: Patterns and Actions summary. @end menu @node Pattern Overview @section Pattern Elements @menu * Regexp Patterns:: Using regexps as patterns. * Expression Patterns:: Any expression can be used as a pattern. * Ranges:: Pairs of patterns specify record ranges. * BEGIN/END:: Specifying initialization and cleanup rules. * BEGINFILE/ENDFILE:: Two special patterns for advanced control. * Empty:: The empty pattern, which matches every record. @end menu @cindex patterns @subentry types of Patterns in @command{awk} control the execution of rules---a rule is executed when its pattern matches the current input record. The following is a summary of the types of @command{awk} patterns: @table @code @item /@var{regular expression}/ A regular expression. It matches when the text of the input record fits the regular expression. (@xref{Regexp}.) @item @var{expression} A single expression. It matches when its value is nonzero (if a number) or non-null (if a string). (@xref{Expression Patterns}.) @item @var{begpat}, @var{endpat} A pair of patterns separated by a comma, specifying a @dfn{range} of records. The range includes both the initial record that matches @var{begpat} and the final record that matches @var{endpat}. (@xref{Ranges}.) @item BEGIN @itemx END Special patterns for you to supply startup or cleanup actions for your @command{awk} program. (@xref{BEGIN/END}.) @item BEGINFILE @itemx ENDFILE Special patterns for you to supply startup or cleanup actions to be done on a per-file basis. (@xref{BEGINFILE/ENDFILE}.) @item @var{empty} The empty pattern matches every input record. (@xref{Empty}.) @end table @node Regexp Patterns @subsection Regular Expressions as Patterns @cindex patterns @subentry regexp constants as @cindex regular expressions @subentry as patterns Regular expressions are one of the first kinds of patterns presented in this book. This kind of pattern is simply a regexp constant in the pattern part of a rule. Its meaning is @samp{$0 ~ /@var{pattern}/}. The pattern matches when the input record matches the regexp. For example: @example /foo|bar|baz/ @{ buzzwords++ @} END @{ print buzzwords, "buzzwords seen" @} @end example @node Expression Patterns @subsection Expressions as Patterns @cindex expressions @subentry as patterns @cindex patterns @subentry expressions as Any @command{awk} expression is valid as an @command{awk} pattern. The pattern matches if the expression's value is nonzero (if a number) or non-null (if a string). The expression is reevaluated each time the rule is tested against a new input record. If the expression uses fields such as @code{$1}, the value depends directly on the new input record's text; otherwise, it depends on only what has happened so far in the execution of the @command{awk} program. @cindex comparison expressions @subentry as patterns @cindex patterns @subentry comparison expressions as Comparison expressions, using the comparison operators described in @ref{Typing and Comparison}, are a very common kind of pattern. Regexp matching and nonmatching are also very common expressions. The left operand of the @samp{~} and @samp{!~} operators is a string. The right operand is either a constant regular expression enclosed in slashes (@code{/@var{regexp}/}), or any expression whose string value is used as a dynamic regular expression (@pxref{Computed Regexps}). The following example prints the second field of each input record whose first field is precisely @samp{li}: @cindex @code{/} (forward slash) @subentry patterns and @cindex forward slash (@code{/}) @subentry patterns and @cindex @code{~} (tilde), @code{~} operator @cindex tilde (@code{~}), @code{~} operator @cindex @code{!} (exclamation point) @subentry @code{!~} operator @cindex exclamation point (@code{!}) @subentry @code{!~} operator @example $ @kbd{awk '$1 == "li" @{ print $2 @}' mail-list} @end example @noindent (There is no output, because there is no person with the exact name @samp{li}.) Contrast this with the following regular expression match, which accepts any record with a first field that contains @samp{li}: @example $ @kbd{awk '$1 ~ /li/ @{ print $2 @}' mail-list} @print{} 555-5553 @print{} 555-6699 @end example @cindex regexp constants @subentry as patterns @cindex patterns @subentry regexp constants as A regexp constant as a pattern is also a special case of an expression pattern. The expression @code{/li/} has the value one if @samp{li} appears in the current input record. Thus, as a pattern, @code{/li/} matches any record containing @samp{li}. @cindex Boolean expressions @subentry as patterns @cindex patterns @subentry Boolean expressions as Boolean expressions are also commonly used as patterns. Whether the pattern matches an input record depends on whether its subexpressions match. For example, the following command prints all the records in @file{mail-list} that contain both @samp{edu} and @samp{li}: @example $ @kbd{awk '/edu/ && /li/' mail-list} @print{} Samuel 555-3430 samuel.lanceolis@@shu.edu A @end example The following command prints all records in @file{mail-list} that contain @emph{either} @samp{edu} or @samp{li} (or both, of course): @example $ @kbd{awk '/edu/ || /li/' mail-list} @print{} Amelia 555-5553 amelia.zodiacusque@@gmail.com F @print{} Broderick 555-0542 broderick.aliquotiens@@yahoo.com R @print{} Fabius 555-1234 fabius.undevicesimus@@ucb.edu F @print{} Julie 555-6699 julie.perscrutabor@@skeeve.com F @print{} Samuel 555-3430 samuel.lanceolis@@shu.edu A @print{} Jean-Paul 555-2127 jeanpaul.campanorum@@nyu.edu R @end example The following command prints all records in @file{mail-list} that do @emph{not} contain the string @samp{li}: @example $ @kbd{awk '! /li/' mail-list} @print{} Anthony 555-3412 anthony.asserturo@@hotmail.com A @print{} Becky 555-7685 becky.algebrarum@@gmail.com A @print{} Bill 555-1675 bill.drowning@@hotmail.com A @print{} Camilla 555-2912 camilla.infusarum@@skynet.be R @print{} Fabius 555-1234 fabius.undevicesimus@@ucb.edu F @group @print{} Martin 555-6480 martin.codicibus@@hotmail.com A @print{} Jean-Paul 555-2127 jeanpaul.campanorum@@nyu.edu R @end group @end example @cindex @code{BEGIN} pattern @subentry Boolean patterns and @cindex @code{END} pattern @subentry Boolean patterns and @cindex @code{BEGINFILE} pattern @subentry Boolean patterns and @cindex @code{ENDFILE} pattern @subentry Boolean patterns and The subexpressions of a Boolean operator in a pattern can be constant regular expressions, comparisons, or any other @command{awk} expressions. Range patterns are not expressions, so they cannot appear inside Boolean patterns. Likewise, the special patterns @code{BEGIN}, @code{END}, @code{BEGINFILE}, and @code{ENDFILE}, which never match any input record, are not expressions and cannot appear inside Boolean patterns. The precedence of the different operators that can appear in patterns is described in @ref{Precedence}. @node Ranges @subsection Specifying Record Ranges with Patterns @cindex range patterns @cindex patterns @subentry ranges in @cindex lines @subentry matching ranges of @cindex @code{,} (comma), in range patterns @cindex comma (@code{,}), in range patterns A @dfn{range pattern} is made of two patterns separated by a comma, in the form @samp{@var{begpat}, @var{endpat}}. It is used to match ranges of consecutive input records. The first pattern, @var{begpat}, controls where the range begins, while @var{endpat} controls where the pattern ends. For example, the following: @example awk '$1 == "on", $1 == "off"' myfile @end example @noindent prints every record in @file{myfile} between @samp{on}/@samp{off} pairs, inclusive. A range pattern starts out by matching @var{begpat} against every input record. When a record matches @var{begpat}, the range pattern is @dfn{turned on}, and the range pattern matches this record as well. As long as the range pattern stays turned on, it automatically matches every input record read. The range pattern also matches @var{endpat} against every input record; when this succeeds, the range pattern is @dfn{turned off} again for the following record. Then the range pattern goes back to checking @var{begpat} against each record. @cindex @code{if} statement @subentry actions, changing The record that turns on the range pattern and the one that turns it off both match the range pattern. If you don't want to operate on these records, you can write @code{if} statements in the rule's action to distinguish them from the records you are interested in. It is possible for a pattern to be turned on and off by the same record. If the record satisfies both conditions, then the action is executed for just that record. For example, suppose there is text between two identical markers (e.g., the @samp{%} symbol), each on its own line, that should be ignored. A first attempt would be to combine a range pattern that describes the delimited text with the @code{next} statement (not discussed yet, @pxref{Next Statement}). This causes @command{awk} to skip any further processing of the current record and start over again with the next input record. Such a program looks like this: @example /^%$/,/^%$/ @{ next @} @{ print @} @end example @noindent @cindex lines @subentry skipping between markers @c @cindex flag variables This program fails because the range pattern is both turned on and turned off by the first line, which just has a @samp{%} on it. To accomplish this task, write the program in the following manner, using a flag: @cindex @code{!} (exclamation point) @subentry @code{!} operator @example /^%$/ @{ skip = ! skip; next @} skip == 1 @{ next @} # skip lines with `skip' set @end example In a range pattern, the comma (@samp{,}) has the lowest precedence of all the operators (i.e., it is evaluated last). Thus, the following program attempts to combine a range pattern with another, simpler test: @example echo Yes | awk '/1/,/2/ || /Yes/' @end example The intent of this program is @samp{(/1/,/2/) || /Yes/}. However, @command{awk} interprets this as @samp{/1/, (/2/ || /Yes/)}. This cannot be changed or worked around; range patterns do not combine with other patterns: @example $ @kbd{echo Yes | gawk '(/1/,/2/) || /Yes/'} @error{} gawk: cmd. line:1: (/1/,/2/) || /Yes/ @error{} gawk: cmd. line:1: ^ syntax error @end example @cindex range patterns @subentry line continuation and @cindex dark corner @subentry range patterns, line continuation and As a minor point of interest, although it is poor style, POSIX allows you to put a newline after the comma in a range pattern. @value{DARKCORNER} @node BEGIN/END @subsection The @code{BEGIN} and @code{END} Special Patterns @cindex @code{BEGIN} pattern @cindex @code{END} pattern All the patterns described so far are for matching input records. The @code{BEGIN} and @code{END} special patterns are different. They supply startup and cleanup actions for @command{awk} programs. @code{BEGIN} and @code{END} rules must have actions; there is no default action for these rules because there is no current record when they run. @code{BEGIN} and @code{END} rules are often referred to as ``@code{BEGIN} and @code{END} blocks'' by longtime @command{awk} programmers. @menu * Using BEGIN/END:: How and why to use BEGIN/END rules. * I/O And BEGIN/END:: I/O issues in BEGIN/END rules. @end menu @node Using BEGIN/END @subsubsection Startup and Cleanup Actions @cindex @code{BEGIN} pattern @cindex @code{END} pattern A @code{BEGIN} rule is executed once only, before the first input record is read. Likewise, an @code{END} rule is executed once only, after all the input is read. For example: @example $ @kbd{awk '} > @kbd{BEGIN @{ print "Analysis of \"li\"" @}} > @kbd{/li/ @{ ++n @}} > @kbd{END @{ print "\"li\" appears in", n, "records." @}' mail-list} @print{} Analysis of "li" @print{} "li" appears in 4 records. @end example @cindex @code{BEGIN} pattern @subentry operators and @cindex @code{END} pattern @subentry operators and This program finds the number of records in the input file @file{mail-list} that contain the string @samp{li}. The @code{BEGIN} rule prints a title for the report. There is no need to use the @code{BEGIN} rule to initialize the counter @code{n} to zero, as @command{awk} does this automatically (@pxref{Variables}). The second rule increments the variable @code{n} every time a record containing the pattern @samp{li} is read. The @code{END} rule prints the value of @code{n} at the end of the run. The special patterns @code{BEGIN} and @code{END} cannot be used in ranges or with Boolean operators (indeed, they cannot be used with any operators). An @command{awk} program may have multiple @code{BEGIN} and/or @code{END} rules. They are executed in the order in which they appear: all the @code{BEGIN} rules at startup and all the @code{END} rules at termination. @code{BEGIN} and @code{END} rules may be intermixed with other rules. This feature was added in the 1987 version of @command{awk} and is included in the POSIX standard. The original (1978) version of @command{awk} required the @code{BEGIN} rule to be placed at the beginning of the program, the @code{END} rule to be placed at the end, and only allowed one of each. This is no longer required, but it is a good idea to follow this template in terms of program organization and readability. Multiple @code{BEGIN} and @code{END} rules are useful for writing library functions, because each library file can have its own @code{BEGIN} and/or @code{END} rule to do its own initialization and/or cleanup. The order in which library functions are named on the command line controls the order in which their @code{BEGIN} and @code{END} rules are executed. Therefore, you have to be careful when writing such rules in library files so that the order in which they are executed doesn't matter. @xref{Options} for more information on using library functions. @xref{Library Functions}, for a number of useful library functions. If an @command{awk} program has only @code{BEGIN} rules and no other rules, then the program exits after the @code{BEGIN} rules are run.@footnote{The original version of @command{awk} kept reading and ignoring input until the end of the file was seen.} However, if an @code{END} rule exists, then the input is read, even if there are no other rules in the program. This is necessary in case the @code{END} rule checks the @code{FNR} and @code{NR} variables. @node I/O And BEGIN/END @subsubsection Input/Output from @code{BEGIN} and @code{END} Rules @cindex input/output @subentry from @code{BEGIN} and @code{END} There are several (sometimes subtle) points to be aware of when doing I/O from a @code{BEGIN} or @code{END} rule. The first has to do with the value of @code{$0} in a @code{BEGIN} rule. Because @code{BEGIN} rules are executed before any input is read, there simply is no input record, and therefore no fields, when executing @code{BEGIN} rules. References to @code{$0} and the fields yield a null string or zero, depending upon the context. One way to give @code{$0} a real value is to execute a @code{getline} command without a variable (@pxref{Getline}). Another way is simply to assign a value to @code{$0}. @cindex Brian Kernighan's @command{awk} @cindex differences in @command{awk} and @command{gawk} @subentry @code{BEGIN}/@code{END} patterns @cindex POSIX @command{awk} @subentry @code{BEGIN}/@code{END} patterns @cindex @code{print} statement @subentry @code{BEGIN}/@code{END} patterns and @cindex @code{BEGIN} pattern @subentry @code{print} statement and @cindex @code{END} pattern @subentry @code{print} statement and The second point is similar to the first, but from the other direction. Traditionally, due largely to implementation issues, @code{$0} and @code{NF} were @emph{undefined} inside an @code{END} rule. The POSIX standard specifies that @code{NF} is available in an @code{END} rule. It contains the number of fields from the last input record. Most probably due to an oversight, the standard does not say that @code{$0} is also preserved, although logically one would think that it should be. In fact, all of BWK @command{awk}, @command{mawk}, and @command{gawk} preserve the value of @code{$0} for use in @code{END} rules. Be aware, however, that some other implementations and many older versions of Unix @command{awk} do not. The third point follows from the first two. The meaning of @samp{print} inside a @code{BEGIN} or @code{END} rule is the same as always: @samp{print $0}. If @code{$0} is the null string, then this prints an empty record. Many longtime @command{awk} programmers use an unadorned @samp{print} in @code{BEGIN} and @code{END} rules, to mean @samp{@w{print ""}}, relying on @code{$0} being null. Although one might generally get away with this in @code{BEGIN} rules, it is a very bad idea in @code{END} rules, at least in @command{gawk}. It is also poor style, because if an empty line is needed in the output, the program should print one explicitly. @cindex @code{next} statement @subentry @code{BEGIN}/@code{END} patterns and @cindex @code{nextfile} statement @subentry @code{BEGIN}/@code{END} patterns and @cindex @code{BEGIN} pattern @subentry @code{next}/@code{nextfile} statements and @cindex @code{END} pattern @subentry @code{next}/@code{nextfile} statements and Finally, the @code{next} and @code{nextfile} statements are not allowed in a @code{BEGIN} rule, because the implicit read-a-record-and-match-against-the-rules loop has not started yet. Similarly, those statements are not valid in an @code{END} rule, because all the input has been read. (@xref{Next Statement} and @ifnotdocbook @pxref{Nextfile Statement}.) @end ifnotdocbook @ifdocbook @ref{Nextfile Statement}.) @end ifdocbook @node BEGINFILE/ENDFILE @subsection The @code{BEGINFILE} and @code{ENDFILE} Special Patterns @cindex @code{BEGINFILE} pattern @cindex @code{ENDFILE} pattern @cindex differences in @command{awk} and @command{gawk} @subentry @code{BEGINFILE}/@code{ENDFILE} patterns This @value{SECTION} describes a @command{gawk}-specific feature. Two special kinds of rule, @code{BEGINFILE} and @code{ENDFILE}, give you ``hooks'' into @command{gawk}'s command-line file processing loop. As with the @code{BEGIN} and @code{END} rules @ifnottex @ifnotdocbook (@pxref{BEGIN/END}), @end ifnotdocbook @end ifnottex @iftex (see the previous @value{SECTION}), @end iftex @ifdocbook (see the previous @value{SECTION}), @end ifdocbook all @code{BEGINFILE} rules in a program are merged, in the order they are read by @command{gawk}, and all @code{ENDFILE} rules are merged as well. The body of the @code{BEGINFILE} rules is executed just before @command{gawk} reads the first record from a file. @code{FILENAME} is set to the name of the current file, and @code{FNR} is set to zero. The @code{BEGINFILE} rule provides you the opportunity to accomplish two tasks that would otherwise be difficult or impossible to perform: @itemize @value{BULLET} @item You can test if the file is readable. Normally, it is a fatal error if a file named on the command line cannot be opened for reading. However, you can bypass the fatal error and move on to the next file on the command line. @cindex @command{gawk} @subentry @code{ERRNO} variable in @cindex @code{ERRNO} variable @subentry with @code{BEGINFILE} pattern @cindex @code{nextfile} statement @subentry @code{BEGINFILE}/@code{ENDFILE} patterns and You do this by checking if the @code{ERRNO} variable is not the empty string; if so, then @command{gawk} was not able to open the file. In this case, your program can execute the @code{nextfile} statement (@pxref{Nextfile Statement}). This causes @command{gawk} to skip the file entirely. Otherwise, @command{gawk} exits with the usual fatal error. @item If you have written extensions that modify the record handling (by inserting an ``input parser''; @pxref{Input Parsers}), you can invoke them at this point, before @command{gawk} has started processing the file. (This is a @emph{very} advanced feature, currently used only by the @uref{https://sourceforge.net/projects/gawkextlib, @code{gawkextlib} project}.) @end itemize The @code{ENDFILE} rule is called when @command{gawk} has finished processing the last record in an input file. For the last input file, it will be called before any @code{END} rules. The @code{ENDFILE} rule is executed even for empty input files. Normally, when an error occurs when reading input in the normal input-processing loop, the error is fatal. However, if an @code{ENDFILE} rule is present, the error becomes non-fatal, and instead @code{ERRNO} is set. This makes it possible to catch and process I/O errors at the level of the @command{awk} program. @cindex @code{next} statement @subentry @code{BEGINFILE}/@code{ENDFILE} patterns and The @code{next} statement (@pxref{Next Statement}) is not allowed inside either a @code{BEGINFILE} or an @code{ENDFILE} rule. The @code{nextfile} statement is allowed only inside a @code{BEGINFILE} rule, not inside an @code{ENDFILE} rule. @cindex @code{getline} command @subentry @code{BEGINFILE}/@code{ENDFILE} patterns and The @code{getline} statement (@pxref{Getline}) is restricted inside both @code{BEGINFILE} and @code{ENDFILE}: only redirected forms of @code{getline} are allowed. @code{BEGINFILE} and @code{ENDFILE} are @command{gawk} extensions. In most other @command{awk} implementations, or if @command{gawk} is in compatibility mode (@pxref{Options}), they are not special. @c FIXME: For 4.2 maybe deal with this? @ignore Date: Tue, 17 May 2011 02:06:10 PDT From: rankin@pactechdata.com (Pat Rankin) Message-Id: <110517015127.20240f4a@pactechdata.com> Subject: BEGINFILE To: arnold@skeeve.com The documentation for BEGINFILE states that FNR is 0, which seems pretty obvious. It doesn't mention what the value of $0 is, and that's not obvious. I think setting it to null before starting the BEGINFILE action would be preferable to leaving whatever was there in the last record of the previous file. ENDFILE can retain the last record in $0. I guess it has to if the END rule's actions see that value too. But the beginning of a new file doesn't just mean that the old one has been closed; the old file is being superseded, so leaving the old data around feels wrong to me. [If the user wants to keep it on hand, he or she can use an ENDFILE rule to grab it before moving on to the next file.] @end ignore @node Empty @subsection The Empty Pattern @cindex empty pattern @cindex patterns @subentry empty An empty (i.e., nonexistent) pattern is considered to match @emph{every} input record. For example, the program: @example awk '@{ print $1 @}' mail-list @end example @noindent prints the first field of every record. @node Using Shell Variables @section Using Shell Variables in Programs @cindex shells @subentry variables @cindex @command{awk} programs @subentry shell variables in @c @cindex shell and @command{awk} interaction @command{awk} programs are often used as components in larger programs written in shell. For example, it is very common to use a shell variable to hold a pattern that the @command{awk} program searches for. There are two ways to get the value of the shell variable into the body of the @command{awk} program. @cindex shells @subentry quoting A common method is to use shell quoting to substitute the variable's value into the program inside the script. For example, consider the following program: @example @group printf "Enter search pattern: " read pattern awk "/$pattern/ "'@{ nmatches++ @} END @{ print nmatches, "found" @}' /path/to/data @end group @end example @noindent The @command{awk} program consists of two pieces of quoted text that are concatenated together to form the program. The first part is double-quoted, which allows substitution of the @code{pattern} shell variable inside the quotes. The second part is single-quoted. Variable substitution via quoting works, but can potentially be messy. It requires a good understanding of the shell's quoting rules (@pxref{Quoting}), and it's often difficult to correctly match up the quotes when reading the program. A better method is to use @command{awk}'s variable assignment feature (@pxref{Assignment Options}) to assign the shell variable's value to an @command{awk} variable. Then use dynamic regexps to match the pattern (@pxref{Computed Regexps}). The following shows how to redo the previous example using this technique: @example printf "Enter search pattern: " read pattern awk -v pat="$pattern" '$0 ~ pat @{ nmatches++ @} END @{ print nmatches, "found" @}' /path/to/data @end example @noindent Now, the @command{awk} program is just one single-quoted string. The assignment @samp{-v pat="$pattern"} still requires double quotes, in case there is whitespace in the value of @code{$pattern}. The @command{awk} variable @code{pat} could be named @code{pattern} too, but that would be more confusing. Using a variable also provides more flexibility, as the variable can be used anywhere inside the program---for printing, as an array subscript, or for any other use---without requiring the quoting tricks at every point in the program. @node Action Overview @section Actions @c @cindex action, definition of @c @cindex curly braces @c @cindex action, curly braces @c @cindex action, separating statements @cindex actions An @command{awk} program or script consists of a series of rules and function definitions interspersed. (Functions are described later. @xref{User-defined}.) A rule contains a pattern and an action, either of which (but not both) may be omitted. The purpose of the @dfn{action} is to tell @command{awk} what to do once a match for the pattern is found. Thus, in outline, an @command{awk} program generally looks like this: @display [@var{pattern}] @code{@{ @var{action} @}} @var{pattern} [@code{@{ @var{action} @}}] @dots{} @code{function @var{name}(@var{args}) @{ @dots{} @}} @dots{} @end display @cindex @code{@{@}} (braces) @subentry actions and @cindex braces (@code{@{@}}) @subentry actions and @cindex separators @subentry for statements in actions @cindex newlines @subentry separating statements in actions @cindex @code{;} (semicolon) @subentry separating statements in actions @cindex semicolon (@code{;}) @subentry separating statements in actions An action consists of one or more @command{awk} @dfn{statements}, enclosed in braces (@samp{@{@r{@dots{}}@}}). Each statement specifies one thing to do. The statements are separated by newlines or semicolons. The braces around an action must be used even if the action contains only one statement, or if it contains no statements at all. However, if you omit the action entirely, omit the braces as well. An omitted action is equivalent to @samp{@{ print $0 @}}: @example /foo/ @{ @} @ii{match @code{foo}, do nothing --- empty action} /foo/ @ii{match @code{foo}, print the record --- omitted action} @end example The following types of statements are supported in @command{awk}: @table @asis @cindex side effects @subentry statements @item Expressions Call functions or assign values to variables (@pxref{Expressions}). Executing this kind of statement simply computes the value of the expression. This is useful when the expression has side effects (@pxref{Assignment Ops}). @item Control statements Specify the control flow of @command{awk} programs. The @command{awk} language gives you C-like constructs (@code{if}, @code{for}, @code{while}, and @code{do}) as well as a few special ones (@pxref{Statements}). @item Compound statements Enclose one or more statements in braces. A compound statement is used in order to put several statements together in the body of an @code{if}, @code{while}, @code{do}, or @code{for} statement. @item Input statements Use the @code{getline} command (@pxref{Getline}). Also supplied in @command{awk} are the @code{next} statement (@pxref{Next Statement}) and the @code{nextfile} statement (@pxref{Nextfile Statement}). @item Output statements Such as @code{print} and @code{printf}. @xref{Printing}. @item Deletion statements For deleting array elements. @xref{Delete}. @end table @node Statements @section Control Statements in Actions @cindex control statements @cindex statements @subentry control, in actions @cindex actions @subentry control statements in @dfn{Control statements}, such as @code{if}, @code{while}, and so on, control the flow of execution in @command{awk} programs. Most of @command{awk}'s control statements are patterned after similar statements in C. @cindex compound statements, control statements and @cindex statements @subentry compound, control statements and @cindex body @subentry in actions @cindex @code{@{@}} (braces) @subentry statements, grouping @cindex braces (@code{@{@}}) @subentry statements, grouping @cindex newlines @subentry separating statements in actions @cindex @code{;} (semicolon) @subentry separating statements in actions @cindex semicolon (@code{;}) @subentry separating statements in actions All the control statements start with special keywords, such as @code{if} and @code{while}, to distinguish them from simple expressions. Many control statements contain other statements. For example, the @code{if} statement contains another statement that may or may not be executed. The contained statement is called the @dfn{body}. To include more than one statement in the body, group them into a single @dfn{compound statement} with braces, separating them with newlines or semicolons. @menu * If Statement:: Conditionally execute some @command{awk} statements. * While Statement:: Loop until some condition is satisfied. * Do Statement:: Do specified action while looping until some condition is satisfied. * For Statement:: Another looping statement, that provides initialization and increment clauses. * Switch Statement:: Switch/case evaluation for conditional execution of statements based on a value. * Break Statement:: Immediately exit the innermost enclosing loop. * Continue Statement:: Skip to the end of the innermost enclosing loop. * Next Statement:: Stop processing the current input record. * Nextfile Statement:: Stop processing the current file. * Exit Statement:: Stop execution of @command{awk}. @end menu @node If Statement @subsection The @code{if}-@code{else} Statement @cindex @code{if} statement The @code{if}-@code{else} statement is @command{awk}'s decision-making statement. It looks like this: @display @code{if (@var{condition}) @var{then-body}} [@code{else @var{else-body}}] @end display @noindent The @var{condition} is an expression that controls what the rest of the statement does. If the @var{condition} is true, @var{then-body} is executed; otherwise, @var{else-body} is executed. The @code{else} part of the statement is optional. The condition is considered false if its value is zero or the null string; otherwise, the condition is true. Refer to the following: @example @group if (x % 2 == 0) print "x is even" else print "x is odd" @end group @end example In this example, if the expression @samp{x % 2 == 0} is true (i.e., if the value of @code{x} is evenly divisible by two), then the first @code{print} statement is executed; otherwise, the second @code{print} statement is executed. If the @code{else} keyword appears on the same line as @var{then-body} and @var{then-body} is not a compound statement (i.e., not surrounded by braces), then a semicolon must separate @var{then-body} from the @code{else}. To illustrate this, the previous example can be rewritten as: @example if (x % 2 == 0) print "x is even"; else print "x is odd" @end example @noindent If the @samp{;} is left out, @command{awk} can't interpret the statement and it produces a syntax error. Don't actually write programs this way, because a human reader might fail to see the @code{else} if it is not the first thing on its line. @node While Statement @subsection The @code{while} Statement @cindex @code{while} statement @cindex loops @cindex loops @subentry @code{while} @cindex loops @seealso{@code{while} statement} In programming, a @dfn{loop} is a part of a program that can be executed two or more times in succession. The @code{while} statement is the simplest looping statement in @command{awk}. It repeatedly executes a statement as long as a condition is true. For example: @example while (@var{condition}) @var{body} @end example @cindex body @subentry in loops @noindent @var{body} is a statement called the @dfn{body} of the loop, and @var{condition} is an expression that controls how long the loop keeps running. The first thing the @code{while} statement does is test the @var{condition}. If the @var{condition} is true, it executes the statement @var{body}. @ifinfo (The @var{condition} is true when the value is not zero and not a null string.) @end ifinfo After @var{body} has been executed, @var{condition} is tested again, and if it is still true, @var{body} executes again. This process repeats until the @var{condition} is no longer true. If the @var{condition} is initially false, the body of the loop never executes and @command{awk} continues with the statement following the loop. This example prints the first three fields of each record, one per line: @example awk ' @{ i = 1 while (i <= 3) @{ print $i i++ @} @}' inventory-shipped @end example @noindent The body of this loop is a compound statement enclosed in braces, containing two statements. The loop works in the following manner: first, the value of @code{i} is set to one. Then, the @code{while} statement tests whether @code{i} is less than or equal to three. This is true when @code{i} equals one, so the @code{i}th field is printed. Then the @samp{i++} increments the value of @code{i} and the loop repeats. The loop terminates when @code{i} reaches four. A newline is not required between the condition and the body; however, using one makes the program clearer unless the body is a compound statement or else is very simple. The newline after the open brace that begins the compound statement is not required either, but the program is harder to read without it. @node Do Statement @subsection The @code{do}-@code{while} Statement @cindex @code{do}-@code{while} statement @cindex loops @subentry @code{do}-@code{while} The @code{do} loop is a variation of the @code{while} looping statement. The @code{do} loop executes the @var{body} once and then repeats the @var{body} as long as the @var{condition} is true. It looks like this: @example do @var{body} while (@var{condition}) @end example Even if the @var{condition} is false at the start, the @var{body} executes at least once (and only once, unless executing @var{body} makes @var{condition} true). Contrast this with the corresponding @code{while} statement: @example while (@var{condition}) @var{body} @end example @noindent This statement does not execute the @var{body} even once if the @var{condition} is false to begin with. The following is an example of a @code{do} statement: @example @{ i = 1 do @{ print $0 i++ @} while (i <= 10) @} @end example @noindent This program prints each input record 10 times. However, it isn't a very realistic example, because in this case an ordinary @code{while} would do just as well. This situation reflects actual experience; only occasionally is there a real use for a @code{do} statement. @node For Statement @subsection The @code{for} Statement @cindex @code{for} statement @cindex loops @subentry @code{for} @subentry iterative The @code{for} statement makes it more convenient to count iterations of a loop. The general form of the @code{for} statement looks like this: @example for (@var{initialization}; @var{condition}; @var{increment}) @var{body} @end example @noindent The @var{initialization}, @var{condition}, and @var{increment} parts are arbitrary @command{awk} expressions, and @var{body} stands for any @command{awk} statement. The @code{for} statement starts by executing @var{initialization}. Then, as long as the @var{condition} is true, it repeatedly executes @var{body} and then @var{increment}. Typically, @var{initialization} sets a variable to either zero or one, @var{increment} adds one to it, and @var{condition} compares it against the desired number of iterations. For example: @example awk ' @{ for (i = 1; i <= 3; i++) print $i @}' inventory-shipped @end example @noindent This prints the first three fields of each input record, with one input field per output line. It isn't possible to set more than one variable in the @var{initialization} part without using a multiple assignment statement such as @samp{x = y = 0}. This makes sense only if all the initial values are equal. (But it is possible to initialize additional variables by writing their assignments as separate statements preceding the @code{for} loop.) @c @cindex comma operator, not supported The same is true of the @var{increment} part. Incrementing additional variables requires separate statements at the end of the loop. The C compound expression, using C's comma operator, is useful in this context, but it is not supported in @command{awk}. Most often, @var{increment} is an increment expression, as in the previous example. But this is not required; it can be any expression whatsoever. For example, the following statement prints all the powers of two between 1 and 100: @example for (i = 1; i <= 100; i *= 2) print i @end example If there is nothing to be done, any of the three expressions in the parentheses following the @code{for} keyword may be omitted. Thus, @w{@samp{for (; x > 0;)}} is equivalent to @w{@samp{while (x > 0)}}. If the @var{condition} is omitted, it is treated as true, effectively yielding an @dfn{infinite loop} (i.e., a loop that never terminates). In most cases, a @code{for} loop is an abbreviation for a @code{while} loop, as shown here: @example @var{initialization} while (@var{condition}) @{ @var{body} @var{increment} @} @end example @cindex loops @subentry @code{continue} statement and @noindent The only exception is when the @code{continue} statement (@pxref{Continue Statement}) is used inside the loop. Changing a @code{for} statement to a @code{while} statement in this way can change the effect of the @code{continue} statement inside the loop. The @command{awk} language has a @code{for} statement in addition to a @code{while} statement because a @code{for} loop is often both less work to type and more natural to think of. Counting the number of iterations is very common in loops. It can be easier to think of this counting as part of looping rather than as something to do inside the loop. @cindex @code{in} operator There is an alternative version of the @code{for} loop, for iterating over all the indices of an array: @example for (i in array) @var{do something with} array[i] @end example @noindent @xref{Scanning an Array} for more information on this version of the @code{for} loop. @node Switch Statement @subsection The @code{switch} Statement @cindex @code{switch} statement @cindex @code{case} keyword @cindex @code{default} keyword This @value{SECTION} describes a @command{gawk}-specific feature. If @command{gawk} is in compatibility mode (@pxref{Options}), it is not available. The @code{switch} statement allows the evaluation of an expression and the execution of statements based on a @code{case} match. Case statements are checked for a match in the order they are defined. If no suitable @code{case} is found, the @code{default} section is executed, if supplied. Each @code{case} contains a single constant, be it numeric, string, or regexp. The @code{switch} expression is evaluated, and then each @code{case}'s constant is compared against the result in turn. The type of constant determines the comparison: numeric or string do the usual comparisons. A regexp constant does a regular expression match against the string value of the original expression. The general form of the @code{switch} statement looks like this: @example switch (@var{expression}) @{ case @var{value or regular expression}: @var{case-body} default: @var{default-body} @} @end example Control flow in the @code{switch} statement works as it does in C. Once a match to a given case is made, the case statement bodies execute until a @code{break}, @code{continue}, @code{next}, @code{nextfile}, or @code{exit} is encountered, or the end of the @code{switch} statement itself. For example: @example while ((c = getopt(ARGC, ARGV, "aksx")) != -1) @{ switch (c) @{ case "a": # report size of all files all_files = TRUE; break case "k": BLOCK_SIZE = 1024 # 1K block size break case "s": # do sums only sum_only = TRUE break case "x": # don't cross filesystems fts_flags = or(fts_flags, FTS_XDEV) break case "?": default: usage() break @} @} @end example Note that if none of the statements specified here halt execution of a matched @code{case} statement, execution falls through to the next @code{case} until execution halts. In this example, the @code{case} for @code{"?"} falls through to the @code{default} case, which is to call a function named @code{usage()}. (The @code{getopt()} function being called here is described in @ref{Getopt Function}.) @node Break Statement @subsection The @code{break} Statement @cindex @code{break} statement @cindex loops @subentry exiting @cindex loops @subentry @code{break} statement and The @code{break} statement jumps out of the innermost @code{for}, @code{while}, or @code{do} loop that encloses it. The following example finds the smallest divisor of any integer, and also identifies prime numbers: @example @group # find smallest divisor of num @{ num = $1 for (divisor = 2; divisor * divisor <= num; divisor++) @{ if (num % divisor == 0) break @} @end group @group if (num % divisor == 0) printf "Smallest divisor of %d is %d\n", num, divisor else printf "%d is prime\n", num @} @end group @end example When the remainder is zero in the first @code{if} statement, @command{awk} immediately @dfn{breaks out} of the containing @code{for} loop. This means that @command{awk} proceeds immediately to the statement following the loop and continues processing. (This is very different from the @code{exit} statement, which stops the entire @command{awk} program. @xref{Exit Statement}.) The following program illustrates how the @var{condition} of a @code{for} or @code{while} statement could be replaced with a @code{break} inside an @code{if}: @example # find smallest divisor of num @{ num = $1 for (divisor = 2; ; divisor++) @{ if (num % divisor == 0) @{ printf "Smallest divisor of %d is %d\n", num, divisor break @} if (divisor * divisor > num) @{ printf "%d is prime\n", num break @} @} @} @end example The @code{break} statement is also used to break out of the @code{switch} statement. This is discussed in @ref{Switch Statement}. @c @cindex @code{break}, outside of loops @c @cindex historical features @c @cindex @command{awk} language, POSIX version @cindex POSIX @command{awk} @subentry @code{break} statement and @cindex dark corner @subentry @code{break} statement @cindex @command{gawk} @subentry @code{break} statement in @cindex Brian Kernighan's @command{awk} The @code{break} statement has no meaning when used outside the body of a loop or @code{switch}. However, although it was never documented, historical implementations of @command{awk} treated the @code{break} statement outside of a loop as if it were a @code{next} statement (@pxref{Next Statement}). @value{DARKCORNER} Recent versions of BWK @command{awk} no longer allow this usage, nor does @command{gawk}. @node Continue Statement @subsection The @code{continue} Statement @cindex @code{continue} statement Similar to @code{break}, the @code{continue} statement is used only inside @code{for}, @code{while}, and @code{do} loops. It skips over the rest of the loop body, causing the next cycle around the loop to begin immediately. Contrast this with @code{break}, which jumps out of the loop altogether. The @code{continue} statement in a @code{for} loop directs @command{awk} to skip the rest of the body of the loop and resume execution with the increment-expression of the @code{for} statement. The following program illustrates this fact: @example BEGIN @{ for (x = 0; x <= 20; x++) @{ if (x == 5) continue printf "%d ", x @} print "" @} @end example @noindent This program prints all the numbers from 0 to 20---except for 5, for which the @code{printf} is skipped. Because the increment @samp{x++} is not skipped, @code{x} does not remain stuck at 5. Contrast the @code{for} loop from the previous example with the following @code{while} loop: @example BEGIN @{ x = 0 while (x <= 20) @{ if (x == 5) continue printf "%d ", x x++ @} print "" @} @end example @noindent This program loops forever once @code{x} reaches 5, because the increment (@samp{x++}) is never reached. @c @cindex @code{continue}, outside of loops @c @cindex historical features @c @cindex @command{awk} language, POSIX version @cindex POSIX @command{awk} @subentry @code{continue} statement and @cindex dark corner @subentry @code{continue} statement @cindex @command{gawk} @subentry @code{continue} statement in @cindex Brian Kernighan's @command{awk} The @code{continue} statement has no special meaning with respect to the @code{switch} statement, nor does it have any meaning when used outside the body of a loop. Historical versions of @command{awk} treated a @code{continue} statement outside a loop the same way they treated a @code{break} statement outside a loop: as if it were a @code{next} statement @ifset FOR_PRINT (discussed in the following @value{SECTION}). @end ifset @ifclear FOR_PRINT (@pxref{Next Statement}). @end ifclear @value{DARKCORNER} Recent versions of BWK @command{awk} no longer work this way, nor does @command{gawk}. @node Next Statement @subsection The @code{next} Statement @cindex @code{next} statement The @code{next} statement forces @command{awk} to immediately stop processing the current record and go on to the next record. This means that no further rules are executed for the current record, and the rest of the current rule's action isn't executed. Contrast this with the effect of the @code{getline} function (@pxref{Getline}). That also causes @command{awk} to read the next record immediately, but it does not alter the flow of control in any way (i.e., the rest of the current action executes with a new input record). @cindex @command{awk} programs @subentry execution of At the highest level, @command{awk} program execution is a loop that reads an input record and then tests each rule's pattern against it. If you think of this loop as a @code{for} statement whose body contains the rules, then the @code{next} statement is analogous to a @code{continue} statement. It skips to the end of the body of this implicit loop and executes the increment (which reads another record). For example, suppose an @command{awk} program works only on records with four fields, and it shouldn't fail when given bad input. To avoid complicating the rest of the program, write a ``weed out'' rule near the beginning, in the following manner: @example NF != 4 @{ printf("%s:%d: skipped: NF != 4\n", FILENAME, FNR) > "/dev/stderr" next @} @end example @noindent Because of the @code{next} statement, the program's subsequent rules won't see the bad record. The error message is redirected to the standard error output stream, as error messages should be. For more detail, see @ref{Special Files}. If the @code{next} statement causes the end of the input to be reached, then the code in any @code{END} rules is executed. @xref{BEGIN/END}. The @code{next} statement is not allowed inside @code{BEGINFILE} and @code{ENDFILE} rules. @xref{BEGINFILE/ENDFILE}. @c @cindex @code{next}, inside a user-defined function @cindex @command{awk} @subentry language, POSIX version @cindex @code{BEGIN} pattern @subentry @code{next}/@code{nextfile} statements and @cindex @code{END} pattern @subentry @code{next}/@code{nextfile} statements and @cindex POSIX @command{awk} @subentry @code{next}/@code{nextfile} statements and @cindex @code{next} statement @subentry user-defined functions and @cindex functions @subentry user-defined @subentry @code{next}/@code{nextfile} statements and According to the POSIX standard, the behavior is undefined if the @code{next} statement is used in a @code{BEGIN} or @code{END} rule. @command{gawk} treats it as a syntax error. Although POSIX does not disallow it, most other @command{awk} implementations don't allow the @code{next} statement inside function bodies (@pxref{User-defined}). Just as with any other @code{next} statement, a @code{next} statement inside a function body reads the next record and starts processing it with the first rule in the program. @node Nextfile Statement @subsection The @code{nextfile} Statement @cindex @code{nextfile} statement The @code{nextfile} statement is similar to the @code{next} statement. However, instead of abandoning processing of the current record, the @code{nextfile} statement instructs @command{awk} to stop processing the current @value{DF}. Upon execution of the @code{nextfile} statement, @code{FILENAME} is updated to the name of the next @value{DF} listed on the command line, @code{FNR} is reset to one, and processing starts over with the first rule in the program. If the @code{nextfile} statement causes the end of the input to be reached, then the code in any @code{END} rules is executed. An exception to this is when @code{nextfile} is invoked during execution of any statement in an @code{END} rule; in this case, it causes the program to stop immediately. @xref{BEGIN/END}. The @code{nextfile} statement is useful when there are many @value{DF}s to process but it isn't necessary to process every record in every file. Without @code{nextfile}, in order to move on to the next @value{DF}, a program would have to continue scanning the unwanted records. The @code{nextfile} statement accomplishes this much more efficiently. In @command{gawk}, execution of @code{nextfile} causes additional things to happen: any @code{ENDFILE} rules are executed if @command{gawk} is not currently in an @code{END} or @code{BEGINFILE} rule, @code{ARGIND} is incremented, and any @code{BEGINFILE} rules are executed. (@code{ARGIND} hasn't been introduced yet. @xref{Built-in Variables}.) With @command{gawk}, @code{nextfile} is useful inside a @code{BEGINFILE} rule to skip over a file that would otherwise cause @command{gawk} to exit with a fatal error. In this case, @code{ENDFILE} rules are not executed. @xref{BEGINFILE/ENDFILE}. Although it might seem that @samp{close(FILENAME)} would accomplish the same as @code{nextfile}, this isn't true. @code{close()} is reserved for closing files, pipes, and coprocesses that are opened with redirections. It is not related to the main processing that @command{awk} does with the files listed in @code{ARGV}. @quotation NOTE For many years, @code{nextfile} was a common extension. In September 2012, it was accepted for inclusion into the POSIX standard. See @uref{http://austingroupbugs.net/view.php?id=607, the Austin Group website}. @end quotation @cindex functions @subentry user-defined @subentry @code{next}/@code{nextfile} statements and @cindex @code{nextfile} statement @subentry user-defined functions and @cindex Brian Kernighan's @command{awk} @cindex @command{mawk} utility The current version of BWK @command{awk} and @command{mawk} also support @code{nextfile}. However, they don't allow the @code{nextfile} statement inside function bodies (@pxref{User-defined}). @command{gawk} does; a @code{nextfile} inside a function body reads the first record from the next file and starts processing it with the first rule in the program, just as any other @code{nextfile} statement. @node Exit Statement @subsection The @code{exit} Statement @cindex @code{exit} statement The @code{exit} statement causes @command{awk} to immediately stop executing the current rule and to stop processing input; any remaining input is ignored. The @code{exit} statement is written as follows: @display @code{exit} [@var{return code}] @end display @cindex @code{BEGIN} pattern @subentry @code{exit} statement and @cindex @code{END} pattern @subentry @code{exit} statement and When an @code{exit} statement is executed from a @code{BEGIN} rule, the program stops processing everything immediately. No input records are read. However, if an @code{END} rule is present, as part of executing the @code{exit} statement, the @code{END} rule is executed (@pxref{BEGIN/END}). If @code{exit} is used in the body of an @code{END} rule, it causes the program to stop immediately. An @code{exit} statement that is not part of a @code{BEGIN} or @code{END} rule stops the execution of any further automatic rules for the current record, skips reading any remaining input records, and executes the @code{END} rule if there is one. @command{gawk} also skips any @code{ENDFILE} rules; they do not execute. In such a case, if you don't want the @code{END} rule to do its job, set a variable to a nonzero value before the @code{exit} statement and check that variable in the @code{END} rule. @xref{Assert Function} for an example that does this. @cindex dark corner @subentry @code{exit} statement If an argument is supplied to @code{exit}, its value is used as the exit status code for the @command{awk} process. If no argument is supplied, @code{exit} causes @command{awk} to return a ``success'' status. In the case where an argument is supplied to a first @code{exit} statement, and then @code{exit} is called a second time from an @code{END} rule with no argument, @command{awk} uses the previously supplied exit value. @value{DARKCORNER} @xref{Exit Status} for more information. @cindex programming conventions @subentry @code{exit} statement For example, suppose an error condition occurs that is difficult or impossible to handle. Conventionally, programs report this by exiting with a nonzero status. An @command{awk} program can do this using an @code{exit} statement with a nonzero argument, as shown in the following example: @example @group BEGIN @{ if (("date" | getline date_now) <= 0) @{ print "Can't get system date" > "/dev/stderr" exit 1 @} @end group @group print "current date is", date_now close("date") @} @end group @end example @quotation NOTE For full portability, exit values should be between zero and 126, inclusive. Negative values, and values of 127 or greater, may not produce consistent results across different operating systems. @end quotation @node Built-in Variables @section Predefined Variables @cindex predefined variables @cindex variables @subentry predefined Most @command{awk} variables are available to use for your own purposes; they never change unless your program assigns values to them, and they never affect anything unless your program examines them. However, a few variables in @command{awk} have special built-in meanings. @command{awk} examines some of these automatically, so that they enable you to tell @command{awk} how to do certain things. Others are set automatically by @command{awk}, so that they carry information from the internal workings of @command{awk} to your program. @cindex @command{gawk} @subentry predefined variables and This @value{SECTION} documents all of @command{gawk}'s predefined variables, most of which are also documented in the @value{CHAPTER}s describing their areas of activity. @menu * User-modified:: Built-in variables that you change to control @command{awk}. * Auto-set:: Built-in variables where @command{awk} gives you information. * ARGC and ARGV:: Ways to use @code{ARGC} and @code{ARGV}. @end menu @node User-modified @subsection Built-in Variables That Control @command{awk} @cindex predefined variables @subentry user-modifiable @cindex user-modifiable variables The following is an alphabetical list of variables that you can change to control how @command{awk} does certain things. The variables that are specific to @command{gawk} are marked with a pound sign (@samp{#}). These variables are @command{gawk} extensions. In other @command{awk} implementations or if @command{gawk} is in compatibility mode (@pxref{Options}), they are not special. (Any exceptions are noted in the description of each variable.) @table @code @cindex @code{BINMODE} variable @cindex binary input/output @cindex input/output @subentry binary @cindex differences in @command{awk} and @command{gawk} @subentry @code{BINMODE} variable @item BINMODE # On non-POSIX systems, this variable specifies use of binary mode for all I/O. Numeric values of one, two, or three specify that input files, output files, or all files, respectively, should use binary I/O. A numeric value less than zero is treated as zero, and a numeric value greater than three is treated as three. Alternatively, string values of @code{"r"} or @code{"w"} specify that input files and output files, respectively, should use binary I/O. A string value of @code{"rw"} or @code{"wr"} indicates that all files should use binary I/O. Any other string value is treated the same as @code{"rw"}, but causes @command{gawk} to generate a warning message. @code{BINMODE} is described in more detail in @ref{PC Using}. @command{mawk} (@pxref{Other Versions}) also supports this variable, but only using numeric values. @cindex @code{CONVFMT} variable @cindex POSIX @command{awk} @subentry @code{CONVFMT} variable and @cindex numbers @subentry converting @subentry to strings @cindex strings @subentry converting @subentry numbers to @item @code{CONVFMT} A string that controls the conversion of numbers to strings (@pxref{Conversion}). It works by being passed, in effect, as the first argument to the @code{sprintf()} function (@pxref{String Functions}). Its default value is @code{"%.6g"}. @code{CONVFMT} was introduced by the POSIX standard. @cindex @command{gawk} @subentry @code{FIELDWIDTHS} variable in @cindex @code{FIELDWIDTHS} variable @cindex differences in @command{awk} and @command{gawk} @subentry @code{FIELDWIDTHS} variable @cindex field separator @subentry @code{FIELDWIDTHS} variable and @cindex separators @subentry field @subentry @code{FIELDWIDTHS} variable and @item FIELDWIDTHS # A space-separated list of columns that tells @command{gawk} how to split input with fixed columnar boundaries. Starting in @value{PVERSION} 4.2, each field width may optionally be preceded by a colon-separated value specifying the number of characters to skip before the field starts. Assigning a value to @code{FIELDWIDTHS} overrides the use of @code{FS} and @code{FPAT} for field splitting. @xref{Constant Size} for more information. @cindex @command{gawk} @subentry @code{FPAT} variable in @cindex @code{FPAT} variable @cindex differences in @command{awk} and @command{gawk} @subentry @code{FPAT} variable @cindex field separator @subentry @code{FPAT} variable and @cindex separators @subentry field @subentry @code{FPAT} variable and @item FPAT # A regular expression (as a string) that tells @command{gawk} to create the fields based on text that matches the regular expression. Assigning a value to @code{FPAT} overrides the use of @code{FS} and @code{FIELDWIDTHS} for field splitting. @xref{Splitting By Content} for more information. @cindex @code{FS} variable @cindex separators @subentry field @cindex field separator @item FS The input field separator (@pxref{Field Separators}). The value is a single-character string or a multicharacter regular expression that matches the separations between fields in an input record. If the value is the null string (@code{""}), then each character in the record becomes a separate field. (This behavior is a @command{gawk} extension. POSIX @command{awk} does not specify the behavior when @code{FS} is the null string. Nonetheless, some other versions of @command{awk} also treat @code{""} specially.) The default value is @w{@code{" "}}, a string consisting of a single space. As a special exception, this value means that any sequence of spaces, TABs, and/or newlines is a single separator. It also causes spaces, TABs, and newlines at the beginning and end of a record to be ignored. You can set the value of @code{FS} on the command line using the @option{-F} option: @example awk -F, '@var{program}' @var{input-files} @end example @cindex @command{gawk} @subentry field separators and If @command{gawk} is using @code{FIELDWIDTHS} or @code{FPAT} for field splitting, assigning a value to @code{FS} causes @command{gawk} to return to the normal, @code{FS}-based field splitting. An easy way to do this is to simply say @samp{FS = FS}, perhaps with an explanatory comment. @cindex @command{gawk} @subentry @code{IGNORECASE} variable in @cindex @code{IGNORECASE} variable @cindex differences in @command{awk} and @command{gawk} @subentry @code{IGNORECASE} variable @cindex case sensitivity @subentry string comparisons and @cindex case sensitivity @subentry regexps and @cindex regular expressions @subentry case sensitivity @item IGNORECASE # If @code{IGNORECASE} is nonzero or non-null, then all string comparisons and all regular expression matching are case-independent. This applies to regexp matching with @samp{~} and @samp{!~}, the @code{gensub()}, @code{gsub()}, @code{index()}, @code{match()}, @code{patsplit()}, @code{split()}, and @code{sub()} functions, record termination with @code{RS}, and field splitting with @code{FS} and @code{FPAT}. However, the value of @code{IGNORECASE} does @emph{not} affect array subscripting and it does not affect field splitting when using a single-character field separator. @xref{Case-sensitivity}. @cindex @command{gawk} @subentry @code{LINT} variable in @cindex @code{LINT} variable @cindex differences in @command{awk} and @command{gawk} @subentry @code{LINT} variable @cindex lint checking @item LINT # When this variable is true (nonzero or non-null), @command{gawk} behaves as if the @option{--lint} command-line option is in effect (@pxref{Options}). With a value of @code{"fatal"}, lint warnings become fatal errors. With a value of @code{"invalid"}, only warnings about things that are actually invalid are issued. (This is not fully implemented yet.) Any other true value prints nonfatal warnings. Assigning a false value to @code{LINT} turns off the lint warnings. This variable is a @command{gawk} extension. It is not special in other @command{awk} implementations. Unlike with the other special variables, changing @code{LINT} does affect the production of lint warnings, even if @command{gawk} is in compatibility mode. Much as the @option{--lint} and @option{--traditional} options independently control different aspects of @command{gawk}'s behavior, the control of lint warnings during program execution is independent of the flavor of @command{awk} being executed. @cindex @code{OFMT} variable @cindex numbers @subentry converting @subentry to strings @cindex strings @subentry converting @subentry numbers to @item OFMT A string that controls conversion of numbers to strings (@pxref{Conversion}) for printing with the @code{print} statement. It works by being passed as the first argument to the @code{sprintf()} function (@pxref{String Functions}). Its default value is @code{"%.6g"}. Earlier versions of @command{awk} used @code{OFMT} to specify the format for converting numbers to strings in general expressions; this is now done by @code{CONVFMT}. @cindex @code{print} statement @subentry @code{OFMT} variable and @cindex @code{OFS} variable @cindex separators @subentry field @cindex field separator @item OFS The output field separator (@pxref{Output Separators}). It is output between the fields printed by a @code{print} statement. Its default value is @w{@code{" "}}, a string consisting of a single space. @cindex @code{ORS} variable @item ORS The output record separator. It is output at the end of every @code{print} statement. Its default value is @code{"\n"}, the newline character. (@xref{Output Separators}.) @cindex @code{PREC} variable @item PREC # The working precision of arbitrary-precision floating-point numbers, 53 bits by default (@pxref{Setting precision}). @cindex @code{ROUNDMODE} variable @item ROUNDMODE # The rounding mode to use for arbitrary-precision arithmetic on numbers, by default @code{"N"} (@code{roundTiesToEven} in the IEEE 754 standard; @pxref{Setting the rounding mode}). @cindex @code{RS} variable @cindex separators @subentry for records @cindex record separators @item @code{RS} The input record separator. Its default value is a string containing a single newline character, which means that an input record consists of a single line of text. It can also be the null string, in which case records are separated by runs of blank lines. If it is a regexp, records are separated by matches of the regexp in the input text. (@xref{Records}.) The ability for @code{RS} to be a regular expression is a @command{gawk} extension. In most other @command{awk} implementations, or if @command{gawk} is in compatibility mode (@pxref{Options}), just the first character of @code{RS}'s value is used. @cindex @code{SUBSEP} variable @cindex separators @subentry subscript @cindex subscript separators @item @code{SUBSEP} The subscript separator. It has the default value of @code{"\034"} and is used to separate the parts of the indices of a multidimensional array. Thus, the expression @samp{@w{foo["A", "B"]}} really accesses @code{foo["A\034B"]} (@pxref{Multidimensional}). @cindex @command{gawk} @subentry @code{TEXTDOMAIN} variable in @cindex @code{TEXTDOMAIN} variable @cindex differences in @command{awk} and @command{gawk} @subentry @code{TEXTDOMAIN} variable @cindex internationalization @subentry localization @item TEXTDOMAIN # Used for internationalization of programs at the @command{awk} level. It sets the default text domain for specially marked string constants in the source text, as well as for the @code{dcgettext()}, @code{dcngettext()}, and @code{bindtextdomain()} functions (@pxref{Internationalization}). The default value of @code{TEXTDOMAIN} is @code{"messages"}. @end table @node Auto-set @subsection Built-in Variables That Convey Information @cindex predefined variables @subentry conveying information @cindex variables @subentry predefined @subentry conveying information The following is an alphabetical list of variables that @command{awk} sets automatically on certain occasions in order to provide information to your program. The variables that are specific to @command{gawk} are marked with a pound sign (@samp{#}). These variables are @command{gawk} extensions. In other @command{awk} implementations or if @command{gawk} is in compatibility mode (@pxref{Options}), they are not special: @c @asis for docbook @table @asis @cindex @code{ARGC}/@code{ARGV} variables @cindex arguments @subentry command-line @cindex command line @subentry arguments @item @code{ARGC}, @code{ARGV} The command-line arguments available to @command{awk} programs are stored in an array called @code{ARGV}. @code{ARGC} is the number of command-line arguments present. @xref{Other Arguments}. Unlike most @command{awk} arrays, @code{ARGV} is indexed from 0 to @code{ARGC} @minus{} 1. In the following example: @example @group $ @kbd{awk 'BEGIN @{} > @kbd{for (i = 0; i < ARGC; i++)} > @kbd{print ARGV[i]} > @kbd{@}' inventory-shipped mail-list} @print{} awk @print{} inventory-shipped @print{} mail-list @end group @end example @noindent @code{ARGV[0]} contains @samp{awk}, @code{ARGV[1]} contains @samp{inventory-shipped}, and @code{ARGV[2]} contains @samp{mail-list}. The value of @code{ARGC} is three, one more than the index of the last element in @code{ARGV}, because the elements are numbered from zero. @cindex programming conventions @subentry @code{ARGC}/@code{ARGV} variables The names @code{ARGC} and @code{ARGV}, as well as the convention of indexing the array from 0 to @code{ARGC} @minus{} 1, are derived from the C language's method of accessing command-line arguments. @cindex dark corner @subentry value of @code{ARGV[0]} The value of @code{ARGV[0]} can vary from system to system. Also, you should note that the program text is @emph{not} included in @code{ARGV}, nor are any of @command{awk}'s command-line options. @xref{ARGC and ARGV} for information about how @command{awk} uses these variables. @value{DARKCORNER} @cindex @code{ARGIND} variable @cindex differences in @command{awk} and @command{gawk} @subentry @code{ARGIND} variable @item @code{ARGIND #} The index in @code{ARGV} of the current file being processed. Every time @command{gawk} opens a new @value{DF} for processing, it sets @code{ARGIND} to the index in @code{ARGV} of the @value{FN}. When @command{gawk} is processing the input files, @samp{FILENAME == ARGV[ARGIND]} is always true. @cindex files @subentry processing, @code{ARGIND} variable and This variable is useful in file processing; it allows you to tell how far along you are in the list of @value{DF}s as well as to distinguish between successive instances of the same @value{FN} on the command line. @cindex file names @subentry distinguishing While you can change the value of @code{ARGIND} within your @command{awk} program, @command{gawk} automatically sets it to a new value when it opens the next file. @cindex @code{ENVIRON} array @cindex environment variables @subentry in @code{ENVIRON} array @item @code{ENVIRON} An associative array containing the values of the environment. The array indices are the environment variable names; the elements are the values of the particular environment variables. For example, @code{ENVIRON["HOME"]} might be @code{/home/arnold}. For POSIX @command{awk}, changing this array does not affect the environment passed on to any programs that @command{awk} may spawn via redirection or the @code{system()} function. However, beginning with @value{PVERSION} 4.2, if not in POSIX compatibility mode, @command{gawk} does update its own environment when @code{ENVIRON} is changed, thus changing the environment seen by programs that it creates. You should therefore be especially careful if you modify @code{ENVIRON["PATH"]}, which is the search path for finding executable programs. This can also affect the running @command{gawk} program, since some of the built-in functions may pay attention to certain environment variables. The most notable instance of this is @code{mktime()} (@pxref{Time Functions}), which pays attention the value of the @env{TZ} environment variable on many systems. Some operating systems may not have environment variables. On such systems, the @code{ENVIRON} array is empty (except for @w{@code{ENVIRON["AWKPATH"]}} and @w{@code{ENVIRON["AWKLIBPATH"]}}; @pxref{AWKPATH Variable} and @ifdocbook @ref{AWKLIBPATH Variable}). @end ifdocbook @ifnotdocbook @pxref{AWKLIBPATH Variable}). @end ifnotdocbook @cindex @command{gawk} @subentry @code{ERRNO} variable in @cindex @code{ERRNO} variable @cindex differences in @command{awk} and @command{gawk} @subentry @code{ERRNO} variable @cindex error handling @subentry @code{ERRNO} variable and @item @code{ERRNO #} If a system error occurs during a redirection for @code{getline}, during a read for @code{getline}, or during a @code{close()} operation, then @code{ERRNO} contains a string describing the error. In addition, @command{gawk} clears @code{ERRNO} before opening each command-line input file. This enables checking if the file is readable inside a @code{BEGINFILE} pattern (@pxref{BEGINFILE/ENDFILE}). Otherwise, @code{ERRNO} works similarly to the C variable @code{errno}. Except for the case just mentioned, @command{gawk} @emph{never} clears it (sets it to zero or @code{""}). Thus, you should only expect its value to be meaningful when an I/O operation returns a failure value, such as @code{getline} returning @minus{}1. You are, of course, free to clear it yourself before doing an I/O operation. If the value of @code{ERRNO} corresponds to a system error in the C @code{errno} variable, then @code{PROCINFO["errno"]} will be set to the value of @code{errno}. For non-system errors, @code{PROCINFO["errno"]} will be zero. @cindex @code{FILENAME} variable @cindex dark corner @subentry @code{FILENAME} variable @item @code{FILENAME} The name of the current input file. When no @value{DF}s are listed on the command line, @command{awk} reads from the standard input and @code{FILENAME} is set to @code{"-"}. @code{FILENAME} changes each time a new file is read (@pxref{Reading Files}). Inside a @code{BEGIN} rule, the value of @code{FILENAME} is @code{""}, because there are no input files being processed yet.@footnote{Some early implementations of Unix @command{awk} initialized @code{FILENAME} to @code{"-"}, even if there were @value{DF}s to be processed. This behavior was incorrect and should not be relied upon in your programs.} @value{DARKCORNER} Note, though, that using @code{getline} (@pxref{Getline}) inside a @code{BEGIN} rule can give @code{FILENAME} a value. @cindex @code{FNR} variable @item @code{FNR} The current record number in the current file. @command{awk} increments @code{FNR} each time it reads a new record (@pxref{Records}). @command{awk} resets @code{FNR} to zero each time it starts a new input file. @cindex @code{NF} variable @item @code{NF} The number of fields in the current input record. @code{NF} is set each time a new record is read, when a new field is created, or when @code{$0} changes (@pxref{Fields}). Unlike most of the variables described in this @value{SUBSECTION}, assigning a value to @code{NF} has the potential to affect @command{awk}'s internal workings. In particular, assignments to @code{NF} can be used to create fields in or remove fields from the current record. @xref{Changing Fields}. @cindex @code{FUNCTAB} array @cindex @command{gawk} @subentry @code{FUNCTAB} array in @cindex differences in @command{awk} and @command{gawk} @subentry @code{FUNCTAB} variable @item @code{FUNCTAB #} An array whose indices and corresponding values are the names of all the built-in, user-defined, and extension functions in the program. @quotation NOTE Attempting to use the @code{delete} statement with the @code{FUNCTAB} array causes a fatal error. Any attempt to assign to an element of @code{FUNCTAB} also causes a fatal error. @end quotation @cindex @code{NR} variable @item @code{NR} The number of input records @command{awk} has processed since the beginning of the program's execution (@pxref{Records}). @command{awk} increments @code{NR} each time it reads a new record. @cindex @command{gawk} @subentry @code{PROCINFO} array in @cindex @code{PROCINFO} array @cindex differences in @command{awk} and @command{gawk} @subentry @code{PROCINFO} array @item @code{PROCINFO #} The elements of this array provide access to information about the running @command{awk} program. The following elements (listed alphabetically) are guaranteed to be available: @table @code @item PROCINFO["argv"] @cindex command line @subentry arguments The @code{PROCINFO["argv"]} array contains all of the command-line arguments (after glob expansion and redirection processing on platforms where that must be done manually by the program) with subscripts ranging from 0 through @code{argc} @minus{} 1. For example, @code{PROCINFO["argv"][0]} will contain the name by which @command{gawk} was invoked. Here is an example of how this feature may be used: @example gawk ' BEGIN @{ for (i = 0; i < length(PROCINFO["argv"]); i++) print i, PROCINFO["argv"][i] @}' @end example Please note that this differs from the standard @code{ARGV} array which does not include command-line arguments that have already been processed by @command{gawk} (@pxref{ARGC and ARGV}). @cindex effective group ID of @command{gawk} user @item PROCINFO["egid"] The value of the @code{getegid()} system call. @item PROCINFO["errno"] The value of the C @code{errno} variable when @code{ERRNO} is set to the associated error message. @item PROCINFO["euid"] @cindex effective user ID of @command{gawk} user The value of the @code{geteuid()} system call. @item PROCINFO["FS"] This is @code{"FS"} if field splitting with @code{FS} is in effect, @code{"FIELDWIDTHS"} if field splitting with @code{FIELDWIDTHS} is in effect, @code{"FPAT"} if field matching with @code{FPAT} is in effect, or @code{"API"} if field splitting is controlled by an API input parser. @item PROCINFO["gid"] @cindex group ID of @command{gawk} user The value of the @code{getgid()} system call. @item PROCINFO["identifiers"] @cindex program identifiers A subarray, indexed by the names of all identifiers used in the text of the @command{awk} program. An @dfn{identifier} is simply the name of a variable (be it scalar or array), built-in function, user-defined function, or extension function. For each identifier, the value of the element is one of the following: @table @code @item "array" The identifier is an array. @item "builtin" The identifier is a built-in function. @item "extension" The identifier is an extension function loaded via @code{@@load} or @option{-l}. @item "scalar" The identifier is a scalar. @item "untyped" The identifier is untyped (could be used as a scalar or an array; @command{gawk} doesn't know yet). @item "user" The identifier is a user-defined function. @end table @noindent The values indicate what @command{gawk} knows about the identifiers after it has finished parsing the program; they are @emph{not} updated while the program runs. @item PROCINFO["platform"] @cindex platform running on @cindex @code{PROCINFO} array @subentry platform running on This element gives a string indicating the platform for which @command{gawk} was compiled. The value will be one of the following: @c nested table @table @code @item "djgpp" @itemx "mingw" Microsoft Windows, using either DJGPP or MinGW, respectively. @item "os2" OS/2. @item "os390" OS/390. @item "posix" GNU/Linux, Cygwin, Mac OS X, and legacy Unix systems. @item "vms" OpenVMS or Vax/VMS. @end table @item PROCINFO["pgrpid"] @cindex process group ID of @command{gawk} process The process group ID of the current process. @item PROCINFO["pid"] @cindex process ID of @command{gawk} process The process ID of the current process. @item PROCINFO["ppid"] @cindex parent process ID of @command{gawk} process The parent process ID of the current process. @item PROCINFO["strftime"] The default time format string for @code{strftime()}. Assigning a new value to this element changes the default. @xref{Time Functions}. @item PROCINFO["uid"] The value of the @code{getuid()} system call. @item PROCINFO["version"] @cindex version of @subentry @command{gawk} @cindex @command{gawk} @subentry version of The version of @command{gawk}. @end table The following additional elements in the array are available to provide information about the MPFR and GMP libraries if your version of @command{gawk} supports arbitrary-precision arithmetic (@pxref{Arbitrary Precision Arithmetic}): @table @code @item PROCINFO["gmp_version"] @cindex version of @subentry GNU MP library The version of the GNU MP library. @cindex version of @subentry GNU MPFR library @item PROCINFO["mpfr_version"] The version of the GNU MPFR library. @item PROCINFO["prec_max"] @cindex maximum precision supported by MPFR library The maximum precision supported by MPFR. @item PROCINFO["prec_min"] @cindex minimum precision required by MPFR library The minimum precision required by MPFR. @end table The following additional elements in the array are available to provide information about the version of the extension API, if your version of @command{gawk} supports dynamic loading of extension functions (@pxref{Dynamic Extensions}): @table @code @item PROCINFO["api_major"] @cindex version of @subentry @command{gawk} extension API @cindex extension API @subentry version number The major version of the extension API. @item PROCINFO["api_minor"] The minor version of the extension API. @end table @cindex supplementary groups of @command{gawk} process On some systems, there may be elements in the array, @code{"group1"} through @code{"group@var{N}"} for some @var{N}. @var{N} is the number of supplementary groups that the process has. Use the @code{in} operator to test for these elements (@pxref{Reference to Elements}). The following elements allow you to change @command{gawk}'s behavior: @table @code @item PROCINFO["NONFATAL"] If this element exists, then I/O errors for all redirections become nonfatal. @xref{Nonfatal}. @item PROCINFO["@var{name}", "NONFATAL"] Make I/O errors for @var{name} be nonfatal. @xref{Nonfatal}. @item PROCINFO["@var{command}", "pty"] For two-way communication to @var{command}, use a pseudo-tty instead of setting up a two-way pipe. @xref{Two-way I/O} for more information. @item PROCINFO["@var{input_name}", "READ_TIMEOUT"] Set a timeout for reading from input redirection @var{input_name}. @xref{Read Timeout} for more information. @item PROCINFO["@var{input_name}", "RETRY"] If an I/O error that may be retried occurs when reading data from @var{input_name}, and this array entry exists, then @code{getline} returns @minus{}2 instead of following the default behavior of returning @minus{}1 and configuring @var{input_name} to return no further data. An I/O error that may be retried is one where @code{errno} has the value @code{EAGAIN}, @code{EWOULDBLOCK}, @code{EINTR}, or @code{ETIMEDOUT}. This may be useful in conjunction with @code{PROCINFO["@var{input_name}", "READ_TIMEOUT"]} or situations where a file descriptor has been configured to behave in a non-blocking fashion. @xref{Retrying Input} for more information. @item PROCINFO["sorted_in"] If this element exists in @code{PROCINFO}, its value controls the order in which array indices will be processed by @samp{for (@var{indx} in @var{array})} loops. This is an advanced feature, so we defer the full description until later; see @ref{Controlling Scanning}. @end table @cindex @code{RLENGTH} variable @item @code{RLENGTH} The length of the substring matched by the @code{match()} function (@pxref{String Functions}). @code{RLENGTH} is set by invoking the @code{match()} function. Its value is the length of the matched string, or @minus{}1 if no match is found. @cindex @code{RSTART} variable @item @code{RSTART} The start index in characters of the substring that is matched by the @code{match()} function (@pxref{String Functions}). @code{RSTART} is set by invoking the @code{match()} function. Its value is the position of the string where the matched substring starts, or zero if no match was found. @cindex @command{gawk} @subentry @code{RT} variable in @cindex @code{RT} variable @cindex differences in @command{awk} and @command{gawk} @subentry @code{RS}/@code{RT} variables @item @code{RT #} The input text that matched the text denoted by @code{RS}, the record separator. It is set every time a record is read. @cindex @command{gawk} @subentry @code{SYMTAB} array in @cindex @code{SYMTAB} array @cindex differences in @command{awk} and @command{gawk} @subentry @code{SYMTAB} variable @item @code{SYMTAB #} An array whose indices are the names of all defined global variables and arrays in the program. @code{SYMTAB} makes @command{gawk}'s symbol table visible to the @command{awk} programmer. It is built as @command{gawk} parses the program and is complete before the program starts to run. The array may be used for indirect access to read or write the value of a variable: @example foo = 5 SYMTAB["foo"] = 4 print foo # prints 4 @end example @noindent The @code{isarray()} function (@pxref{Type Functions}) may be used to test if an element in @code{SYMTAB} is an array. Also, you may not use the @code{delete} statement with the @code{SYMTAB} array. Prior to @value{PVERSION} 5.0 of @command{gawk}, you could use an index for @code{SYMTAB} that was not a predefined identifier: @example SYMTAB["xxx"] = 5 print SYMTAB["xxx"] @end example @noindent This no longer works, instead producing a fatal error, as it led to rampant confusion. @cindex Schorr, Andrew The @code{SYMTAB} array is more interesting than it looks. Andrew Schorr points out that it effectively gives @command{awk} data pointers. Consider his example: @example @group # Indirect multiply of any variable by amount, return result function multiply(variable, amount) @{ return SYMTAB[variable] *= amount @} @end group @end example @noindent You would use it like this: @example BEGIN @{ answer = 10.5 multiply("answer", 4) print "The answer is", answer @} @end example @noindent When run, this produces: @example $ @kbd{gawk -f answer.awk} @print{} The answer is 42 @end example @quotation NOTE In order to avoid severe time-travel paradoxes,@footnote{Not to mention difficult implementation issues.} neither @code{FUNCTAB} nor @code{SYMTAB} is available as an element within the @code{SYMTAB} array. @end quotation @end table @cindex sidebar @subentry Changing @code{NR} and @code{FNR} @ifdocbook @docbook Changing @code{NR} and @code{FNR} @end docbook @cindex @code{NR} variable @subentry changing @cindex @code{FNR} variable @subentry changing @cindex dark corner @subentry @code{FNR}/@code{NR} variables @command{awk} increments @code{NR} and @code{FNR} each time it reads a record, instead of setting them to the absolute value of the number of records read. This means that a program can change these variables and their new values are incremented for each record. @value{DARKCORNER} The following example shows this: @example $ @kbd{echo '1} > @kbd{2} > @kbd{3} > @kbd{4' | awk 'NR == 2 @{ NR = 17 @}} > @kbd{@{ print NR @}'} @print{} 1 @print{} 17 @print{} 18 @print{} 19 @end example @noindent Before @code{FNR} was added to the @command{awk} language (@pxref{V7/SVR3.1}), many @command{awk} programs used this feature to track the number of records in a file by resetting @code{NR} to zero when @code{FILENAME} changed. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Changing @code{NR} and @code{FNR}} @cindex @code{NR} variable @subentry changing @cindex @code{FNR} variable @subentry changing @cindex dark corner @subentry @code{FNR}/@code{NR} variables @command{awk} increments @code{NR} and @code{FNR} each time it reads a record, instead of setting them to the absolute value of the number of records read. This means that a program can change these variables and their new values are incremented for each record. @value{DARKCORNER} The following example shows this: @example $ @kbd{echo '1} > @kbd{2} > @kbd{3} > @kbd{4' | awk 'NR == 2 @{ NR = 17 @}} > @kbd{@{ print NR @}'} @print{} 1 @print{} 17 @print{} 18 @print{} 19 @end example @noindent Before @code{FNR} was added to the @command{awk} language (@pxref{V7/SVR3.1}), many @command{awk} programs used this feature to track the number of records in a file by resetting @code{NR} to zero when @code{FILENAME} changed. @end cartouche @end ifnotdocbook @node ARGC and ARGV @subsection Using @code{ARGC} and @code{ARGV} @cindex @code{ARGC}/@code{ARGV} variables @subentry how to use @cindex arguments @subentry command-line @cindex command line @subentry arguments @ref{Auto-set} presented the following program describing the information contained in @code{ARGC} and @code{ARGV}: @example @group $ @kbd{awk 'BEGIN @{} > @kbd{for (i = 0; i < ARGC; i++)} > @kbd{print ARGV[i]} > @kbd{@}' inventory-shipped mail-list} @print{} awk @print{} inventory-shipped @print{} mail-list @end group @end example @noindent In this example, @code{ARGV[0]} contains @samp{awk}, @code{ARGV[1]} contains @samp{inventory-shipped}, and @code{ARGV[2]} contains @samp{mail-list}. Notice that the @command{awk} program is not entered in @code{ARGV}. The other command-line options, with their arguments, are also not entered. This includes variable assignments done with the @option{-v} option (@pxref{Options}). Normal variable assignments on the command line @emph{are} treated as arguments and do show up in the @code{ARGV} array. Given the following program in a file named @file{showargs.awk}: @example BEGIN @{ printf "A=%d, B=%d\n", A, B for (i = 0; i < ARGC; i++) printf "\tARGV[%d] = %s\n", i, ARGV[i] @} END @{ printf "A=%d, B=%d\n", A, B @} @end example @noindent Running it produces the following: @example $ @kbd{awk -v A=1 -f showargs.awk B=2 /dev/null} @print{} A=1, B=0 @print{} ARGV[0] = awk @print{} ARGV[1] = B=2 @print{} ARGV[2] = /dev/null @print{} A=1, B=2 @end example A program can alter @code{ARGC} and the elements of @code{ARGV}. Each time @command{awk} reaches the end of an input file, it uses the next element of @code{ARGV} as the name of the next input file. By storing a different string there, a program can change which files are read. Use @code{"-"} to represent the standard input. Storing additional elements and incrementing @code{ARGC} causes additional files to be read. If the value of @code{ARGC} is decreased, that eliminates input files from the end of the list. By recording the old value of @code{ARGC} elsewhere, a program can treat the eliminated arguments as something other than @value{FN}s. To eliminate a file from the middle of the list, store the null string (@code{""}) into @code{ARGV} in place of the file's name. As a special feature, @command{awk} ignores @value{FN}s that have been replaced with the null string. Another option is to use the @code{delete} statement to remove elements from @code{ARGV} (@pxref{Delete}). All of these actions are typically done in the @code{BEGIN} rule, before actual processing of the input begins. @xref{Split Program} and @ifnotdocbook @pxref{Tee Program} @end ifnotdocbook @ifdocbook @ref{Tee Program} @end ifdocbook for examples of each way of removing elements from @code{ARGV}. To actually get options into an @command{awk} program, end the @command{awk} options with @option{--} and then supply the @command{awk} program's options, in the following manner: @example awk -f myprog.awk -- -v -q file1 file2 @dots{} @end example The following fragment processes @code{ARGV} in order to examine, and then remove, the previously mentioned command-line options: @example BEGIN @{ for (i = 1; i < ARGC; i++) @{ if (ARGV[i] == "-v") verbose = 1 else if (ARGV[i] == "-q") debug = 1 else if (ARGV[i] ~ /^-./) @{ e = sprintf("%s: unrecognized option -- %c", ARGV[0], substr(ARGV[i], 2, 1)) print e > "/dev/stderr" @} else break delete ARGV[i] @} @} @end example @cindex differences in @command{awk} and @command{gawk} @subentry @code{ARGC}/@code{ARGV} variables Ending the @command{awk} options with @option{--} isn't necessary in @command{gawk}. Unless @option{--posix} has been specified, @command{gawk} silently puts any unrecognized options into @code{ARGV} for the @command{awk} program to deal with. As soon as it sees an unknown option, @command{gawk} stops looking for other options that it might otherwise recognize. The previous command line with @command{gawk} would be: @example gawk -f myprog.awk -q -v file1 file2 @dots{} @end example @noindent Because @option{-q} is not a valid @command{gawk} option, it and the following @option{-v} are passed on to the @command{awk} program. (@xref{Getopt Function} for an @command{awk} library function that parses command-line options.) When designing your program, you should choose options that don't conflict with @command{gawk}'s, because it will process any options that it accepts before passing the rest of the command line on to your program. Using @samp{#!} with the @option{-E} option may help (@pxref{Executable Scripts} and @ifnotdocbook @pxref{Options}). @end ifnotdocbook @ifdocbook @ref{Options}). @end ifdocbook @node Pattern Action Summary @section Summary @itemize @value{BULLET} @item Pattern--action pairs make up the basic elements of an @command{awk} program. Patterns are either normal expressions, range expressions, or regexp constants; one of the special keywords @code{BEGIN}, @code{END}, @code{BEGINFILE}, or @code{ENDFILE}; or empty. The action executes if the current record matches the pattern. Empty (missing) patterns match all records. @item I/O from @code{BEGIN} and @code{END} rules has certain constraints. This is also true, only more so, for @code{BEGINFILE} and @code{ENDFILE} rules. The latter two give you ``hooks'' into @command{gawk}'s file processing, allowing you to recover from a file that otherwise would cause a fatal error (such as a file that cannot be opened). @item Shell variables can be used in @command{awk} programs by careful use of shell quoting. It is easier to pass a shell variable into @command{awk} by using the @option{-v} option and an @command{awk} variable. @item Actions consist of statements enclosed in curly braces. Statements are built up from expressions, control statements, compound statements, input and output statements, and deletion statements. @item The control statements in @command{awk} are @code{if}-@code{else}, @code{while}, @code{for}, and @code{do}-@code{while}. @command{gawk} adds the @code{switch} statement. There are two flavors of @code{for} statement: one for performing general looping, and the other for iterating through an array. @item @code{break} and @code{continue} let you exit early or start the next iteration of a loop (or get out of a @code{switch}). @item @code{next} and @code{nextfile} let you read the next record and start over at the top of your program or skip to the next input file and start over, respectively. @item The @code{exit} statement terminates your program. When executed from an action (or function body), it transfers control to the @code{END} statements. From an @code{END} statement body, it exits immediately. You may pass an optional numeric value to be used as @command{awk}'s exit status. @item Some predefined variables provide control over @command{awk}, mainly for I/O. Other variables convey information from @command{awk} to your program. @item @code{ARGC} and @code{ARGV} make the command-line arguments available to your program. Manipulating them from a @code{BEGIN} rule lets you control how @command{awk} will process the provided @value{DF}s. @end itemize @node Arrays @chapter Arrays in @command{awk} @cindex arrays An @dfn{array} is a table of values called @dfn{elements}. The elements of an array are distinguished by their @dfn{indices}. Indices may be either numbers or strings. This @value{CHAPTER} describes how arrays work in @command{awk}, how to use array elements, how to scan through every element in an array, and how to remove array elements. It also describes how @command{awk} simulates multidimensional arrays, as well as some of the less obvious points about array usage. The @value{CHAPTER} moves on to discuss @command{gawk}'s facility for sorting arrays, and ends with a brief description of @command{gawk}'s ability to support true arrays of arrays. @menu * Array Basics:: The basics of arrays. * Numeric Array Subscripts:: How to use numbers as subscripts in @command{awk}. * Uninitialized Subscripts:: Using Uninitialized variables as subscripts. * Delete:: The @code{delete} statement removes an element from an array. * Multidimensional:: Emulating multidimensional arrays in @command{awk}. * Arrays of Arrays:: True multidimensional arrays. * Arrays Summary:: Summary of arrays. @end menu @node Array Basics @section The Basics of Arrays This @value{SECTION} presents the basics: working with elements in arrays one at a time, and traversing all of the elements in an array. @menu * Array Intro:: Introduction to Arrays * Reference to Elements:: How to examine one element of an array. * Assigning Elements:: How to change an element of an array. * Array Example:: Basic Example of an Array * Scanning an Array:: A variation of the @code{for} statement. It loops through the indices of an array's existing elements. * Controlling Scanning:: Controlling the order in which arrays are scanned. @end menu @node Array Intro @subsection Introduction to Arrays @cindex Wall, Larry @quotation @i{Doing linear scans over an associative array is like trying to club someone to death with a loaded Uzi.} @author Larry Wall @end quotation The @command{awk} language provides one-dimensional arrays for storing groups of related strings or numbers. Every @command{awk} array must have a name. Array names have the same syntax as variable names; any valid variable name would also be a valid array name. But one name cannot be used in both ways (as an array and as a variable) in the same @command{awk} program. Arrays in @command{awk} superficially resemble arrays in other programming languages, but there are fundamental differences. In @command{awk}, it isn't necessary to specify the size of an array before starting to use it. Additionally, any number or string, not just consecutive integers, may be used as an array index. In most other languages, arrays must be @dfn{declared} before use, including a specification of how many elements or components they contain. In such languages, the declaration causes a contiguous block of memory to be allocated for that many elements. Usually, an index in the array must be a nonnegative integer. For example, the index zero specifies the first element in the array, which is actually stored at the beginning of the block of memory. Index one specifies the second element, which is stored in memory right after the first element, and so on. It is impossible to add more elements to the array, because it has room only for as many elements as given in the declaration. (Some languages allow arbitrary starting and ending indices---e.g., @samp{15 .. 27}---but the size of the array is still fixed when the array is declared.) @c 1/2015: Do not put the numeric values into @code. Array element @c values are no different than scalar variable values. A contiguous array of four elements might look like @ifnotdocbook @ref{figure-array-elements}, @end ifnotdocbook @ifdocbook @inlineraw{docbook, }, @end ifdocbook conceptually, if the element values are eight, @code{"foo"}, @code{""}, and 30. @ifnotdocbook @float Figure,figure-array-elements @caption{A contiguous array} @center @image{array-elements, , , A Contiguous Array} @end float @end ifnotdocbook @docbook

A contiguous array
@end docbook @noindent Only the values are stored; the indices are implicit from the order of the values. Here, eight is the value at index zero, because eight appears in the position with zero elements before it. @cindex arrays @subentry indexing @cindex indexing arrays @cindex associative arrays @cindex arrays @subentry associative Arrays in @command{awk} are different---they are @dfn{associative}. This means that each array is a collection of pairs---an index and its corresponding array element value: @ifnotdocbook @c extra empty column to indent it right @multitable @columnfractions .1 .1 .1 @headitem @tab Index @tab Value @item @tab @code{3} @tab @code{30} @item @tab @code{1} @tab @code{"foo"} @item @tab @code{0} @tab @code{8} @item @tab @code{2} @tab @code{""} @end multitable @end ifnotdocbook @docbook Index Value 3 30 1 "foo" 0 8 2 "" @end docbook @noindent The pairs are shown in jumbled order because their order is irrelevant.@footnote{The ordering will vary among @command{awk} implementations, which typically use hash tables to store array elements and values.} One advantage of associative arrays is that new pairs can be added at any time. For example, suppose a tenth element is added to the array whose value is @w{@code{"number ten"}}. The result is: @ifnotdocbook @c extra empty column to indent it right @multitable @columnfractions .1 .1 .2 @headitem @tab Index @tab Value @item @tab @code{10} @tab @code{"number ten"} @item @tab @code{3} @tab @code{30} @item @tab @code{1} @tab @code{"foo"} @item @tab @code{0} @tab @code{8} @item @tab @code{2} @tab @code{""} @end multitable @end ifnotdocbook @docbook Index Value 10 "number ten" 3 30 1 "foo" 0 8 2 "" @end docbook @noindent @cindex sparse arrays @cindex arrays @subentry sparse Now the array is @dfn{sparse}, which just means some indices are missing. It has elements 0--3 and 10, but doesn't have elements 4, 5, 6, 7, 8, or 9. Another consequence of associative arrays is that the indices don't have to be nonnegative integers. Any number, or even a string, can be an index. For example, the following is an array that translates words from English to French: @ifnotdocbook @multitable @columnfractions .1 .1 .1 @headitem @tab Index @tab Value @item @tab @code{"dog"} @tab @code{"chien"} @item @tab @code{"cat"} @tab @code{"chat"} @item @tab @code{"one"} @tab @code{"un"} @item @tab @code{1} @tab @code{"un"} @end multitable @end ifnotdocbook @docbook Index Value "dog" "chien" "cat" "chat" "one" "un" 1 "un" @end docbook @noindent Here we decided to translate the number one in both spelled-out and numeric form---thus illustrating that a single array can have both numbers and strings as indices. (In fact, array subscripts are always strings. There are some subtleties to how numbers work when used as array subscripts; this is discussed in more detail in @ref{Numeric Array Subscripts}.) Here, the number @code{1} isn't double-quoted, because @command{awk} automatically converts it to a string. @cindex @command{gawk} @subentry @code{IGNORECASE} variable in @cindex case sensitivity @subentry array indices and @cindex arrays @subentry @code{IGNORECASE} variable and @cindex @code{IGNORECASE} variable @subentry array indices and The value of @code{IGNORECASE} has no effect upon array subscripting. The identical string value used to store an array element must be used to retrieve it. When @command{awk} creates an array (e.g., with the @code{split()} built-in function), that array's indices are consecutive integers starting at one. (@xref{String Functions}.) @command{awk}'s arrays are efficient---the time to access an element is independent of the number of elements in the array. @node Reference to Elements @subsection Referring to an Array Element @cindex arrays @subentry referencing elements @cindex array members @cindex elements in arrays The principal way to use an array is to refer to one of its elements. An @dfn{array reference} is an expression as follows: @example @var{array}[@var{index-expression}] @end example @noindent Here, @var{array} is the name of an array. The expression @var{index-expression} is the index of the desired element of the array. @c 1/2015: Having the 4.3 in @samp is a little iffy. It's essentially @c an expression though, so leave be. It's to early in the discussion @c to mention that it's really a string. The value of the array reference is the current value of that array element. For example, @code{foo[4.3]} is an expression referencing the element of array @code{foo} at index @samp{4.3}. @cindex arrays @subentry unassigned elements @cindex unassigned array elements @cindex empty array elements A reference to an array element that has no recorded value yields a value of @code{""}, the null string. This includes elements that have not been assigned any value as well as elements that have been deleted (@pxref{Delete}). @cindex non-existent array elements @cindex arrays @subentry elements @subentry that don't exist @quotation NOTE A reference to an element that does not exist @emph{automatically} creates that array element, with the null string as its value. (In some cases, this is unfortunate, because it might waste memory inside @command{awk}.) Novice @command{awk} programmers often make the mistake of checking if an element exists by checking if the value is empty: @example # Check if "foo" exists in a: @ii{Incorrect!} if (a["foo"] != "") @dots{} @end example @noindent This is incorrect for two reasons. First, it @emph{creates} @code{a["foo"]} if it didn't exist before! Second, it is valid (if a bit unusual) to set an array element equal to the empty string. @end quotation @c @cindex arrays, @code{in} operator and @cindex @code{in} operator @subentry testing if array element exists To determine whether an element exists in an array at a certain index, use the following expression: @example @var{indx} in @var{array} @end example @cindex side effects @subentry array indexing @noindent This expression tests whether the particular index @var{indx} exists, without the side effect of creating that element if it is not present. The expression has the value one (true) if @code{@var{array}[@var{indx}]} exists and zero (false) if it does not exist. (We use @var{indx} here, because @samp{index} is the name of a built-in function.) For example, this statement tests whether the array @code{frequencies} contains the index @samp{2}: @example @group if (2 in frequencies) print "Subscript 2 is present." @end group @end example Note that this is @emph{not} a test of whether the array @code{frequencies} contains an element whose @emph{value} is two. There is no way to do that except to scan all the elements. Also, this @emph{does not} create @code{frequencies[2]}, while the following (incorrect) alternative does: @example @group if (frequencies[2] != "") print "Subscript 2 is present." @end group @end example @node Assigning Elements @subsection Assigning Array Elements @cindex arrays @subentry elements @subentry assigning values @cindex elements in arrays @subentry assigning values Array elements can be assigned values just like @command{awk} variables: @example @var{array}[@var{index-expression}] = @var{value} @end example @noindent @var{array} is the name of an array. The expression @var{index-expression} is the index of the element of the array that is assigned a value. The expression @var{value} is the value to assign to that element of the array. @node Array Example @subsection Basic Array Example @cindex arrays @subentry example of using The following program takes a list of lines, each beginning with a line number, and prints them out in order of line number. The line numbers are not in order when they are first read---instead, they are scrambled. This program sorts the lines by making an array using the line numbers as subscripts. The program then prints out the lines in sorted order of their numbers. It is a very simple program and gets confused upon encountering repeated numbers, gaps, or lines that don't begin with a number: @example @c file eg/misc/arraymax.awk @{ if ($1 > max) max = $1 arr[$1] = $0 @} END @{ for (x = 1; x <= max; x++) print arr[x] @} @c endfile @end example The first rule keeps track of the largest line number seen so far; it also stores each line into the array @code{arr}, at an index that is the line's number. The second rule runs after all the input has been read, to print out all the lines. When this program is run with the following input: @example @group @c file eg/misc/arraymax.data 5 I am the Five man 2 Who are you? The new number two! 4 . . . And four on the floor 1 Who is number one? 3 I three you. @c endfile @end group @end example @noindent Its output is: @example @group 1 Who is number one? 2 Who are you? The new number two! 3 I three you. 4 . . . And four on the floor 5 I am the Five man @end group @end example If a line number is repeated, the last line with a given number overrides the others. Gaps in the line numbers can be handled with an easy improvement to the program's @code{END} rule, as follows: @example @group END @{ for (x = 1; x <= max; x++) if (x in arr) print arr[x] @} @end group @end example @node Scanning an Array @subsection Scanning All Elements of an Array @cindex elements in arrays @subentry scanning @cindex scanning arrays @cindex arrays @subentry scanning @cindex loops @subentry @code{for} @subentry array scanning In programs that use arrays, it is often necessary to use a loop that executes once for each element of an array. In other languages, where arrays are contiguous and indices are limited to nonnegative integers, this is easy: all the valid indices can be found by counting from the lowest index up to the highest. This technique won't do the job in @command{awk}, because any number or string can be an array index. So @command{awk} has a special kind of @code{for} statement for scanning an array: @example @group for (@var{var} in @var{array}) @var{body} @end group @end example @noindent @cindex @code{in} operator @subentry use in loops This loop executes @var{body} once for each index in @var{array} that the program has previously used, with the variable @var{var} set to that index. @cindex arrays @subentry @code{for} statement and @cindex @code{for} statement @subentry looping over arrays The following program uses this form of the @code{for} statement. The first rule scans the input records and notes which words appear (at least once) in the input, by storing a one into the array @code{used} with the word as the index. The second rule scans the elements of @code{used} to find all the distinct words that appear in the input. It prints each word that is more than 10 characters long and also prints the number of such words. @xref{String Functions} for more information on the built-in function @code{length()}. @example @group # Record a 1 for each word that is used at least once @{ for (i = 1; i <= NF; i++) used[$i] = 1 @} @end group @group # Find number of distinct words more than 10 characters long END @{ for (x in used) @{ if (length(x) > 10) @{ ++num_long_words print x @} @} print num_long_words, "words longer than 10 characters" @} @end group @end example @noindent @xref{Word Sorting} for a more detailed example of this type. @cindex arrays @subentry elements @subentry order of access by @code{in} operator @cindex elements in arrays @subentry order of access by @code{in} operator @cindex @code{in} operator @subentry order of array access The order in which elements of the array are accessed by this statement is determined by the internal arrangement of the array elements within @command{awk} and in standard @command{awk} cannot be controlled or changed. This can lead to problems if new elements are added to @var{array} by statements in the loop body; it is not predictable whether the @code{for} loop will reach them. Similarly, changing @var{var} inside the loop may produce strange results. It is best to avoid such things. As a point of information, @command{gawk} sets up the list of elements to be iterated over before the loop starts, and does not change it. But not all @command{awk} versions do so. Consider this program, named @file{loopcheck.awk}: @example BEGIN @{ a["here"] = "here" a["is"] = "is" a["a"] = "a" a["loop"] = "loop" for (i in a) @{ j++ a[j] = j print i @} @} @end example Here is what happens when run with @command{gawk} (and @command{mawk}): @example $ @kbd{gawk -f loopcheck.awk} @print{} here @print{} loop @print{} a @print{} is @end example Contrast this to BWK @command{awk}: @example $ @kbd{nawk -f loopcheck.awk} @print{} loop @print{} here @print{} is @print{} a @print{} 1 @end example @node Controlling Scanning @subsection Using Predefined Array Scanning Orders with @command{gawk} This @value{SUBSECTION} describes a feature that is specific to @command{gawk}. By default, when a @code{for} loop traverses an array, the order is undefined, meaning that the @command{awk} implementation determines the order in which the array is traversed. This order is usually based on the internal implementation of arrays and will vary from one version of @command{awk} to the next. @cindex array scanning order, controlling @cindex controlling array scanning order Often, though, you may wish to do something simple, such as ``traverse the array by comparing the indices in ascending order,'' or ``traverse the array by comparing the values in descending order.'' @command{gawk} provides two mechanisms that give you this control: @itemize @value{BULLET} @item Set @code{PROCINFO["sorted_in"]} to one of a set of predefined values. We describe this now. @item Set @code{PROCINFO["sorted_in"]} to the name of a user-defined function to use for comparison of array elements. This advanced feature is described later in @ref{Array Sorting}. @end itemize @cindex @code{PROCINFO} array @subentry values of @code{sorted_in} The following special values for @code{PROCINFO["sorted_in"]} are available: @table @code @item "@@unsorted" Array elements are processed in arbitrary order, which is the default @command{awk} behavior. @item "@@ind_str_asc" Order by indices in ascending order compared as strings; this is the most basic sort. (Internally, array indices are always strings, so with @samp{a[2*5] = 1} the index is @code{"10"} rather than numeric 10.) @item "@@ind_num_asc" Order by indices in ascending order but force them to be treated as numbers in the process. Any index with a non-numeric value will end up positioned as if it were zero. @item "@@val_type_asc" Order by element values in ascending order (rather than by indices). Ordering is by the type assigned to the element (@pxref{Typing and Comparison}). All numeric values come before all string values, which in turn come before all subarrays. (Subarrays have not been described yet; @pxref{Arrays of Arrays}.) If you choose to use this feature in traversing @code{FUNCTAB} (@pxref{Auto-set}), then the order is built-in functions first (@pxref{Built-in}), then user-defined functions (@pxref{User-defined}) next, and finally functions loaded from an extension (@pxref{Dynamic Extensions}). @item "@@val_str_asc" Order by element values in ascending order (rather than by indices). Scalar values are compared as strings. If the string values are identical, the index string values are compared instead. When comparing non-scalar values, @code{"@@val_type_asc"} sort ordering is used, so subarrays, if present, come out last. @item "@@val_num_asc" Order by element values in ascending order (rather than by indices). Scalar values are compared as numbers. Non-scalar values are compared using @code{"@@val_type_asc"} sort ordering, so subarrays, if present, come out last. When numeric values are equal, the string values are used to provide an ordering: this guarantees consistent results across different versions of the C @code{qsort()} function,@footnote{When two elements compare as equal, the C @code{qsort()} function does not guarantee that they will maintain their original relative order after sorting. Using the string value to provide a unique ordering when the numeric values are equal ensures that @command{gawk} behaves consistently across different environments.} which @command{gawk} uses internally to perform the sorting. If the string values are also identical, the index string values are compared instead. @item "@@ind_str_desc" Like @code{"@@ind_str_asc"}, but the string indices are ordered from high to low. @item "@@ind_num_desc" Like @code{"@@ind_num_asc"}, but the numeric indices are ordered from high to low. @item "@@val_type_desc" Like @code{"@@val_type_asc"}, but the element values, based on type, are ordered from high to low. Subarrays, if present, come out first. @item "@@val_str_desc" Like @code{"@@val_str_asc"}, but the element values, treated as strings, are ordered from high to low. If the string values are identical, the index string values are compared instead. When comparing non-scalar values, @code{"@@val_type_desc"} sort ordering is used, so subarrays, if present, come out first. @item "@@val_num_desc" Like @code{"@@val_num_asc"}, but the element values, treated as numbers, are ordered from high to low. If the numeric values are equal, the string values are compared instead. If they are also identical, the index string values are compared instead. Non-scalar values are compared using @code{"@@val_type_desc"} sort ordering, so subarrays, if present, come out first. @end table The array traversal order is determined before the @code{for} loop starts to run. Changing @code{PROCINFO["sorted_in"]} in the loop body does not affect the loop. For example: @example $ @kbd{gawk '} > @kbd{BEGIN @{} > @kbd{ a[4] = 4} > @kbd{ a[3] = 3} > @kbd{ for (i in a)} > @kbd{ print i, a[i]} > @kbd{@}'} @print{} 4 4 @print{} 3 3 $ @kbd{gawk '} > @kbd{BEGIN @{} > @kbd{ PROCINFO["sorted_in"] = "@@ind_str_asc"} > @kbd{ a[4] = 4} > @kbd{ a[3] = 3} > @kbd{ for (i in a)} > @kbd{ print i, a[i]} > @kbd{@}'} @print{} 3 3 @print{} 4 4 @end example When sorting an array by element values, if a value happens to be a subarray then it is considered to be greater than any string or numeric value, regardless of what the subarray itself contains, and all subarrays are treated as being equal to each other. Their order relative to each other is determined by their index strings. Here are some additional things to bear in mind about sorted array traversal: @itemize @value{BULLET} @item The value of @code{PROCINFO["sorted_in"]} is global. That is, it affects all array traversal @code{for} loops. If you need to change it within your own code, you should see if it's defined and save and restore the value: @example @dots{} if ("sorted_in" in PROCINFO) @{ save_sorted = PROCINFO["sorted_in"] PROCINFO["sorted_in"] = "@@val_str_desc" # or whatever @} @dots{} if (save_sorted) PROCINFO["sorted_in"] = save_sorted @end example @item As already mentioned, the default array traversal order is represented by @code{"@@unsorted"}. You can also get the default behavior by assigning the null string to @code{PROCINFO["sorted_in"]} or by just deleting the @code{"sorted_in"} element from the @code{PROCINFO} array with the @code{delete} statement. (The @code{delete} statement hasn't been described yet; @pxref{Delete}.) @end itemize In addition, @command{gawk} provides built-in functions for sorting arrays; see @ref{Array Sorting Functions}. @node Numeric Array Subscripts @section Using Numbers to Subscript Arrays @cindex numbers @subentry as array subscripts @cindex array subscripts @subentry numbers as @cindex arrays @subentry numeric subscripts @cindex subscripts in arrays @subentry numbers as @cindex @code{CONVFMT} variable @subentry array subscripts and An important aspect to remember about arrays is that @emph{array subscripts are always strings}. When a numeric value is used as a subscript, it is converted to a string value before being used for subscripting (@pxref{Conversion}). This means that the value of the predefined variable @code{CONVFMT} can affect how your program accesses elements of an array. For example: @example xyz = 12.153 data[xyz] = 1 CONVFMT = "%2.2f" if (xyz in data) printf "%s is in data\n", xyz else printf "%s is not in data\n", xyz @end example @noindent This prints @samp{12.15 is not in data}. The first statement gives @code{xyz} a numeric value. Assigning to @code{data[xyz]} subscripts @code{data} with the string value @code{"12.153"} (using the default conversion value of @code{CONVFMT}, @code{"%.6g"}). Thus, the array element @code{data["12.153"]} is assigned the value one. The program then changes the value of @code{CONVFMT}. The test @samp{(xyz in data)} generates a new string value from @code{xyz}---this time @code{"12.15"}---because the value of @code{CONVFMT} only allows two significant digits. This test fails, because @code{"12.15"} is different from @code{"12.153"}. @cindex converting @subentry integer array subscripts to strings @cindex integer array indices According to the rules for conversions (@pxref{Conversion}), integer values always convert to strings as integers, no matter what the value of @code{CONVFMT} may happen to be. So the usual case of the following works: @example for (i = 1; i <= maxsub; i++) @ii{do something with} array[i] @end example The ``integer values always convert to strings as integers'' rule has an additional consequence for array indexing. Octal and hexadecimal constants @ifnotdocbook (@pxref{Nondecimal-numbers}) @end ifnotdocbook @ifdocbook (covered in @ref{Nondecimal-numbers}) @end ifdocbook are converted internally into numbers, and their original form is forgotten. This means, for example, that @code{array[17]}, @code{array[021]}, and @code{array[0x11]} all refer to the same element! As with many things in @command{awk}, the majority of the time things work as you would expect them to. But it is useful to have a precise knowledge of the actual rules, as they can sometimes have a subtle effect on your programs. @node Uninitialized Subscripts @section Using Uninitialized Variables as Subscripts @cindex variables @subentry uninitialized, as array subscripts @cindex uninitialized variables, as array subscripts @cindex subscripts in arrays @subentry uninitialized variables as @cindex arrays @subentry subscripts, uninitialized variables as Suppose it's necessary to write a program to print the input data in reverse order. A reasonable attempt to do so (with some test data) might look like this: @example $ @kbd{echo 'line 1} > @kbd{line 2} > @kbd{line 3' | awk '@{ l[lines] = $0; ++lines @}} > @kbd{END @{} > @kbd{for (i = lines - 1; i >= 0; i--)} > @kbd{print l[i]} > @kbd{@}'} @print{} line 3 @print{} line 2 @end example Unfortunately, the very first line of input data did not appear in the output! Upon first glance, we would think that this program should have worked. The variable @code{lines} is uninitialized, and uninitialized variables have the numeric value zero. So, @command{awk} should have printed the value of @code{l[0]}. The issue here is that subscripts for @command{awk} arrays are @emph{always} strings. Uninitialized variables, when used as strings, have the value @code{""}, not zero. Thus, @samp{line 1} ends up stored in @code{l[""]}. The following version of the program works correctly: @example @{ l[lines++] = $0 @} END @{ for (i = lines - 1; i >= 0; i--) print l[i] @} @end example Here, the @samp{++} forces @code{lines} to be numeric, thus making the ``old value'' numeric zero. This is then converted to @code{"0"} as the array subscript. @cindex array subscripts @subentry null string as @cindex null strings @subentry as array subscripts @cindex dark corner @subentry array subscripts @cindex lint checking @subentry array subscripts Even though it is somewhat unusual, the null string (@code{""}) is a valid array subscript. @value{DARKCORNER} @command{gawk} warns about the use of the null string as a subscript if @option{--lint} is provided on the command line (@pxref{Options}). @node Delete @section The @code{delete} Statement @cindex @code{delete} statement @cindex deleting @subentry elements in arrays @cindex arrays @subentry elements @subentry deleting @cindex elements in arrays @subentry deleting To remove an individual element of an array, use the @code{delete} statement: @example delete @var{array}[@var{index-expression}] @end example Once an array element has been deleted, any value the element once had is no longer available. It is as if the element had never been referred to or been given a value. The following is an example of deleting elements in an array: @example for (i in frequencies) delete frequencies[i] @end example @noindent This example removes all the elements from the array @code{frequencies}. Once an element is deleted, a subsequent @code{for} statement to scan the array does not report that element and using the @code{in} operator to check for the presence of that element returns zero (i.e., false): @example delete foo[4] if (4 in foo) print "This will never be printed" @end example @cindex null strings @subentry deleting array elements and It is important to note that deleting an element is @emph{not} the same as assigning it a null value (the empty string, @code{""}). For example: @example @group foo[4] = "" if (4 in foo) print "This is printed, even though foo[4] is empty" @end group @end example @cindex lint checking @subentry array subscripts It is not an error to delete an element that does not exist. However, if @option{--lint} is provided on the command line (@pxref{Options}), @command{gawk} issues a warning message when an element that is not in the array is deleted. @cindex common extensions @subentry @code{delete} to delete entire arrays @cindex extensions @subentry common @subentry @code{delete} to delete entire arrays @cindex arrays @subentry deleting entire contents @cindex deleting @subentry entire arrays @cindex @code{delete} @var{array} @cindex differences in @command{awk} and @command{gawk} @subentry array elements, deleting All the elements of an array may be deleted with a single statement by leaving off the subscript in the @code{delete} statement, as follows: @example delete @var{array} @end example Using this version of the @code{delete} statement is about three times more efficient than the equivalent loop that deletes each element one at a time. This form of the @code{delete} statement is also supported by BWK @command{awk} and @command{mawk}, as well as by a number of other implementations. @cindex Brian Kernighan's @command{awk} @quotation NOTE For many years, using @code{delete} without a subscript was a common extension. In September 2012, it was accepted for inclusion into the POSIX standard. See @uref{http://austingroupbugs.net/view.php?id=544, the Austin Group website}. @end quotation @cindex portability @subentry deleting array elements @cindex Brennan, Michael The following statement provides a portable but nonobvious way to clear out an array:@footnote{Thanks to Michael Brennan for pointing this out.} @example split("", array) @end example @cindex @code{split()} function @subentry array elements, deleting The @code{split()} function (@pxref{String Functions}) clears out the target array first. This call asks it to split apart the null string. Because there is no data to split out, the function simply clears the array and then returns. @quotation CAUTION Deleting all the elements from an array does not change its type; you cannot clear an array and then use the array's name as a scalar (i.e., a regular variable). For example, the following does not work: @example a[1] = 3 delete a a = 3 @end example @end quotation @node Multidimensional @section Multidimensional Arrays @menu * Multiscanning:: Scanning multidimensional arrays. @end menu @cindex subscripts in arrays @subentry multidimensional @cindex arrays @subentry multidimensional A @dfn{multidimensional array} is an array in which an element is identified by a sequence of indices instead of a single index. For example, a two-dimensional array requires two indices. The usual way (in many languages, including @command{awk}) to refer to an element of a two-dimensional array named @code{grid} is with @code{grid[@var{x},@var{y}]}. @cindex @code{SUBSEP} variable @subentry multidimensional arrays and Multidimensional arrays are supported in @command{awk} through concatenation of indices into one string. @command{awk} converts the indices into strings (@pxref{Conversion}) and concatenates them together, with a separator between them. This creates a single string that describes the values of the separate indices. The combined string is used as a single index into an ordinary, one-dimensional array. The separator used is the value of the built-in variable @code{SUBSEP}. For example, suppose we evaluate the expression @samp{foo[5,12] = "value"} when the value of @code{SUBSEP} is @code{"@@"}. The numbers 5 and 12 are converted to strings and concatenated with an @samp{@@} between them, yielding @code{"5@@12"}; thus, the array element @code{foo["5@@12"]} is set to @code{"value"}. Once the element's value is stored, @command{awk} has no record of whether it was stored with a single index or a sequence of indices. The two expressions @samp{foo[5,12]} and @w{@samp{foo[5 SUBSEP 12]}} are always equivalent. The default value of @code{SUBSEP} is the string @code{"\034"}, which contains a nonprinting character that is unlikely to appear in an @command{awk} program or in most input data. The usefulness of choosing an unlikely character comes from the fact that index values that contain a string matching @code{SUBSEP} can lead to combined strings that are ambiguous. Suppose that @code{SUBSEP} is @code{"@@"}; then @w{@samp{foo["a@@b", "c"]}} and @w{@samp{foo["a", "b@@c"]}} are indistinguishable because both are actually stored as @samp{foo["a@@b@@c"]}. @cindex @code{in} operator @subentry index existence in multidimensional arrays To test whether a particular index sequence exists in a multidimensional array, use the same operator (@code{in}) that is used for single-dimensional arrays. Write the whole sequence of indices in parentheses, separated by commas, as the left operand: @example if ((@var{subscript1}, @var{subscript2}, @dots{}) in @var{array}) @dots{} @end example Here is an example that treats its input as a two-dimensional array of fields; it rotates this array 90 degrees clockwise and prints the result. It assumes that all lines have the same number of elements: @example @{ if (max_nf < NF) max_nf = NF max_nr = NR for (x = 1; x <= NF; x++) vector[x, NR] = $x @} END @{ for (x = 1; x <= max_nf; x++) @{ for (y = max_nr; y >= 1; --y) printf("%s ", vector[x, y]) printf("\n") @} @} @end example @noindent When given the input: @example @group 1 2 3 4 5 6 2 3 4 5 6 1 3 4 5 6 1 2 4 5 6 1 2 3 @end group @end example @noindent the program produces the following output: @example @group 4 3 2 1 5 4 3 2 6 5 4 3 1 6 5 4 2 1 6 5 3 2 1 6 @end group @end example @node Multiscanning @subsection Scanning Multidimensional Arrays There is no special @code{for} statement for scanning a ``multidimensional'' array. There cannot be one, because, in truth, @command{awk} does not have multidimensional arrays or elements---there is only a multidimensional @emph{way of accessing} an array. @cindex subscripts in arrays @subentry multidimensional @subentry scanning @cindex arrays @subentry multidimensional @subentry scanning @cindex scanning multidimensional arrays However, if your program has an array that is always accessed as multidimensional, you can get the effect of scanning it by combining the scanning @code{for} statement (@pxref{Scanning an Array}) with the built-in @code{split()} function (@pxref{String Functions}). It works in the following manner: @example for (combined in array) @{ split(combined, separate, SUBSEP) @dots{} @} @end example @noindent This sets the variable @code{combined} to each concatenated combined index in the array, and splits it into the individual indices by breaking it apart where the value of @code{SUBSEP} appears. The individual indices then become the elements of the array @code{separate}. Thus, if a value is previously stored in @code{array[1, "foo"]}, then an element with index @code{"1\034foo"} exists in @code{array}. (Recall that the default value of @code{SUBSEP} is the character with code 034.) Sooner or later, the @code{for} statement finds that index and does an iteration with the variable @code{combined} set to @code{"1\034foo"}. Then the @code{split()} function is called as follows: @example split("1\034foo", separate, "\034") @end example @noindent The result is to set @code{separate[1]} to @code{"1"} and @code{separate[2]} to @code{"foo"}. Presto! The original sequence of separate indices is recovered. @node Arrays of Arrays @section Arrays of Arrays @cindex arrays @subentry arrays of arrays @command{gawk} goes beyond standard @command{awk}'s multidimensional array access and provides true arrays of arrays. Elements of a subarray are referred to by their own indices enclosed in square brackets, just like the elements of the main array. For example, the following creates a two-element subarray at index @code{1} of the main array @code{a}: @example a[1][1] = 1 a[1][2] = 2 @end example This simulates a true two-dimensional array. Each subarray element can contain another subarray as a value, which in turn can hold other arrays as well. In this way, you can create arrays of three or more dimensions. The indices can be any @command{awk} expressions, including scalars separated by commas (i.e., a regular @command{awk} simulated multidimensional subscript). So the following is valid in @command{gawk}: @example a[1][3][1, "name"] = "barney" @end example Each subarray and the main array can be of different length. In fact, the elements of an array or its subarray do not all have to have the same type. This means that the main array and any of its subarrays can be nonrectangular, or jagged in structure. You can assign a scalar value to the index @code{4} of the main array @code{a}, even though @code{a[1]} is itself an array and not a scalar: @example a[4] = "An element in a jagged array" @end example The terms @dfn{dimension}, @dfn{row}, and @dfn{column} are meaningless when applied to such an array, but we will use ``dimension'' henceforth to imply the maximum number of indices needed to refer to an existing element. The type of any element that has already been assigned cannot be changed by assigning a value of a different type. You have to first delete the current element, which effectively makes @command{gawk} forget about the element at that index: @example delete a[4] a[4][5][6][7] = "An element in a four-dimensional array" @end example @noindent This removes the scalar value from index @code{4} and then inserts a three-level nested subarray containing a scalar. You can also delete an entire subarray or subarray of subarrays: @example delete a[4][5] a[4][5] = "An element in subarray a[4]" @end example But recall that you can not delete the main array @code{a} and then use it as a scalar. The built-in functions that take array arguments can also be used with subarrays. For example, the following code fragment uses @code{length()} (@pxref{String Functions}) to determine the number of elements in the main array @code{a} and its subarrays: @example print length(a), length(a[1]), length(a[1][3]) @end example @noindent This results in the following output for our main array @code{a}: @example 2, 3, 1 @end example @noindent The @samp{@var{subscript} in @var{array}} expression (@pxref{Reference to Elements}) works similarly for both regular @command{awk}-style arrays and arrays of arrays. For example, the tests @samp{1 in a}, @samp{3 in a[1]}, and @samp{(1, "name") in a[1][3]} all evaluate to one (true) for our array @code{a}. The @samp{for (item in array)} statement (@pxref{Scanning an Array}) can be nested to scan all the elements of an array of arrays if it is rectangular in structure. In order to print the contents (scalar values) of a two-dimensional array of arrays (i.e., in which each first-level element is itself an array, not necessarily of the same length), you could use the following code: @example for (i in array) for (j in array[i]) print array[i][j] @end example The @code{isarray()} function (@pxref{Type Functions}) lets you test if an array element is itself an array: @example for (i in array) @{ if (isarray(array[i])) @{ for (j in array[i]) @{ print array[i][j] @} @} else print array[i] @} @end example If the structure of a jagged array of arrays is known in advance, you can often devise workarounds using control statements. For example, the following code prints the elements of our main array @code{a}: @example @group for (i in a) @{ for (j in a[i]) @{ if (j == 3) @{ for (k in a[i][j]) print a[i][j][k] @end group @group @} else print a[i][j] @} @} @end group @end example @noindent @xref{Walking Arrays} for a user-defined function that ``walks'' an arbitrarily dimensioned array of arrays. Recall that a reference to an uninitialized array element yields a value of @code{""}, the null string. This has one important implication when you intend to use a subarray as an argument to a function, as illustrated by the following example: @example $ @kbd{gawk 'BEGIN @{ split("a b c d", b[1]); print b[1][1] @}'} @error{} gawk: cmd. line:1: fatal: split: second argument is not an array @end example The way to work around this is to first force @code{b[1]} to be an array by creating an arbitrary index: @example $ @kbd{gawk 'BEGIN @{ b[1][1] = ""; split("a b c d", b[1]); print b[1][1] @}'} @print{} a @end example @node Arrays Summary @section Summary @itemize @value{BULLET} @item Standard @command{awk} provides one-dimensional associative arrays (arrays indexed by string values). All arrays are associative; numeric indices are converted automatically to strings. @item Array elements are referenced as @code{@var{array}[@var{indx}]}. Referencing an element creates it if it did not exist previously. @item The proper way to see if an array has an element with a given index is to use the @code{in} operator: @samp{@var{indx} in @var{array}}. @item Use @samp{for (@var{indx} in @var{array}) @dots{}} to scan through all the individual elements of an array. In the body of the loop, @var{indx} takes on the value of each element's index in turn. @item The order in which a @samp{for (@var{indx} in @var{array})} loop traverses an array is undefined in POSIX @command{awk} and varies among implementations. @command{gawk} lets you control the order by assigning special predefined values to @code{PROCINFO["sorted_in"]}. @item Use @samp{delete @var{array}[@var{indx}]} to delete an individual element. To delete all of the elements in an array, use @samp{delete @var{array}}. This latter feature has been a common extension for many years and is now standard, but may not be supported by all commercial versions of @command{awk}. @item Standard @command{awk} simulates multidimensional arrays by separating subscript values with commas. The values are concatenated into a single string, separated by the value of @code{SUBSEP}. The fact that such a subscript was created in this way is not retained; thus, changing @code{SUBSEP} may have unexpected consequences. You can use @samp{(@var{sub1}, @var{sub2}, @dots{}) in @var{array}} to see if such a multidimensional subscript exists in @var{array}. @item @command{gawk} provides true arrays of arrays. You use a separate set of square brackets for each dimension in such an array: @code{data[row][col]}, for example. Array elements may thus be either scalar values (number or string) or other arrays. @item Use the @code{isarray()} built-in function to determine if an array element is itself a subarray. @end itemize @node Functions @chapter Functions @cindex functions @subentry built-in @cindex built-in functions This @value{CHAPTER} describes @command{awk}'s built-in functions, which fall into three categories: numeric, string, and I/O. @command{gawk} provides additional groups of functions to work with values that represent time, do bit manipulation, sort arrays, provide type information, and internationalize and localize programs. Besides the built-in functions, @command{awk} has provisions for writing new functions that the rest of a program can use. The second half of this @value{CHAPTER} describes these @dfn{user-defined} functions. Finally, we explore indirect function calls, a @command{gawk}-specific extension that lets you determine at runtime what function is to be called. @menu * Built-in:: Summarizes the built-in functions. * User-defined:: Describes User-defined functions in detail. * Indirect Calls:: Choosing the function to call at runtime. * Functions Summary:: Summary of functions. @end menu @node Built-in @section Built-in Functions @dfn{Built-in} functions are always available for your @command{awk} program to call. This @value{SECTION} defines all the built-in functions in @command{awk}; some of these are mentioned in other @value{SECTION}s but are summarized here for your convenience. @menu * Calling Built-in:: How to call built-in functions. * Numeric Functions:: Functions that work with numbers, including @code{int()}, @code{sin()} and @code{rand()}. * String Functions:: Functions for string manipulation, such as @code{split()}, @code{match()} and @code{sprintf()}. * I/O Functions:: Functions for files and shell commands. * Time Functions:: Functions for dealing with timestamps. * Bitwise Functions:: Functions for bitwise operations. * Type Functions:: Functions for type information. * I18N Functions:: Functions for string translation. @end menu @node Calling Built-in @subsection Calling Built-in Functions To call one of @command{awk}'s built-in functions, write the name of the function followed by arguments in parentheses. For example, @samp{atan2(y + z, 1)} is a call to the function @code{atan2()} and has two arguments. @cindex programming conventions @subentry functions @subentry calling @cindex whitespace @subentry functions, calling Whitespace is ignored between the built-in function name and the opening parenthesis, but nonetheless it is good practice to avoid using whitespace there. User-defined functions do not permit whitespace in this way, and it is easier to avoid mistakes by following a simple convention that always works---no whitespace after a function name. @cindex troubleshooting @subentry @command{gawk} @subentry fatal errors, function arguments @cindex @command{gawk} @subentry function arguments and @cindex differences in @command{awk} and @command{gawk} @subentry function arguments Each built-in function accepts a certain number of arguments. In some cases, arguments can be omitted. The defaults for omitted arguments vary from function to function and are described under the individual functions. In some @command{awk} implementations, extra arguments given to built-in functions are ignored. However, in @command{gawk}, it is a fatal error to give extra arguments to a built-in function. When a function is called, expressions that create the function's actual parameters are evaluated completely before the call is performed. For example, in the following code fragment: @example i = 4 j = sqrt(i++) @end example @cindex evaluation order @subentry functions @cindex functions @subentry built-in @subentry evaluation order @cindex built-in functions @subentry evaluation order @noindent the variable @code{i} is incremented to the value five before @code{sqrt()} is called with a value of four for its actual parameter. The order of evaluation of the expressions used for the function's parameters is undefined. Thus, avoid writing programs that assume that parameters are evaluated from left to right or from right to left. For example: @example i = 5 j = atan2(++i, i *= 2) @end example If the order of evaluation is left to right, then @code{i} first becomes six, and then 12, and @code{atan2()} is called with the two arguments six and 12. But if the order of evaluation is right to left, @code{i} first becomes 10, then 11, and @code{atan2()} is called with the two arguments 11 and 10. @node Numeric Functions @subsection Numeric Functions @cindex numeric @subentry functions The following list describes all of the built-in functions that work with numbers. Optional parameters are enclosed in square brackets@w{ ([ ]):} @c @asis for docbook @table @asis @item @code{atan2(@var{y}, @var{x})} @cindexawkfunc{atan2} @cindex arctangent Return the arctangent of @code{@var{y} / @var{x}} in radians. You can use @samp{pi = atan2(0, -1)} to retrieve the value of @value{PI}. @item @code{cos(@var{x})} @cindexawkfunc{cos} @cindex cosine Return the cosine of @var{x}, with @var{x} in radians. @item @code{exp(@var{x})} @cindexawkfunc{exp} @cindex exponent Return the exponential of @var{x} (@code{e ^ @var{x}}) or report an error if @var{x} is out of range. The range of values @var{x} can have depends on your machine's floating-point representation. @item @code{int(@var{x})} @cindexawkfunc{int} @cindex round to nearest integer Return the nearest integer to @var{x}, located between @var{x} and zero and truncated toward zero. For example, @code{int(3)} is 3, @code{int(3.9)} is 3, @code{int(-3.9)} is @minus{}3, and @code{int(-3)} is @minus{}3 as well. @ifset INTDIV @item @code{intdiv0(@var{numerator}, @var{denominator}, @var{result})} @cindexawkfunc{intdiv0} @cindex intdiv0 Perform integer division, similar to the standard C @code{div()} function. First, truncate @code{numerator} and @code{denominator} towards zero, creating integer values. Clear the @code{result} array, and then set @code{result["quotient"]} to the result of @samp{numerator / denominator}, truncated towards zero to an integer, and set @code{result["remainder"]} to the result of @samp{numerator % denominator}, truncated towards zero to an integer. Attempting division by zero causes a fatal error. The function returns zero upon success, and @minus{}1 upon error. This function is primarily intended for use with arbitrary length integers; it avoids creating MPFR arbitrary precision floating-point values (@pxref{Arbitrary Precision Integers}). This function is a @code{gawk} extension. It is not available in compatibility mode (@pxref{Options}). @end ifset @item @code{log(@var{x})} @cindexawkfunc{log} @cindex logarithm Return the natural logarithm of @var{x}, if @var{x} is positive; otherwise, return @code{NaN} (``not a number'') on IEEE 754 systems. Additionally, @command{gawk} prints a warning message when @code{x} is negative. @cindex Beebe, Nelson H.F.@: @item @code{rand()} @cindexawkfunc{rand} @cindex random numbers @subentry @code{rand()}/@code{srand()} functions Return a random number. The values of @code{rand()} are uniformly distributed between zero and one. The value could be zero but is never one.@footnote{The C version of @code{rand()} on many Unix systems is known to produce fairly poor sequences of random numbers. However, nothing requires that an @command{awk} implementation use the C @code{rand()} to implement the @command{awk} version of @code{rand()}. In fact, for many years, @command{gawk} used the BSD @code{random()} function, which is considerably better than @code{rand()}, to produce random numbers. From @value{PVERSION} 4.1.4, courtesy of Nelson H.F.@: Beebe, @command{gawk} uses the Bayes-Durham shuffle buffer algorithm which considerably extends the period of the random number generator, and eliminates short-range and long-range correlations that might exist in the original generator.} Often random integers are needed instead. Following is a user-defined function that can be used to obtain a random nonnegative integer less than @var{n}: @example function randint(n) @{ return int(n * rand()) @} @end example @noindent The multiplication produces a random number greater than or equal to zero and less than @code{n}. Using @code{int()}, this result is made into an integer between zero and @code{n} @minus{} 1, inclusive. The following example uses a similar function to produce random integers between one and @var{n}. This program prints a new random number for each input record: @example # Function to roll a simulated die. function roll(n) @{ return 1 + int(rand() * n) @} # Roll 3 six-sided dice and # print total number of points. @{ printf("%d points\n", roll(6) + roll(6) + roll(6)) @} @end example @cindex seeding random number generator @cindex random numbers @subentry seed of @quotation CAUTION In most @command{awk} implementations, including @command{gawk}, @code{rand()} starts generating numbers from the same starting number, or @dfn{seed}, each time you run @command{awk}.@footnote{@command{mawk} uses a different seed each time.} Thus, a program generates the same results each time you run it. The numbers are random within one @command{awk} run but predictable from run to run. This is convenient for debugging, but if you want a program to do different things each time it is used, you must change the seed to a value that is different in each run. To do this, use @code{srand()}. @end quotation @item @code{sin(@var{x})} @cindexawkfunc{sin} @cindex sine Return the sine of @var{x}, with @var{x} in radians. @item @code{sqrt(@var{x})} @cindexawkfunc{sqrt} @cindex square root Return the positive square root of @var{x}. @command{gawk} prints a warning message if @var{x} is negative. Thus, @code{sqrt(4)} is 2. @item @code{srand(}[@var{x}]@code{)} @cindexawkfunc{srand} Set the starting point, or seed, for generating random numbers to the value @var{x}. Each seed value leads to a particular sequence of random numbers.@footnote{Computer-generated random numbers really are not truly random. They are technically known as @dfn{pseudorandom}. This means that although the numbers in a sequence appear to be random, you can in fact generate the same sequence of random numbers over and over again.} Thus, if the seed is set to the same value a second time, the same sequence of random numbers is produced again. @quotation CAUTION Different @command{awk} implementations use different random-number generators internally. Don't expect the same @command{awk} program to produce the same series of random numbers when executed by different versions of @command{awk}. @end quotation If the argument @var{x} is omitted, as in @samp{srand()}, then the current date and time of day are used for a seed. This is the way to get random numbers that are truly unpredictable. The return value of @code{srand()} is the previous seed. This makes it easy to keep track of the seeds in case you need to consistently reproduce sequences of random numbers. POSIX does not specify the initial seed; it differs among @command{awk} implementations. @end table @node String Functions @subsection String-Manipulation Functions @cindex string-manipulation functions The functions in this @value{SECTION} look at or change the text of one or more strings. @command{gawk} understands locales (@pxref{Locales}) and does all string processing in terms of @emph{characters}, not @emph{bytes}. This distinction is particularly important to understand for locales where one character may be represented by multiple bytes. Thus, for example, @code{length()} returns the number of characters in a string, and not the number of bytes used to represent those characters. Similarly, @code{index()} works with character indices, and not byte indices. @quotation CAUTION A number of functions deal with indices into strings. For these functions, the first character of a string is at position (index) one. This is different from C and the languages descended from it, where the first character is at position zero. You need to remember this when doing index calculations, particularly if you are used to C. @end quotation In the following list, optional parameters are enclosed in square brackets@w{ ([ ]).} Several functions perform string substitution; the full discussion is provided in the description of the @code{sub()} function, which comes toward the end, because the list is presented alphabetically. Those functions that are specific to @command{gawk} are marked with a pound sign (@samp{#}). They are not available in compatibility mode (@pxref{Options}): @menu * Gory Details:: More than you want to know about @samp{\} and @samp{&} with @code{sub()}, @code{gsub()}, and @code{gensub()}. @end menu @c @asis for docbook @table @asis @item @code{asort(}@var{source} [@code{,} @var{dest} [@code{,} @var{how} ] ]@code{) #} @itemx @code{asorti(}@var{source} [@code{,} @var{dest} [@code{,} @var{how} ] ]@code{) #} @cindexgawkfunc{asorti} @cindex sort array @cindex arrays @subentry elements @subentry retrieving number of @cindexgawkfunc{asort} @cindex sort array indices These two functions are similar in behavior, so they are described together. @quotation NOTE The following description ignores the third argument, @var{how}, as it requires understanding features that we have not discussed yet. Thus, the discussion here is a deliberate simplification. (We do provide all the details later on; see @ref{Array Sorting Functions} for the full story.) @end quotation Both functions return the number of elements in the array @var{source}. For @command{asort()}, @command{gawk} sorts the values of @var{source} and replaces the indices of the sorted values of @var{source} with sequential integers starting with one. If the optional array @var{dest} is specified, then @var{source} is duplicated into @var{dest}. @var{dest} is then sorted, leaving the indices of @var{source} unchanged. @cindex @command{gawk} @subentry @code{IGNORECASE} variable in When comparing strings, @code{IGNORECASE} affects the sorting (@pxref{Array Sorting Functions}). If the @var{source} array contains subarrays as values (@pxref{Arrays of Arrays}), they will come last, after all scalar values. Subarrays are @emph{not} recursively sorted. For example, if the contents of @code{a} are as follows: @example a["last"] = "de" a["first"] = "sac" a["middle"] = "cul" @end example @noindent A call to @code{asort()}: @example asort(a) @end example @noindent results in the following contents of @code{a}: @example @group a[1] = "cul" a[2] = "de" a[3] = "sac" @end group @end example The @code{asorti()} function works similarly to @code{asort()}; however, the @emph{indices} are sorted, instead of the values. Thus, in the previous example, starting with the same initial set of indices and values in @code{a}, calling @samp{asorti(a)} would yield: @example a[1] = "first" a[2] = "last" a[3] = "middle" @end example @quotation NOTE Due to implementation limitations, you may not use either @code{SYMTAB} or @code{FUNCTAB} as arguments to these functions, even if providing a second array to use for the actual sorting. Attempting to do so produces a fatal error. This restriction may be lifted in the future. @end quotation @item @code{gensub(@var{regexp}, @var{replacement}, @var{how}} [@code{, @var{target}}]@code{) #} @cindexgawkfunc{gensub} @cindex search and replace in strings @cindex substitute in string Search the target string @var{target} for matches of the regular expression @var{regexp}. If @var{how} is a string beginning with @samp{g} or @samp{G} (short for ``global''), then replace all matches of @var{regexp} with @var{replacement}. Otherwise, treat @var{how} as a number indicating which match of @var{regexp} to replace. Treat numeric values less than one as if they were one. If no @var{target} is supplied, use @code{$0}. Return the modified string as the result of the function. The original target string is @emph{not} changed. @code{gensub()} is a general substitution function. Its purpose is to provide more features than the standard @code{sub()} and @code{gsub()} functions. @code{gensub()} provides an additional feature that is not available in @code{sub()} or @code{gsub()}: the ability to specify components of a regexp in the replacement text. This is done by using parentheses in the regexp to mark the components and then specifying @samp{\@var{N}} in the replacement text, where @var{N} is a digit from 1 to 9. For example: @example $ @kbd{gawk '} > @kbd{BEGIN @{} > @kbd{a = "abc def"} > @kbd{b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a)} > @kbd{print b} > @kbd{@}'} @print{} def abc @end example @noindent As with @code{sub()}, you must type two backslashes in order to get one into the string. In the replacement text, the sequence @samp{\0} represents the entire matched text, as does the character @samp{&}. The following example shows how you can use the third argument to control which match of the regexp should be changed: @example $ @kbd{echo a b c a b c |} > @kbd{gawk '@{ print gensub(/a/, "AA", 2) @}'} @print{} a b c AA b c @end example In this case, @code{$0} is the default target string. @code{gensub()} returns the new string as its result, which is passed directly to @code{print} for printing. @c @cindex automatic warnings @c @cindex warnings, automatic If the @var{how} argument is a string that does not begin with @samp{g} or @samp{G}, or if it is a number that is less than or equal to zero, only one substitution is performed. If @var{how} is zero, @command{gawk} issues a warning message. If @var{regexp} does not match @var{target}, @code{gensub()}'s return value is the original unchanged value of @var{target}. @item @code{gsub(@var{regexp}, @var{replacement}} [@code{, @var{target}}]@code{)} @cindexawkfunc{gsub} Search @var{target} for @emph{all} of the longest, leftmost, @emph{nonoverlapping} matching substrings it can find and replace them with @var{replacement}. The @samp{g} in @code{gsub()} stands for ``global,'' which means replace everywhere. For example: @example @{ gsub(/Britain/, "United Kingdom"); print @} @end example @noindent replaces all occurrences of the string @samp{Britain} with @samp{United Kingdom} for all input records. The @code{gsub()} function returns the number of substitutions made. If the variable to search and alter (@var{target}) is omitted, then the entire input record (@code{$0}) is used. As in @code{sub()}, the characters @samp{&} and @samp{\} are special, and the third argument must be assignable. @item @code{index(@var{in}, @var{find})} @cindexawkfunc{index} @cindex search for substring @cindex find substring in string Search the string @var{in} for the first occurrence of the string @var{find}, and return the position in characters where that occurrence begins in the string @var{in}. Consider the following example: @example $ @kbd{awk 'BEGIN @{ print index("peanut", "an") @}'} @print{} 3 @end example @noindent If @var{find} is not found, @code{index()} returns zero. @cindex dark corner @subentry regexp as second argument to @code{index()} With BWK @command{awk} and @command{gawk}, it is a fatal error to use a regexp constant for @var{find}. Other implementations allow it, simply treating the regexp constant as an expression meaning @samp{$0 ~ /regexp/}. @value{DARKCORNER} @item @code{length(}[@var{string}]@code{)} @cindexawkfunc{length} @cindex string @subentry length @cindex length of string Return the number of characters in @var{string}. If @var{string} is a number, the length of the digit string representing that number is returned. For example, @code{length("abcde")} is five. By contrast, @code{length(15 * 35)} works out to three. In this example, @iftex @math{15 @cdot 35 = 525}, @end iftex @ifnottex @ifnotdocbook 15 * 35 = 525, @end ifnotdocbook @end ifnottex @docbook 15 ⋅ 35 = 525, @end docbook and 525 is then converted to the string @code{"525"}, which has three characters. @cindex length of input record @cindex input record, length of If no argument is supplied, @code{length()} returns the length of @code{$0}. @c @cindex historical features @cindex portability @subentry @code{length()} function @cindex POSIX @command{awk} @subentry functions and @subentry @code{length()} @quotation NOTE In older versions of @command{awk}, the @code{length()} function could be called without any parentheses. Doing so is considered poor practice, although the 2008 POSIX standard explicitly allows it, to support historical practice. For programs to be maximally portable, always supply the parentheses. @end quotation @cindex dark corner @subentry @code{length()} function If @code{length()} is called with a variable that has not been used, @command{gawk} forces the variable to be a scalar. Other implementations of @command{awk} leave the variable without a type. @value{DARKCORNER} Consider: @example $ @kbd{gawk 'BEGIN @{ print length(x) ; x[1] = 1 @}'} @print{} 0 @error{} gawk: fatal: attempt to use scalar `x' as array $ @kbd{nawk 'BEGIN @{ print length(x) ; x[1] = 1 @}'} @print{} 0 @end example @noindent If @option{--lint} has been specified on the command line, @command{gawk} issues a warning about this. @cindex common extensions @subentry @code{length()} applied to an array @cindex extensions @subentry common @subentry @code{length()} applied to an array @cindex differences in @command{awk} and @command{gawk} @subentry @code{length()} function @cindex number of array elements @cindex arrays @subentry number of elements With @command{gawk} and several other @command{awk} implementations, when given an array argument, the @code{length()} function returns the number of elements in the array. @value{COMMONEXT} This is less useful than it might seem at first, as the array is not guaranteed to be indexed from one to the number of elements in it. If @option{--lint} is provided on the command line (@pxref{Options}), @command{gawk} warns that passing an array argument is not portable. If @option{--posix} is supplied, using an array argument is a fatal error (@pxref{Arrays}). @item @code{match(@var{string}, @var{regexp}} [@code{, @var{array}}]@code{)} @cindexawkfunc{match} @cindex string @subentry regular expression match of @cindex match regexp in string Search @var{string} for the longest, leftmost substring matched by the regular expression @var{regexp} and return the character position (index) at which that substring begins (one, if it starts at the beginning of @var{string}). If no match is found, return zero. The @var{regexp} argument may be either a regexp constant (@code{/}@dots{}@code{/}) or a string constant (@code{"}@dots{}@code{"}). In the latter case, the string is treated as a regexp to be matched. @xref{Computed Regexps} for a discussion of the difference between the two forms, and the implications for writing your program correctly. The order of the first two arguments is the opposite of most other string functions that work with regular expressions, such as @code{sub()} and @code{gsub()}. It might help to remember that for @code{match()}, the order is the same as for the @samp{~} operator: @samp{@var{string} ~ @var{regexp}}. @cindex @code{RSTART} variable @subentry @code{match()} function and @cindex @code{RLENGTH} variable @subentry @code{match()} function and @cindex @code{match()} function @subentry @code{RSTART}/@code{RLENGTH} variables @cindex @code{match()} function @subentry side effects @cindex side effects @subentry @code{match()} function The @code{match()} function sets the predefined variable @code{RSTART} to the index. It also sets the predefined variable @code{RLENGTH} to the length in characters of the matched substring. If no match is found, @code{RSTART} is set to zero, and @code{RLENGTH} to @minus{}1. For example: @example @c file eg/misc/findpat.awk @{ if ($1 == "FIND") regex = $2 else @{ where = match($0, regex) if (where != 0) print "Match of", regex, "found at", where, "in", $0 @} @} @c endfile @end example @noindent This program looks for lines that match the regular expression stored in the variable @code{regex}. This regular expression can be changed. If the first word on a line is @samp{FIND}, @code{regex} is changed to be the second word on that line. Therefore, if given: @example @c file eg/misc/findpat.data FIND ru+n My program runs but not very quickly FIND Melvin JF+KM This line is property of Reality Engineering Co. Melvin was here. @c endfile @end example @noindent @command{awk} prints: @example Match of ru+n found at 12 in My program runs Match of Melvin found at 1 in Melvin was here. @end example @cindex differences in @command{awk} and @command{gawk} @subentry @code{match()} function If @var{array} is present, it is cleared, and then the zeroth element of @var{array} is set to the entire portion of @var{string} matched by @var{regexp}. If @var{regexp} contains parentheses, the integer-indexed elements of @var{array} are set to contain the portion of @var{string} matching the corresponding parenthesized subexpression. For example: @example $ @kbd{echo foooobazbarrrrr |} > @kbd{gawk '@{ match($0, /(fo+).+(bar*)/, arr)} > @kbd{print arr[1], arr[2] @}'} @print{} foooo barrrrr @end example In addition, multidimensional subscripts are available providing the start index and length of each matched subexpression: @example $ @kbd{echo foooobazbarrrrr |} > @kbd{gawk '@{ match($0, /(fo+).+(bar*)/, arr)} > @kbd{print arr[1], arr[2]} > @kbd{print arr[1, "start"], arr[1, "length"]} > @kbd{print arr[2, "start"], arr[2, "length"]} > @kbd{@}'} @print{} foooo barrrrr @print{} 1 5 @print{} 9 7 @end example There may not be subscripts for the start and index for every parenthesized subexpression, because they may not all have matched text; thus, they should be tested for with the @code{in} operator (@pxref{Reference to Elements}). @cindex troubleshooting @subentry @code{match()} function The @var{array} argument to @code{match()} is a @command{gawk} extension. In compatibility mode (@pxref{Options}), using a third argument is a fatal error. @item @code{patsplit(@var{string}, @var{array}} [@code{, @var{fieldpat}} [@code{, @var{seps}} ] ]@code{) #} @cindexgawkfunc{patsplit} @cindex split string into array Divide @var{string} into pieces (or ``fields'') defined by @var{fieldpat} and store the pieces in @var{array} and the separator strings in the @var{seps} array. The first piece is stored in @code{@var{array}[1]}, the second piece in @code{@var{array}[2]}, and so forth. The third argument, @var{fieldpat}, is a regexp describing the fields in @var{string} (just as @code{FPAT} is a regexp describing the fields in input records). It may be either a regexp constant or a string. If @var{fieldpat} is omitted, the value of @code{FPAT} is used. @code{patsplit()} returns the number of elements created. @code{@var{seps}[@var{i}]} is the possibly null separator string after @code{@var{array}[@var{i}]}. The possibly null leading separator will be in @code{@var{seps}[0]}. So a non-null @var{string} with @var{n} fields will have @var{n+1} separators. A null @var{string} will not have neither fields nor separators. The @code{patsplit()} function splits strings into pieces in a manner similar to the way input lines are split into fields using @code{FPAT} (@pxref{Splitting By Content}). Before splitting the string, @code{patsplit()} deletes any previously existing elements in the arrays @var{array} and @var{seps}. @item @code{split(@var{string}, @var{array}} [@code{, @var{fieldsep}} [@code{, @var{seps}} ] ]@code{)} @cindexawkfunc{split} Divide @var{string} into pieces separated by @var{fieldsep} and store the pieces in @var{array} and the separator strings in the @var{seps} array. The first piece is stored in @code{@var{array}[1]}, the second piece in @code{@var{array}[2]}, and so forth. The string value of the third argument, @var{fieldsep}, is a regexp describing where to split @var{string} (much as @code{FS} can be a regexp describing where to split input records). If @var{fieldsep} is omitted, the value of @code{FS} is used. @code{split()} returns the number of elements created. @var{seps} is a @command{gawk} extension, with @code{@var{seps}[@var{i}]} being the separator string between @code{@var{array}[@var{i}]} and @code{@var{array}[@var{i}+1]}. If @var{fieldsep} is a single space, then any leading whitespace goes into @code{@var{seps}[0]} and any trailing whitespace goes into @code{@var{seps}[@var{n}]}, where @var{n} is the return value of @code{split()} (i.e., the number of elements in @var{array}). The @code{split()} function splits strings into pieces in the same way that input lines are split into fields. For example: @example split("cul-de-sac", a, "-", seps) @end example @noindent @cindex strings @subentry splitting, example splits the string @code{"cul-de-sac"} into three fields using @samp{-} as the separator. It sets the contents of the array @code{a} as follows: @example a[1] = "cul" a[2] = "de" a[3] = "sac" @end example and sets the contents of the array @code{seps} as follows: @example seps[1] = "-" seps[2] = "-" @end example @noindent The value returned by this call to @code{split()} is three. @cindex differences in @command{awk} and @command{gawk} @subentry @code{split()} function As with input field-splitting, when the value of @var{fieldsep} is @w{@code{" "}}, leading and trailing whitespace is ignored in values assigned to the elements of @var{array} but not in @var{seps}, and the elements are separated by runs of whitespace. Also, as with input field splitting, if @var{fieldsep} is the null string, each individual character in the string is split into its own array element. @value{COMMONEXT} Additionally, if @var{fieldsep} is a single-character string, that string acts as the separator, even if its value is a regular expression metacharacter. Note, however, that @code{RS} has no effect on the way @code{split()} works. Even though @samp{RS = ""} causes the newline character to also be an input field separator, this does not affect how @code{split()} splits strings. @cindex dark corner @subentry @code{split()} function Modern implementations of @command{awk}, including @command{gawk}, allow the third argument to be a regexp constant (@w{@code{/}@dots{}@code{/}}) as well as a string. @value{DARKCORNER} The POSIX standard allows this as well. @xref{Computed Regexps} for a discussion of the difference between using a string constant or a regexp constant, and the implications for writing your program correctly. Before splitting the string, @code{split()} deletes any previously existing elements in the arrays @var{array} and @var{seps}. If @var{string} is null, the array has no elements. (So this is a portable way to delete an entire array with one statement. @xref{Delete}.) If @var{string} does not match @var{fieldsep} at all (but is not null), @var{array} has one element only. The value of that element is the original @var{string}. @cindex POSIX mode In POSIX mode (@pxref{Options}), the fourth argument is not allowed. @item @code{sprintf(@var{format}, @var{expression1}, @dots{})} @cindexawkfunc{sprintf} @cindex formatting @subentry strings Return (without printing) the string that @code{printf} would have printed out with the same arguments (@pxref{Printf}). For example: @example pival = sprintf("pi = %.2f (approx.)", 22/7) @end example @noindent assigns the string @w{@samp{pi = 3.14 (approx.)}} to the variable @code{pival}. @cindexgawkfunc{strtonum} @cindex converting @subentry string to numbers @item @code{strtonum(@var{str}) #} Examine @var{str} and return its numeric value. If @var{str} begins with a leading @samp{0}, @code{strtonum()} assumes that @var{str} is an octal number. If @var{str} begins with a leading @samp{0x} or @samp{0X}, @code{strtonum()} assumes that @var{str} is a hexadecimal number. For example: @example $ @kbd{echo 0x11 |} > @kbd{gawk '@{ printf "%d\n", strtonum($1) @}'} @print{} 17 @end example Using the @code{strtonum()} function is @emph{not} the same as adding zero to a string value; the automatic coercion of strings to numbers works only for decimal data, not for octal or hexadecimal.@footnote{Unless you use the @option{--non-decimal-data} option, which isn't recommended. @xref{Nondecimal Data} for more information.} Note also that @code{strtonum()} uses the current locale's decimal point for recognizing numbers (@pxref{Locales}). @item @code{sub(@var{regexp}, @var{replacement}} [@code{, @var{target}}]@code{)} @cindexawkfunc{sub} @cindex replace in string Search @var{target}, which is treated as a string, for the leftmost, longest substring matched by the regular expression @var{regexp}. Modify the entire string by replacing the matched text with @var{replacement}. The modified string becomes the new value of @var{target}. Return the number of substitutions made (zero or one). The @var{regexp} argument may be either a regexp constant (@code{/}@dots{}@code{/}) or a string constant (@code{"}@dots{}@code{"}). In the latter case, the string is treated as a regexp to be matched. @xref{Computed Regexps} for a discussion of the difference between the two forms, and the implications for writing your program correctly. This function is peculiar because @var{target} is not simply used to compute a value, and not just any expression will do---it must be a variable, field, or array element so that @code{sub()} can store a modified value there. If this argument is omitted, then the default is to use and alter @code{$0}.@footnote{Note that this means that the record will first be regenerated using the value of @code{OFS} if any fields have been changed, and that the fields will be updated after the substitution, even if the operation is a ``no-op'' such as @samp{sub(/^/, "")}.} For example: @example str = "water, water, everywhere" sub(/at/, "ith", str) @end example @noindent sets @code{str} to @w{@samp{wither, water, everywhere}}, by replacing the leftmost longest occurrence of @samp{at} with @samp{ith}. If the special character @samp{&} appears in @var{replacement}, it stands for the precise substring that was matched by @var{regexp}. (If the regexp can match more than one string, then this precise substring may vary.) For example: @example @{ sub(/candidate/, "& and his wife"); print @} @end example @noindent changes the first occurrence of @samp{candidate} to @samp{candidate and his wife} on each input line. Here is another example: @example $ @kbd{awk 'BEGIN @{} > @kbd{str = "daabaaa"} > @kbd{sub(/a+/, "C&C", str)} > @kbd{print str} > @kbd{@}'} @print{} dCaaCbaaa @end example @noindent This shows how @samp{&} can represent a nonconstant string and also illustrates the ``leftmost, longest'' rule in regexp matching (@pxref{Leftmost Longest}). The effect of this special character (@samp{&}) can be turned off by putting a backslash before it in the string. As usual, to insert one backslash in the string, you must write two backslashes. Therefore, write @samp{\\&} in a string constant to include a literal @samp{&} in the replacement. For example, the following shows how to replace the first @samp{|} on each line with an @samp{&}: @example @{ sub(/\|/, "\\&"); print @} @end example @cindex @code{sub()} function @subentry arguments of @cindex @code{gsub()} function @subentry arguments of @cindex side effects @subentry @code{sub()} function @cindex side effects @subentry @code{gsub()} function As mentioned, the third argument to @code{sub()} must be a variable, field, or array element. Some versions of @command{awk} allow the third argument to be an expression that is not an lvalue. In such a case, @code{sub()} still searches for the pattern and returns zero or one, but the result of the substitution (if any) is thrown away because there is no place to put it. Such versions of @command{awk} accept expressions like the following: @example sub(/USA/, "United States", "the USA and Canada") @end example @noindent @cindex troubleshooting @subentry @code{gsub()}/@code{sub()} functions For historical compatibility, @command{gawk} accepts such erroneous code. However, using any other nonchangeable object as the third parameter causes a fatal error and your program will not run. Finally, if the @var{regexp} is not a regexp constant, it is converted into a string, and then the value of that string is treated as the regexp to match. @item @code{substr(@var{string}, @var{start}} [@code{, @var{length}} ]@code{)} @cindexawkfunc{substr} @cindex substring Return a @var{length}-character-long substring of @var{string}, starting at character number @var{start}. The first character of a string is character number one.@footnote{This is different from C and C++, in which the first character is number zero.} For example, @code{substr("washington", 5, 3)} returns @code{"ing"}. If @var{length} is not present, @code{substr()} returns the whole suffix of @var{string} that begins at character number @var{start}. For example, @code{substr("washington", 5)} returns @code{"ington"}. The whole suffix is also returned if @var{length} is greater than the number of characters remaining in the string, counting from character @var{start}. @cindex Brian Kernighan's @command{awk} If @var{start} is less than one, @code{substr()} treats it as if it was one. (POSIX doesn't specify what to do in this case: BWK @command{awk} acts this way, and therefore @command{gawk} does too.) If @var{start} is greater than the number of characters in the string, @code{substr()} returns the null string. Similarly, if @var{length} is present but less than or equal to zero, the null string is returned. @cindex troubleshooting @subentry @code{substr()} function The string returned by @code{substr()} @emph{cannot} be assigned. Thus, it is a mistake to attempt to change a portion of a string, as shown in the following example: @example string = "abcdef" # try to get "abCDEf", won't work substr(string, 3, 3) = "CDE" @end example @noindent It is also a mistake to use @code{substr()} as the third argument of @code{sub()} or @code{gsub()}: @example gsub(/xyz/, "pdq", substr($0, 5, 20)) # WRONG @end example @cindex portability @subentry @code{substr()} function (Some commercial versions of @command{awk} treat @code{substr()} as assignable, but doing so is not portable.) If you need to replace bits and pieces of a string, combine @code{substr()} with string concatenation, in the following manner: @example string = "abcdef" @dots{} string = substr(string, 1, 2) "CDE" substr(string, 6) @end example @cindex case sensitivity @subentry converting case @cindex strings @subentry converting letter case @item @code{tolower(@var{string})} @cindexawkfunc{tolower} @cindex converting @subentry string to lower case Return a copy of @var{string}, with each uppercase character in the string replaced with its corresponding lowercase character. Nonalphabetic characters are left unchanged. For example, @code{tolower("MiXeD cAsE 123")} returns @code{"mixed case 123"}. @item @code{toupper(@var{string})} @cindexawkfunc{toupper} @cindex converting @subentry string to upper case Return a copy of @var{string}, with each lowercase character in the string replaced with its corresponding uppercase character. Nonalphabetic characters are left unchanged. For example, @code{toupper("MiXeD cAsE 123")} returns @code{"MIXED CASE 123"}. @end table @cindex sidebar @subentry Matching the Null String @ifdocbook @docbook Matching the Null String @end docbook @cindex matching @subentry null strings @cindex null strings @subentry matching @cindex @code{*} (asterisk) @subentry @code{*} operator @subentry null strings, matching @cindex asterisk (@code{*}) @subentry @code{*} operator @subentry null strings, matching In @command{awk}, the @samp{*} operator can match the null string. This is particularly important for the @code{sub()}, @code{gsub()}, and @code{gensub()} functions. For example: @example $ @kbd{echo abc | awk '@{ gsub(/m*/, "X"); print @}'} @print{} XaXbXcX @end example @noindent Although this makes a certain amount of sense, it can be surprising. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Matching the Null String} @cindex matching @subentry null strings @cindex null strings @subentry matching @cindex @code{*} (asterisk) @subentry @code{*} operator @subentry null strings, matching @cindex asterisk (@code{*}) @subentry @code{*} operator @subentry null strings, matching In @command{awk}, the @samp{*} operator can match the null string. This is particularly important for the @code{sub()}, @code{gsub()}, and @code{gensub()} functions. For example: @example $ @kbd{echo abc | awk '@{ gsub(/m*/, "X"); print @}'} @print{} XaXbXcX @end example @noindent Although this makes a certain amount of sense, it can be surprising. @end cartouche @end ifnotdocbook @node Gory Details @subsubsection More about @samp{\} and @samp{&} with @code{sub()}, @code{gsub()}, and @code{gensub()} @cindex escape processing @subentry @code{gsub()}/@code{gensub()}/@code{sub()} functions @cindex @code{sub()} function @subentry escape processing @cindex @code{gsub()} function @subentry escape processing @cindex @code{gensub()} function (@command{gawk}) @subentry escape processing @cindex @code{\} (backslash) @subentry @code{gsub()}/@code{gensub()}/@code{sub()} functions and @cindex backslash (@code{\}) @subentry @code{gsub()}/@code{gensub()}/@code{sub()} functions and @cindex @code{&} (ampersand) @subentry @code{gsub()}/@code{gensub()}/@code{sub()} functions and @cindex ampersand (@code{&}) @subentry @code{gsub()}/@code{gensub()}/@code{sub()} functions and @quotation CAUTION This subsubsection has been reported to cause headaches. You might want to skip it upon first reading. @end quotation When using @code{sub()}, @code{gsub()}, or @code{gensub()}, and trying to get literal backslashes and ampersands into the replacement text, you need to remember that there are several levels of @dfn{escape processing} going on. First, there is the @dfn{lexical} level, which is when @command{awk} reads your program and builds an internal copy of it to execute. Then there is the runtime level, which is when @command{awk} actually scans the replacement string to determine what to generate. @cindex Brian Kernighan's @command{awk} At both levels, @command{awk} looks for a defined set of characters that can come after a backslash. At the lexical level, it looks for the escape sequences listed in @ref{Escape Sequences}. Thus, for every @samp{\} that @command{awk} processes at the runtime level, you must type two backslashes at the lexical level. When a character that is not valid for an escape sequence follows the @samp{\}, BWK @command{awk} and @command{gawk} both simply remove the initial @samp{\} and put the next character into the string. Thus, for example, @code{"a\qb"} is treated as @code{"aqb"}. At the runtime level, the various functions handle sequences of @samp{\} and @samp{&} differently. The situation is (sadly) somewhat complex. Historically, the @code{sub()} and @code{gsub()} functions treated the two-character sequence @samp{\&} specially; this sequence was replaced in the generated text with a single @samp{&}. Any other @samp{\} within the @var{replacement} string that did not precede an @samp{&} was passed through unchanged. This is illustrated in @ref{table-sub-escapes}. @c Thank to Karl Berry for help with the TeX stuff. @float Table,table-sub-escapes @caption{Historical escape sequence processing for @code{sub()} and @code{gsub()}} @tex \vbox{\bigskip % We need more characters for escape and tab ... \catcode`_ = 0 \catcode`! = 4 % ... since this table has lots of &'s and \'s, so we unspecialize them. \catcode`\& = \other \catcode`\\ = \other _halign{_hfil#!_qquad_hfil#!_qquad#_hfil_cr You type!@code{sub()} sees!@code{sub()} generates_cr _hrulefill!_hrulefill!_hrulefill_cr @code{\&}! @code{&}!The matched text_cr @code{\\&}! @code{\&}!A literal @samp{&}_cr @code{\\\&}! @code{\&}!A literal @samp{&}_cr @code{\\\\&}! @code{\\&}!A literal @samp{\&}_cr @code{\\\\\&}! @code{\\&}!A literal @samp{\&}_cr @code{\\\\\\&}! @code{\\\&}!A literal @samp{\\&}_cr @code{\\q}! @code{\q}!A literal @samp{\q}_cr } _bigskip} @end tex @ifdocbook @multitable @columnfractions .20 .20 .60 @headitem You type @tab @code{sub()} sees @tab @code{sub()} generates @item @code{\&} @tab @code{&} @tab The matched text @item @code{\\&} @tab @code{\&} @tab A literal @samp{&} @item @code{\\\&} @tab @code{\&} @tab A literal @samp{&} @item @code{\\\\&} @tab @code{\\&} @tab A literal @samp{\&} @item @code{\\\\\&} @tab @code{\\&} @tab A literal @samp{\&} @item @code{\\\\\\&} @tab @code{\\\&} @tab A literal @samp{\\&} @item @code{\\q} @tab @code{\q} @tab A literal @samp{\q} @end multitable @end ifdocbook @ifnottex @ifnotdocbook @display You type @code{sub()} sees @code{sub()} generates -------- ---------- --------------- @code{\&} @code{&} The matched text @code{\\&} @code{\&} A literal @samp{&} @code{\\\&} @code{\&} A literal @samp{&} @code{\\\\&} @code{\\&} A literal @samp{\&} @code{\\\\\&} @code{\\&} A literal @samp{\&} @code{\\\\\\&} @code{\\\&} A literal @samp{\\&} @code{\\q} @code{\q} A literal @samp{\q} @end display @end ifnotdocbook @end ifnottex @end float @noindent This table shows the lexical-level processing, where an odd number of backslashes becomes an even number at the runtime level, as well as the runtime processing done by @code{sub()}. (For the sake of simplicity, the rest of the following tables only show the case of even numbers of backslashes entered at the lexical level.) The problem with the historical approach is that there is no way to get a literal @samp{\} followed by the matched text. Several editions of the POSIX standard attempted to fix this problem but weren't successful. The details are irrelevant at this point in time. At one point, the @command{gawk} maintainer submitted proposed text for a revised standard that reverts to rules that correspond more closely to the original existing practice. The proposed rules have special cases that make it possible to produce a @samp{\} preceding the matched text. This is shown in @ref{table-sub-proposed}. @float Table,table-sub-proposed @caption{@command{gawk} rules for @code{sub()} and backslash} @tex \vbox{\bigskip % We need more characters for escape and tab ... \catcode`_ = 0 \catcode`! = 4 % ... since this table has lots of &'s and \'s, so we unspecialize them. \catcode`\& = \other \catcode`\\ = \other _halign{_hfil#!_qquad_hfil#!_qquad#_hfil_cr You type!@code{sub()} sees!@code{sub()} generates_cr _hrulefill!_hrulefill!_hrulefill_cr @code{\\\\\\&}! @code{\\\&}!A literal @samp{\&}_cr @code{\\\\&}! @code{\\&}!A literal @samp{\}, followed by the matched text_cr @code{\\&}! @code{\&}!A literal @samp{&}_cr @code{\\q}! @code{\q}!A literal @samp{\q}_cr @code{\\\\}! @code{\\}!@code{\\}_cr } _bigskip} @end tex @ifdocbook @multitable @columnfractions .20 .20 .60 @headitem You type @tab @code{sub()} sees @tab @code{sub()} generates @item @code{\\\\\\&} @tab @code{\\\&} @tab A literal @samp{\&} @item @code{\\\\&} @tab @code{\\&} @tab A literal @samp{\}, followed by the matched text @item @code{\\&} @tab @code{\&} @tab A literal @samp{&} @item @code{\\q} @tab @code{\q} @tab A literal @samp{\q} @item @code{\\\\} @tab @code{\\} @tab @code{\\} @end multitable @end ifdocbook @ifnottex @ifnotdocbook @display You type @code{sub()} sees @code{sub()} generates -------- ---------- --------------- @code{\\\\\\&} @code{\\\&} A literal @samp{\&} @code{\\\\&} @code{\\&} A literal @samp{\}, followed by the matched text @code{\\&} @code{\&} A literal @samp{&} @code{\\q} @code{\q} A literal @samp{\q} @code{\\\\} @code{\\} @code{\\} @end display @end ifnotdocbook @end ifnottex @end float In a nutshell, at the runtime level, there are now three special sequences of characters (@samp{\\\&}, @samp{\\&}, and @samp{\&}) whereas historically there was only one. However, as in the historical case, any @samp{\} that is not part of one of these three sequences is not special and appears in the output literally. @command{gawk} 3.0 and 3.1 follow these rules for @code{sub()} and @code{gsub()}. The POSIX standard took much longer to be revised than was expected. In addition, the @command{gawk} maintainer's proposal was lost during the standardization process. The final rules are somewhat simpler. The results are similar except for one case. @cindex POSIX @command{awk} @subentry functions and @subentry @code{gsub()}/@code{sub()} The POSIX rules state that @samp{\&} in the replacement string produces a literal @samp{&}, @samp{\\} produces a literal @samp{\}, and @samp{\} followed by anything else is not special; the @samp{\} is placed straight into the output. These rules are presented in @ref{table-posix-sub}. @float Table,table-posix-sub @caption{POSIX rules for @code{sub()} and @code{gsub()}} @tex \vbox{\bigskip % We need more characters for escape and tab ... \catcode`_ = 0 \catcode`! = 4 % ... since this table has lots of &'s and \'s, so we unspecialize them. \catcode`\& = \other \catcode`\\ = \other _halign{_hfil#!_qquad_hfil#!_qquad#_hfil_cr You type!@code{sub()} sees!@code{sub()} generates_cr _hrulefill!_hrulefill!_hrulefill_cr @code{\\\\\\&}! @code{\\\&}!A literal @samp{\&}_cr @code{\\\\&}! @code{\\&}!A literal @samp{\}, followed by the matched text_cr @code{\\&}! @code{\&}!A literal @samp{&}_cr @code{\\q}! @code{\q}!A literal @samp{\q}_cr @code{\\\\}! @code{\\}!@code{\}_cr } _bigskip} @end tex @ifdocbook @multitable @columnfractions .20 .20 .60 @headitem You type @tab @code{sub()} sees @tab @code{sub()} generates @item @code{\\\\\\&} @tab @code{\\\&} @tab A literal @samp{\&} @item @code{\\\\&} @tab @code{\\&} @tab A literal @samp{\}, followed by the matched text @item @code{\\&} @tab @code{\&} @tab A literal @samp{&} @item @code{\\q} @tab @code{\q} @tab A literal @samp{\q} @item @code{\\\\} @tab @code{\\} @tab @code{\} @end multitable @end ifdocbook @ifnottex @ifnotdocbook @display You type @code{sub()} sees @code{sub()} generates -------- ---------- --------------- @code{\\\\\\&} @code{\\\&} A literal @samp{\&} @code{\\\\&} @code{\\&} A literal @samp{\}, followed by the matched text @code{\\&} @code{\&} A literal @samp{&} @code{\\q} @code{\q} A literal @samp{\q} @code{\\\\} @code{\\} @code{\} @end display @end ifnotdocbook @end ifnottex @end float The only case where the difference is noticeable is the last one: @samp{\\\\} is seen as @samp{\\} and produces @samp{\} instead of @samp{\\}. Starting with @value{PVERSION} 3.1.4, @command{gawk} followed the POSIX rules when @option{--posix} was specified (@pxref{Options}). Otherwise, it continued to follow the proposed rules, as that had been its behavior for many years. When @value{PVERSION} 4.0.0 was released, the @command{gawk} maintainer made the POSIX rules the default, breaking well over a decade's worth of backward compatibility.@footnote{This was rather naive of him, despite there being a note in this @value{SECTION} indicating that the next major version would move to the POSIX rules.} Needless to say, this was a bad idea, and as of @value{PVERSION} 4.0.1, @command{gawk} resumed its historical behavior, and only follows the POSIX rules when @option{--posix} is given. The rules for @code{gensub()} are considerably simpler. At the runtime level, whenever @command{gawk} sees a @samp{\}, if the following character is a digit, then the text that matched the corresponding parenthesized subexpression is placed in the generated output. Otherwise, no matter what character follows the @samp{\}, it appears in the generated text and the @samp{\} does not, as shown in @ref{table-gensub-escapes}. @float Table,table-gensub-escapes @caption{Escape sequence processing for @code{gensub()}} @tex \vbox{\bigskip % We need more characters for escape and tab ... \catcode`_ = 0 \catcode`! = 4 % ... since this table has lots of &'s and \'s, so we unspecialize them. \catcode`\& = \other \catcode`\\ = \other _halign{_hfil#!_qquad_hfil#!_qquad#_hfil_cr You type!@code{gensub()} sees!@code{gensub()} generates_cr _hrulefill!_hrulefill!_hrulefill_cr @code{&}! @code{&}!The matched text_cr @code{\\&}! @code{\&}!A literal @samp{&}_cr @code{\\\\}! @code{\\}!A literal @samp{\}_cr @code{\\\\&}! @code{\\&}!A literal @samp{\}, then the matched text_cr @code{\\\\\\&}! @code{\\\&}!A literal @samp{\&}_cr @code{\\q}! @code{\q}!A literal @samp{q}_cr } _bigskip} @end tex @ifdocbook @multitable @columnfractions .20 .20 .60 @headitem You type @tab @code{gensub()} sees @tab @code{gensub()} generates @item @code{&} @tab @code{&} @tab The matched text @item @code{\\&} @tab @code{\&} @tab A literal @samp{&} @item @code{\\\\} @tab @code{\\} @tab A literal @samp{\} @item @code{\\\\&} @tab @code{\\&} @tab A literal @samp{\}, then the matched text @item @code{\\\\\\&} @tab @code{\\\&} @tab A literal @samp{\&} @item @code{\\q} @tab @code{\q} @tab A literal @samp{q} @end multitable @end ifdocbook @ifnottex @ifnotdocbook @display You type @code{gensub()} sees @code{gensub()} generates -------- ------------- ------------------ @code{&} @code{&} The matched text @code{\\&} @code{\&} A literal @samp{&} @code{\\\\} @code{\\} A literal @samp{\} @code{\\\\&} @code{\\&} A literal @samp{\}, then the matched text @code{\\\\\\&} @code{\\\&} A literal @samp{\&} @code{\\q} @code{\q} A literal @samp{q} @end display @end ifnotdocbook @end ifnottex @end float Because of the complexity of the lexical- and runtime-level processing and the special cases for @code{sub()} and @code{gsub()}, we recommend the use of @command{gawk} and @code{gensub()} when you have to do substitutions. @node I/O Functions @subsection Input/Output Functions @cindex input/output @subentry functions The following functions relate to input/output (I/O). Optional parameters are enclosed in square brackets ([ ]): @table @asis @item @code{close(}@var{filename} [@code{,} @var{how}]@code{)} @cindexawkfunc{close} @cindex files @subentry closing @cindex close file or coprocess Close the file @var{filename} for input or output. Alternatively, the argument may be a shell command that was used for creating a coprocess, or for redirecting to or from a pipe; then the coprocess or pipe is closed. @xref{Close Files And Pipes} for more information. When closing a coprocess, it is occasionally useful to first close one end of the two-way pipe and then to close the other. This is done by providing a second argument to @code{close()}. This second argument (@var{how}) should be one of the two string values @code{"to"} or @code{"from"}, indicating which end of the pipe to close. Case in the string does not matter. @xref{Two-way I/O}, which discusses this feature in more detail and gives an example. Note that the second argument to @code{close()} is a @command{gawk} extension; it is not available in compatibility mode (@pxref{Options}). @item @code{fflush(}[@var{filename}]@code{)} @cindexawkfunc{fflush} @cindex flush buffered output Flush any buffered output associated with @var{filename}, which is either a file opened for writing or a shell command for redirecting output to a pipe or coprocess. @cindex buffers @subentry flushing @cindex output @subentry buffering Many utility programs @dfn{buffer} their output (i.e., they save information to write to a disk file or the screen in memory until there is enough for it to be worthwhile to send the data to the output device). This is often more efficient than writing every little bit of information as soon as it is ready. However, sometimes it is necessary to force a program to @dfn{flush} its buffers (i.e., write the information to its destination, even if a buffer is not full). This is the purpose of the @code{fflush()} function---@command{gawk} also buffers its output, and the @code{fflush()} function forces @command{gawk} to flush its buffers. @cindex extensions @subentry common @subentry @code{fflush()} function @cindex Brian Kernighan's @command{awk} Brian Kernighan added @code{fflush()} to his @command{awk} in April 1992. For two decades, it was a common extension. In December 2012, it was accepted for inclusion into the POSIX standard. See @uref{http://austingroupbugs.net/view.php?id=634, the Austin Group website}. POSIX standardizes @code{fflush()} as follows: if there is no argument, or if the argument is the null string (@w{@code{""}}), then @command{awk} flushes the buffers for @emph{all} open output files and pipes. @quotation NOTE Prior to @value{PVERSION} 4.0.2, @command{gawk} would flush only the standard output if there was no argument, and flush all output files and pipes if the argument was the null string. This was changed in order to be compatible with Brian Kernighan's @command{awk}, in the hope that standardizing this feature in POSIX would then be easier (which indeed proved to be the case). With @command{gawk}, you can use @samp{fflush("/dev/stdout")} if you wish to flush only the standard output. @end quotation @c @cindex automatic warnings @c @cindex warnings, automatic @cindex troubleshooting @subentry @code{fflush()} function @code{fflush()} returns zero if the buffer is successfully flushed; otherwise, it returns a nonzero value. (@command{gawk} returns @minus{}1.) In the case where all buffers are flushed, the return value is zero only if all buffers were flushed successfully. Otherwise, it is @minus{}1, and @command{gawk} warns about the problem @var{filename}. @command{gawk} also issues a warning message if you attempt to flush a file or pipe that was opened for reading (such as with @code{getline}), or if @var{filename} is not an open file, pipe, or coprocess. In such a case, @code{fflush()} returns @minus{}1, as well. @c end the table to let the sidebar take up the full width of the page. @end table @cindex sidebar @subentry Interactive Versus Noninteractive Buffering @ifdocbook @docbook Interactive Versus Noninteractive Buffering @end docbook @cindex buffering @subentry interactive vs.@: noninteractive As a side point, buffering issues can be even more confusing if your program is @dfn{interactive} (i.e., communicating with a user sitting at a keyboard).@footnote{A program is interactive if the standard output is connected to a terminal device. On modern systems, this means your keyboard and screen.} @c Thanks to Walter.Mecky@dresdnerbank.de for this example, and for @c motivating me to write this section. Interactive programs generally @dfn{line buffer} their output (i.e., they write out every line). Noninteractive programs wait until they have a full buffer, which may be many lines of output. Here is an example of the difference: @example $ @kbd{awk '@{ print $1 + $2 @}'} @kbd{1 1} @print{} 2 @kbd{2 3} @print{} 5 @kbd{Ctrl-d} @end example @noindent Each line of output is printed immediately. Compare that behavior with this example: @example $ @kbd{awk '@{ print $1 + $2 @}' | cat} @kbd{1 1} @kbd{2 3} @kbd{Ctrl-d} @print{} 2 @print{} 5 @end example @noindent Here, no output is printed until after the @kbd{Ctrl-d} is typed, because it is all buffered and sent down the pipe to @command{cat} in one shot. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Interactive Versus Noninteractive Buffering} @cindex buffering @subentry interactive vs.@: noninteractive As a side point, buffering issues can be even more confusing if your program is @dfn{interactive} (i.e., communicating with a user sitting at a keyboard).@footnote{A program is interactive if the standard output is connected to a terminal device. On modern systems, this means your keyboard and screen.} @c Thanks to Walter.Mecky@dresdnerbank.de for this example, and for @c motivating me to write this section. Interactive programs generally @dfn{line buffer} their output (i.e., they write out every line). Noninteractive programs wait until they have a full buffer, which may be many lines of output. Here is an example of the difference: @example $ @kbd{awk '@{ print $1 + $2 @}'} @kbd{1 1} @print{} 2 @kbd{2 3} @print{} 5 @kbd{Ctrl-d} @end example @noindent Each line of output is printed immediately. Compare that behavior with this example: @example $ @kbd{awk '@{ print $1 + $2 @}' | cat} @kbd{1 1} @kbd{2 3} @kbd{Ctrl-d} @print{} 2 @print{} 5 @end example @noindent Here, no output is printed until after the @kbd{Ctrl-d} is typed, because it is all buffered and sent down the pipe to @command{cat} in one shot. @end cartouche @end ifnotdocbook @table @asis @item @code{system(@var{command})} @cindexawkfunc{system} @cindex invoke shell command @cindex interacting with other programs Execute the operating system command @var{command} and then return to the @command{awk} program. Return @var{command}'s exit status (see further on). For example, if the following fragment of code is put in your @command{awk} program: @example END @{ system("date | mail -s 'awk run done' root") @} @end example @noindent the system administrator is sent mail when the @command{awk} program finishes processing input and begins its end-of-input processing. Note that redirecting @code{print} or @code{printf} into a pipe is often enough to accomplish your task. If you need to run many commands, it is more efficient to simply print them down a pipeline to the shell: @example while (@var{more stuff to do}) print @var{command} | "/bin/sh" close("/bin/sh") @end example @noindent @cindex troubleshooting @subentry @code{system()} function @cindex @option{--sandbox} option @subentry disabling @code{system()} function However, if your @command{awk} program is interactive, @code{system()} is useful for running large self-contained programs, such as a shell or an editor. Some operating systems cannot implement the @code{system()} function. @code{system()} causes a fatal error if it is not supported. @quotation NOTE When @option{--sandbox} is specified, the @code{system()} function is disabled (@pxref{Options}). @end quotation On POSIX systems, a command's exit status is a 16-bit number. The exit value passed to the C @code{exit()} function is held in the high-order eight bits. The low-order bits indicate if the process was killed by a signal (bit 7) and if so, the guilty signal number (bits 0--6). Traditionally, @command{awk}'s @code{system()} function has simply returned the exit status value divided by 256. In the normal case this gives the exit status but in the case of death-by-signal it yields a fractional floating-point value.@footnote{In private correspondence, Dr.@: Kernighan has indicated to me that the way this was done was probably a mistake.} POSIX states that @command{awk}'s @code{system()} should return the full 16-bit value. @command{gawk} steers a middle ground. The return values are summarized in @ref{table-system-return-values}. @float Table,table-system-return-values @caption{Return values from @code{system()}} @multitable @columnfractions .40 .60 @headitem Situation @tab Return value from @code{system()} @item @option{--traditional} @tab C @code{system()}'s value divided by 256 @item @option{--posix} @tab C @code{system()}'s value @item Normal exit of command @tab Command's exit status @item Death by signal of command @tab 256 + number of murderous signal @item Death by signal of command with core dump @tab 512 + number of murderous signal @item Some kind of error @tab @minus{}1 @end multitable @end float @end table As of August, 2018, BWK @command{awk} now follows @command{gawk}'s behavior for the return value of @code{system()}. @cindex sidebar @subentry Controlling Output Buffering with @code{system()} @ifdocbook @docbook Controlling Output Buffering with @code{system()} @end docbook @cindex buffers @subentry flushing @cindex buffering @subentry input/output @cindex output @subentry buffering The @code{fflush()} function provides explicit control over output buffering for individual files and pipes. However, its use is not portable to many older @command{awk} implementations. An alternative method to flush output buffers is to call @code{system()} with a null string as its argument: @example system("") # flush output @end example @noindent @command{gawk} treats this use of the @code{system()} function as a special case and is smart enough not to run a shell (or other command interpreter) with the empty command. Therefore, with @command{gawk}, this idiom is not only useful, it is also efficient. Although this method should work with other @command{awk} implementations, it does not necessarily avoid starting an unnecessary shell. (Other implementations may only flush the buffer associated with the standard output and not necessarily all buffered output.) If you think about what a programmer expects, it makes sense that @code{system()} should flush any pending output. The following program: @example BEGIN @{ print "first print" system("echo system echo") print "second print" @} @end example @noindent must print: @example first print system echo second print @end example @noindent and not: @example system echo first print second print @end example If @command{awk} did not flush its buffers before calling @code{system()}, you would see the latter (undesirable) output. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Controlling Output Buffering with @code{system()}} @cindex buffers @subentry flushing @cindex buffering @subentry input/output @cindex output @subentry buffering The @code{fflush()} function provides explicit control over output buffering for individual files and pipes. However, its use is not portable to many older @command{awk} implementations. An alternative method to flush output buffers is to call @code{system()} with a null string as its argument: @example system("") # flush output @end example @noindent @command{gawk} treats this use of the @code{system()} function as a special case and is smart enough not to run a shell (or other command interpreter) with the empty command. Therefore, with @command{gawk}, this idiom is not only useful, it is also efficient. Although this method should work with other @command{awk} implementations, it does not necessarily avoid starting an unnecessary shell. (Other implementations may only flush the buffer associated with the standard output and not necessarily all buffered output.) If you think about what a programmer expects, it makes sense that @code{system()} should flush any pending output. The following program: @example BEGIN @{ print "first print" system("echo system echo") print "second print" @} @end example @noindent must print: @example first print system echo second print @end example @noindent and not: @example system echo first print second print @end example If @command{awk} did not flush its buffers before calling @code{system()}, you would see the latter (undesirable) output. @end cartouche @end ifnotdocbook @node Time Functions @subsection Time Functions @cindex time functions @cindex timestamps @cindex log files, timestamps in @cindex files @subentry log, timestamps in @cindex @command{gawk} @subentry timestamps @cindex POSIX @command{awk} @subentry timestamps and @command{awk} programs are commonly used to process log files containing timestamp information, indicating when a particular log record was written. Many programs log their timestamps in the form returned by the @code{time()} system call, which is the number of seconds since a particular epoch. On POSIX-compliant systems, it is the number of seconds since 1970-01-01 00:00:00 UTC, not counting leap @ifclear FOR_PRINT seconds.@footnote{@xref{Glossary}, especially the entries ``Epoch'' and ``UTC.''} @end ifclear @ifset FOR_PRINT seconds. @end ifset All known POSIX-compliant systems support timestamps from 0 through @iftex @math{2^{31} - 1}, @end iftex @ifinfo 2^31 - 1, @end ifinfo @ifnottex @ifnotinfo 2@sup{31} @minus{} 1, @end ifnotinfo @end ifnottex which is sufficient to represent times through 2038-01-19 03:14:07 UTC. Many systems support a wider range of timestamps, including negative timestamps that represent times before the epoch. @cindex @command{date} utility @subentry GNU @cindex time @subentry retrieving In order to make it easier to process such log files and to produce useful reports, @command{gawk} provides the following functions for working with timestamps. They are @command{gawk} extensions; they are not specified in the POSIX standard.@footnote{The GNU @command{date} utility can also do many of the things described here. Its use may be preferable for simple time-related operations in shell scripts.} However, recent versions of @command{mawk} (@pxref{Other Versions}) also support these functions. Optional parameters are enclosed in square brackets ([ ]): @c @asis for docbook @table @asis @item @code{mktime(@var{datespec}} [@code{, @var{utc-flag}} ]@code{)} @cindexgawkfunc{mktime} @cindex generate time values Turn @var{datespec} into a timestamp in the same form as is returned by @code{systime()}. It is similar to the function of the same name in ISO C. The argument, @var{datespec}, is a string of the form @w{@code{"@var{YYYY} @var{MM} @var{DD} @var{HH} @var{MM} @var{SS} [@var{DST}]"}}. The string consists of six or seven numbers representing, respectively, the full year including century, the month from 1 to 12, the day of the month from 1 to 31, the hour of the day from 0 to 23, the minute from 0 to 59, the second from 0 to 60,@footnote{Occasionally there are minutes in a year with a leap second, which is why the seconds can go up to 60.} and an optional daylight-savings flag. The values of these numbers need not be within the ranges specified; for example, an hour of @minus{}1 means 1 hour before midnight. The origin-zero Gregorian calendar is assumed, with year 0 preceding year 1 and year @minus{}1 preceding year 0. If @var{utc-flag} is present and is either nonzero or non-null, the time is assumed to be in the UTC time zone; otherwise, the time is assumed to be in the local time zone. If the @var{DST} daylight-savings flag is positive, the time is assumed to be daylight savings time; if zero, the time is assumed to be standard time; and if negative (the default), @code{mktime()} attempts to determine whether daylight savings time is in effect for the specified time. If @var{datespec} does not contain enough elements or if the resulting time is out of range, @code{mktime()} returns @minus{}1. @cindex @command{gawk} @subentry @code{PROCINFO} array in @cindex @code{PROCINFO} array @item @code{strftime(}[@var{format} [@code{,} @var{timestamp} [@code{,} @var{utc-flag}] ] ]@code{)} @cindexgawkfunc{strftime} @cindex format time string Format the time specified by @var{timestamp} based on the contents of the @var{format} string and return the result. It is similar to the function of the same name in ISO C. If @var{utc-flag} is present and is either nonzero or non-null, the value is formatted as UTC (Coordinated Universal Time, formerly GMT or Greenwich Mean Time). Otherwise, the value is formatted for the local time zone. The @var{timestamp} is in the same format as the value returned by the @code{systime()} function. If no @var{timestamp} argument is supplied, @command{gawk} uses the current time of day as the timestamp. Without a @var{format} argument, @code{strftime()} uses the value of @code{PROCINFO["strftime"]} as the format string (@pxref{Built-in Variables}). The default string value is @code{@w{"%a %b %e %H:%M:%S %Z %Y"}}. This format string produces output that is equivalent to that of the @command{date} utility. You can assign a new value to @code{PROCINFO["strftime"]} to change the default format; see the following list for the various format directives. @item @code{systime()} @cindexgawkfunc{systime} @cindex timestamps @cindex current system time Return the current time as the number of seconds since the system epoch. On POSIX systems, this is the number of seconds since 1970-01-01 00:00:00 UTC, not counting leap seconds. It may be a different number on other systems. @end table The @code{systime()} function allows you to compare a timestamp from a log file with the current time of day. In particular, it is easy to determine how long ago a particular record was logged. It also allows you to produce log records using the ``seconds since the epoch'' format. @cindex converting @subentry dates to timestamps @cindex dates @subentry converting to timestamps @cindex timestamps @subentry converting dates to The @code{mktime()} function allows you to convert a textual representation of a date and time into a timestamp. This makes it easy to do before/after comparisons of dates and times, particularly when dealing with date and time data coming from an external source, such as a log file. The @code{strftime()} function allows you to easily turn a timestamp into human-readable information. It is similar in nature to the @code{sprintf()} function (@pxref{String Functions}), in that it copies nonformat specification characters verbatim to the returned string, while substituting date and time values for format specifications in the @var{format} string. @cindex format specifiers @subentry @code{strftime()} function (@command{gawk}) @code{strftime()} is guaranteed by the 1999 ISO C standard@footnote{Unfortunately, not every system's @code{strftime()} necessarily supports all of the conversions listed here.} to support the following date format specifications: @table @code @item %a The locale's abbreviated weekday name. @item %A The locale's full weekday name. @item %b The locale's abbreviated month name. @item %B The locale's full month name. @item %c The locale's ``appropriate'' date and time representation. (This is @samp{%A %B %d %T %Y} in the @code{"C"} locale.) @item %C The century part of the current year. This is the year divided by 100 and truncated to the next lower integer. @item %d The day of the month as a decimal number (01--31). @item %D Equivalent to specifying @samp{%m/%d/%y}. @item %e The day of the month, padded with a space if it is only one digit. @item %F Equivalent to specifying @samp{%Y-%m-%d}. This is the ISO 8601 date format. @item %g The year modulo 100 of the ISO 8601 week number, as a decimal number (00--99). For example, January 1, 2012, is in week 53 of 2011. Thus, the year of its ISO 8601 week number is 2011, even though its year is 2012. Similarly, December 31, 2012, is in week 1 of 2013. Thus, the year of its ISO week number is 2013, even though its year is 2012. @item %G The full year of the ISO week number, as a decimal number. @item %h Equivalent to @samp{%b}. @item %H The hour (24-hour clock) as a decimal number (00--23). @item %I The hour (12-hour clock) as a decimal number (01--12). @item %j The day of the year as a decimal number (001--366). @item %m The month as a decimal number (01--12). @item %M The minute as a decimal number (00--59). @item %n A newline character (ASCII LF). @item %p The locale's equivalent of the AM/PM designations associated with a 12-hour clock. @item %r The locale's 12-hour clock time. (This is @samp{%I:%M:%S %p} in the @code{"C"} locale.) @item %R Equivalent to specifying @samp{%H:%M}. @item %S The second as a decimal number (00--60). @item %t A TAB character. @item %T Equivalent to specifying @samp{%H:%M:%S}. @item %u The weekday as a decimal number (1--7). Monday is day one. @item %U The week number of the year (with the first Sunday as the first day of week one) as a decimal number (00--53). @cindex ISO @subentry ISO 8601 date and time standard @item %V The week number of the year (with the first Monday as the first day of week one) as a decimal number (01--53). The method for determining the week number is as specified by ISO 8601. (To wit: if the week containing January 1 has four or more days in the new year, then it is week one; otherwise it is the last week [52 or 53] of the previous year and the next week is week one.) @item %w The weekday as a decimal number (0--6). Sunday is day zero. @item %W The week number of the year (with the first Monday as the first day of week one) as a decimal number (00--53). @item %x The locale's ``appropriate'' date representation. (This is @samp{%A %B %d %Y} in the @code{"C"} locale.) @item %X The locale's ``appropriate'' time representation. (This is @samp{%T} in the @code{"C"} locale.) @item %y The year modulo 100 as a decimal number (00--99). @item %Y The full year as a decimal number (e.g., 2015). @c @cindex RFC 822 @c @cindex RFC 1036 @item %z The time zone offset in a @samp{+@var{HHMM}} format (e.g., the format necessary to produce RFC 822/RFC 1036 date headers). @item %Z The time zone name or abbreviation; no characters if no time zone is determinable. @item %Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH @itemx %OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy ``Alternative representations'' for the specifications that use only the second letter (@samp{%c}, @samp{%C}, and so on).@footnote{If you don't understand any of this, don't worry about it; these facilities are meant to make it easier to ``internationalize'' programs. Other internationalization features are described in @ref{Internationalization}.} (These facilitate compliance with the POSIX @command{date} utility.) @item %% A literal @samp{%}. @end table If a conversion specifier is not one of those just listed, the behavior is undefined.@footnote{This is because ISO C leaves the behavior of the C version of @code{strftime()} undefined and @command{gawk} uses the system's version of @code{strftime()} if it's there. Typically, the conversion specifier either does not appear in the returned string or appears literally.} For systems that are not yet fully standards-compliant, @command{gawk} supplies a copy of @code{strftime()} from the GNU C Library. It supports all of the just-listed format specifications. If that version is used to compile @command{gawk} (@pxref{Installation}), then the following additional format specifications are available: @table @code @item %k The hour (24-hour clock) as a decimal number (0--23). Single-digit numbers are padded with a space. @item %l The hour (12-hour clock) as a decimal number (1--12). Single-digit numbers are padded with a space. @ignore @item %N The ``Emperor/Era'' name. Equivalent to @samp{%C}. @item %o The ``Emperor/Era'' year. Equivalent to @samp{%y}. @end ignore @item %s The time as a decimal timestamp in seconds since the epoch. @ignore @item %v The date in VMS format (e.g., @samp{20-JUN-1991}). @end ignore @end table Additionally, the alternative representations are recognized but their normal representations are used. @cindex @code{date} utility @subentry POSIX @cindex POSIX @command{awk} @subentry @code{date} utility and The following example is an @command{awk} implementation of the POSIX @command{date} utility. Normally, the @command{date} utility prints the current date and time of day in a well-known format. However, if you provide an argument to it that begins with a @samp{+}, @command{date} copies nonformat specifier characters to the standard output and interprets the current time according to the format specifiers in the string. For example: @example $ @kbd{date '+Today is %A, %B %d, %Y.'} @print{} Today is Monday, September 22, 2014. @end example Here is the @command{gawk} version of the @command{date} utility. It has a shell ``wrapper'' to handle the @option{-u} option, which requires that @command{date} run as if the time zone is set to UTC: @example #! /bin/sh # # date --- approximate the POSIX 'date' command case $1 in -u) TZ=UTC0 # use UTC export TZ shift ;; esac gawk 'BEGIN @{ format = PROCINFO["strftime"] exitval = 0 if (ARGC > 2) exitval = 1 else if (ARGC == 2) @{ format = ARGV[1] if (format ~ /^\+/) format = substr(format, 2) # remove leading + @} print strftime(format) exit exitval @}' "$@@" @end example @node Bitwise Functions @subsection Bit-Manipulation Functions @cindex bit-manipulation functions @cindex bitwise @subentry operations @cindex AND bitwise operation @cindex OR bitwise operation @cindex XOR bitwise operation @cindex operations, bitwise @quotation @i{I can explain it for you, but I can't understand it for you.} @author Anonymous @end quotation Many languages provide the ability to perform @dfn{bitwise} operations on two integer numbers. In other words, the operation is performed on each successive pair of bits in the operands. Three common operations are bitwise AND, OR, and XOR. The operations are described in @ref{table-bitwise-ops}. @c 11/2014: Postprocessing turns the docbook informaltable @c into a table. Hurray for scripting! @float Table,table-bitwise-ops @caption{Bitwise operations} @ifnottex @ifnotdocbook @verbatim Bit operator | AND | OR | XOR |---+---+---+---+---+--- Operands | 0 | 1 | 0 | 1 | 0 | 1 ----------+---+---+---+---+---+--- 0 | 0 0 | 0 1 | 0 1 1 | 0 1 | 1 1 | 1 0 @end verbatim @end ifnotdocbook @end ifnottex @tex \centerline{ \vbox{\bigskip % space above the table (about 1 linespace) % Because we have vertical rules, we can't let TeX insert interline space % in its usual way. \offinterlineskip \halign{\strut\hfil#\quad\hfil % operands &\vrule#&\quad#\quad % rule, 0 (of and) &\vrule#&\quad#\quad % rule, 1 (of and) &\vrule# % rule between and and or &\quad#\quad % 0 (of or) &\vrule#&\quad#\quad % rule, 1 (of of) &\vrule# % rule between or and xor &\quad#\quad % 0 of xor &\vrule#&\quad#\quad % rule, 1 of xor \cr &\omit&\multispan{11}\hfil\bf Bit operator\hfil\cr \noalign{\smallskip} & &\multispan3\hfil AND\hfil&&\multispan3\hfil OR\hfil &&\multispan3\hfil XOR\hfil\cr \bf Operands&&0&&1&&0&&1&&0&&1\cr \noalign{\hrule} \omit&height 2pt&&\omit&&&&\omit&&&&\omit\cr \noalign{\hrule height0pt}% without this the rule does not extend; why? 0&&0&\omit&0&&0&\omit&1&&0&\omit&1\cr 1&&0&\omit&1&&1&\omit&1&&1&\omit&0\cr }}} @end tex @docbook Bit operator AND OR XOR Operands 0 1 0 1 0 1 0 0 0 0 1 0 1 1 0 1 1 1 1 0 @end docbook @end float @cindex bitwise @subentry complement @cindex complement, bitwise As you can see, the result of an AND operation is 1 only when @emph{both} bits are 1. The result of an OR operation is 1 if @emph{either} bit is 1. The result of an XOR operation is 1 if either bit is 1, but not both. The next operation is the @dfn{complement}; the complement of 1 is 0 and the complement of 0 is 1. Thus, this operation ``flips'' all the bits of a given value. @cindex bitwise @subentry shift @cindex left shift, bitwise @cindex right shift, bitwise @cindex shift, bitwise Finally, two other common operations are to shift the bits left or right. For example, if you have a bit string @samp{10111001} and you shift it right by three bits, you end up with @samp{00010111}.@footnote{This example shows that zeros come in on the left side. For @command{gawk}, this is always true, but in some languages, it's possible to have the left side fill with ones.} If you start over again with @samp{10111001} and shift it left by three bits, you end up with @samp{11001000}. The following list describes @command{gawk}'s built-in functions that implement the bitwise operations. Optional parameters are enclosed in square brackets ([ ]): @cindex @command{gawk} @subentry bitwise operations in @table @asis @cindexgawkfunc{and} @cindex bitwise @subentry AND @item @code{and(}@var{v1}@code{,} @var{v2} [@code{,} @dots{}]@code{)} Return the bitwise AND of the arguments. There must be at least two. @cindexgawkfunc{compl} @cindex bitwise @subentry complement @item @code{compl(@var{val})} Return the bitwise complement of @var{val}. @cindexgawkfunc{lshift} @item @code{lshift(@var{val}, @var{count})} Return the value of @var{val}, shifted left by @var{count} bits. @cindexgawkfunc{or} @cindex bitwise @subentry OR @item @code{or(}@var{v1}@code{,} @var{v2} [@code{,} @dots{}]@code{)} Return the bitwise OR of the arguments. There must be at least two. @cindexgawkfunc{rshift} @item @code{rshift(@var{val}, @var{count})} Return the value of @var{val}, shifted right by @var{count} bits. @cindexgawkfunc{xor} @cindex bitwise @subentry XOR @item @code{xor(}@var{v1}@code{,} @var{v2} [@code{,} @dots{}]@code{)} Return the bitwise XOR of the arguments. There must be at least two. @end table @quotation CAUTION Beginning with @command{gawk} @value{PVERSION} 4.2, negative operands are not allowed for any of these functions. A negative operand produces a fatal error. See the sidebar ``Beware The Smoke and Mirrors!'' for more information as to why. @end quotation Here is a user-defined function (@pxref{User-defined}) that illustrates the use of these functions: @cindex @code{bits2str()} user-defined function @cindex user-defined @subentry function @subentry @code{bits2str()} @cindex @code{testbits.awk} program @example @group @c file eg/lib/bits2str.awk # bits2str --- turn an integer into readable ones and zeros function bits2str(bits, data, mask) @{ if (bits == 0) return "0" mask = 1 for (; bits != 0; bits = rshift(bits, 1)) data = (and(bits, mask) ? "1" : "0") data while ((length(data) % 8) != 0) data = "0" data return data @} @c endfile @end group @c this is a hack to make testbits.awk self-contained @ignore @c file eg/prog/testbits.awk # bits2str --- turn an integer into readable ones and zeros function bits2str(bits, data, mask) @{ if (bits == 0) return "0" mask = 1 for (; bits != 0; bits = rshift(bits, 1)) data = (and(bits, mask) ? "1" : "0") data while ((length(data) % 8) != 0) data = "0" data return data @} @c endfile @end ignore @c file eg/prog/testbits.awk BEGIN @{ printf "123 = %s\n", bits2str(123) printf "0123 = %s\n", bits2str(0123) printf "0x99 = %s\n", bits2str(0x99) comp = compl(0x99) printf "compl(0x99) = %#x = %s\n", comp, bits2str(comp) shift = lshift(0x99, 2) printf "lshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift) shift = rshift(0x99, 2) printf "rshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift) @} @c endfile @end example @noindent This program produces the following output when run: @example $ @kbd{gawk -f testbits.awk} @print{} 123 = 01111011 @print{} 0123 = 01010011 @print{} 0x99 = 10011001 @print{} compl(0x99) = 0x3fffffffffff66 = @print{} 00111111111111111111111111111111111111111111111101100110 @print{} lshift(0x99, 2) = 0x264 = 0000001001100100 @print{} rshift(0x99, 2) = 0x26 = 00100110 @end example @cindex converting @subentry string to numbers @cindex strings @subentry converting @cindex numbers @subentry converting @cindex converting @subentry numbers to strings @cindex numbers @subentry as string of bits The @code{bits2str()} function turns a binary number into a string. Initializing @code{mask} to one creates a binary value where the rightmost bit is set to one. Using this mask, the function repeatedly checks the rightmost bit. ANDing the mask with the value indicates whether the rightmost bit is one or not. If so, a @code{"1"} is concatenated onto the front of the string. Otherwise, a @code{"0"} is added. The value is then shifted right by one bit and the loop continues until there are no more one bits. If the initial value is zero, it returns a simple @code{"0"}. Otherwise, at the end, it pads the value with zeros to represent multiples of 8-bit quantities. This is typical in modern computers. The main code in the @code{BEGIN} rule shows the difference between the decimal and octal values for the same numbers (@pxref{Nondecimal-numbers}), and then demonstrates the results of the @code{compl()}, @code{lshift()}, and @code{rshift()} functions. @cindex sidebar @subentry Beware The Smoke and Mirrors! @ifdocbook @docbook Beware The Smoke and Mirrors! @end docbook It other languages, bitwise operations are performed on integer values, not floating-point values. As a general statement, such operations work best when performed on unsigned integers. @command{gawk} attempts to treat the arguments to the bitwise functions as unsigned integers. For this reason, negative arguments produce a fatal error. In normal operation, for all of these functions, first the double-precision floating-point value is converted to the widest C unsigned integer type, then the bitwise operation is performed. If the result cannot be represented exactly as a C @code{double}, leading nonzero bits are removed one by one until it can be represented exactly. The result is then converted back into a C @code{double}.@footnote{If you don't understand this paragraph, the upshot is that @command{gawk} can only store a particular range of integer values; numbers outside that range are reduced to fit within the range.} However, when using arbitrary precision arithmetic with the @option{-M} option (@pxref{Arbitrary Precision Arithmetic}), the results may differ. This is particularly noticeable with the @code{compl()} function: @example $ @kbd{gawk 'BEGIN @{ print compl(42) @}'} @print{} 9007199254740949 $ @kbd{gawk -M 'BEGIN @{ print compl(42) @}'} @print{} -43 @end example What's going on becomes clear when printing the results in hexadecimal: @example $ @kbd{gawk 'BEGIN @{ printf "%#x\n", compl(42) @}'} @print{} 0x1fffffffffffd5 $ @kbd{gawk -M 'BEGIN @{ printf "%#x\n", compl(42) @}'} @print{} 0xffffffffffffffd5 @end example When using the @option{-M} option, under the hood, @command{gawk} uses GNU MP arbitrary precision integers which have at least 64 bits of precision. When not using @option{-M}, @command{gawk} stores integral values in regular double-precision floating point, which only maintain 53 bits of precision. Furthermore, the GNU MP library treats (or at least seems to treat) the leading bit as a sign bit; thus the result with @option{-M} in this case is a negative number. In short, using @command{gawk} for any but the simplest kind of bitwise operations is probably a bad idea; caveat emptor! @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{Beware The Smoke and Mirrors!} It other languages, bitwise operations are performed on integer values, not floating-point values. As a general statement, such operations work best when performed on unsigned integers. @command{gawk} attempts to treat the arguments to the bitwise functions as unsigned integers. For this reason, negative arguments produce a fatal error. In normal operation, for all of these functions, first the double-precision floating-point value is converted to the widest C unsigned integer type, then the bitwise operation is performed. If the result cannot be represented exactly as a C @code{double}, leading nonzero bits are removed one by one until it can be represented exactly. The result is then converted back into a C @code{double}.@footnote{If you don't understand this paragraph, the upshot is that @command{gawk} can only store a particular range of integer values; numbers outside that range are reduced to fit within the range.} However, when using arbitrary precision arithmetic with the @option{-M} option (@pxref{Arbitrary Precision Arithmetic}), the results may differ. This is particularly noticeable with the @code{compl()} function: @example $ @kbd{gawk 'BEGIN @{ print compl(42) @}'} @print{} 9007199254740949 $ @kbd{gawk -M 'BEGIN @{ print compl(42) @}'} @print{} -43 @end example What's going on becomes clear when printing the results in hexadecimal: @example $ @kbd{gawk 'BEGIN @{ printf "%#x\n", compl(42) @}'} @print{} 0x1fffffffffffd5 $ @kbd{gawk -M 'BEGIN @{ printf "%#x\n", compl(42) @}'} @print{} 0xffffffffffffffd5 @end example When using the @option{-M} option, under the hood, @command{gawk} uses GNU MP arbitrary precision integers which have at least 64 bits of precision. When not using @option{-M}, @command{gawk} stores integral values in regular double-precision floating point, which only maintain 53 bits of precision. Furthermore, the GNU MP library treats (or at least seems to treat) the leading bit as a sign bit; thus the result with @option{-M} in this case is a negative number. In short, using @command{gawk} for any but the simplest kind of bitwise operations is probably a bad idea; caveat emptor! @end cartouche @end ifnotdocbook @node Type Functions @subsection Getting Type Information @command{gawk} provides two functions that let you distinguish the type of a variable. This is necessary for writing code that traverses every element of an array of arrays (@pxref{Arrays of Arrays}), and in other contexts. @table @code @cindexgawkfunc{isarray} @cindex scalar or array @item isarray(@var{x}) Return a true value if @var{x} is an array. Otherwise, return false. @cindexgawkfunc{typeof} @cindex variable type, @code{typeof()} function (@command{gawk}) @cindex type @subentry of variable, @code{typeof()} function (@command{gawk}) @item typeof(@var{x}) Return one of the following strings, depending upon the type of @var{x}: @c nested table @table @code @item "array" @var{x} is an array. @item "regexp" @var{x} is a strongly typed regexp (@pxref{Strong Regexp Constants}). @item "number" @var{x} is a number. @item "string" @var{x} is a string. @item "strnum" @var{x} is a number that started life as user input, such as a field or the result of calling @code{split()}. (I.e., @var{x} has the strnum attribute; @pxref{Variable Typing}.) @item "unassigned" @var{x} is a scalar variable that has not been assigned a value yet. For example: @example BEGIN @{ # creates a[1] but it has no assigned value a[1] print typeof(a[1]) # unassigned @} @end example @item "untyped" @var{x} has not yet been used yet at all; it can become a scalar or an array. The typing could even conceivably differ from run to run of the same program! For example: @example BEGIN @{ print "initially, typeof(v) = ", typeof(v) if ("FOO" in ENVIRON) make_scalar(v) else make_array(v) print "typeof(v) =", typeof(v) @} function make_scalar(p, l) @{ l = p @} function make_array(p) @{ p[1] = 1 @} @end example @end table @end table @code{isarray()} is meant for use in two circumstances. The first is when traversing a multidimensional array: you can test if an element is itself an array or not. The second is inside the body of a user-defined function (not discussed yet; @pxref{User-defined}), to test if a parameter is an array or not. @quotation NOTE While you can use @code{isarray()} at the global level to test variables, doing so makes no sense. Because @emph{you} are the one writing the program, @emph{you} are supposed to know if your variables are arrays or not. @end quotation The @code{typeof()} function is general; it allows you to determine if a variable or function parameter is a scalar (number, string, or strongly typed regexp) or an array. Normally, passing a variable that has never been used to a built-in function causes it to become a scalar variable (unassigned). However, @code{isarray()} and @code{typeof()} are different; they do not change their arguments from untyped to unassigned. @node I18N Functions @subsection String-Translation Functions @cindex @command{gawk} @subentry string-translation functions @cindex functions @subentry string-translation @cindex string-translation functions @cindex internationalization @cindex @command{awk} programs @subentry internationalizing @command{gawk} provides facilities for internationalizing @command{awk} programs. These include the functions described in the following list. The descriptions here are purposely brief. @xref{Internationalization}, for the full story. Optional parameters are enclosed in square brackets ([ ]): @table @asis @cindexgawkfunc{bindtextdomain} @cindex set directory of message catalogs @item @code{bindtextdomain(@var{directory}} [@code{,} @var{domain}]@code{)} Set the directory in which @command{gawk} will look for message translation files, in case they will not or cannot be placed in the ``standard'' locations (e.g., during testing). It returns the directory in which @var{domain} is ``bound.'' The default @var{domain} is the value of @code{TEXTDOMAIN}. If @var{directory} is the null string (@code{""}), then @code{bindtextdomain()} returns the current binding for the given @var{domain}. @cindexgawkfunc{dcgettext} @cindex translate string @item @code{dcgettext(@var{string}} [@code{,} @var{domain} [@code{,} @var{category}] ]@code{)} Return the translation of @var{string} in text domain @var{domain} for locale category @var{category}. The default value for @var{domain} is the current value of @code{TEXTDOMAIN}. The default value for @var{category} is @code{"LC_MESSAGES"}. @cindexgawkfunc{dcngettext} @item @code{dcngettext(@var{string1}, @var{string2}, @var{number}} [@code{,} @var{domain} [@code{,} @var{category}] ]@code{)} Return the plural form used for @var{number} of the translation of @var{string1} and @var{string2} in text domain @var{domain} for locale category @var{category}. @var{string1} is the English singular variant of a message, and @var{string2} is the English plural variant of the same message. The default value for @var{domain} is the current value of @code{TEXTDOMAIN}. The default value for @var{category} is @code{"LC_MESSAGES"}. @end table @node User-defined @section User-Defined Functions @cindex user-defined @subentry functions @cindex functions @subentry user-defined Complicated @command{awk} programs can often be simplified by defining your own functions. User-defined functions can be called just like built-in ones (@pxref{Function Calls}), but it is up to you to define them (i.e., to tell @command{awk} what they should do). @menu * Definition Syntax:: How to write definitions and what they mean. * Function Example:: An example function definition and what it does. * Function Calling:: Calling user-defined functions. * Return Statement:: Specifying the value a function returns. * Dynamic Typing:: How variable types can change at runtime. @end menu @node Definition Syntax @subsection Function Definition Syntax @quotation @i{It's entirely fair to say that the awk syntax for local variable definitions is appallingly awful.} @author Brian Kernighan @end quotation @cindex functions @subentry defining Definitions of functions can appear anywhere between the rules of an @command{awk} program. Thus, the general form of an @command{awk} program is extended to include sequences of rules @emph{and} user-defined function definitions. There is no need to put the definition of a function before all uses of the function. This is because @command{awk} reads the entire program before starting to execute any of it. The definition of a function named @var{name} looks like this: @display @group @code{function} @var{name}@code{(}[@var{parameter-list}]@code{)} @code{@{} @var{body-of-function} @code{@}} @end group @end display @cindex names @subentry functions @cindex functions @subentry names of @cindex naming issues @subentry functions @noindent Here, @var{name} is the name of the function to define. A valid function name is like a valid variable name: a sequence of letters, digits, and underscores that doesn't start with a digit. Here too, only the 52 upper- and lowercase English letters may be used in a function name. Within a single @command{awk} program, any particular name can only be used as a variable, array, or function. @var{parameter-list} is an optional list of the function's arguments and local variable names, separated by commas. When the function is called, the argument names are used to hold the argument values given in the call. A function cannot have two parameters with the same name, nor may it have a parameter with the same name as the function itself. @quotation CAUTION According to the POSIX standard, function parameters cannot have the same name as one of the special predefined variables (@pxref{Built-in Variables}), nor may a function parameter have the same name as another function. Not all versions of @command{awk} enforce these restrictions. @command{gawk} always enforces the first restriction. With @option{--posix} (@pxref{Options}), it also enforces the second restriction. @end quotation Local variables act like the empty string if referenced where a string value is required, and like zero if referenced where a numeric value is required. This is the same as the behavior of regular variables that have never been assigned a value. (There is more to understand about local variables; @pxref{Dynamic Typing}.) The @var{body-of-function} consists of @command{awk} statements. It is the most important part of the definition, because it says what the function should actually @emph{do}. The argument names exist to give the body a way to talk about the arguments; local variables exist to give the body places to keep temporary values. Argument names are not distinguished syntactically from local variable names. Instead, the number of arguments supplied when the function is called determines how many argument variables there are. Thus, if three argument values are given, the first three names in @var{parameter-list} are arguments and the rest are local variables. It follows that if the number of arguments is not the same in all calls to the function, some of the names in @var{parameter-list} may be arguments on some occasions and local variables on others. Another way to think of this is that omitted arguments default to the null string. @cindex programming conventions @subentry functions @subentry writing Usually when you write a function, you know how many names you intend to use for arguments and how many you intend to use as local variables. It is conventional to place some extra space between the arguments and the local variables, in order to document how your function is supposed to be used. @cindex variables @subentry shadowing @cindex shadowing of variable values During execution of the function body, the arguments and local variable values hide, or @dfn{shadow}, any variables of the same names used in the rest of the program. The shadowed variables are not accessible in the function definition, because there is no way to name them while their names have been taken away for the arguments and local variables. All other variables used in the @command{awk} program can be referenced or set normally in the function's body. The arguments and local variables last only as long as the function body is executing. Once the body finishes, you can once again access the variables that were shadowed while the function was running. @cindex recursive functions @cindex functions @subentry recursive The function body can contain expressions that call functions. They can even call this function, either directly or by way of another function. When this happens, we say the function is @dfn{recursive}. The act of a function calling itself is called @dfn{recursion}. All the built-in functions return a value to their caller. User-defined functions can do so also, using the @code{return} statement, which is described in detail in @ref{Return Statement}. Many of the subsequent examples in this @value{SECTION} use the @code{return} statement. @cindex common extensions @subentry @code{func} keyword @cindex extensions @subentry common @subentry @code{func} keyword @c @cindex POSIX @command{awk} @cindex @command{awk} @subentry language, POSIX version @cindex POSIX @command{awk} @subentry @code{function} keyword in In many @command{awk} implementations, including @command{gawk}, the keyword @code{function} may be abbreviated @code{func}. @value{COMMONEXT} However, POSIX only specifies the use of the keyword @code{function}. This actually has some practical implications. If @command{gawk} is in POSIX-compatibility mode (@pxref{Options}), then the following statement does @emph{not} define a function: @example func foo() @{ a = sqrt($1) ; print a @} @end example @noindent Instead, it defines a rule that, for each record, concatenates the value of the variable @samp{func} with the return value of the function @samp{foo}. If the resulting string is non-null, the action is executed. This is probably not what is desired. (@command{awk} accepts this input as syntactically valid, because functions may be used before they are defined in @command{awk} programs.@footnote{This program won't actually run, because @code{foo()} is undefined.}) @cindex portability @subentry functions, defining To ensure that your @command{awk} programs are portable, always use the keyword @code{function} when defining a function. @node Function Example @subsection Function Definition Examples @cindex function definition example Here is an example of a user-defined function, called @code{myprint()}, that takes a number and prints it in a specific format: @example function myprint(num) @{ printf "%6.3g\n", num @} @end example @noindent To illustrate, here is an @command{awk} rule that uses our @code{myprint()} function: @example $3 > 0 @{ myprint($3) @} @end example @noindent This program prints, in our special format, all the third fields that contain a positive number in our input. Therefore, when given the following input: @example 1.2 3.4 5.6 7.8 9.10 11.12 -13.14 15.16 17.18 19.20 21.22 23.24 @end example @noindent this program, using our function to format the results, prints: @example 5.6 21.2 @end example This function deletes all the elements in an array (recall that the extra whitespace signifies the start of the local variable list): @example @group function delarray(a, i) @{ for (i in a) delete a[i] @} @end group @end example When working with arrays, it is often necessary to delete all the elements in an array and start over with a new list of elements (@pxref{Delete}). Instead of having to repeat this loop everywhere that you need to clear out an array, your program can just call @code{delarray()}. (This guarantees portability. The use of @samp{delete @var{array}} to delete the contents of an entire array is a relatively recent@footnote{Late in 2012.} addition to the POSIX standard.) The following is an example of a recursive function. It takes a string as an input parameter and returns the string in reverse order. Recursive functions must always have a test that stops the recursion. In this case, the recursion terminates when the input string is already empty: @c 8/2014: Thanks to Mike Brennan for the improved formulation @cindex @code{rev()} user-defined function @cindex user-defined @subentry function @subentry @code{rev()} @example function rev(str) @{ if (str == "") return "" return (rev(substr(str, 2)) substr(str, 1, 1)) @} @end example If this function is in a file named @file{rev.awk}, it can be tested this way: @example $ @kbd{echo "Don't Panic!" |} > @kbd{gawk -e '@{ print rev($0) @}' -f rev.awk} @print{} !cinaP t'noD @end example The C @code{ctime()} function takes a timestamp and returns it as a string, formatted in a well-known fashion. The following example uses the built-in @code{strftime()} function (@pxref{Time Functions}) to create an @command{awk} version of @code{ctime()}: @cindex @code{ctime()} user-defined function @cindex user-defined @subentry function @subentry @code{ctime()} @example @c file eg/lib/ctime.awk # ctime.awk # # awk version of C ctime(3) function function ctime(ts, format) @{ format = "%a %b %e %H:%M:%S %Z %Y" if (ts == 0) ts = systime() # use current time as default return strftime(format, ts) @} @c endfile @end example You might think that @code{ctime()} could use @code{PROCINFO["strftime"]} for its format string. That would be a mistake, because @code{ctime()} is supposed to return the time formatted in a standard fashion, and user-level code could have changed @code{PROCINFO["strftime"]}. @node Function Calling @subsection Calling User-Defined Functions @cindex functions @subentry user-defined @subentry calling @dfn{Calling a function} means causing the function to run and do its job. A function call is an expression and its value is the value returned by the function. @menu * Calling A Function:: Don't use spaces. * Variable Scope:: Controlling variable scope. * Pass By Value/Reference:: Passing parameters. * Function Caveats:: Other points to know about functions. @end menu @node Calling A Function @subsubsection Writing a Function Call A function call consists of the function name followed by the arguments in parentheses. @command{awk} expressions are what you write in the call for the arguments. Each time the call is executed, these expressions are evaluated, and the values become the actual arguments. For example, here is a call to @code{foo()} with three arguments (the first being a string concatenation): @example foo(x y, "lose", 4 * z) @end example @quotation CAUTION Whitespace characters (spaces and TABs) are not allowed between the function name and the opening parenthesis of the argument list. If you write whitespace by mistake, @command{awk} might think that you mean to concatenate a variable with an expression in parentheses. However, it notices that you used a function name and not a variable name, and reports an error. @end quotation @node Variable Scope @subsubsection Controlling Variable Scope @cindex local variables @subentry in a function @cindex variables @subentry local to a function Unlike in many languages, there is no way to make a variable local to a @code{@{} @dots{} @code{@}} block in @command{awk}, but you can make a variable local to a function. It is good practice to do so whenever a variable is needed only in that function. To make a variable local to a function, simply declare the variable as an argument after the actual function arguments (@pxref{Definition Syntax}). Look at the following example, where variable @code{i} is a global variable used by both functions @code{foo()} and @code{bar()}: @example function bar() @{ for (i = 0; i < 3; i++) print "bar's i=" i @} function foo(j) @{ i = j + 1 print "foo's i=" i bar() print "foo's i=" i @} BEGIN @{ i = 10 print "top's i=" i foo(0) print "top's i=" i @} @end example Running this script produces the following, because the @code{i} in functions @code{foo()} and @code{bar()} and at the top level refer to the same variable instance: @example top's i=10 foo's i=1 bar's i=0 bar's i=1 bar's i=2 foo's i=3 top's i=3 @end example If you want @code{i} to be local to both @code{foo()} and @code{bar()}, do as follows (the extra space before @code{i} is a coding convention to indicate that @code{i} is a local variable, not an argument): @example function bar( i) @{ for (i = 0; i < 3; i++) print "bar's i=" i @} function foo(j, i) @{ i = j + 1 print "foo's i=" i bar() print "foo's i=" i @} BEGIN @{ i = 10 print "top's i=" i foo(0) print "top's i=" i @} @end example Running the corrected script produces the following: @example top's i=10 foo's i=1 bar's i=0 bar's i=1 bar's i=2 foo's i=1 top's i=10 @end example Besides scalar values (strings and numbers), you may also have local arrays. By using a parameter name as an array, @command{awk} treats it as an array, and it is local to the function. In addition, recursive calls create new arrays. Consider this example: @example @group function some_func(p1, a) @{ if (p1++ > 3) return @end group a[p1] = p1 some_func(p1) printf("At level %d, index %d %s found in a\n", p1, (p1 - 1), (p1 - 1) in a ? "is" : "is not") printf("At level %d, index %d %s found in a\n", p1, p1, p1 in a ? "is" : "is not") print "" @} BEGIN @{ some_func(1) @} @end example When run, this program produces the following output: @example At level 4, index 3 is not found in a At level 4, index 4 is found in a At level 3, index 2 is not found in a At level 3, index 3 is found in a At level 2, index 1 is not found in a At level 2, index 2 is found in a @end example @node Pass By Value/Reference @subsubsection Passing Function Arguments by Value Or by Reference In @command{awk}, when you declare a function, there is no way to declare explicitly whether the arguments are passed @dfn{by value} or @dfn{by reference}. Instead, the passing convention is determined at runtime when the function is called, according to the following rule: if the argument is an array variable, then it is passed by reference. Otherwise, the argument is passed by value. @cindex call by value Passing an argument by value means that when a function is called, it is given a @emph{copy} of the value of this argument. The caller may use a variable as the expression for the argument, but the called function does not know this---it only knows what value the argument had. For example, if you write the following code: @example foo = "bar" z = myfunc(foo) @end example @noindent then you should not think of the argument to @code{myfunc()} as being ``the variable @code{foo}.'' Instead, think of the argument as the string value @code{"bar"}. If the function @code{myfunc()} alters the values of its local variables, this has no effect on any other variables. Thus, if @code{myfunc()} does this: @example @group function myfunc(str) @{ print str str = "zzz" print str @} @end group @end example @noindent to change its first argument variable @code{str}, it does @emph{not} change the value of @code{foo} in the caller. The role of @code{foo} in calling @code{myfunc()} ended when its value (@code{"bar"}) was computed. If @code{str} also exists outside of @code{myfunc()}, the function body cannot alter this outer value, because it is shadowed during the execution of @code{myfunc()} and cannot be seen or changed from there. @cindex call by reference @cindex arrays @subentry as parameters to functions @cindex functions @subentry arrays as parameters to However, when arrays are the parameters to functions, they are @emph{not} copied. Instead, the array itself is made available for direct manipulation by the function. This is usually termed @dfn{call by reference}. Changes made to an array parameter inside the body of a function @emph{are} visible outside that function. @quotation NOTE Changing an array parameter inside a function can be very dangerous if you do not watch what you are doing. For example: @example function changeit(array, ind, nvalue) @{ array[ind] = nvalue @} BEGIN @{ a[1] = 1; a[2] = 2; a[3] = 3 changeit(a, 2, "two") printf "a[1] = %s, a[2] = %s, a[3] = %s\n", a[1], a[2], a[3] @} @end example @noindent prints @samp{a[1] = 1, a[2] = two, a[3] = 3}, because @code{changeit()} stores @code{"two"} in the second element of @code{a}. @end quotation @node Function Caveats @subsubsection Other Points About Calling Functions @cindex undefined functions @cindex functions @subentry undefined Some @command{awk} implementations allow you to call a function that has not been defined. They only report a problem at runtime, when the program actually tries to call the function. For example: @example BEGIN @{ if (0) foo() else bar() @} function bar() @{ @dots{} @} # note that `foo' is not defined @end example @noindent Because the @samp{if} statement will never be true, it is not really a problem that @code{foo()} has not been defined. Usually, though, it is a problem if a program calls an undefined function. @cindex lint checking @subentry undefined functions If @option{--lint} is specified (@pxref{Options}), @command{gawk} reports calls to undefined functions. @cindex portability @subentry @code{next} statement in user-defined functions Some @command{awk} implementations generate a runtime error if you use either the @code{next} statement or the @code{nextfile} statement (@pxref{Next Statement}, and @ifdocbook @ref{Nextfile Statement}) @end ifdocbook @ifnotdocbook @pxref{Nextfile Statement}) @end ifnotdocbook inside a user-defined function. @command{gawk} does not have this limitation. You can call a function and pass it more parameters than it was declared with, like so: @example function foo(p1, p2) @{ @dots{} @} BEGIN @{ foo(1, 2, 3, 4) @} @end example Doing so is bad practice, however. The called function cannot do anything with the additional values being passed to it, so @command{awk} evaluates the expressions but then just throws them away. More importantly, such a call is confusing for whoever will next read your program.@footnote{Said person might even be you, sometime in the future, at which point you will wonder, ``what was I thinking?!?''} Function parameters generally are input items that influence the computation performed by the function. Calling a function with more parameters than it accepts gives the false impression that those values are important to the function, when in fact they are not. Because this is such a bad practice, @command{gawk} @emph{unconditionally} issues a warning whenever it executes such a function call. (If you don't like the warning, fix your code! It's incorrect, after all.) @node Return Statement @subsection The @code{return} Statement @cindex @code{return} statement, user-defined functions As seen in several earlier examples, the body of a user-defined function can contain a @code{return} statement. This statement returns control to the calling part of the @command{awk} program. It can also be used to return a value for use in the rest of the @command{awk} program. It looks like this: @display @code{return} [@var{expression}] @end display The @var{expression} part is optional. Due most likely to an oversight, POSIX does not define what the return value is if you omit the @var{expression}. Technically speaking, this makes the returned value undefined, and therefore, unpredictable. In practice, though, all versions of @command{awk} simply return the null string, which acts like zero if used in a numeric context. A @code{return} statement without an @var{expression} is assumed at the end of every function definition. So, if control reaches the end of the function body, then technically the function returns an unpredictable value. In practice, it returns the empty string. @command{awk} does @emph{not} warn you if you use the return value of such a function. Sometimes, you want to write a function for what it does, not for what it returns. Such a function corresponds to a @code{void} function in C, C++, or Java, or to a @code{procedure} in Ada. Thus, it may be appropriate to not return any value; simply bear in mind that you should not be using the return value of such a function. The following is an example of a user-defined function that returns a value for the largest number among the elements of an array: @example function maxelt(vec, i, ret) @{ for (i in vec) @{ if (ret == "" || vec[i] > ret) ret = vec[i] @} return ret @} @end example @cindex programming conventions @subentry function parameters @noindent You call @code{maxelt()} with one argument, which is an array name. The local variables @code{i} and @code{ret} are not intended to be arguments; there is nothing to stop you from passing more than one argument to @code{maxelt()} but the results would be strange. The extra space before @code{i} in the function parameter list indicates that @code{i} and @code{ret} are local variables. You should follow this convention when defining functions. The following program uses the @code{maxelt()} function. It loads an array, calls @code{maxelt()}, and then reports the maximum number in that array: @example function maxelt(vec, i, ret) @{ for (i in vec) @{ if (ret == "" || vec[i] > ret) ret = vec[i] @} return ret @} @group # Load all fields of each record into nums. @{ for(i = 1; i <= NF; i++) nums[NR, i] = $i @} @end group END @{ print maxelt(nums) @} @end example Given the following input: @example 1 5 23 8 16 44 3 5 2 8 26 256 291 1396 2962 100 -6 467 998 1101 99385 11 0 225 @end example @noindent the program reports (predictably) that 99,385 is the largest value in the array. @node Dynamic Typing @subsection Functions and Their Effects on Variable Typing @command{awk} is a very fluid language. It is possible that @command{awk} can't tell if an identifier represents a scalar variable or an array until runtime. Here is an annotated sample program: @example function foo(a) @{ a[1] = 1 # parameter is an array @} BEGIN @{ b = 1 foo(b) # invalid: fatal type mismatch foo(x) # x uninitialized, becomes an array dynamically x = 1 # now not allowed, runtime error @} @end example In this example, the first call to @code{foo()} generates a fatal error, so @command{awk} will not report the second error. If you comment out that call, though, then @command{awk} does report the second error. Usually, such things aren't a big issue, but it's worth being aware of them. @node Indirect Calls @section Indirect Function Calls @cindex indirect function calls @cindex function calls @subentry indirect @cindex function pointers @cindex pointers to functions @cindex differences in @command{awk} and @command{gawk} @subentry indirect function calls This section describes an advanced, @command{gawk}-specific extension. Often, you may wish to defer the choice of function to call until runtime. For example, you may have different kinds of records, each of which should be processed differently. Normally, you would have to use a series of @code{if}-@code{else} statements to decide which function to call. By using @dfn{indirect} function calls, you can specify the name of the function to call as a string variable, and then call the function. Let's look at an example. Suppose you have a file with your test scores for the classes you are taking, and you wish to get the sum and the average of your test scores. The first field is the class name. The following fields are the functions to call to process the data, up to a ``marker'' field @samp{data:}. Following the marker, to the end of the record, are the various numeric test scores. Here is the initial file: @example @c file eg/data/class_data1 Biology_101 sum average data: 87.0 92.4 78.5 94.9 Chemistry_305 sum average data: 75.2 98.3 94.7 88.2 English_401 sum average data: 100.0 95.6 87.1 93.4 @c endfile @end example To process the data, you might write initially: @example @{ class = $1 for (i = 2; $i != "data:"; i++) @{ if ($i == "sum") sum() # processes the whole record else if ($i == "average") average() @dots{} # and so on @} @} @end example @noindent This style of programming works, but can be awkward. With @dfn{indirect} function calls, you tell @command{gawk} to use the @emph{value} of a variable as the @emph{name} of the function to call. @cindex @code{@@} (at-sign) @subentry @code{@@}-notation for indirect function calls @cindex at-sign (@code{@@}) @subentry @code{@@}-notation for indirect function calls @cindex indirect function calls @subentry @code{@@}-notation @cindex function calls @subentry indirect @subentry @code{@@}-notation for The syntax is similar to that of a regular function call: an identifier immediately followed by an opening parenthesis, any arguments, and then a closing parenthesis, with the addition of a leading @samp{@@} character: @example the_func = "sum" result = @@the_func() # calls the sum() function @end example Here is a full program that processes the previously shown data, using indirect function calls: @example @c file eg/prog/indirectcall.awk # indirectcall.awk --- Demonstrate indirect function calls @c endfile @ignore @c file eg/prog/indirectcall.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # January 2009 @c endfile @end ignore @c file eg/prog/indirectcall.awk # average --- return the average of the values in fields $first - $last function average(first, last, sum, i) @{ sum = 0; for (i = first; i <= last; i++) sum += $i return sum / (last - first + 1) @} # sum --- return the sum of the values in fields $first - $last function sum(first, last, ret, i) @{ ret = 0; for (i = first; i <= last; i++) ret += $i return ret @} @c endfile @end example These two functions expect to work on fields; thus, the parameters @code{first} and @code{last} indicate where in the fields to start and end. Otherwise, they perform the expected computations and are not unusual: @example @c file eg/prog/indirectcall.awk # For each record, print the class name and the requested statistics @{ class_name = $1 gsub(/_/, " ", class_name) # Replace _ with spaces # find start for (i = 1; i <= NF; i++) @{ if ($i == "data:") @{ start = i + 1 break @} @} printf("%s:\n", class_name) for (i = 2; $i != "data:"; i++) @{ the_function = $i printf("\t%s: <%s>\n", $i, @@the_function(start, NF) "") @} print "" @} @c endfile @end example This is the main processing for each record. It prints the class name (with underscores replaced with spaces). It then finds the start of the actual data, saving it in @code{start}. The last part of the code loops through each function name (from @code{$2} up to the marker, @samp{data:}), calling the function named by the field. The indirect function call itself occurs as a parameter in the call to @code{printf}. (The @code{printf} format string uses @samp{%s} as the format specifier so that we can use functions that return strings, as well as numbers. Note that the result from the indirect call is concatenated with the empty string, in order to force it to be a string value.) Here is the result of running the program: @example $ @kbd{gawk -f indirectcall.awk class_data1} @print{} Biology 101: @print{} sum: <352.8> @print{} average: <88.2> @print{} @print{} Chemistry 305: @print{} sum: <356.4> @print{} average: <89.1> @print{} @print{} English 401: @print{} sum: <376.1> @print{} average: <94.025> @end example The ability to use indirect function calls is more powerful than you may think at first. The C and C++ languages provide ``function pointers,'' which are a mechanism for calling a function chosen at runtime. One of the most well-known uses of this ability is the C @code{qsort()} function, which sorts an array using the famous ``quicksort'' algorithm (see @uref{https://en.wikipedia.org/wiki/Quicksort, the Wikipedia article} for more information). To use this function, you supply a pointer to a comparison function. This mechanism allows you to sort arbitrary data in an arbitrary fashion. We can do something similar using @command{gawk}, like this: @example @c file eg/lib/quicksort.awk # quicksort.awk --- Quicksort algorithm, with user-supplied # comparison function @c endfile @ignore @c file eg/lib/quicksort.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # January 2009 @c endfile @end ignore @c file eg/lib/quicksort.awk # quicksort --- C.A.R. Hoare's quicksort algorithm. See Wikipedia # or almost any algorithms or computer science text. @c endfile @ignore @c file eg/lib/quicksort.awk # # Adapted from K&R-II, page 110 @c endfile @end ignore @c file eg/lib/quicksort.awk function quicksort(data, left, right, less_than, i, last) @{ if (left >= right) # do nothing if array contains fewer return # than two elements quicksort_swap(data, left, int((left + right) / 2)) last = left for (i = left + 1; i <= right; i++) if (@@less_than(data[i], data[left])) quicksort_swap(data, ++last, i) quicksort_swap(data, left, last) quicksort(data, left, last - 1, less_than) quicksort(data, last + 1, right, less_than) @} # quicksort_swap --- helper function for quicksort, should really be inline function quicksort_swap(data, i, j, temp) @{ temp = data[i] data[i] = data[j] data[j] = temp @} @c endfile @end example The @code{quicksort()} function receives the @code{data} array, the starting and ending indices to sort (@code{left} and @code{right}), and the name of a function that performs a ``less than'' comparison. It then implements the quicksort algorithm. To make use of the sorting function, we return to our previous example. The first thing to do is write some comparison functions: @example @c file eg/prog/indirectcall.awk @group # num_lt --- do a numeric less than comparison function num_lt(left, right) @{ return ((left + 0) < (right + 0)) @} @end group # num_ge --- do a numeric greater than or equal to comparison function num_ge(left, right) @{ return ((left + 0) >= (right + 0)) @} @c endfile @end example The @code{num_ge()} function is needed to perform a descending sort; when used to perform a ``less than'' test, it actually does the opposite (greater than or equal to), which yields data sorted in descending order. Next comes a sorting function. It is parameterized with the starting and ending field numbers and the comparison function. It builds an array with the data and calls @code{quicksort()} appropriately, and then formats the results as a single string: @example @c file eg/prog/indirectcall.awk # do_sort --- sort the data according to `compare' # and return it as a string function do_sort(first, last, compare, data, i, retval) @{ delete data for (i = 1; first <= last; first++) @{ data[i] = $first i++ @} quicksort(data, 1, i-1, compare) retval = data[1] for (i = 2; i in data; i++) retval = retval " " data[i] return retval @} @c endfile @end example Finally, the two sorting functions call @code{do_sort()}, passing in the names of the two comparison functions: @example @c file eg/prog/indirectcall.awk @group # sort --- sort the data in ascending order and return it as a string function sort(first, last) @{ return do_sort(first, last, "num_lt") @} @end group @group # rsort --- sort the data in descending order and return it as a string function rsort(first, last) @{ return do_sort(first, last, "num_ge") @} @end group @c endfile @end example Here is an extended version of the @value{DF}: @example @c file eg/data/class_data2 Biology_101 sum average sort rsort data: 87.0 92.4 78.5 94.9 Chemistry_305 sum average sort rsort data: 75.2 98.3 94.7 88.2 English_401 sum average sort rsort data: 100.0 95.6 87.1 93.4 @c endfile @end example Finally, here are the results when the enhanced program is run: @example $ @kbd{gawk -f quicksort.awk -f indirectcall.awk class_data2} @print{} Biology 101: @print{} sum: <352.8> @print{} average: <88.2> @print{} sort: <78.5 87.0 92.4 94.9> @print{} rsort: <94.9 92.4 87.0 78.5> @print{} @print{} Chemistry 305: @print{} sum: <356.4> @print{} average: <89.1> @print{} sort: <75.2 88.2 94.7 98.3> @print{} rsort: <98.3 94.7 88.2 75.2> @print{} @print{} English 401: @print{} sum: <376.1> @print{} average: <94.025> @print{} sort: <87.1 93.4 95.6 100.0> @print{} rsort: <100.0 95.6 93.4 87.1> @end example Another example where indirect functions calls are useful can be found in processing arrays. This is described in @ref{Walking Arrays}. Remember that you must supply a leading @samp{@@} in front of an indirect function call. Starting with @value{PVERSION} 4.1.2 of @command{gawk}, indirect function calls may also be used with built-in functions and with extension functions (@pxref{Dynamic Extensions}). There are some limitations when calling built-in functions indirectly, as follows. @itemize @value{BULLET} @item You cannot pass a regular expression constant to a built-in function through an indirect function call.@footnote{This may change in a future version; recheck the documentation that comes with your version of @command{gawk} to see if it has.} This applies to the @code{sub()}, @code{gsub()}, @code{gensub()}, @code{match()}, @code{split()} and @code{patsplit()} functions. @item If calling @code{sub()} or @code{gsub()}, you may only pass two arguments, since those functions are unusual in that they update their third argument. This means that @code{$0} will be updated. @end itemize @command{gawk} does its best to make indirect function calls efficient. For example, in the following case: @example for (i = 1; i <= n; i++) @@the_func() @end example @noindent @command{gawk} looks up the actual function to call only once. @node Functions Summary @section Summary @itemize @value{BULLET} @item @command{awk} provides built-in functions and lets you define your own functions. @item POSIX @command{awk} provides three kinds of built-in functions: numeric, string, and I/O. @command{gawk} provides functions that sort arrays, work with values representing time, do bit manipulation, determine variable type (array versus scalar), and internationalize and localize programs. @command{gawk} also provides several extensions to some of standard functions, typically in the form of additional arguments. @item Functions accept zero or more arguments and return a value. The expressions that provide the argument values are completely evaluated before the function is called. Order of evaluation is not defined. The return value can be ignored. @item The handling of backslash in @code{sub()} and @code{gsub()} is not simple. It is more straightforward in @command{gawk}'s @code{gensub()} function, but that function still requires care in its use. @item User-defined functions provide important capabilities but come with some syntactic inelegancies. In a function call, there cannot be any space between the function name and the opening left parenthesis of the argument list. Also, there is no provision for local variables, so the convention is to add extra parameters, and to separate them visually from the real parameters by extra whitespace. @item User-defined functions may call other user-defined (and built-in) functions and may call themselves recursively. Function parameters ``hide'' any global variables of the same names. You cannot use the name of a reserved variable (such as @code{ARGC}) as the name of a parameter in user-defined functions. @item Scalar values are passed to user-defined functions by value. Array parameters are passed by reference; any changes made by the function to array parameters are thus visible after the function has returned. @item Use the @code{return} statement to return from a user-defined function. An optional expression becomes the function's return value. Only scalar values may be returned by a function. @item If a variable that has never been used is passed to a user-defined function, how that function treats the variable can set its nature: either scalar or array. @item @command{gawk} provides indirect function calls using a special syntax. By setting a variable to the name of a function, you can determine at runtime what function will be called at that point in the program. This is equivalent to function pointers in C and C++. @end itemize @ifnotinfo @part @value{PART2}Problem Solving with @command{awk} @end ifnotinfo @ifdocbook Part II shows how to use @command{awk} and @command{gawk} for problem solving. There is lots of code here for you to read and learn from. It contains the following chapters: @itemize @value{BULLET} @item @ref{Library Functions} @item @ref{Sample Programs} @end itemize @end ifdocbook @node Library Functions @chapter A Library of @command{awk} Functions @cindex libraries of @command{awk} functions @cindex functions @subentry library @cindex functions @subentry user-defined @subentry library of @ref{User-defined} describes how to write your own @command{awk} functions. Writing functions is important, because it allows you to encapsulate algorithms and program tasks in a single place. It simplifies programming, making program development more manageable and making programs more readable. @cindex Kernighan, Brian @cindex Plauger, P.J.@: In their seminal 1976 book, @cite{Software Tools},@footnote{Sadly, over 35 years later, many of the lessons taught by this book have yet to be learned by a vast number of practicing programmers.} Brian Kernighan and P.J.@: Plauger wrote: @quotation Good Programming is not learned from generalities, but by seeing how significant programs can be made clean, easy to read, easy to maintain and modify, human-engineered, efficient and reliable, by the application of common sense and good programming practices. Careful study and imitation of good programs leads to better writing. @end quotation In fact, they felt this idea was so important that they placed this statement on the cover of their book. Because we believe strongly that their statement is correct, this @value{CHAPTER} and @ref{Sample Programs}, provide a good-sized body of code for you to read and, we hope, to learn from. This @value{CHAPTER} presents a library of useful @command{awk} functions. Many of the sample programs presented later in this @value{DOCUMENT} use these functions. The functions are presented here in a progression from simple to complex. @cindex Texinfo @ref{Extract Program} presents a program that you can use to extract the source code for these example library functions and programs from the Texinfo source for this @value{DOCUMENT}. (This has already been done as part of the @command{gawk} distribution.) @ifclear FOR_PRINT If you have written one or more useful, general-purpose @command{awk} functions and would like to contribute them to the @command{awk} user community, see @ref{How To Contribute}, for more information. @end ifclear @cindex portability @subentry example programs The programs in this @value{CHAPTER} and in @ref{Sample Programs}, freely use @command{gawk}-specific features. Rewriting these programs for different implementations of @command{awk} is pretty straightforward: @itemize @value{BULLET} @item Diagnostic error messages are sent to @file{/dev/stderr}. Use @samp{| "cat 1>&2"} instead of @samp{> "/dev/stderr"} if your system does not have a @file{/dev/stderr}, or if you cannot use @command{gawk}. @item A number of programs use @code{nextfile} (@pxref{Nextfile Statement}) to skip any remaining input in the input file. @item @c 12/2000: Thanks to Nelson Beebe for pointing out the output issue. @cindex case sensitivity @subentry example programs @cindex @code{IGNORECASE} variable @subentry in example programs Finally, some of the programs choose to ignore upper- and lowercase distinctions in their input. They do so by assigning one to @code{IGNORECASE}. You can achieve almost the same effect@footnote{The effects are not identical. Output of the transformed record will be in all lowercase, while @code{IGNORECASE} preserves the original contents of the input record.} by adding the following rule to the beginning of the program: @example # ignore case @{ $0 = tolower($0) @} @end example @noindent Also, verify that all regexp and string constants used in comparisons use only lowercase letters. @end itemize @menu * Library Names:: How to best name private global variables in library functions. * General Functions:: Functions that are of general use. * Data File Management:: Functions for managing command-line data files. * Getopt Function:: A function for processing command-line arguments. * Passwd Functions:: Functions for getting user information. * Group Functions:: Functions for getting group information. * Walking Arrays:: A function to walk arrays of arrays. * Library Functions Summary:: Summary of library functions. * Library Exercises:: Exercises. @end menu @node Library Names @section Naming Library Function Global Variables @cindex names @subentry arrays/variables @cindex names @subentry functions @cindex naming issues @cindex @command{awk} programs @subentry documenting @cindex documentation @subentry of @command{awk} programs Due to the way the @command{awk} language evolved, variables are either @dfn{global} (usable by the entire program) or @dfn{local} (usable just by a specific function). There is no intermediate state analogous to @code{static} variables in C. @cindex variables @subentry global @subentry for library functions @cindex private variables @cindex variables @subentry private Library functions often need to have global variables that they can use to preserve state information between calls to the function---for example, @code{getopt()}'s variable @code{_opti} (@pxref{Getopt Function}). Such variables are called @dfn{private}, as the only functions that need to use them are the ones in the library. When writing a library function, you should try to choose names for your private variables that will not conflict with any variables used by either another library function or a user's main program. For example, a name like @code{i} or @code{j} is not a good choice, because user programs often use variable names like these for their own purposes. @cindex programming conventions @subentry private variable names The example programs shown in this @value{CHAPTER} all start the names of their private variables with an underscore (@samp{_}). Users generally don't use leading underscores in their variable names, so this convention immediately decreases the chances that the variable names will be accidentally shared with the user's program. @cindex @code{_} (underscore) @subentry in names of private variables @cindex underscore (@code{_}) @subentry in names of private variables In addition, several of the library functions use a prefix that helps indicate what function or set of functions use the variables---for example, @code{_pw_byname()} in the user database routines (@pxref{Passwd Functions}). This convention is recommended, as it even further decreases the chance of inadvertent conflict among variable names. Note that this convention is used equally well for variable names and for private function names.@footnote{Although all the library routines could have been rewritten to use this convention, this was not done, in order to show how our own @command{awk} programming style has evolved and to provide some basis for this discussion.} As a final note on variable naming, if a function makes global variables available for use by a main program, it is a good convention to start those variables' names with a capital letter---for example, @code{getopt()}'s @code{Opterr} and @code{Optind} variables (@pxref{Getopt Function}). The leading capital letter indicates that it is global, while the fact that the variable name is not all capital letters indicates that the variable is not one of @command{awk}'s predefined variables, such as @code{FS}. @cindex @option{--dump-variables} option @subentry using for library functions It is also important that @emph{all} variables in library functions that do not need to save state are, in fact, declared local.@footnote{@command{gawk}'s @option{--dump-variables} command-line option is useful for verifying this.} If this is not done, the variables could accidentally be used in the user's program, leading to bugs that are very difficult to track down: @example function lib_func(x, y, l1, l2) @{ @dots{} # some_var should be local but by oversight is not @var{use variable} some_var @dots{} @} @end example @cindex arrays @subentry associative @subentry library functions and @cindex libraries of @command{awk} functions @subentry associative arrays and @cindex functions @subentry library @subentry associative arrays and @cindex Tcl A different convention, common in the Tcl community, is to use a single associative array to hold the values needed by the library function(s), or ``package.'' This significantly decreases the number of actual global names in use. For example, the functions described in @ref{Passwd Functions} might have used array elements @code{@w{PW_data["inited"]}}, @code{@w{PW_data["total"]}}, @code{@w{PW_data["count"]}}, and @code{@w{PW_data["awklib"]}}, instead of @code{@w{_pw_inited}}, @code{@w{_pw_awklib}}, @code{@w{_pw_total}}, and @code{@w{_pw_count}}. The conventions presented in this @value{SECTION} are exactly that: conventions. You are not required to write your programs this way---we merely recommend that you do so. Beginning with @value{PVERSION} 5.0, @command{gawk} provides a powerful mechanism for solving the problems described in this section: @dfn{namespaces}. Namespaces and their use are described in detail in @ref{Namespaces}. @node General Functions @section General Programming This @value{SECTION} presents a number of functions that are of general programming use. @menu * Strtonum Function:: A replacement for the built-in @code{strtonum()} function. * Assert Function:: A function for assertions in @command{awk} programs. * Round Function:: A function for rounding if @code{sprintf()} does not do it correctly. * Cliff Random Function:: The Cliff Random Number Generator. * Ordinal Functions:: Functions for using characters as numbers and vice versa. * Join Function:: A function to join an array into a string. * Getlocaltime Function:: A function to get formatted times. * Readfile Function:: A function to read an entire file at once. * Shell Quoting:: A function to quote strings for the shell. @end menu @node Strtonum Function @subsection Converting Strings to Numbers The @code{strtonum()} function (@pxref{String Functions}) is a @command{gawk} extension. The following function provides an implementation for other versions of @command{awk}: @example @c file eg/lib/strtonum.awk # mystrtonum --- convert string to number @c endfile @ignore @c file eg/lib/strtonum.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # February, 2004 # Revised June, 2014 @c endfile @end ignore @c file eg/lib/strtonum.awk function mystrtonum(str, ret, n, i, k, c) @{ if (str ~ /^0[0-7]*$/) @{ # octal n = length(str) ret = 0 for (i = 1; i <= n; i++) @{ c = substr(str, i, 1) # index() returns 0 if c not in string, # includes c == "0" k = index("1234567", c) ret = ret * 8 + k @} @} else if (str ~ /^0[xX][[:xdigit:]]+$/) @{ # hexadecimal str = substr(str, 3) # lop off leading 0x n = length(str) ret = 0 for (i = 1; i <= n; i++) @{ c = substr(str, i, 1) c = tolower(c) # index() returns 0 if c not in string, # includes c == "0" k = index("123456789abcdef", c) ret = ret * 16 + k @} @} else if (str ~ \ /^[-+]?([0-9]+([.][0-9]*([Ee][0-9]+)?)?|([.][0-9]+([Ee][-+]?[0-9]+)?))$/) @{ # decimal number, possibly floating point ret = str + 0 @} else ret = "NOT-A-NUMBER" return ret @} # BEGIN @{ # gawk test harness # a[1] = "25" # a[2] = ".31" # a[3] = "0123" # a[4] = "0xdeadBEEF" # a[5] = "123.45" # a[6] = "1.e3" # a[7] = "1.32" # a[8] = "1.32E2" # # for (i = 1; i in a; i++) # print a[i], strtonum(a[i]), mystrtonum(a[i]) # @} @c endfile @end example The function first looks for C-style octal numbers (base 8). If the input string matches a regular expression describing octal numbers, then @code{mystrtonum()} loops through each character in the string. It sets @code{k} to the index in @code{"1234567"} of the current octal digit. The return value will either be the same number as the digit, or zero if the character is not there, which will be true for a @samp{0}. This is safe, because the regexp test in the @code{if} ensures that only octal values are converted. Similar logic applies to the code that checks for and converts a hexadecimal value, which starts with @samp{0x} or @samp{0X}. The use of @code{tolower()} simplifies the computation for finding the correct numeric value for each hexadecimal digit. Finally, if the string matches the (rather complicated) regexp for a regular decimal integer or floating-point number, the computation @samp{ret = str + 0} lets @command{awk} convert the value to a number. A commented-out test program is included, so that the function can be tested with @command{gawk} and the results compared to the built-in @code{strtonum()} function. @node Assert Function @subsection Assertions @cindex assertions @cindex @code{assert()} function (C library) @cindex C library functions @subentry @code{assert()} @cindex libraries of @command{awk} functions @subentry assertions @cindex functions @subentry library @subentry assertions @cindex @command{awk} programs @subentry lengthy @subentry assertions When writing large programs, it is often useful to know that a condition or set of conditions is true. Before proceeding with a particular computation, you make a statement about what you believe to be the case. Such a statement is known as an @dfn{assertion}. The C language provides an @code{} header file and corresponding @code{assert()} macro that a programmer can use to make assertions. If an assertion fails, the @code{assert()} macro arranges to print a diagnostic message describing the condition that should have been true but was not, and then it kills the program. In C, using @code{assert()} looks this: @example @group #include int myfunc(int a, double b) @{ assert(a <= 5 && b >= 17.1); @dots{} @} @end group @end example If the assertion fails, the program prints a message similar to this: @example prog.c:5: assertion failed: a <= 5 && b >= 17.1 @end example @cindex @code{assert()} user-defined function @cindex user-defined @subentry function @subentry @code{assert()} The C language makes it possible to turn the condition into a string for use in printing the diagnostic message. This is not possible in @command{awk}, so this @code{assert()} function also requires a string version of the condition that is being tested. Following is the function: @example @c file eg/lib/assert.awk # assert --- assert that a condition is true. Otherwise, exit. @c endfile @ignore @c file eg/lib/assert.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May, 1993 @c endfile @end ignore @c file eg/lib/assert.awk function assert(condition, string) @{ if (! condition) @{ printf("%s:%d: assertion failed: %s\n", FILENAME, FNR, string) > "/dev/stderr" _assert_exit = 1 exit 1 @} @} @group END @{ if (_assert_exit) exit 1 @} @end group @c endfile @end example The @code{assert()} function tests the @code{condition} parameter. If it is false, it prints a message to standard error, using the @code{string} parameter to describe the failed condition. It then sets the variable @code{_assert_exit} to one and executes the @code{exit} statement. The @code{exit} statement jumps to the @code{END} rule. If the @code{END} rule finds @code{_assert_exit} to be true, it exits immediately. The purpose of the test in the @code{END} rule is to keep any other @code{END} rules from running. When an assertion fails, the program should exit immediately. If no assertions fail, then @code{_assert_exit} is still false when the @code{END} rule is run normally, and the rest of the program's @code{END} rules execute. For all of this to work correctly, @file{assert.awk} must be the first source file read by @command{awk}. The function can be used in a program in the following way: @example function myfunc(a, b) @{ assert(a <= 5 && b >= 17.1, "a <= 5 && b >= 17.1") @dots{} @} @end example @noindent If the assertion fails, you see a message similar to the following: @example mydata:1357: assertion failed: a <= 5 && b >= 17.1 @end example @cindex @code{END} pattern @subentry @code{assert()} user-defined function and There is a small problem with this version of @code{assert()}. An @code{END} rule is automatically added to the program calling @code{assert()}. Normally, if a program consists of just a @code{BEGIN} rule, the input files and/or standard input are not read. However, now that the program has an @code{END} rule, @command{awk} attempts to read the input @value{DF}s or standard input (@pxref{Using BEGIN/END}), most likely causing the program to hang as it waits for input. @cindex @code{BEGIN} pattern @subentry @code{assert()} user-defined function and There is a simple workaround to this: make sure that such a @code{BEGIN} rule always ends with an @code{exit} statement. @node Round Function @subsection Rounding Numbers @cindex rounding numbers @cindex numbers @subentry rounding @cindex libraries of @command{awk} functions @subentry rounding numbers @cindex functions @subentry library @subentry rounding numbers @cindex @code{print} statement @subentry @code{sprintf()} function and @cindex @code{printf} statement @subentry @code{sprintf()} function and @cindex @code{sprintf()} function @subentry @code{print}/@code{printf} statements and The way @code{printf} and @code{sprintf()} (@pxref{Printf}) perform rounding often depends upon the system's C @code{sprintf()} subroutine. On many machines, @code{sprintf()} rounding is @dfn{unbiased}, which means it doesn't always round a trailing .5 up, contrary to naive expectations. In unbiased rounding, .5 rounds to even, rather than always up, so 1.5 rounds to 2 but 4.5 rounds to 4. This means that if you are using a format that does rounding (e.g., @code{"%.0f"}), you should check what your system does. The following function does traditional rounding; it might be useful if your @command{awk}'s @code{printf} does unbiased rounding: @cindex @code{round()} user-defined function @cindex user-defined @subentry function @subentry @code{round()} @example @c file eg/lib/round.awk # round.awk --- do normal rounding @c endfile @ignore @c file eg/lib/round.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # August, 1996 @c endfile @end ignore @c file eg/lib/round.awk function round(x, ival, aval, fraction) @{ ival = int(x) # integer part, int() truncates # see if fractional part if (ival == x) # no fraction return ival # ensure no decimals if (x < 0) @{ aval = -x # absolute value ival = int(aval) fraction = aval - ival if (fraction >= .5) return int(x) - 1 # -2.5 --> -3 else return int(x) # -2.3 --> -2 @} else @{ fraction = x - ival if (fraction >= .5) return ival + 1 else return ival @} @} @c endfile @c don't include test harness in the file that gets installed @group # test harness # @{ print $0, round($0) @} @end group @end example @node Cliff Random Function @subsection The Cliff Random Number Generator @cindex random numbers @subentry Cliff @cindex Cliff random numbers @cindex numbers @subentry Cliff random @cindex functions @subentry library @subentry Cliff random numbers The @uref{http://mathworld.wolfram.com/CliffRandomNumberGenerator.html, Cliff random number generator} is a very simple random number generator that ``passes the noise sphere test for randomness by showing no structure.'' It is easily programmed, in less than 10 lines of @command{awk} code: @cindex @code{cliff_rand()} user-defined function @cindex user-defined @subentry function @subentry @code{cliff_rand()} @example @c file eg/lib/cliff_rand.awk # cliff_rand.awk --- generate Cliff random numbers @c endfile @ignore @c file eg/lib/cliff_rand.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # December 2000 @c endfile @end ignore @c file eg/lib/cliff_rand.awk BEGIN @{ _cliff_seed = 0.1 @} function cliff_rand() @{ _cliff_seed = (100 * log(_cliff_seed)) % 1 if (_cliff_seed < 0) _cliff_seed = - _cliff_seed return _cliff_seed @} @c endfile @end example This algorithm requires an initial ``seed'' of 0.1. Each new value uses the current seed as input for the calculation. If the built-in @code{rand()} function (@pxref{Numeric Functions}) isn't random enough, you might try using this function instead. @node Ordinal Functions @subsection Translating Between Characters and Numbers @cindex libraries of @command{awk} functions @subentry character values as numbers @cindex functions @subentry library @subentry character values as numbers @cindex characters @subentry values of as numbers @cindex numbers @subentry as values of characters One commercial implementation of @command{awk} supplies a built-in function, @code{ord()}, which takes a character and returns the numeric value for that character in the machine's character set. If the string passed to @code{ord()} has more than one character, only the first one is used. The inverse of this function is @code{chr()} (from the function of the same name in Pascal), which takes a number and returns the corresponding character. Both functions are written very nicely in @command{awk}; there is no real reason to build them into the @command{awk} interpreter: @cindex @code{ord()} user-defined function @cindex user-defined @subentry function @subentry @code{ord()} @cindex @code{chr()} user-defined function @cindex user-defined @subentry function @subentry @code{chr()} @cindex @code{_ord_init()} user-defined function @cindex user-defined @subentry function @subentry @code{_ord_init()} @example @c file eg/lib/ord.awk # ord.awk --- do ord and chr # Global identifiers: # _ord_: numerical values indexed by characters # _ord_init: function to initialize _ord_ @c endfile @ignore @c file eg/lib/ord.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # 16 January, 1992 # 20 July, 1992, revised @c endfile @end ignore @c file eg/lib/ord.awk BEGIN @{ _ord_init() @} function _ord_init( low, high, i, t) @{ low = sprintf("%c", 7) # BEL is ascii 7 if (low == "\a") @{ # regular ascii low = 0 high = 127 @} else if (sprintf("%c", 128 + 7) == "\a") @{ # ascii, mark parity low = 128 high = 255 @} else @{ # ebcdic(!) low = 0 high = 255 @} for (i = low; i <= high; i++) @{ t = sprintf("%c", i) _ord_[t] = i @} @} @c endfile @end example @cindex character sets (machine character encodings) @cindex ASCII @cindex EBCDIC @cindex Unicode @cindex mark parity Some explanation of the numbers used by @code{_ord_init()} is worthwhile. The most prominent character set in use today is ASCII.@footnote{This is changing; many systems use Unicode, a very large character set that includes ASCII as a subset. On systems with full Unicode support, a character can occupy up to 32 bits, making simple tests such as used here prohibitively expensive.} Although an 8-bit byte can hold 256 distinct values (from 0 to 255), ASCII only defines characters that use the values from 0 to 127.@footnote{ASCII has been extended in many countries to use the values from 128 to 255 for country-specific characters. If your system uses these extensions, you can simplify @code{_ord_init()} to loop from 0 to 255.} In the now distant past, at least one minicomputer manufacturer @c Pr1me, blech used ASCII, but with mark parity, meaning that the leftmost bit in the byte is always 1. This means that on those systems, characters have numeric values from 128 to 255. Finally, large mainframe systems use the EBCDIC character set, which uses all 256 values. There are other character sets in use on some older systems, but they are not really worth worrying about: @example @c file eg/lib/ord.awk function ord(str, c) @{ # only first character is of interest c = substr(str, 1, 1) return _ord_[c] @} function chr(c) @{ # force c to be numeric by adding 0 return sprintf("%c", c + 0) @} @c endfile #### test code #### # BEGIN @{ # for (;;) @{ # printf("enter a character: ") # if (getline var <= 0) # break # printf("ord(%s) = %d\n", var, ord(var)) # @} # @} @c endfile @end example An obvious improvement to these functions is to move the code for the @code{@w{_ord_init}} function into the body of the @code{BEGIN} rule. It was written this way initially for ease of development. There is a ``test program'' in a @code{BEGIN} rule, to test the function. It is commented out for production use. @node Join Function @subsection Merging an Array into a String @cindex libraries of @command{awk} functions @subentry merging arrays into strings @cindex functions @subentry library @subentry merging arrays into strings @cindex strings @subentry merging arrays into @cindex arrays @subentry merging into strings When doing string processing, it is often useful to be able to join all the strings in an array into one long string. The following function, @code{join()}, accomplishes this task. It is used later in several of the application programs (@pxref{Sample Programs}). Good function design is important; this function needs to be general, but it should also have a reasonable default behavior. It is called with an array as well as the beginning and ending indices of the elements in the array to be merged. This assumes that the array indices are numeric---a reasonable assumption, as the array was likely created with @code{split()} (@pxref{String Functions}): @cindex @code{join()} user-defined function @cindex user-defined @subentry function @subentry @code{join()} @example @c file eg/lib/join.awk # join.awk --- join an array into a string @c endfile @ignore @c file eg/lib/join.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 @c endfile @end ignore @c file eg/lib/join.awk function join(array, start, end, sep, result, i) @{ if (sep == "") sep = " " else if (sep == SUBSEP) # magic value sep = "" result = array[start] for (i = start + 1; i <= end; i++) result = result sep array[i] return result @} @c endfile @end example An optional additional argument is the separator to use when joining the strings back together. If the caller supplies a nonempty value, @code{join()} uses it; if it is not supplied, it has a null value. In this case, @code{join()} uses a single space as a default separator for the strings. If the value is equal to @code{SUBSEP}, then @code{join()} joins the strings with no separator between them. @code{SUBSEP} serves as a ``magic'' value to indicate that there should be no separation between the component strings.@footnote{It would be nice if @command{awk} had an assignment operator for concatenation. The lack of an explicit operator for concatenation makes string operations more difficult than they really need to be.} @node Getlocaltime Function @subsection Managing the Time of Day @cindex libraries of @command{awk} functions @subentry managing @subentry time @cindex functions @subentry library @subentry managing time @cindex timestamps @subentry formatted @cindex time @subentry managing The @code{systime()} and @code{strftime()} functions described in @ref{Time Functions} provide the minimum functionality necessary for dealing with the time of day in human-readable form. Although @code{strftime()} is extensive, the control formats are not necessarily easy to remember or intuitively obvious when reading a program. The following function, @code{getlocaltime()}, populates a user-supplied array with preformatted time information. It returns a string with the current time formatted in the same way as the @command{date} utility: @cindex @code{getlocaltime()} user-defined function @cindex user-defined @subentry function @subentry @code{getlocaltime()} @example @c file eg/lib/gettime.awk # getlocaltime.awk --- get the time of day in a usable format @c endfile @ignore @c file eg/lib/gettime.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain, May 1993 # @c endfile @end ignore @c file eg/lib/gettime.awk # Returns a string in the format of output of date(1) # Populates the array argument time with individual values: # time["second"] -- seconds (0 - 59) # time["minute"] -- minutes (0 - 59) # time["hour"] -- hours (0 - 23) # time["althour"] -- hours (0 - 12) # time["monthday"] -- day of month (1 - 31) # time["month"] -- month of year (1 - 12) # time["monthname"] -- name of the month # time["shortmonth"] -- short name of the month # time["year"] -- year modulo 100 (0 - 99) # time["fullyear"] -- full year # time["weekday"] -- day of week (Sunday = 0) # time["altweekday"] -- day of week (Monday = 0) # time["dayname"] -- name of weekday # time["shortdayname"] -- short name of weekday # time["yearday"] -- day of year (0 - 365) # time["timezone"] -- abbreviation of timezone name # time["ampm"] -- AM or PM designation # time["weeknum"] -- week number, Sunday first day # time["altweeknum"] -- week number, Monday first day function getlocaltime(time, ret, now, i) @{ # get time once, avoids unnecessary system calls now = systime() # return date(1)-style output ret = strftime("%a %b %e %H:%M:%S %Z %Y", now) # clear out target array delete time # fill in values, force numeric values to be # numeric by adding 0 time["second"] = strftime("%S", now) + 0 time["minute"] = strftime("%M", now) + 0 time["hour"] = strftime("%H", now) + 0 time["althour"] = strftime("%I", now) + 0 time["monthday"] = strftime("%d", now) + 0 time["month"] = strftime("%m", now) + 0 time["monthname"] = strftime("%B", now) time["shortmonth"] = strftime("%b", now) time["year"] = strftime("%y", now) + 0 time["fullyear"] = strftime("%Y", now) + 0 time["weekday"] = strftime("%w", now) + 0 time["altweekday"] = strftime("%u", now) + 0 time["dayname"] = strftime("%A", now) time["shortdayname"] = strftime("%a", now) time["yearday"] = strftime("%j", now) + 0 time["timezone"] = strftime("%Z", now) time["ampm"] = strftime("%p", now) time["weeknum"] = strftime("%U", now) + 0 time["altweeknum"] = strftime("%W", now) + 0 return ret @} @c endfile @end example The string indices are easier to use and read than the various formats required by @code{strftime()}. The @code{alarm} program presented in @ref{Alarm Program} uses this function. A more general design for the @code{getlocaltime()} function would have allowed the user to supply an optional timestamp value to use instead of the current time. @node Readfile Function @subsection Reading a Whole File at Once Often, it is convenient to have the entire contents of a file available in memory as a single string. A straightforward but naive way to do that might be as follows: @example function readfile1(file, tmp, contents) @{ if ((getline tmp < file) < 0) return contents = tmp RT while ((getline tmp < file) > 0) contents = contents tmp RT close(file) return contents @} @end example This function reads from @code{file} one record at a time, building up the full contents of the file in the local variable @code{contents}. It works, but is not necessarily efficient. The following function, based on a suggestion by Denis Shirokov, reads the entire contents of the named file in one shot: @cindex @code{readfile()} user-defined function @cindex user-defined @subentry function @subentry @code{readfile()} @example @c file eg/lib/readfile.awk # readfile.awk --- read an entire file at once @c endfile @ignore @c file eg/lib/readfile.awk # # Original idea by Denis Shirokov, cosmogen@@gmail.com, April 2013 # @c endfile @end ignore @c file eg/lib/readfile.awk function readfile(file, tmp, save_rs) @{ save_rs = RS RS = "^$" getline tmp < file close(file) RS = save_rs return tmp @} @c endfile @end example It works by setting @code{RS} to @samp{^$}, a regular expression that will never match if the file has contents. @command{gawk} reads data from the file into @code{tmp}, attempting to match @code{RS}. The match fails after each read, but fails quickly, such that @command{gawk} fills @code{tmp} with the entire contents of the file. (@xref{Records} for information on @code{RT} and @code{RS}.) In the case that @code{file} is empty, the return value is the null string. Thus, calling code may use something like: @example contents = readfile("/some/path") if (length(contents) == 0) # file was empty @dots{} @end example This tests the result to see if it is empty or not. An equivalent test would be @samp{@w{contents == ""}}. @xref{Extension Sample Readfile} for an extension function that also reads an entire file into memory. @node Shell Quoting @subsection Quoting Strings to Pass to the Shell @c included by permission @ignore Date: Sun, 27 Jul 2014 17:16:16 -0700 Message-ID: Subject: Useful awk function From: Mike Brennan To: Arnold Robbins @end ignore Michael Brennan offers the following programming pattern, which he uses frequently: @example #! /bin/sh awkp=' @dots{} ' @var{input_program} | awk "$awkp" | /bin/sh @end example For example, a program of his named @command{flac-edit} has this form: @example $ @kbd{flac-edit -song="Whoope! That's Great" file.flac} @end example It generates the following output, which is to be piped to the shell (@file{/bin/sh}): @example chmod +w file.flac metaflac --remove-tag=TITLE file.flac LANG=en_US.88591 metaflac --set-tag=TITLE='Whoope! That'"'"'s Great' file.flac chmod -w file.flac @end example Note the need for shell quoting. The function @code{shell_quote()} does it. @code{SINGLE} is the one-character string @code{"'"} and @code{QSINGLE} is the three-character string @code{"\"'\""}: @example @c file eg/lib/shellquote.awk # shell_quote --- quote an argument for passing to the shell @c endfile @ignore @c file eg/lib/shellquote.awk # # Michael Brennan # brennan@@madronabluff.com # September 2014 @c endfile @end ignore @c file eg/lib/shellquote.awk function shell_quote(s, # parameter SINGLE, QSINGLE, i, X, n, ret) # locals @{ if (s == "") return "\"\"" SINGLE = "\x27" # single quote QSINGLE = "\"\x27\"" n = split(s, X, SINGLE) ret = SINGLE X[1] SINGLE for (i = 2; i <= n; i++) ret = ret QSINGLE SINGLE X[i] SINGLE return ret @} @c endfile @end example @node Data File Management @section @value{DDF} Management @cindex files @subentry managing @cindex libraries of @command{awk} functions @subentry managing @subentry data files @cindex functions @subentry library @subentry managing data files This @value{SECTION} presents functions that are useful for managing command-line @value{DF}s. @menu * Filetrans Function:: A function for handling data file transitions. * Rewind Function:: A function for rereading the current file. * File Checking:: Checking that data files are readable. * Empty Files:: Checking for zero-length files. * Ignoring Assigns:: Treating assignments as file names. @end menu @node Filetrans Function @subsection Noting @value{DDF} Boundaries @cindex files @subentry managing @subentry data file boundaries @cindex files @subentry initialization and cleanup The @code{BEGIN} and @code{END} rules are each executed exactly once, at the beginning and end of your @command{awk} program, respectively (@pxref{BEGIN/END}). We (the @command{gawk} authors) once had a user who mistakenly thought that the @code{BEGIN} rules were executed at the beginning of each @value{DF} and the @code{END} rules were executed at the end of each @value{DF}. When informed that this was not the case, the user requested that we add new special patterns to @command{gawk}, named @code{BEGIN_FILE} and @code{END_FILE}, that would have the desired behavior. He even supplied us the code to do so. Adding these special patterns to @command{gawk} wasn't necessary; the job can be done cleanly in @command{awk} itself, as illustrated by the following library program. It arranges to call two user-supplied functions, @code{beginfile()} and @code{endfile()}, at the beginning and end of each @value{DF}. Besides solving the problem in only nine(!) lines of code, it does so @emph{portably}; this works with any implementation of @command{awk}: @example # transfile.awk # # Give the user a hook for filename transitions # # The user must supply functions beginfile() and endfile() # that each take the name of the file being started or # finished, respectively. @c # @c # Arnold Robbins, arnold@@skeeve.com, Public Domain @c # January 1992 FILENAME != _oldfilename @{ if (_oldfilename != "") endfile(_oldfilename) _oldfilename = FILENAME beginfile(FILENAME) @} END @{ endfile(FILENAME) @} @end example This file must be loaded before the user's ``main'' program, so that the rule it supplies is executed first. This rule relies on @command{awk}'s @code{FILENAME} variable, which automatically changes for each new @value{DF}. The current @value{FN} is saved in a private variable, @code{_oldfilename}. If @code{FILENAME} does not equal @code{_oldfilename}, then a new @value{DF} is being processed and it is necessary to call @code{endfile()} for the old file. Because @code{endfile()} should only be called if a file has been processed, the program first checks to make sure that @code{_oldfilename} is not the null string. The program then assigns the current @value{FN} to @code{_oldfilename} and calls @code{beginfile()} for the file. Because, like all @command{awk} variables, @code{_oldfilename} is initialized to the null string, this rule executes correctly even for the first @value{DF}. The program also supplies an @code{END} rule to do the final processing for the last file. Because this @code{END} rule comes before any @code{END} rules supplied in the ``main'' program, @code{endfile()} is called first. Once again, the value of multiple @code{BEGIN} and @code{END} rules should be clear. @cindex @code{beginfile()} user-defined function @cindex user-defined @subentry function @subentry @code{beginfile()} @cindex @code{endfile()} user-defined function @cindex user-defined @subentry function @subentry @code{endfile()} If the same @value{DF} occurs twice in a row on the command line, then @code{endfile()} and @code{beginfile()} are not executed at the end of the first pass and at the beginning of the second pass. The following version solves the problem: @example @c file eg/lib/ftrans.awk # ftrans.awk --- handle datafile transitions # # user supplies beginfile() and endfile() functions @c endfile @ignore @c file eg/lib/ftrans.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # November 1992 @c endfile @end ignore @c file eg/lib/ftrans.awk FNR == 1 @{ if (_filename_ != "") endfile(_filename_) _filename_ = FILENAME beginfile(FILENAME) @} END @{ endfile(_filename_) @} @c endfile @end example @ref{Wc Program} shows how this library function can be used and how it simplifies writing the main program. @cindex sidebar @subentry So Why Does @command{gawk} Have @code{BEGINFILE} and @code{ENDFILE}? @ifdocbook @docbook So Why Does @command{gawk} Have @code{BEGINFILE} and @code{ENDFILE}? @end docbook You are probably wondering, if @code{beginfile()} and @code{endfile()} functions can do the job, why does @command{gawk} have @code{BEGINFILE} and @code{ENDFILE} patterns? Good question. Normally, if @command{awk} cannot open a file, this causes an immediate fatal error. In this case, there is no way for a user-defined function to deal with the problem, as the mechanism for calling it relies on the file being open and at the first record. Thus, the main reason for @code{BEGINFILE} is to give you a ``hook'' to catch files that cannot be processed. @code{ENDFILE} exists for symmetry, and because it provides an easy way to do per-file cleanup processing. For more information, refer to @ref{BEGINFILE/ENDFILE}. @docbook @end docbook @end ifdocbook @ifnotdocbook @cartouche @center @b{So Why Does @command{gawk} Have @code{BEGINFILE} and @code{ENDFILE}?} You are probably wondering, if @code{beginfile()} and @code{endfile()} functions can do the job, why does @command{gawk} have @code{BEGINFILE} and @code{ENDFILE} patterns? Good question. Normally, if @command{awk} cannot open a file, this causes an immediate fatal error. In this case, there is no way for a user-defined function to deal with the problem, as the mechanism for calling it relies on the file being open and at the first record. Thus, the main reason for @code{BEGINFILE} is to give you a ``hook'' to catch files that cannot be processed. @code{ENDFILE} exists for symmetry, and because it provides an easy way to do per-file cleanup processing. For more information, refer to @ref{BEGINFILE/ENDFILE}. @end cartouche @end ifnotdocbook @node Rewind Function @subsection Rereading the Current File @cindex files @subentry reading Another request for a new built-in function was for a function that would make it possible to reread the current file. The requesting user didn't want to have to use @code{getline} (@pxref{Getline}) inside a loop. However, as long as you are not in the @code{END} rule, it is quite easy to arrange to immediately close the current input file and then start over with it from the top. For lack of a better name, we'll call the function @code{rewind()}: @cindex @code{rewind()} user-defined function @cindex user-defined @subentry function @subentry @code{rewind()} @example @c file eg/lib/rewind.awk # rewind.awk --- rewind the current file and start over @c endfile @ignore @c file eg/lib/rewind.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # September 2000 @c endfile @end ignore @c file eg/lib/rewind.awk function rewind( i) @{ # shift remaining arguments up for (i = ARGC; i > ARGIND; i--) ARGV[i] = ARGV[i-1] # make sure gawk knows to keep going ARGC++ # make current file next to get done ARGV[ARGIND+1] = FILENAME # do it nextfile @} @c endfile @end example The @code{rewind()} function relies on the @code{ARGIND} variable (@pxref{Auto-set}), which is specific to @command{gawk}. It also relies on the @code{nextfile} keyword (@pxref{Nextfile Statement}). Because of this, you should not call it from an @code{ENDFILE} rule. (This isn't necessary anyway, because @command{gawk} goes to the next file as soon as an @code{ENDFILE} rule finishes!) You need to be careful calling @code{rewind()}. You can end up causing infinite recursion if you don't pay attention. Here is an example use: @example $ @kbd{cat data} @print{} a @print{} b @print{} c @print{} d @print{} e $ cat @kbd{test.awk} @print{} FNR == 3 && ! rewound @{ @print{} rewound = 1 @print{} rewind() @print{} @} @print{} @print{} @{ print FILENAME, FNR, $0 @} $ @kbd{gawk -f rewind.awk -f test.awk data } @print{} data 1 a @print{} data 2 b @print{} data 1 a @print{} data 2 b @print{} data 3 c @group @print{} data 4 d @print{} data 5 e @end group @end example @node File Checking @subsection Checking for Readable @value{DDF}s @cindex troubleshooting @subentry readable data files @cindex readable data files, checking @cindex files @subentry skipping Normally, if you give @command{awk} a @value{DF} that isn't readable, it stops with a fatal error. There are times when you might want to just ignore such files and keep going.@footnote{The @code{BEGINFILE} special pattern (@pxref{BEGINFILE/ENDFILE}) provides an alternative mechanism for dealing with files that can't be opened. However, the code here provides a portable solution.} You can do this by prepending the following program to your @command{awk} program: @cindex @code{readable.awk} program @example @c file eg/lib/readable.awk # readable.awk --- library file to skip over unreadable files @c endfile @ignore @c file eg/lib/readable.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # October 2000 # December 2010 @c endfile @end ignore @c file eg/lib/readable.awk BEGIN @{ for (i = 1; i < ARGC; i++) @{ if (ARGV[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/ \ || ARGV[i] == "-" || ARGV[i] == "/dev/stdin") continue # assignment or standard input else if ((getline junk < ARGV[i]) < 0) # unreadable delete ARGV[i] else close(ARGV[i]) @} @} @c endfile @end example @cindex troubleshooting @subentry @code{getline} command This works, because the @code{getline} won't be fatal. Removing the element from @code{ARGV} with @code{delete} skips the file (because it's no longer in the list). See also @ref{ARGC and ARGV}. Because @command{awk} variable names only allow the English letters, the regular expression check purposely does not use character classes such as @samp{[:alpha:]} and @samp{[:alnum:]} (@pxref{Bracket Expressions}). @node Empty Files @subsection Checking for Zero-Length Files All known @command{awk} implementations silently skip over zero-length files. This is a by-product of @command{awk}'s implicit read-a-record-and-match-against-the-rules loop: when @command{awk} tries to read a record from an empty file, it immediately receives an end-of-file indication, closes the file, and proceeds on to the next command-line @value{DF}, @emph{without} executing any user-level @command{awk} program code. Using @command{gawk}'s @code{ARGIND} variable (@pxref{Built-in Variables}), it is possible to detect when an empty @value{DF} has been skipped. Similar to the library file presented in @ref{Filetrans Function}, the following library file calls a function named @code{zerofile()} that the user must provide. The arguments passed are the @value{FN} and the position in @code{ARGV} where it was found: @cindex @code{zerofile.awk} program @example @c file eg/lib/zerofile.awk # zerofile.awk --- library file to process empty input files @c endfile @ignore @c file eg/lib/zerofile.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # June 2003 @c endfile @end ignore @c file eg/lib/zerofile.awk BEGIN @{ Argind = 0 @} ARGIND > Argind + 1 @{ for (Argind++; Argind < ARGIND; Argind++) zerofile(ARGV[Argind], Argind) @} ARGIND != Argind @{ Argind = ARGIND @} END @{ if (ARGIND > Argind) for (Argind++; Argind <= ARGIND; Argind++) zerofile(ARGV[Argind], Argind) @} @c endfile @end example The user-level variable @code{Argind} allows the @command{awk} program to track its progress through @code{ARGV}. Whenever the program detects that @code{ARGIND} is greater than @samp{Argind + 1}, it means that one or more empty files were skipped. The action then calls @code{zerofile()} for each such file, incrementing @code{Argind} along the way. The @samp{Argind != ARGIND} rule simply keeps @code{Argind} up to date in the normal case. Finally, the @code{END} rule catches the case of any empty files at the end of the command-line arguments. Note that the test in the condition of the @code{for} loop uses the @samp{<=} operator, not @samp{<}. @node Ignoring Assigns @subsection Treating Assignments as @value{FFN}s @cindex assignments as file names @cindex file names @subentry assignments as Occasionally, you might not want @command{awk} to process command-line variable assignments (@pxref{Assignment Options}). In particular, if you have a @value{FN} that contains an @samp{=} character, @command{awk} treats the @value{FN} as an assignment and does not process it. Some users have suggested an additional command-line option for @command{gawk} to disable command-line assignments. However, some simple programming with a library file does the trick: @cindex @code{noassign.awk} program @example @c file eg/lib/noassign.awk # noassign.awk --- library file to avoid the need for a # special option that disables command-line assignments @c endfile @ignore @c file eg/lib/noassign.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # October 1999 @c endfile @end ignore @c file eg/lib/noassign.awk function disable_assigns(argc, argv, i) @{ for (i = 1; i < argc; i++) if (argv[i] ~ /^[a-zA-Z_][a-zA-Z0-9_]*=.*/) argv[i] = ("./" argv[i]) @} BEGIN @{ if (No_command_assign) disable_assigns(ARGC, ARGV) @} @c endfile @end example You then run your program this way: @example awk -v No_command_assign=1 -f noassign.awk -f yourprog.awk * @end example The function works by looping through the arguments. It prepends @samp{./} to any argument that matches the form of a variable assignment, turning that argument into a @value{FN}. The use of @code{No_command_assign} allows you to disable command-line assignments at invocation time, by giving the variable a true value. When not set, it is initially zero (i.e., false), so the command-line arguments are left alone. @node Getopt Function @section Processing Command-Line Options @cindex libraries of @command{awk} functions @subentry command-line options @cindex functions @subentry library @subentry command-line options @cindex command line @subentry options @subentry processing @cindex options @subentry command-line @subentry processing @cindex functions @subentry library @subentry C library @cindex arguments @subentry processing Most utilities on POSIX-compatible systems take options on the command line that can be used to change the way a program behaves. @command{awk} is an example of such a program (@pxref{Options}). Often, options take @dfn{arguments} (i.e., data that the program needs to correctly obey the command-line option). For example, @command{awk}'s @option{-F} option requires a string to use as the field separator. The first occurrence on the command line of either @option{--} or a string that does not begin with @samp{-} ends the options. @cindex @code{getopt()} function (C library) @cindex C library functions @subentry @code{getopt()} Modern Unix systems provide a C function named @code{getopt()} for processing command-line arguments. The programmer provides a string describing the one-letter options. If an option requires an argument, it is followed in the string with a colon. @code{getopt()} is also passed the count and values of the command-line arguments and is called in a loop. @code{getopt()} processes the command-line arguments for option letters. Each time around the loop, it returns a single character representing the next option letter that it finds, or @samp{?} if it finds an invalid option. When it returns @minus{}1, there are no options left on the command line. When using @code{getopt()}, options that do not take arguments can be grouped together. Furthermore, options that take arguments require that the argument be present. The argument can immediately follow the option letter, or it can be a separate command-line argument. Given a hypothetical program that takes three command-line options, @option{-a}, @option{-b}, and @option{-c}, where @option{-b} requires an argument, all of the following are valid ways of invoking the program: @example prog -a -b foo -c data1 data2 data3 prog -ac -bfoo -- data1 data2 data3 prog -acbfoo data1 data2 data3 @end example Notice that when the argument is grouped with its option, the rest of the argument is considered to be the option's argument. In this example, @option{-acbfoo} indicates that all of the @option{-a}, @option{-b}, and @option{-c} options were supplied, and that @samp{foo} is the argument to the @option{-b} option. @code{getopt()} provides four external variables that the programmer can use: @table @code @item optind The index in the argument value array (@code{argv}) where the first nonoption command-line argument can be found. @item optarg The string value of the argument to an option. @item opterr Usually @code{getopt()} prints an error message when it finds an invalid option. Setting @code{opterr} to zero disables this feature. (An application might want to print its own error message.) @item optopt The letter representing the command-line option. @end table The following C fragment shows how @code{getopt()} might process command-line arguments for @command{awk}: @example int main(int argc, char *argv[]) @{ @dots{} /* print our own message */ opterr = 0; while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) @{ switch (c) @{ case 'f': /* file */ @dots{} break; case 'F': /* field separator */ @dots{} break; case 'v': /* variable assignment */ @dots{} break; case 'W': /* extension */ @dots{} break; case '?': default: usage(); break; @} @} @dots{} @} @end example The GNU project's version of the original Unix utilities popularized the use of long command line options. For example, @option{--help} in addition to @option{-h}. Arguments to long options are either provided as separate command line arguments (@samp{--source '@var{program-text}'}) or separated from the option with an @samp{=} sign (@samp{--source='@var{program-text}'}). As a side point, @command{gawk} actually uses the GNU @code{getopt_long()} function to process both normal and GNU-style long options (@pxref{Options}). The abstraction provided by @code{getopt()} is very useful and is quite handy in @command{awk} programs as well. Following is an @command{awk} version of @code{getopt()} that accepts both short and long options. This function highlights one of the greatest weaknesses in @command{awk}, which is that it is very poor at manipulating single characters. The function needs repeated calls to @code{substr()} in order to access individual characters (@pxref{String Functions}).@footnote{This function was written before @command{gawk} acquired the ability to split strings into single characters using @code{""} as the separator. We have left it alone, as using @code{substr()} is more portable.} The discussion that follows walks through the code a bit at a time: @cindex @code{getopt()} user-defined function @cindex user-defined @subentry function @subentry @code{getopt()} @example @c file eg/lib/getopt.awk # getopt.awk --- Do C library getopt(3) function in awk # Also supports long options. @c endfile @ignore @c file eg/lib/getopt.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # # Initial version: March, 1991 # Revised: May, 1993 # Long options added by Greg Minshall, January 2020 @c endfile @end ignore @c file eg/lib/getopt.awk # External variables: # Optind -- index in ARGV of first nonoption argument # Optarg -- string value of argument to current option # Opterr -- if nonzero, print our own diagnostic # Optopt -- current option letter # Returns: # -1 at end of options # "?" for unrecognized option # a string representing the current option # Private Data: # _opti -- index in multiflag option, e.g., -abc @c endfile @end example The function starts out with comments presenting a list of the global variables it uses, what the return values are, what they mean, and any global variables that are ``private'' to this library function. Such documentation is essential for any program, and particularly for library functions. The @code{getopt()} function first checks that it was indeed called with a string of options (the @code{options} parameter). If both @code{options} and @code{longoptions} have a zero length, @code{getopt()} immediately returns @minus{}1: @cindex @code{getopt()} user-defined function @cindex user-defined @subentry function @subentry @code{getopt()} @example @c file eg/lib/getopt.awk function getopt(argc, argv, options, longopts, thisopt, i, j) @{ if (length(options) == 0 && length(longopts) == 0) return -1 # no options given @group if (argv[Optind] == "--") @{ # all done Optind++ _opti = 0 return -1 @end group @} else if (argv[Optind] !~ /^-[^:[:space:]]/) @{ _opti = 0 return -1 @} @c endfile @end example The next thing to check for is the end of the options. A @option{--} ends the command-line options, as does any command-line argument that does not begin with a @samp{-} (unless it is an argument to a preceding option). @code{Optind} steps through the array of command-line arguments; it retains its value across calls to @code{getopt()}, because it is a global variable. The regular expression @code{@w{/^-[^:[:space:]/}} checks for a @samp{-} followed by anything that is not whitespace and not a colon. If the current command-line argument does not match this pattern, it is not an option, and it ends option processing. Now, we check to see if we are processing a short (single letter) option, or a long option (indicated by two dashes, e.g., @samp{--filename}). If it is a short option, we continue on: @example @c file eg/lib/getopt.awk if (argv[Optind] !~ /^--/) @{ # if this is a short option if (_opti == 0) _opti = 2 thisopt = substr(argv[Optind], _opti, 1) Optopt = thisopt i = index(options, thisopt) if (i == 0) @{ if (Opterr) printf("%c -- invalid option\n", thisopt) > "/dev/stderr" if (_opti >= length(argv[Optind])) @{ Optind++ _opti = 0 @} else _opti++ return "?" @} @c endfile @end example The @code{_opti} variable tracks the position in the current command-line argument (@code{argv[Optind]}). If multiple options are grouped together with one @samp{-} (e.g., @option{-abx}), it is necessary to return them to the user one at a time. If @code{_opti} is equal to zero, it is set to two, which is the index in the string of the next character to look at (we skip the @samp{-}, which is at position one). The variable @code{thisopt} holds the character, obtained with @code{substr()}. It is saved in @code{Optopt} for the main program to use. If @code{thisopt} is not in the @code{options} string, then it is an invalid option. If @code{Opterr} is nonzero, @code{getopt()} prints an error message on the standard error that is similar to the message from the C version of @code{getopt()}. Because the option is invalid, it is necessary to skip it and move on to the next option character. If @code{_opti} is greater than or equal to the length of the current command-line argument, it is necessary to move on to the next argument, so @code{Optind} is incremented and @code{_opti} is reset to zero. Otherwise, @code{Optind} is left alone and @code{_opti} is merely incremented. In any case, because the option is invalid, @code{getopt()} returns @code{"?"}. The main program can examine @code{Optopt} if it needs to know what the invalid option letter actually is. Continuing on: @example @c file eg/lib/getopt.awk if (substr(options, i + 1, 1) == ":") @{ # get option argument if (length(substr(argv[Optind], _opti + 1)) > 0) Optarg = substr(argv[Optind], _opti + 1) else Optarg = argv[++Optind] _opti = 0 @} else Optarg = "" @c endfile @end example If the option requires an argument, the option letter is followed by a colon in the @code{options} string. If there are remaining characters in the current command-line argument (@code{argv[Optind]}), then the rest of that string is assigned to @code{Optarg}. Otherwise, the next command-line argument is used (@samp{-xFOO} versus @samp{@w{-x FOO}}). In either case, @code{_opti} is reset to zero, because there are no more characters left to examine in the current command-line argument. Continuing: @example @c file eg/lib/getopt.awk if (_opti == 0 || _opti >= length(argv[Optind])) @{ Optind++ _opti = 0 @} else _opti++ return thisopt @c endfile @end example Finally, for a short option, if @code{_opti} is either zero or greater than the length of the current command-line argument, it means this element in @code{argv} is through being processed, so @code{Optind} is incremented to point to the next element in @code{argv}. If neither condition is true, then only @code{_opti} is incremented, so that the next option letter can be processed on the next call to @code{getopt()}. On the other hand, if the earlier test found that this was a long option, we take a different branch: @example @c file eg/lib/getopt.awk @} else @{ j = index(argv[Optind], "=") if (j > 0) thisopt = substr(argv[Optind], 3, j - 3) else thisopt = substr(argv[Optind], 3) Optopt = thisopt @c endfile @end example First, we search this option for a possible embedded equal sign, as the specification of long options allows an argument to an option @samp{--someopt:} to be specified as @samp{--someopt=answer} as well as @samp{@w{--someopt answer}}. @example @c file eg/lib/getopt.awk i = match(longopts, "(^|,)" thisopt "($|[,:])") if (i == 0) @{ if (Opterr) printf("%s -- invalid option\n", thisopt) > "/dev/stderr" Optind++ return "?" @} @c endfile @end example Next, we try to find the current option in @code{longopts}. The regular expression given to @code{match()}, @code{@w{"(^|,)" thisopt "($|[,:])"}}, matches this option at the beginning of @code{longopts}, or at the beginning of a subsequent long option (the previous long option would have been terminated by a comma), and, in any case, either at the end of the @code{longopts} string (@samp{$}), or followed by a comma (separating this option from a subsequent option) or a colon (indicating this long option takes an argument (@samp{@w{[,:]}}). Using this regular expression, we check to see if the current option might possibly be in @code{longopts} (if @code{longopts} is not specified, this test will also fail). In case of an error, we possibly print an error message and then return @code{"?"}. Continuing on: @example @c file eg/lib/getopt.awk if (substr(longopts, i+1+length(thisopt), 1) == ":") @{ if (j > 0) Optarg = substr(argv[Optind], j + 1) else Optarg = argv[++Optind] @} else Optarg = "" @c endfile @end example We now check to see if this option takes an argument and, if so, we set @code{Optarg} to the value of that argument (either a value after an equal sign specified on the command line, immediately adjoining the long option string, or as the next argument on the command line). @example @c file eg/lib/getopt.awk Optind++ return thisopt @} @} @c endfile @end example We increase @code{Optind} (which we already increased once if a required argument was separated from its option by an equal sign), and return the long option (minus its leading dashes). The @code{BEGIN} rule initializes both @code{Opterr} and @code{Optind} to one. @code{Opterr} is set to one, because the default behavior is for @code{getopt()} to print a diagnostic message upon seeing an invalid option. @code{Optind} is set to one, because there's no reason to look at the program name, which is in @code{ARGV[0]}: @example @c file eg/lib/getopt.awk BEGIN @{ Opterr = 1 # default is to diagnose Optind = 1 # skip ARGV[0] # test program if (_getopt_test) @{ _myshortopts = "ab:cd" _mylongopts = "longa,longb:,otherc,otherd" while ((_go_c = getopt(ARGC, ARGV, _myshortopts, _mylongopts)) != -1) printf("c = <%s>, Optarg = <%s>\n", _go_c, Optarg) printf("non-option arguments:\n") for (; Optind < ARGC; Optind++) printf("\tARGV[%d] = <%s>\n", Optind, ARGV[Optind]) @} @} @c endfile @end example The rest of the @code{BEGIN} rule is a simple test program. Here are the results of some sample runs of the test program: @example $ @kbd{awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x} @print{} c =
, Optarg = <> @print{} c = , Optarg = <> @print{} c = , Optarg = @print{} non-option arguments: @print{} ARGV[3] = @print{} ARGV[4] = <-x> $ @kbd{awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc} @print{} c = , Optarg = <> @error{} x -- invalid option @print{} c = , Optarg = <> @print{} non-option arguments: @print{} ARGV[4] = @print{} ARGV[5] = $ @kbd{awk -f getopt.awk -v _getopt_test=1 -- -a \} > @kbd{--longa -b xx --longb=foo=bar --otherd --otherc arg1 arg2} @print{} c = , Optarg = <> @print{} c = , Optarg = <> @print{} c = , Optarg = @print{} c = , Optarg = @print{} c = , Optarg = <> @print{} c = , Optarg = <> @print{} non-option arguments: @print{} ARGV[8] = @print{} ARGV[9] = @end example In all the runs, the first @option{--} terminates the arguments to @command{awk}, so that it does not try to interpret the @option{-a}, etc., as its own options. @quotation NOTE After @code{getopt()} is through, user-level code must clear out all the elements of @code{ARGV} from 1 to @code{Optind}, so that @command{awk} does not try to process the command-line options as @value{FN}s. @end quotation Using @samp{#!} with the @option{-E} option may help avoid conflicts between your program's options and @command{gawk}'s options, as @option{-E} causes @command{gawk} to abandon processing of further options (@pxref{Executable Scripts} and @ifnotdocbook @pxref{Options}). @end ifnotdocbook @ifdocbook @ref{Options}). @end ifdocbook Several of the sample programs presented in @ref{Sample Programs}, use @code{getopt()} to process their arguments. @node Passwd Functions @section Reading the User Database @cindex libraries of @command{awk} functions @subentry user database, reading @cindex functions @subentry library @subentry user database, reading @cindex user database, reading @cindex database @subentry users, reading @cindex @code{PROCINFO} array The @code{PROCINFO} array (@pxref{Built-in Variables}) provides access to the current user's real and effective user and group ID numbers, and, if available, the user's supplementary group set. However, because these are numbers, they do not provide very useful information to the average user. There needs to be some way to find the user information associated with the user and group ID numbers. This @value{SECTION} presents a suite of functions for retrieving information from the user database. @xref{Group Functions} for a similar suite that retrieves information from the group database. @cindex @code{getpwent()} function (C library) @cindex C library functions @subentry @code{getpwent()} @cindex @code{getpwent()} user-defined function @cindex user-defined @subentry function @subentry @code{getpwent()} @cindex users, information about @subentry retrieving @cindex login information @cindex account information @cindex password file @cindex files @subentry password The POSIX standard does not define the file where user information is kept. Instead, it provides the @code{} header file and several C language subroutines for obtaining user information. The primary function is @code{getpwent()}, for ``get password entry.'' The ``password'' comes from the original user database file, @file{/etc/passwd}, which stores user information along with the encrypted passwords (hence the name). @cindex @command{pwcat} program Although an @command{awk} program could simply read @file{/etc/passwd} directly, this file may not contain complete information about the system's set of users.@footnote{It is often the case that password information is stored in a network database.} To be sure you are able to produce a readable and complete version of the user database, it is necessary to write a small C program that calls @code{getpwent()}. @code{getpwent()} is defined as returning a pointer to a @code{struct passwd}. Each time it is called, it returns the next entry in the database. When there are no more entries, it returns @code{NULL}, the null pointer. When this happens, the C program should call @code{endpwent()} to close the database. Following is @command{pwcat}, a C program that ``cats'' the password database: @example @c file eg/lib/pwcat.c /* * pwcat.c * * Generate a printable version of the password database. */ @c endfile @ignore @c file eg/lib/pwcat.c /* * Arnold Robbins, arnold@@skeeve.com, May 1993 * Public Domain * December 2010, move to ANSI C definition for main(). */ #if HAVE_CONFIG_H #include #endif @c endfile @end ignore @c file eg/lib/pwcat.c #include #include @c endfile @ignore @c file eg/lib/pwcat.c #if defined (STDC_HEADERS) #include #endif @c endfile @end ignore @c file eg/lib/pwcat.c int main(int argc, char **argv) @{ struct passwd *p; while ((p = getpwent()) != NULL) @c endfile @ignore @c file eg/lib/pwcat.c #ifdef HAVE_STRUCT_PASSWD_PW_PASSWD @c endfile @end ignore @c file eg/lib/pwcat.c printf("%s:%s:%ld:%ld:%s:%s:%s\n", p->pw_name, p->pw_passwd, (long) p->pw_uid, (long) p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell); @c endfile @ignore @c file eg/lib/pwcat.c #else printf("%s:*:%ld:%ld:%s:%s\n", p->pw_name, (long) p->pw_uid, (long) p->pw_gid, p->pw_dir, p->pw_shell); #endif @c endfile @end ignore @c file eg/lib/pwcat.c endpwent(); return 0; @} @c endfile @end example If you don't understand C, don't worry about it. The output from @command{pwcat} is the user database, in the traditional @file{/etc/passwd} format of colon-separated fields. The fields are: @table @asis @item Login name The user's login name. @item Encrypted password The user's encrypted password. This may not be available on some systems. @item User-ID The user's numeric user ID number. (On some systems, it's a C @code{long}, and not an @code{int}. Thus, we cast it to @code{long} for all cases.) @item Group-ID The user's numeric group ID number. (Similar comments about @code{long} versus @code{int} apply here.) @item Full name The user's full name, and perhaps other information associated with the user. @item Home directory The user's login (or ``home'') directory (familiar to shell programmers as @code{$HOME}). @item Login shell The program that is run when the user logs in. This is usually a shell, such as Bash. @end table A few lines representative of @command{pwcat}'s output are as follows: @cindex Jacobs, Andrew @cindex Robbins @subentry Arnold @cindex Robbins @subentry Miriam @example $ @kbd{pwcat} @print{} root:x:0:1:Operator:/:/bin/sh @print{} nobody:*:65534:65534::/: @print{} daemon:*:1:1::/: @print{} sys:*:2:2::/:/bin/csh @print{} bin:*:3:3::/bin: @print{} arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh @print{} miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh @print{} andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh @dots{} @end example With that introduction, following is a group of functions for getting user information. There are several functions here, corresponding to the C functions of the same names: @cindex @code{_pw_init()} user-defined function @cindex user-defined @subentry function @subentry @code{_pw_init()} @example @c file eg/lib/passwdawk.in # passwd.awk --- access password file information @c endfile @ignore @c file eg/lib/passwdawk.in # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 # Revised October 2000 # Revised December 2010 @c endfile @end ignore @c file eg/lib/passwdawk.in BEGIN @{ # tailor this to suit your system _pw_awklib = "/usr/local/libexec/awk/" @} function _pw_init( oldfs, oldrs, olddol0, pwcat, using_fw, using_fpat) @{ if (_pw_inited) return oldfs = FS oldrs = RS olddol0 = $0 using_fw = (PROCINFO["FS"] == "FIELDWIDTHS") using_fpat = (PROCINFO["FS"] == "FPAT") FS = ":" RS = "\n" pwcat = _pw_awklib "pwcat" while ((pwcat | getline) > 0) @{ _pw_byname[$1] = $0 _pw_byuid[$3] = $0 _pw_bycount[++_pw_total] = $0 @} close(pwcat) _pw_count = 0 _pw_inited = 1 FS = oldfs if (using_fw) FIELDWIDTHS = FIELDWIDTHS else if (using_fpat) FPAT = FPAT RS = oldrs $0 = olddol0 @} @c endfile @end example @cindex @code{BEGIN} pattern @subentry @code{pwcat} program The @code{BEGIN} rule sets a private variable to the directory where @command{pwcat} is stored. Because it is used to help out an @command{awk} library routine, we have chosen to put it in @file{/usr/local/libexec/awk}; however, you might want it to be in a different directory on your system. The function @code{_pw_init()} fills three copies of the user information into three associative arrays. The arrays are indexed by username (@code{_pw_byname}), by user ID number (@code{_pw_byuid}), and by order of occurrence (@code{_pw_bycount}). The variable @code{_pw_inited} is used for efficiency, as @code{_pw_init()} needs to be called only once. @cindex @code{PROCINFO} array @subentry testing the field splitting @cindex @code{getline} command @subentry @code{_pw_init()} function Because this function uses @code{getline} to read information from @command{pwcat}, it first saves the values of @code{FS}, @code{RS}, and @code{$0}. It notes in the variable @code{using_fw} whether field splitting with @code{FIELDWIDTHS} is in effect or not. Doing so is necessary, as these functions could be called from anywhere within a user's program, and the user may have his or her own way of splitting records and fields. This makes it possible to restore the correct field-splitting mechanism later. The test can only be true for @command{gawk}. It is false if using @code{FS} or @code{FPAT}, or on some other @command{awk} implementation. The code that checks for using @code{FPAT}, using @code{using_fpat} and @code{PROCINFO["FS"]}, is similar. The main part of the function uses a loop to read database lines, split the lines into fields, and then store the lines into each array as necessary. When the loop is done, @code{@w{_pw_init()}} cleans up by closing the pipeline, setting @code{@w{_pw_inited}} to one, and restoring @code{FS} (and @code{FIELDWIDTHS} or @code{FPAT} if necessary), @code{RS}, and @code{$0}. The use of @code{@w{_pw_count}} is explained shortly. @cindex @code{getpwnam()} function (C library) @cindex C library functions @subentry @code{getpwnam()} The @code{getpwnam()} function takes a username as a string argument. If that user is in the database, it returns the appropriate line. Otherwise, it relies on the array reference to a nonexistent element to create the element with the null string as its value: @cindex @code{getpwnam()} user-defined function @cindex user-defined @subentry function @subentry @code{getpwnam()} @example @group @c file eg/lib/passwdawk.in function getpwnam(name) @{ _pw_init() return _pw_byname[name] @} @c endfile @end group @end example @cindex @code{getpwuid()} function (C library) @cindex C library functions @subentry @code{getpwuid()} Similarly, the @code{getpwuid()} function takes a user ID number argument. If that user number is in the database, it returns the appropriate line. Otherwise, it returns the null string: @cindex @code{getpwuid()} user-defined function @cindex user-defined @subentry function @subentry @code{getpwuid()} @example @c file eg/lib/passwdawk.in function getpwuid(uid) @{ _pw_init() return _pw_byuid[uid] @} @c endfile @end example @cindex @code{getpwent()} function (C library) @cindex C library functions @subentry @code{getpwent()} The @code{getpwent()} function simply steps through the database, one entry at a time. It uses @code{_pw_count} to track its current position in the @code{_pw_bycount} array: @cindex @code{getpwent()} user-defined function @cindex user-defined @subentry function @subentry @code{getpwent()} @example @c file eg/lib/passwdawk.in function getpwent() @{ _pw_init() if (_pw_count < _pw_total) return _pw_bycount[++_pw_count] return "" @} @c endfile @end example @cindex @code{endpwent()} function (C library) @cindex C library functions @subentry @code{endpwent()} The @code{@w{endpwent()}} function resets @code{@w{_pw_count}} to zero, so that subsequent calls to @code{getpwent()} start over again: @cindex @code{endpwent()} user-defined function @cindex user-defined @subentry function @subentry @code{endpwent()} @example @c file eg/lib/passwdawk.in function endpwent() @{ _pw_count = 0 @} @c endfile @end example A conscious design decision in this suite is that each subroutine calls @code{@w{_pw_init()}} to initialize the database arrays. The overhead of running a separate process to generate the user database, and the I/O to scan it, are only incurred if the user's main program actually calls one of these functions. If this library file is loaded along with a user's program, but none of the routines are ever called, then there is no extra runtime overhead. (The alternative is move the body of @code{@w{_pw_init()}} into a @code{BEGIN} rule, which always runs @command{pwcat}. This simplifies the code but runs an extra process that may never be needed.) In turn, calling @code{_pw_init()} is not too expensive, because the @code{_pw_inited} variable keeps the program from reading the data more than once. If you are worried about squeezing every last cycle out of your @command{awk} program, the check of @code{_pw_inited} could be moved out of @code{_pw_init()} and duplicated in all the other functions. In practice, this is not necessary, as most @command{awk} programs are I/O-bound, and such a change would clutter up the code. The @command{id} program in @ref{Id Program} uses these functions. @node Group Functions @section Reading the Group Database @cindex libraries of @command{awk} functions @subentry group database, reading @cindex functions @subentry library @subentry group database, reading @cindex group database, reading @cindex database @subentry group, reading @cindex @code{PROCINFO} array @subentry group membership and @cindex @code{getgrent()} function (C library) @cindex C library functions @subentry @code{getgrent()} @cindex @code{getgrent()} user-defined function @cindex user-defined @subentry function @subentry @code{getgrent()} @cindex groups, information about @cindex account information @cindex group file @cindex files @subentry group Much of the discussion presented in @ref{Passwd Functions} applies to the group database as well. Although there has traditionally been a well-known file (@file{/etc/group}) in a well-known format, the POSIX standard only provides a set of C library routines (@code{} and @code{getgrent()}) for accessing the information. Even though this file may exist, it may not have complete information. Therefore, as with the user database, it is necessary to have a small C program that generates the group database as its output. @command{grcat}, a C program that ``cats'' the group database, is as follows: @cindex @command{grcat} program @example @c file eg/lib/grcat.c /* * grcat.c * * Generate a printable version of the group database. */ @c endfile @ignore @c file eg/lib/grcat.c /* * Arnold Robbins, arnold@@skeeve.com, May 1993 * Public Domain * December 2010, move to ANSI C definition for main(). */ #if HAVE_CONFIG_H #include #endif #if defined (STDC_HEADERS) #include #endif #ifndef HAVE_GETGRENT int main() { return 0; } #else @c endfile @end ignore @c file eg/lib/grcat.c #include #include int main(int argc, char **argv) @{ struct group *g; int i; while ((g = getgrent()) != NULL) @{ @c endfile @ignore @c file eg/lib/grcat.c #ifdef HAVE_STRUCT_GROUP_GR_PASSWD @c endfile @end ignore @c file eg/lib/grcat.c printf("%s:%s:%ld:", g->gr_name, g->gr_passwd, (long) g->gr_gid); @c endfile @ignore @c file eg/lib/grcat.c #else printf("%s:*:%ld:", g->gr_name, (long) g->gr_gid); #endif @c endfile @end ignore @c file eg/lib/grcat.c for (i = 0; g->gr_mem[i] != NULL; i++) @{ printf("%s", g->gr_mem[i]); @group if (g->gr_mem[i+1] != NULL) putchar(','); @} @end group putchar('\n'); @} endgrent(); return 0; @} @c endfile @ignore @c file eg/lib/grcat.c #endif /* HAVE_GETGRENT */ @c endfile @end ignore @end example Each line in the group database represents one group. The fields are separated with colons and represent the following information: @table @asis @item Group Name The group's name. @item Group Password The group's encrypted password. In practice, this field is never used; it is usually empty or set to @samp{*}. @item Group ID Number The group's numeric group ID number; the association of name to number must be unique within the file. (On some systems it's a C @code{long}, and not an @code{int}. Thus, we cast it to @code{long} for all cases.) @item Group Member List A comma-separated list of usernames. These users are members of the group. Modern Unix systems allow users to be members of several groups simultaneously. If your system does, then there are elements @code{"group1"} through @code{"group@var{N}"} in @code{PROCINFO} for those group ID numbers. (Note that @code{PROCINFO} is a @command{gawk} extension; @pxref{Built-in Variables}.) @end table Here is what running @command{grcat} might produce: @example $ @kbd{grcat} @print{} wheel:*:0:arnold @print{} nogroup:*:65534: @print{} daemon:*:1: @print{} kmem:*:2: @print{} staff:*:10:arnold,miriam,andy @print{} other:*:20: @dots{} @end example Here are the functions for obtaining information from the group database. There are several, modeled after the C library functions of the same names: @cindex @code{getline} command @subentry @code{_gr_init()} user-defined function @cindex @code{_gr_init()} user-defined function @cindex user-defined @subentry function @subentry @code{_gr_init()} @example @c file eg/lib/groupawk.in # group.awk --- functions for dealing with the group file @c endfile @ignore @c file eg/lib/groupawk.in # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 # Revised October 2000 # Revised December 2010 @c endfile @end ignore @c line break on _gr_init for smallbook @c file eg/lib/groupawk.in BEGIN @{ # Change to suit your system _gr_awklib = "/usr/local/libexec/awk/" @} function _gr_init( oldfs, oldrs, olddol0, grcat, using_fw, using_fpat, n, a, i) @{ if (_gr_inited) return oldfs = FS oldrs = RS olddol0 = $0 using_fw = (PROCINFO["FS"] == "FIELDWIDTHS") using_fpat = (PROCINFO["FS"] == "FPAT") FS = ":" RS = "\n" grcat = _gr_awklib "grcat" while ((grcat | getline) > 0) @{ if ($1 in _gr_byname) _gr_byname[$1] = _gr_byname[$1] "," $4 else _gr_byname[$1] = $0 if ($3 in _gr_bygid) _gr_bygid[$3] = _gr_bygid[$3] "," $4 else _gr_bygid[$3] = $0 n = split($4, a, "[ \t]*,[ \t]*") for (i = 1; i <= n; i++) if (a[i] in _gr_groupsbyuser) _gr_groupsbyuser[a[i]] = _gr_groupsbyuser[a[i]] " " $1 else _gr_groupsbyuser[a[i]] = $1 _gr_bycount[++_gr_count] = $0 @} close(grcat) _gr_count = 0 _gr_inited++ FS = oldfs if (using_fw) FIELDWIDTHS = FIELDWIDTHS else if (using_fpat) FPAT = FPAT RS = oldrs $0 = olddol0 @} @c endfile @end example The @code{BEGIN} rule sets a private variable to the directory where @command{grcat} is stored. Because it is used to help out an @command{awk} library routine, we have chosen to put it in @file{/usr/local/libexec/awk}. You might want it to be in a different directory on your system. These routines follow the same general outline as the user database routines (@pxref{Passwd Functions}). The @code{@w{_gr_inited}} variable is used to ensure that the database is scanned no more than once. The @code{@w{_gr_init()}} function first saves @code{FS}, @code{RS}, and @code{$0}, and then sets @code{FS} and @code{RS} to the correct values for scanning the group information. It also takes care to note whether @code{FIELDWIDTHS} or @code{FPAT} is being used, and to restore the appropriate field-splitting mechanism. The group information is stored in several associative arrays. The arrays are indexed by group name (@code{@w{_gr_byname}}), by group ID number (@code{@w{_gr_bygid}}), and by position in the database (@code{@w{_gr_bycount}}). There is an additional array indexed by username (@code{@w{_gr_groupsbyuser}}), which is a space-separated list of groups to which each user belongs. Unlike in the user database, it is possible to have multiple records in the database for the same group. This is common when a group has a large number of members. A pair of such entries might look like the following: @example tvpeople:*:101:johnny,jay,arsenio tvpeople:*:101:david,conan,tom,joan @end example For this reason, @code{_gr_init()} looks to see if a group name or group ID number is already seen. If so, the usernames are simply concatenated onto the previous list of users.@footnote{There is a subtle problem with the code just presented. Suppose that the first time there were no names. This code adds the names with a leading comma. It also doesn't check that there is a @code{$4}.} Finally, @code{_gr_init()} closes the pipeline to @command{grcat}, restores @code{FS} (and @code{FIELDWIDTHS} or @code{FPAT}, if necessary), @code{RS}, and @code{$0}, initializes @code{_gr_count} to zero (it is used later), and makes @code{_gr_inited} nonzero. @cindex @code{getgrnam()} function (C library) @cindex C library functions @subentry @code{getgrnam()} The @code{getgrnam()} function takes a group name as its argument, and if that group exists, it is returned. Otherwise, it relies on the array reference to a nonexistent element to create the element with the null string as its value: @cindex @code{getgrnam()} user-defined function @cindex user-defined @subentry function @subentry @code{getgrnam()} @example @c file eg/lib/groupawk.in function getgrnam(group) @{ _gr_init() return _gr_byname[group] @} @c endfile @end example @cindex @code{getgrgid()} function (C library) @cindex C library functions @subentry @code{getgrgid()} The @code{getgrgid()} function is similar; it takes a numeric group ID and looks up the information associated with that group ID: @cindex @code{getgrgid()} user-defined function @cindex user-defined @subentry function @subentry @code{getgrgid()} @example @c file eg/lib/groupawk.in function getgrgid(gid) @{ _gr_init() return _gr_bygid[gid] @} @c endfile @end example @cindex @code{getgruser()} function (C library) @cindex C library functions @subentry @code{getgruser()} The @code{getgruser()} function does not have a C counterpart. It takes a username and returns the list of groups that have the user as a member: @cindex @code{getgruser()} user-defined function @cindex user-defined @subentry function @subentry @code{getgruser()} @example @c file eg/lib/groupawk.in function getgruser(user) @{ _gr_init() return _gr_groupsbyuser[user] @} @c endfile @end example @cindex @code{getgrent()} function (C library) @cindex C library functions @subentry @code{getgrent()} The @code{getgrent()} function steps through the database one entry at a time. It uses @code{_gr_count} to track its position in the list: @cindex @code{getgrent()} user-defined function @cindex user-defined @subentry function @subentry @code{getgrent()} @example @c file eg/lib/groupawk.in function getgrent() @{ _gr_init() if (++_gr_count in _gr_bycount) return _gr_bycount[_gr_count] @group return "" @} @end group @c endfile @end example @cindex @code{endgrent()} function (C library) @cindex C library functions @subentry @code{endgrent()} The @code{endgrent()} function resets @code{_gr_count} to zero so that @code{getgrent()} can start over again: @cindex @code{endgrent()} user-defined function @cindex user-defined @subentry function @subentry @code{endgrent()} @example @c file eg/lib/groupawk.in function endgrent() @{ _gr_count = 0 @} @c endfile @end example As with the user database routines, each function calls @code{_gr_init()} to initialize the arrays. Doing so only incurs the extra overhead of running @command{grcat} if these functions are used (as opposed to moving the body of @code{_gr_init()} into a @code{BEGIN} rule). Most of the work is in scanning the database and building the various associative arrays. The functions that the user calls are themselves very simple, relying on @command{awk}'s associative arrays to do work. The @command{id} program in @ref{Id Program} uses these functions. @node Walking Arrays @section Traversing Arrays of Arrays @ref{Arrays of Arrays} described how @command{gawk} provides arrays of arrays. In particular, any element of an array may be either a scalar or another array. The @code{isarray()} function (@pxref{Type Functions}) lets you distinguish an array from a scalar. The following function, @code{walk_array()}, recursively traverses an array, printing the element indices and values. You call it with the array and a string representing the name of the array: @cindex @code{walk_array()} user-defined function @cindex user-defined @subentry function @subentry @code{walk_array()} @example @c file eg/lib/walkarray.awk function walk_array(arr, name, i) @{ for (i in arr) @{ if (isarray(arr[i])) walk_array(arr[i], (name "[" i "]")) else printf("%s[%s] = %s\n", name, i, arr[i]) @} @} @c endfile @end example @noindent It works by looping over each element of the array. If any given element is itself an array, the function calls itself recursively, passing the subarray and a new string representing the current index. Otherwise, the function simply prints the element's name, index, and value. Here is a main program to demonstrate: @example BEGIN @{ a[1] = 1 a[2][1] = 21 a[2][2] = 22 a[3] = 3 a[4][1][1] = 411 a[4][2] = 42 walk_array(a, "a") @} @end example When run, the program produces the following output: @example $ @kbd{gawk -f walk_array.awk} @print{} a[1] = 1 @print{} a[2][1] = 21 @print{} a[2][2] = 22 @print{} a[3] = 3 @print{} a[4][1][1] = 411 @print{} a[4][2] = 42 @end example The function just presented simply prints the name and value of each scalar array element. However, it is easy to generalize it, by passing in the name of a function to call when walking an array. The modified function looks like this: @example @c file eg/lib/processarray.awk function process_array(arr, name, process, do_arrays, i, new_name) @{ for (i in arr) @{ new_name = (name "[" i "]") if (isarray(arr[i])) @{ if (do_arrays) @@process(new_name, arr[i]) process_array(arr[i], new_name, process, do_arrays) @} else @@process(new_name, arr[i]) @} @} @c endfile @end example The arguments are as follows: @table @code @item arr The array. @item name The name of the array (a string). @item process The name of the function to call. @item do_arrays If this is true, the function can handle elements that are subarrays. @end table If subarrays are to be processed, that is done before walking them further. When run with the following scaffolding, the function produces the same results as does the earlier version of @code{walk_array()}: @example BEGIN @{ a[1] = 1 a[2][1] = 21 a[2][2] = 22 a[3] = 3 a[4][1][1] = 411 a[4][2] = 42 process_array(a, "a", "do_print", 0) @} function do_print(name, element) @{ printf "%s = %s\n", name, element @} @end example @node Library Functions Summary @section Summary @itemize @value{BULLET} @item Reading programs is an excellent way to learn Good Programming. The functions and programs provided in this @value{CHAPTER} and the next are intended to serve that purpose. @item When writing general-purpose library functions, put some thought into how to name any global variables so that they won't conflict with variables from a user's program. @item The functions presented here fit into the following categories: @c nested list @table @asis @item General problems Number-to-string conversion, testing assertions, rounding, random number generation, converting characters to numbers, joining strings, getting easily usable time-of-day information, and reading a whole file in one shot @item Managing @value{DF}s Noting @value{DF} boundaries, rereading the current file, checking for readable files, checking for zero-length files, and treating assignments as @value{FN}s @item Processing command-line options An @command{awk} version of the standard C @code{getopt()} function @item Reading the user and group databases Two sets of routines that parallel the C library versions @item Traversing arrays of arrays Two functions that traverse an array of arrays to any depth @end table @c end nested list @end itemize @c EXCLUDE START @node Library Exercises @section Exercises @enumerate @item In @ref{Empty Files}, we presented the @file{zerofile.awk} program, which made use of @command{gawk}'s @code{ARGIND} variable. Can this problem be solved without relying on @code{ARGIND}? If so, how? @ignore # zerofile2.awk --- same thing, portably BEGIN @{ ARGIND = Argind = 0 for (i = 1; i < ARGC; i++) Fnames[ARGV[i]]++ @} FNR == 1 @{ while (ARGV[ARGIND] != FILENAME) ARGIND++ Seen[FILENAME]++ if (Seen[FILENAME] == Fnames[FILENAME]) do ARGIND++ while (ARGV[ARGIND] != FILENAME) @} ARGIND > Argind + 1 @{ for (Argind++; Argind < ARGIND; Argind++) zerofile(ARGV[Argind], Argind) @} ARGIND != Argind @{ Argind = ARGIND @} END @{ if (ARGIND < ARGC - 1) ARGIND = ARGC - 1 if (ARGIND > Argind) for (Argind++; Argind <= ARGIND; Argind++) zerofile(ARGV[Argind], Argind) @} @end ignore @item As a related challenge, revise that code to handle the case where an intervening value in @code{ARGV} is a variable assignment. @ignore @c June 13 2015: Antonio points out that this is answered in the text. Ooops. @item @ref{Walking Arrays} presented a function that walked a multidimensional array to print it out. However, walking an array and processing each element is a general-purpose operation. Generalize the @code{walk_array()} function by adding an additional parameter named @code{process}. Then, inside the loop, instead of printing the array element's index and value, use the indirect function call syntax (@pxref{Indirect Calls}) on @code{process}, passing it the index and the value. When calling @code{walk_array()}, you would pass the name of a user-defined function that expects to receive an index and a value, and then processes the element. Test your new version by printing the array; you should end up with output identical to that of the original version. @end ignore @end enumerate @c EXCLUDE END @node Sample Programs @chapter Practical @command{awk} Programs @cindex @command{awk} programs @subentry examples of @c FULLXREF ON @ref{Library Functions}, presents the idea that reading programs in a language contributes to learning that language. This @value{CHAPTER} continues that theme, presenting a potpourri of @command{awk} programs for your reading enjoyment. @c FULLXREF OFF @ifnotinfo There are three @value{SECTION}s. The first describes how to run the programs presented in this @value{CHAPTER}. The second presents @command{awk} versions of several common POSIX utilities. These are programs that you are hopefully already familiar with, and therefore whose problems are understood. By reimplementing these programs in @command{awk}, you can focus on the @command{awk}-related aspects of solving the programming problems. The third is a grab bag of interesting programs. These solve a number of different data-manipulation and management problems. Many of the programs are short, which emphasizes @command{awk}'s ability to do a lot in just a few lines of code. @end ifnotinfo Many of these programs use library functions presented in @ref{Library Functions}. @menu * Running Examples:: How to run these examples. * Clones:: Clones of common utilities. * Miscellaneous Programs:: Some interesting @command{awk} programs. * Programs Summary:: Summary of programs. * Programs Exercises:: Exercises. @end menu @node Running Examples @section Running the Example Programs To run a given program, you would typically do something like this: @example awk -f @var{program} -- @var{options} @var{files} @end example @noindent Here, @var{program} is the name of the @command{awk} program (such as @file{cut.awk}), @var{options} are any command-line options for the program that start with a @samp{-}, and @var{files} are the actual @value{DF}s. If your system supports the @samp{#!} executable interpreter mechanism (@pxref{Executable Scripts}), you can instead run your program directly: @example cut.awk -c1-8 myfiles > results @end example If your @command{awk} is not @command{gawk}, you may instead need to use this: @example cut.awk -- -c1-8 myfiles > results @end example @node Clones @section Reinventing Wheels for Fun and Profit @cindex POSIX @subentry programs, implementing in @command{awk} This @value{SECTION} presents a number of POSIX utilities implemented in @command{awk}. Reinventing these programs in @command{awk} is often enjoyable, because the algorithms can be very clearly expressed, and the code is usually very concise and simple. This is true because @command{awk} does so much for you. It should be noted that these programs are not necessarily intended to replace the installed versions on your system. Nor may all of these programs be fully compliant with the most recent POSIX standard. This is not a problem; their purpose is to illustrate @command{awk} language programming for ``real-world'' tasks. The programs are presented in alphabetical order. @menu * Cut Program:: The @command{cut} utility. * Egrep Program:: The @command{egrep} utility. * Id Program:: The @command{id} utility. * Split Program:: The @command{split} utility. * Tee Program:: The @command{tee} utility. * Uniq Program:: The @command{uniq} utility. * Wc Program:: The @command{wc} utility. @end menu @node Cut Program @subsection Cutting Out Fields and Columns @cindex @command{cut} utility @cindex @command{cut} utility @cindex fields @subentry cutting @cindex columns @subentry cutting The @command{cut} utility selects, or ``cuts,'' characters or fields from its standard input and sends them to its standard output. Fields are separated by TABs by default, but you may supply a command-line option to change the field @dfn{delimiter} (i.e., the field-separator character). @command{cut}'s definition of fields is less general than @command{awk}'s. A common use of @command{cut} might be to pull out just the login names of logged-on users from the output of @command{who}. For example, the following pipeline generates a sorted, unique list of the logged-on users: @example who | cut -c1-8 | sort | uniq @end example The options for @command{cut} are: @table @code @item -c @var{list} Use @var{list} as the list of characters to cut out. Items within the list may be separated by commas, and ranges of characters can be separated with dashes. The list @samp{1-8,15,22-35} specifies characters 1 through 8, 15, and 22 through 35. @item -f @var{list} Use @var{list} as the list of fields to cut out. @item -d @var{delim} Use @var{delim} as the field-separator character instead of the TAB character. @item -s Suppress printing of lines that do not contain the field delimiter. @end table The @command{awk} implementation of @command{cut} uses the @code{getopt()} library function (@pxref{Getopt Function}) and the @code{join()} library function (@pxref{Join Function}). The program begins with a comment describing the options, the library functions needed, and a @code{usage()} function that prints out a usage message and exits. @code{usage()} is called if invalid arguments are supplied: @cindex @code{cut.awk} program @example @c file eg/prog/cut.awk # cut.awk --- implement cut in awk @c endfile @ignore @c file eg/prog/cut.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/cut.awk # Options: # -f list Cut fields # -d c Field delimiter character # -c list Cut characters # # -s Suppress lines without the delimiter # # Requires getopt() and join() library functions @group function usage() @{ print("usage: cut [-f list] [-d c] [-s] [files...]") > "/dev/stderr" print("usage: cut [-c list] [files...]") > "/dev/stderr" exit 1 @} @end group @c endfile @end example @cindex @code{BEGIN} pattern @subentry running @command{awk} programs and @cindex @code{FS} variable @subentry running @command{awk} programs and Next comes a @code{BEGIN} rule that parses the command-line options. It sets @code{FS} to a single TAB character, because that is @command{cut}'s default field separator. The rule then sets the output field separator to be the same as the input field separator. A loop using @code{getopt()} steps through the command-line options. Exactly one of the variables @code{by_fields} or @code{by_chars} is set to true, to indicate that processing should be done by fields or by characters, respectively. When cutting by characters, the output field separator is set to the null string: @example @c file eg/prog/cut.awk BEGIN @{ FS = "\t" # default OFS = FS while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) @{ if (c == "f") @{ by_fields = 1 fieldlist = Optarg @} else if (c == "c") @{ by_chars = 1 fieldlist = Optarg OFS = "" @} else if (c == "d") @{ if (length(Optarg) > 1) @{ printf("cut: using first character of %s" \ " for delimiter\n", Optarg) > "/dev/stderr" Optarg = substr(Optarg, 1, 1) @} fs = FS = Optarg OFS = FS if (FS == " ") # defeat awk semantics FS = "[ ]" @} else if (c == "s") suppress = 1 else usage() @} # Clear out options for (i = 1; i < Optind; i++) ARGV[i] = "" @c endfile @end example @cindex field separator @subentry spaces as The code must take special care when the field delimiter is a space. Using a single space (@code{@w{" "}}) for the value of @code{FS} is incorrect---@command{awk} would separate fields with runs of spaces, TABs, and/or newlines, and we want them to be separated with individual spaces. To this end, we save the original space character in the variable @code{fs} for later use; after setting @code{FS} to @code{"[ ]"} we can't use it directly to see if the field delimiter character is in the string. Also remember that after @code{getopt()} is through (as described in @ref{Getopt Function}), we have to clear out all the elements of @code{ARGV} from 1 to @code{Optind}, so that @command{awk} does not try to process the command-line options as @value{FN}s. After dealing with the command-line options, the program verifies that the options make sense. Only one or the other of @option{-c} and @option{-f} should be used, and both require a field list. Then the program calls either @code{set_fieldlist()} or @code{set_charlist()} to pull apart the list of fields or characters: @example @c file eg/prog/cut.awk if (by_fields && by_chars) usage() if (by_fields == 0 && by_chars == 0) by_fields = 1 # default @group if (fieldlist == "") @{ print "cut: needs list for -c or -f" > "/dev/stderr" exit 1 @} @end group if (by_fields) set_fieldlist() else set_charlist() @} @c endfile @end example @code{set_fieldlist()} splits the field list apart at the commas into an array. Then, for each element of the array, it looks to see if the element is actually a range, and if so, splits it apart. The function checks the range to make sure that the first number is smaller than the second. Each number in the list is added to the @code{flist} array, which simply lists the fields that will be printed. Normal field splitting is used. The program lets @command{awk} handle the job of doing the field splitting: @example @c file eg/prog/cut.awk function set_fieldlist( n, m, i, j, k, f, g) @{ n = split(fieldlist, f, ",") j = 1 # index in flist for (i = 1; i <= n; i++) @{ if (index(f[i], "-") != 0) @{ # a range m = split(f[i], g, "-") @group if (m != 2 || g[1] >= g[2]) @{ printf("cut: bad field list: %s\n", f[i]) > "/dev/stderr" exit 1 @} @end group for (k = g[1]; k <= g[2]; k++) flist[j++] = k @} else flist[j++] = f[i] @} nfields = j - 1 @} @c endfile @end example The @code{set_charlist()} function is more complicated than @code{set_fieldlist()}. The idea here is to use @command{gawk}'s @code{FIELDWIDTHS} variable (@pxref{Constant Size}), which describes constant-width input. When using a character list, that is exactly what we have. Setting up @code{FIELDWIDTHS} is more complicated than simply listing the fields that need to be printed. We have to keep track of the fields to print and also the intervening characters that have to be skipped. For example, suppose you wanted characters 1 through 8, 15, and 22 through 35. You would use @samp{-c 1-8,15,22-35}. The necessary value for @code{FIELDWIDTHS} is @code{@w{"8 6 1 6 14"}}. This yields five fields, and the fields to print are @code{$1}, @code{$3}, and @code{$5}. The intermediate fields are @dfn{filler}, which is stuff in between the desired data. @code{flist} lists the fields to print, and @code{t} tracks the complete field list, including filler fields: @example @c file eg/prog/cut.awk function set_charlist( field, i, j, f, g, n, m, t, filler, last, len) @{ field = 1 # count total fields n = split(fieldlist, f, ",") j = 1 # index in flist for (i = 1; i <= n; i++) @{ if (index(f[i], "-") != 0) @{ # range m = split(f[i], g, "-") if (m != 2 || g[1] >= g[2]) @{ printf("cut: bad character list: %s\n", f[i]) > "/dev/stderr" exit 1 @} len = g[2] - g[1] + 1 if (g[1] > 1) # compute length of filler filler = g[1] - last - 1 else filler = 0 @group if (filler) t[field++] = filler @end group t[field++] = len # length of field last = g[2] flist[j++] = field - 1 @} else @{ if (f[i] > 1) filler = f[i] - last - 1 else filler = 0 if (filler) t[field++] = filler t[field++] = 1 last = f[i] flist[j++] = field - 1 @} @} FIELDWIDTHS = join(t, 1, field - 1) nfields = j - 1 @} @c endfile @end example Next is the rule that processes the data. If the @option{-s} option is given, then @code{suppress} is true. The first @code{if} statement makes sure that the input record does have the field separator. If @command{cut} is processing fields, @code{suppress} is true, and the field separator character is not in the record, then the record is skipped. If the record is valid, then @command{gawk} has split the data into fields, either using the character in @code{FS} or using fixed-length fields and @code{FIELDWIDTHS}. The loop goes through the list of fields that should be printed. The corresponding field is printed if it contains data. If the next field also has data, then the separator character is written out between the fields: @example @c file eg/prog/cut.awk @{ if (by_fields && suppress && index($0, fs) == 0) next for (i = 1; i <= nfields; i++) @{ if ($flist[i] != "") @{ printf "%s", $flist[i] if (i < nfields && $flist[i+1] != "") printf "%s", OFS @} @} print "" @} @c endfile @end example This version of @command{cut} relies on @command{gawk}'s @code{FIELDWIDTHS} variable to do the character-based cutting. It is possible in other @command{awk} implementations to use @code{substr()} (@pxref{String Functions}), but it is also extremely painful. The @code{FIELDWIDTHS} variable supplies an elegant solution to the problem of picking the input line apart by characters. @node Egrep Program @subsection Searching for Regular Expressions in Files @cindex regular expressions @subentry searching for @cindex searching @subentry files for regular expressions @cindex files @subentry searching for regular expressions @cindex @command{egrep} utility The @command{egrep} utility searches files for patterns. It uses regular expressions that are almost identical to those available in @command{awk} (@pxref{Regexp}). You invoke it as follows: @display @command{egrep} [@var{options}] @code{'@var{pattern}'} @var{files} @dots{} @end display The @var{pattern} is a regular expression. In typical usage, the regular expression is quoted to prevent the shell from expanding any of the special characters as @value{FN} wildcards. Normally, @command{egrep} prints the lines that matched. If multiple @value{FN}s are provided on the command line, each output line is preceded by the name of the file and a colon. The options to @command{egrep} are as follows: @table @code @item -c Print out a count of the lines that matched the pattern, instead of the lines themselves. @item -s Be silent. No output is produced and the exit value indicates whether the pattern was matched. @item -v Invert the sense of the test. @command{egrep} prints the lines that do @emph{not} match the pattern and exits successfully if the pattern is not matched. @item -i Ignore case distinctions in both the pattern and the input data. @item -l Only print (list) the names of the files that matched, not the lines that matched. @item -e @var{pattern} Use @var{pattern} as the regexp to match. The purpose of the @option{-e} option is to allow patterns that start with a @samp{-}. @end table This version uses the @code{getopt()} library function (@pxref{Getopt Function}) and the file transition library program (@pxref{Filetrans Function}). The program begins with a descriptive comment and then a @code{BEGIN} rule that processes the command-line arguments with @code{getopt()}. The @option{-i} (ignore case) option is particularly easy with @command{gawk}; we just use the @code{IGNORECASE} predefined variable (@pxref{Built-in Variables}): @cindex @code{egrep.awk} program @example @c file eg/prog/egrep.awk # egrep.awk --- simulate egrep in awk # @c endfile @ignore @c file eg/prog/egrep.awk # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/egrep.awk # Options: # -c count of lines # -s silent - use exit value # -v invert test, success if no match # -i ignore case # -l print filenames only # -e argument is pattern # # Requires getopt and file transition library functions BEGIN @{ while ((c = getopt(ARGC, ARGV, "ce:svil")) != -1) @{ if (c == "c") count_only++ else if (c == "s") no_print++ else if (c == "v") invert++ else if (c == "i") IGNORECASE = 1 else if (c == "l") filenames_only++ else if (c == "e") pattern = Optarg else usage() @} @c endfile @end example Next comes the code that handles the @command{egrep}-specific behavior. If no pattern is supplied with @option{-e}, the first nonoption on the command line is used. The @command{awk} command-line arguments up to @code{ARGV[Optind]} are cleared, so that @command{awk} won't try to process them as files. If no files are specified, the standard input is used, and if multiple files are specified, we make sure to note this so that the @value{FN}s can precede the matched lines in the output: @example @c file eg/prog/egrep.awk if (pattern == "") pattern = ARGV[Optind++] for (i = 1; i < Optind; i++) ARGV[i] = "" if (Optind >= ARGC) @{ ARGV[1] = "-" ARGC = 2 @} else if (ARGC - Optind > 1) do_filenames++ # if (IGNORECASE) # pattern = tolower(pattern) @} @c endfile @end example The last two lines are commented out, as they are not needed in @command{gawk}. They should be uncommented if you have to use another version of @command{awk}. The next set of lines should be uncommented if you are not using @command{gawk}. This rule translates all the characters in the input line into lowercase if the @option{-i} option is specified.@footnote{It also introduces a subtle bug; if a match happens, we output the translated line, not the original.} The rule is commented out as it is not necessary with @command{gawk}: @example @c file eg/prog/egrep.awk #@{ # if (IGNORECASE) # $0 = tolower($0) #@} @c endfile @end example The @code{beginfile()} function is called by the rule in @file{ftrans.awk} when each new file is processed. In this case, it is very simple; all it does is initialize a variable @code{fcount} to zero. @code{fcount} tracks how many lines in the current file matched the pattern. Naming the parameter @code{junk} shows we know that @code{beginfile()} is called with a parameter, but that we're not interested in its value: @example @c file eg/prog/egrep.awk function beginfile(junk) @{ fcount = 0 @} @c endfile @end example The @code{endfile()} function is called after each file has been processed. It affects the output only when the user wants a count of the number of lines that matched. @code{no_print} is true only if the exit status is desired. @code{count_only} is true if line counts are desired. @command{egrep} therefore only prints line counts if printing and counting are enabled. The output format must be adjusted depending upon the number of files to process. Finally, @code{fcount} is added to @code{total}, so that we know the total number of lines that matched the pattern: @example @c file eg/prog/egrep.awk function endfile(file) @{ if (! no_print && count_only) @{ if (do_filenames) print file ":" fcount else print fcount @} @group total += fcount @} @end group @c endfile @end example The @code{BEGINFILE} and @code{ENDFILE} special patterns (@pxref{BEGINFILE/ENDFILE}) could be used, but then the program would be @command{gawk}-specific. Additionally, this example was written before @command{gawk} acquired @code{BEGINFILE} and @code{ENDFILE}. The following rule does most of the work of matching lines. The variable @code{matches} is true if the line matched the pattern. If the user wants lines that did not match, the sense of @code{matches} is inverted using the @samp{!} operator. @code{fcount} is incremented with the value of @code{matches}, which is either one or zero, depending upon a successful or unsuccessful match. If the line does not match, the @code{next} statement just moves on to the next record. A number of additional tests are made, but they are only done if we are not counting lines. First, if the user only wants the exit status (@code{no_print} is true), then it is enough to know that @emph{one} line in this file matched, and we can skip on to the next file with @code{nextfile}. Similarly, if we are only printing @value{FN}s, we can print the @value{FN}, and then skip to the next file with @code{nextfile}. Finally, each line is printed, with a leading @value{FN} and colon if necessary: @cindex @code{!} (exclamation point) @subentry @code{!} operator @cindex exclamation point (@code{!}) @subentry @code{!} operator @example @c file eg/prog/egrep.awk @{ matches = ($0 ~ pattern) if (invert) matches = ! matches fcount += matches # 1 or 0 if (! matches) next if (! count_only) @{ if (no_print) nextfile if (filenames_only) @{ print FILENAME nextfile @} if (do_filenames) print FILENAME ":" $0 else print @} @} @c endfile @end example The @code{END} rule takes care of producing the correct exit status. If there are no matches, the exit status is one; otherwise, it is zero: @example @c file eg/prog/egrep.awk END @{ exit (total == 0) @} @c endfile @end example The @code{usage()} function prints a usage message in case of invalid options, and then exits: @example @c file eg/prog/egrep.awk function usage() @{ print("Usage: egrep [-csvil] [-e pat] [files ...]") > "/dev/stderr" print("\n\tegrep [-csvil] pat [files ...]") > "/dev/stderr" exit 1 @} @c endfile @end example @node Id Program @subsection Printing Out User Information @cindex printing @subentry user information @cindex users, information about @subentry printing @cindex @command{id} utility The @command{id} utility lists a user's real and effective user ID numbers, real and effective group ID numbers, and the user's group set, if any. @command{id} only prints the effective user ID and group ID if they are different from the real ones. If possible, @command{id} also supplies the corresponding user and group names. The output might look like this: @example $ @kbd{id} @print{} uid=1000(arnold) gid=1000(arnold) groups=1000(arnold),4(adm),7(lp),27(sudo) @end example @cindex @code{PROCINFO} array @subentry user and group ID numbers and This information is part of what is provided by @command{gawk}'s @code{PROCINFO} array (@pxref{Built-in Variables}). However, the @command{id} utility provides a more palatable output than just individual numbers. Here is a simple version of @command{id} written in @command{awk}. It uses the user database library functions (@pxref{Passwd Functions}) and the group database library functions (@pxref{Group Functions}) from @ref{Library Functions}. The program is fairly straightforward. All the work is done in the @code{BEGIN} rule. The user and group ID numbers are obtained from @code{PROCINFO}. The code is repetitive. The entry in the user database for the real user ID number is split into parts at the @samp{:}. The name is the first field. Similar code is used for the effective user ID number and the group numbers: @cindex @code{id.awk} program @example @c file eg/prog/id.awk # id.awk --- implement id in awk # # Requires user and group library functions @c endfile @ignore @c file eg/prog/id.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 # Revised February 1996 # Revised May 2014 # Revised September 2014 @c endfile @end ignore @c file eg/prog/id.awk # output is: # uid=12(foo) euid=34(bar) gid=3(baz) \ # egid=5(blat) groups=9(nine),2(two),1(one) @group BEGIN @{ uid = PROCINFO["uid"] euid = PROCINFO["euid"] gid = PROCINFO["gid"] egid = PROCINFO["egid"] @end group printf("uid=%d", uid) pw = getpwuid(uid) pr_first_field(pw) @group if (euid != uid) @{ printf(" euid=%d", euid) pw = getpwuid(euid) @end group @group pr_first_field(pw) @} @end group printf(" gid=%d", gid) pw = getgrgid(gid) pr_first_field(pw) if (egid != gid) @{ printf(" egid=%d", egid) pw = getgrgid(egid) pr_first_field(pw) @} for (i = 1; ("group" i) in PROCINFO; i++) @{ if (i == 1) printf(" groups=") group = PROCINFO["group" i] printf("%d", group) pw = getgrgid(group) pr_first_field(pw) if (("group" (i+1)) in PROCINFO) printf(",") @} print "" @} function pr_first_field(str, a) @{ if (str != "") @{ split(str, a, ":") printf("(%s)", a[1]) @} @} @c endfile @end example The test in the @code{for} loop is worth noting. Any supplementary groups in the @code{PROCINFO} array have the indices @code{"group1"} through @code{"group@var{N}"} for some @var{N} (i.e., the total number of supplementary groups). However, we don't know in advance how many of these groups there are. This loop works by starting at one, concatenating the value with @code{"group"}, and then using @code{in} to see if that value is in the array (@pxref{Reference to Elements}). Eventually, @code{i} is incremented past the last group in the array and the loop exits. The loop is also correct if there are @emph{no} supplementary groups; then the condition is false the first time it's tested, and the loop body never executes. The @code{pr_first_field()} function simply isolates out some code that is used repeatedly, making the whole program shorter and cleaner. In particular, moving the check for the empty string into this function saves several lines of code. @node Split Program @subsection Splitting a Large File into Pieces @c FIXME: One day, update to current POSIX version of split @cindex files @subentry splitting @cindex @code{split} utility The @command{split} program splits large text files into smaller pieces. Usage is as follows:@footnote{This is the traditional usage. The POSIX usage is different, but not relevant for what the program aims to demonstrate.} @display @command{split} [@code{-@var{count}}] [@var{file}] [@var{prefix}] @end display By default, the output files are named @file{xaa}, @file{xab}, and so on. Each file has 1,000 lines in it, with the likely exception of the last file. To change the number of lines in each file, supply a number on the command line preceded with a minus sign (e.g., @samp{-500} for files with 500 lines in them instead of 1,000). To change the names of the output files to something like @file{myfileaa}, @file{myfileab}, and so on, supply an additional argument that specifies the @value{FN} prefix. Here is a version of @command{split} in @command{awk}. It uses the @code{ord()} and @code{chr()} functions presented in @ref{Ordinal Functions}. The program first sets its defaults, and then tests to make sure there are not too many arguments. It then looks at each argument in turn. The first argument could be a minus sign followed by a number. If it is, this happens to look like a negative number, so it is made positive, and that is the count of lines. The @value{DF} name is skipped over and the final argument is used as the prefix for the output @value{FN}s: @cindex @code{split.awk} program @example @c file eg/prog/split.awk # split.awk --- do split in awk # # Requires ord() and chr() library functions @c endfile @ignore @c file eg/prog/split.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 # Revised slightly, May 2014 @c endfile @end ignore @c file eg/prog/split.awk # usage: split [-count] [file] [outname] BEGIN @{ outfile = "x" # default count = 1000 if (ARGC > 4) usage() i = 1 if (i in ARGV && ARGV[i] ~ /^-[[:digit:]]+$/) @{ count = -ARGV[i] ARGV[i] = "" i++ @} # test argv in case reading from stdin instead of file if (i in ARGV) i++ # skip datafile name @group if (i in ARGV) @{ outfile = ARGV[i] ARGV[i] = "" @} @end group @group s1 = s2 = "a" out = (outfile s1 s2) @} @end group @c endfile @end example The next rule does most of the work. @code{tcount} (temporary count) tracks how many lines have been printed to the output file so far. If it is greater than @code{count}, it is time to close the current file and start a new one. @code{s1} and @code{s2} track the current suffixes for the @value{FN}. If they are both @samp{z}, the file is just too big. Otherwise, @code{s1} moves to the next letter in the alphabet and @code{s2} starts over again at @samp{a}: @c else on separate line here for page breaking @example @c file eg/prog/split.awk @{ if (++tcount > count) @{ close(out) if (s2 == "z") @{ if (s1 == "z") @{ printf("split: %s is too large to split\n", FILENAME) > "/dev/stderr" exit 1 @} s1 = chr(ord(s1) + 1) s2 = "a" @} @group else s2 = chr(ord(s2) + 1) @end group out = (outfile s1 s2) tcount = 1 @} print > out @} @c endfile @end example @noindent The @code{usage()} function simply prints an error message and exits: @example @c file eg/prog/split.awk function usage() @{ print("usage: split [-num] [file] [outname]") > "/dev/stderr" exit 1 @} @c endfile @end example This program is a bit sloppy; it relies on @command{awk} to automatically close the last file instead of doing it in an @code{END} rule. It also assumes that letters are contiguous in the character set, which isn't true for EBCDIC systems. @ifset FOR_PRINT You might want to consider how to eliminate the use of @code{ord()} and @code{chr()}; this can be done in such a way as to solve the EBCDIC issue as well. @end ifset @node Tee Program @subsection Duplicating Output into Multiple Files @cindex files @subentry multiple, duplicating output into @cindex output @subentry duplicating into files @cindex @code{tee} utility The @code{tee} program is known as a ``pipe fitting.'' @code{tee} copies its standard input to its standard output and also duplicates it to the files named on the command line. Its usage is as follows: @display @command{tee} [@option{-a}] @var{file} @dots{} @end display The @option{-a} option tells @code{tee} to append to the named files, instead of truncating them and starting over. The @code{BEGIN} rule first makes a copy of all the command-line arguments into an array named @code{copy}. @code{ARGV[0]} is not needed, so it is not copied. @code{tee} cannot use @code{ARGV} directly, because @command{awk} attempts to process each @value{FN} in @code{ARGV} as input data. @cindex flag variables If the first argument is @option{-a}, then the flag variable @code{append} is set to true, and both @code{ARGV[1]} and @code{copy[1]} are deleted. If @code{ARGC} is less than two, then no @value{FN}s were supplied and @code{tee} prints a usage message and exits. Finally, @command{awk} is forced to read the standard input by setting @code{ARGV[1]} to @code{"-"} and @code{ARGC} to two: @cindex @code{tee.awk} program @example @c file eg/prog/tee.awk # tee.awk --- tee in awk # # Copy standard input to all named output files. # Append content if -a option is supplied. # @c endfile @ignore @c file eg/prog/tee.awk # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 # Revised December 1995 @c endfile @end ignore @c file eg/prog/tee.awk BEGIN @{ for (i = 1; i < ARGC; i++) copy[i] = ARGV[i] if (ARGV[1] == "-a") @{ append = 1 delete ARGV[1] delete copy[1] ARGC-- @} if (ARGC < 2) @{ print "usage: tee [-a] file ..." > "/dev/stderr" exit 1 @} ARGV[1] = "-" ARGC = 2 @} @c endfile @end example The following single rule does all the work. Because there is no pattern, it is executed for each line of input. The body of the rule simply prints the line into each file on the command line, and then to the standard output: @example @c file eg/prog/tee.awk @{ # moving the if outside the loop makes it run faster if (append) for (i in copy) print >> copy[i] else for (i in copy) print > copy[i] print @} @c endfile @end example @noindent It is also possible to write the loop this way: @example @group for (i in copy) if (append) print >> copy[i] @end group @group else print > copy[i] @end group @end example @noindent This is more concise, but it is also less efficient. The @samp{if} is tested for each record and for each output file. By duplicating the loop body, the @samp{if} is only tested once for each input record. If there are @var{N} input records and @var{M} output files, the first method only executes @var{N} @samp{if} statements, while the second executes @var{N}@code{*}@var{M} @samp{if} statements. Finally, the @code{END} rule cleans up by closing all the output files: @example @c file eg/prog/tee.awk END @{ for (i in copy) close(copy[i]) @} @c endfile @end example @node Uniq Program @subsection Printing Nonduplicated Lines of Text @c FIXME: One day, update to current POSIX version of uniq @cindex printing @subentry unduplicated lines of text @cindex text, printing @subentry unduplicated lines of @cindex @command{uniq} utility The @command{uniq} utility reads sorted lines of data on its standard input, and by default removes duplicate lines. In other words, it only prints unique lines---hence the name. @command{uniq} has a number of options. The usage is as follows: @display @command{uniq} [@option{-udc} [@code{-@var{n}}]] [@code{+@var{n}}] [@var{inputfile} [@var{outputfile}]] @end display The options for @command{uniq} are: @table @code @item -d Print only repeated (duplicated) lines. @item -u Print only nonrepeated (unique) lines. @item -c Count lines. This option overrides @option{-d} and @option{-u}. Both repeated and nonrepeated lines are counted. @item -@var{n} Skip @var{n} fields before comparing lines. The definition of fields is similar to @command{awk}'s default: nonwhitespace characters separated by runs of spaces and/or TABs. @item +@var{n} Skip @var{n} characters before comparing lines. Any fields specified with @samp{-@var{n}} are skipped first. @item @var{inputfile} Data is read from the input file named on the command line, instead of from the standard input. @item @var{outputfile} The generated output is sent to the named output file, instead of to the standard output. @end table Normally @command{uniq} behaves as if both the @option{-d} and @option{-u} options are provided. @command{uniq} uses the @code{getopt()} library function (@pxref{Getopt Function}) and the @code{join()} library function (@pxref{Join Function}). The program begins with a @code{usage()} function and then a brief outline of the options and their meanings in comments. The @code{BEGIN} rule deals with the command-line arguments and options. It uses a trick to get @code{getopt()} to handle options of the form @samp{-25}, treating such an option as the option letter @samp{2} with an argument of @samp{5}. If indeed two or more digits are supplied (@code{Optarg} looks like a number), @code{Optarg} is concatenated with the option digit and then the result is added to zero to make it into a number. If there is only one digit in the option, then @code{Optarg} is not needed. In this case, @code{Optind} must be decremented so that @code{getopt()} processes it next time. This code is admittedly a bit tricky. If no options are supplied, then the default is taken, to print both repeated and nonrepeated lines. The output file, if provided, is assigned to @code{outputfile}. Early on, @code{outputfile} is initialized to the standard output, @file{/dev/stdout}: @cindex @code{uniq.awk} program @example @c file eg/prog/uniq.awk @group # uniq.awk --- do uniq in awk # # Requires getopt() and join() library functions @end group @c endfile @ignore @c file eg/prog/uniq.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/uniq.awk function usage() @{ print("Usage: uniq [-udc [-n]] [+n] [ in [ out ]]") > "/dev/stderr" exit 1 @} # -c count lines. overrides -d and -u # -d only repeated lines # -u only nonrepeated lines # -n skip n fields # +n skip n characters, skip fields first BEGIN @{ count = 1 outputfile = "/dev/stdout" opts = "udc0:1:2:3:4:5:6:7:8:9:" while ((c = getopt(ARGC, ARGV, opts)) != -1) @{ if (c == "u") non_repeated_only++ else if (c == "d") repeated_only++ else if (c == "c") do_count++ else if (index("0123456789", c) != 0) @{ # getopt() requires args to options # this messes us up for things like -5 if (Optarg ~ /^[[:digit:]]+$/) fcount = (c Optarg) + 0 else @{ fcount = c + 0 Optind-- @} @} else usage() @} @group if (ARGV[Optind] ~ /^\+[[:digit:]]+$/) @{ charcount = substr(ARGV[Optind], 2) + 0 Optind++ @} @end group for (i = 1; i < Optind; i++) ARGV[i] = "" if (repeated_only == 0 && non_repeated_only == 0) repeated_only = non_repeated_only = 1 if (ARGC - Optind == 2) @{ outputfile = ARGV[ARGC - 1] ARGV[ARGC - 1] = "" @} @} @c endfile @end example The following function, @code{are_equal()}, compares the current line, @code{$0}, to the previous line, @code{last}. It handles skipping fields and characters. If no field count and no character count are specified, @code{are_equal()} returns one or zero depending upon the result of a simple string comparison of @code{last} and @code{$0}. Otherwise, things get more complicated. If fields have to be skipped, each line is broken into an array using @code{split()} (@pxref{String Functions}); the desired fields are then joined back into a line using @code{join()}. The joined lines are stored in @code{clast} and @code{cline}. If no fields are skipped, @code{clast} and @code{cline} are set to @code{last} and @code{$0}, respectively. Finally, if characters are skipped, @code{substr()} is used to strip off the leading @code{charcount} characters in @code{clast} and @code{cline}. The two strings are then compared and @code{are_equal()} returns the result: @example @c file eg/prog/uniq.awk @group function are_equal( n, m, clast, cline, alast, aline) @{ if (fcount == 0 && charcount == 0) return (last == $0) @end group if (fcount > 0) @{ n = split(last, alast) m = split($0, aline) clast = join(alast, fcount+1, n) cline = join(aline, fcount+1, m) @} else @{ clast = last cline = $0 @} if (charcount) @{ clast = substr(clast, charcount + 1) cline = substr(cline, charcount + 1) @} @group return (clast == cline) @} @end group @c endfile @end example The following two rules are the body of the program. The first one is executed only for the very first line of data. It sets @code{last} equal to @code{$0}, so that subsequent lines of text have something to be compared to. The second rule does the work. The variable @code{equal} is one or zero, depending upon the results of @code{are_equal()}'s comparison. If @command{uniq} is counting repeated lines, and the lines are equal, then it increments the @code{count} variable. Otherwise, it prints the line and resets @code{count}, because the two lines are not equal. If @command{uniq} is not counting, and if the lines are equal, @code{count} is incremented. Nothing is printed, as the point is to remove duplicates. Otherwise, if @command{uniq} is counting repeated lines and more than one line is seen, or if @command{uniq} is counting nonrepeated lines and only one line is seen, then the line is printed, and @code{count} is reset. Finally, similar logic is used in the @code{END} rule to print the final line of input data: @example @c file eg/prog/uniq.awk NR == 1 @{ last = $0 next @} @{ equal = are_equal() if (do_count) @{ # overrides -d and -u if (equal) count++ else @{ printf("%4d %s\n", count, last) > outputfile last = $0 count = 1 # reset @} next @} if (equal) count++ else @{ if ((repeated_only && count > 1) || (non_repeated_only && count == 1)) print last > outputfile last = $0 count = 1 @} @} END @{ if (do_count) printf("%4d %s\n", count, last) > outputfile @group else if ((repeated_only && count > 1) || (non_repeated_only && count == 1)) print last > outputfile close(outputfile) @} @end group @c endfile @end example As a side note, this program does not follow our recommended convention of naming global variables with a leading capital letter. Doing that would make the program a little easier to follow. @ifset FOR_PRINT The logic for choosing which lines to print represents a @dfn{state machine}, which is ``a device that can be in one of a set number of stable conditions depending on its previous condition and on the present values of its inputs.''@footnote{This is the definition returned from entering @code{define: state machine} into Google.} Brian Kernighan suggests that ``an alternative approach to state machines is to just read the input into an array, then use indexing. It's almost always easier code, and for most inputs where you would use this, just as fast.'' Consider how to rewrite the logic to follow this suggestion. @end ifset @node Wc Program @subsection Counting Things @c FIXME: One day, update to current POSIX version of wc @cindex counting words, lines, and characters @cindex input files @subentry counting elements in @cindex words @subentry counting @cindex characters @subentry counting @cindex lines @subentry counting @cindex @command{wc} utility The @command{wc} (word count) utility counts lines, words, and characters in one or more input files. Its usage is as follows: @display @command{wc} [@option{-lwc}] [@var{files} @dots{}] @end display If no files are specified on the command line, @command{wc} reads its standard input. If there are multiple files, it also prints total counts for all the files. The options and their meanings are as follows: @table @code @item -l Count only lines. @item -w Count only words. A ``word'' is a contiguous sequence of nonwhitespace characters, separated by spaces and/or TABs. Luckily, this is the normal way @command{awk} separates fields in its input data. @item -c Count only characters. @end table Implementing @command{wc} in @command{awk} is particularly elegant, because @command{awk} does a lot of the work for us; it splits lines into words (i.e., fields) and counts them, it counts lines (i.e., records), and it can easily tell us how long a line is. This program uses the @code{getopt()} library function (@pxref{Getopt Function}) and the file-transition functions (@pxref{Filetrans Function}). This version has one notable difference from traditional versions of @command{wc}: it always prints the counts in the order lines, words, and characters. Traditional versions note the order of the @option{-l}, @option{-w}, and @option{-c} options on the command line, and print the counts in that order. The @code{BEGIN} rule does the argument processing. The variable @code{print_total} is true if more than one file is named on the command line: @cindex @code{wc.awk} program @example @c file eg/prog/wc.awk # wc.awk --- count lines, words, characters @c endfile @ignore @c file eg/prog/wc.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/wc.awk # Options: # -l only count lines # -w only count words # -c only count characters # # Default is to count lines, words, characters # # Requires getopt() and file transition library functions BEGIN @{ # let getopt() print a message about # invalid options. we ignore them while ((c = getopt(ARGC, ARGV, "lwc")) != -1) @{ if (c == "l") do_lines = 1 else if (c == "w") do_words = 1 else if (c == "c") do_chars = 1 @} for (i = 1; i < Optind; i++) ARGV[i] = "" # if no options, do all if (! do_lines && ! do_words && ! do_chars) do_lines = do_words = do_chars = 1 print_total = (ARGC - i > 1) @} @c endfile @end example The @code{beginfile()} function is simple; it just resets the counts of lines, words, and characters to zero, and saves the current @value{FN} in @code{fname}: @example @c file eg/prog/wc.awk function beginfile(file) @{ lines = words = chars = 0 fname = FILENAME @} @c endfile @end example The @code{endfile()} function adds the current file's numbers to the running totals of lines, words, and characters. It then prints out those numbers for the file that was just read. It relies on @code{beginfile()} to reset the numbers for the following @value{DF}: @example @c file eg/prog/wc.awk function endfile(file) @{ tlines += lines twords += words tchars += chars if (do_lines) printf "\t%d", lines @group if (do_words) printf "\t%d", words @end group if (do_chars) printf "\t%d", chars printf "\t%s\n", fname @} @c endfile @end example There is one rule that is executed for each line. It adds the length of the record, plus one, to @code{chars}.@footnote{Because @command{gawk} understands multibyte locales, this code counts characters, not bytes.} Adding one plus the record length is needed because the newline character separating records (the value of @code{RS}) is not part of the record itself, and thus not included in its length. Next, @code{lines} is incremented for each line read, and @code{words} is incremented by the value of @code{NF}, which is the number of ``words'' on this line: @example @c file eg/prog/wc.awk # do per line @{ chars += length($0) + 1 # get newline lines++ words += NF @} @c endfile @end example Finally, the @code{END} rule simply prints the totals for all the files: @example @c file eg/prog/wc.awk END @{ if (print_total) @{ if (do_lines) printf "\t%d", tlines if (do_words) printf "\t%d", twords if (do_chars) printf "\t%d", tchars print "\ttotal" @} @} @c endfile @end example @node Miscellaneous Programs @section A Grab Bag of @command{awk} Programs This @value{SECTION} is a large ``grab bag'' of miscellaneous programs. We hope you find them both interesting and enjoyable. @menu * Dupword Program:: Finding duplicated words in a document. * Alarm Program:: An alarm clock. * Translate Program:: A program similar to the @command{tr} utility. * Labels Program:: Printing mailing labels. * Word Sorting:: A program to produce a word usage count. * History Sorting:: Eliminating duplicate entries from a history file. * Extract Program:: Pulling out programs from Texinfo source files. * Simple Sed:: A Simple Stream Editor. * Igawk Program:: A wrapper for @command{awk} that includes files. * Anagram Program:: Finding anagrams from a dictionary. * Signature Program:: People do amazing things with too much time on their hands. @end menu @node Dupword Program @subsection Finding Duplicated Words in a Document @cindex words @subentry duplicate, searching for @cindex searching @subentry for words @cindex documents, searching A common error when writing large amounts of prose is to accidentally duplicate words. Typically you will see this in text as something like ``the the program does the following@dots{}'' When the text is online, often the duplicated words occur at the end of one line and the @iftex the @end iftex beginning of another, making them very difficult to spot. @c as here! This program, @file{dupword.awk}, scans through a file one line at a time and looks for adjacent occurrences of the same word. It also saves the last word on a line (in the variable @code{prev}) for comparison with the first word on the next line. @cindex Texinfo The first two statements make sure that the line is all lowercase, so that, for example, ``The'' and ``the'' compare equal to each other. The next statement replaces nonalphanumeric and nonwhitespace characters with spaces, so that punctuation does not affect the comparison either. The characters are replaced with spaces so that formatting controls don't create nonsense words (e.g., the Texinfo @samp{@@code@{NF@}} becomes @samp{codeNF} if punctuation is simply deleted). The record is then resplit into fields, yielding just the actual words on the line, and ensuring that there are no empty fields. If there are no fields left after removing all the punctuation, the current record is skipped. Otherwise, the program loops through each word, comparing it to the previous one: @cindex @code{dupword.awk} program @example @c file eg/prog/dupword.awk # dupword.awk --- find duplicate words in text @c endfile @ignore @c file eg/prog/dupword.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # December 1991 # Revised October 2000 @c endfile @end ignore @c file eg/prog/dupword.awk @{ $0 = tolower($0) gsub(/[^[:alnum:][:blank:]]/, " "); $0 = $0 # re-split if (NF == 0) next if ($1 == prev) printf("%s:%d: duplicate %s\n", FILENAME, FNR, $1) for (i = 2; i <= NF; i++) if ($i == $(i-1)) printf("%s:%d: duplicate %s\n", FILENAME, FNR, $i) prev = $NF @} @c endfile @end example @node Alarm Program @subsection An Alarm Clock Program @cindex insomnia, cure for @cindex Robbins @subentry Arnold @quotation @i{Nothing cures insomnia like a ringing alarm clock.} @author Arnold Robbins @end quotation @cindex Quanstrom, Erik @ignore Date: Sat, 15 Feb 2014 16:47:09 -0500 Subject: Re: 9atom install question Message-ID: From: Erik Quanstrom To: Aharon Robbins yes. - erik Aharon Robbins wrote: >> sleep is for web developers. > >Can I quote you, in the gawk manual? > >Thanks, > >Arnold @end ignore @quotation @i{Sleep is for web developers.} @author Erik Quanstrom @end quotation @cindex time @subentry alarm clock example program @cindex alarm clock example program The following program is a simple ``alarm clock'' program. You give it a time of day and an optional message. At the specified time, it prints the message on the standard output. In addition, you can give it the number of times to repeat the message as well as a delay between repetitions. This program uses the @code{getlocaltime()} function from @ref{Getlocaltime Function}. @cindex ASCII All the work is done in the @code{BEGIN} rule. The first part is argument checking and setting of defaults: the delay, the count, and the message to print. If the user supplied a message without the ASCII BEL character (known as the ``alert'' character, @code{"\a"}), then it is added to the message. (On many systems, printing the ASCII BEL generates an audible alert. Thus, when the alarm goes off, the system calls attention to itself in case the user is not looking at the computer.) Just for a change, this program uses a @code{switch} statement (@pxref{Switch Statement}), but the processing could be done with a series of @code{if}-@code{else} statements instead. Here is the program: @cindex @code{alarm.awk} program @example @c file eg/prog/alarm.awk # alarm.awk --- set an alarm # # Requires getlocaltime() library function @c endfile @ignore @c file eg/prog/alarm.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 # Revised December 2010 @c endfile @end ignore @c file eg/prog/alarm.awk # usage: alarm time [ "message" [ count [ delay ] ] ] BEGIN @{ # Initial argument sanity checking usage1 = "usage: alarm time ['message' [count [delay]]]" usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1]) if (ARGC < 2) @{ print usage1 > "/dev/stderr" print usage2 > "/dev/stderr" exit 1 @} switch (ARGC) @{ case 5: delay = ARGV[4] + 0 # fall through case 4: count = ARGV[3] + 0 # fall through case 3: message = ARGV[2] break default: if (ARGV[1] !~ /[[:digit:]]?[[:digit:]]:[[:digit:]]@{2@}/) @{ print usage1 > "/dev/stderr" print usage2 > "/dev/stderr" exit 1 @} break @} # set defaults for once we reach the desired time if (delay == 0) delay = 180 # 3 minutes @group if (count == 0) count = 5 @end group if (message == "") message = sprintf("\aIt is now %s!\a", ARGV[1]) else if (index(message, "\a") == 0) message = "\a" message "\a" @c endfile @end example The next @value{SECTION} of code turns the alarm time into hours and minutes, converts it (if necessary) to a 24-hour clock, and then turns that time into a count of the seconds since midnight. Next it turns the current time into a count of seconds since midnight. The difference between the two is how long to wait before setting off the alarm: @example @c file eg/prog/alarm.awk # split up alarm time split(ARGV[1], atime, ":") hour = atime[1] + 0 # force numeric minute = atime[2] + 0 # force numeric # get current broken down time getlocaltime(now) # if time given is 12-hour hours and it's after that # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m., # then add 12 to real hour if (hour < 12 && now["hour"] > hour) hour += 12 # set target time in seconds since midnight target = (hour * 60 * 60) + (minute * 60) # get current time in seconds since midnight current = (now["hour"] * 60 * 60) + \ (now["minute"] * 60) + now["second"] # how long to sleep for naptime = target - current if (naptime <= 0) @{ print "alarm: time is in the past!" > "/dev/stderr" exit 1 @} @c endfile @end example @cindex @command{sleep} utility Finally, the program uses the @code{system()} function (@pxref{I/O Functions}) to call the @command{sleep} utility. The @command{sleep} utility simply pauses for the given number of seconds. If the exit status is not zero, the program assumes that @command{sleep} was interrupted and exits. If @command{sleep} exited with an OK status (zero), then the program prints the message in a loop, again using @command{sleep} to delay for however many seconds are necessary: @example @c file eg/prog/alarm.awk # zzzzzz..... go away if interrupted if (system(sprintf("sleep %d", naptime)) != 0) exit 1 # time to notify! command = sprintf("sleep %d", delay) for (i = 1; i <= count; i++) @{ print message # if sleep command interrupted, go away if (system(command) != 0) break @} exit 0 @} @c endfile @end example @node Translate Program @subsection Transliterating Characters @cindex characters @subentry transliterating @cindex @command{tr} utility The system @command{tr} utility transliterates characters. For example, it is often used to map uppercase letters into lowercase for further processing: @example @var{generate data} | tr 'A-Z' 'a-z' | @var{process data} @dots{} @end example @command{tr} requires two lists of characters.@footnote{On some older systems, including Solaris, the system version of @command{tr} may require that the lists be written as range expressions enclosed in square brackets (@samp{[a-z]}) and quoted, to prevent the shell from attempting a @value{FN} expansion. This is not a feature.} When processing the input, the first character in the first list is replaced with the first character in the second list, the second character in the first list is replaced with the second character in the second list, and so on. If there are more characters in the ``from'' list than in the ``to'' list, the last character of the ``to'' list is used for the remaining characters in the ``from'' list. Once upon a time, @c early or mid-1989! a user proposed adding a transliteration function to @command{gawk}. @c Wishing to avoid gratuitous new features, @c at least theoretically The following program was written to prove that character transliteration could be done with a user-level function. This program is not as complete as the system @command{tr} utility, but it does most of the job. The @command{translate} program was written long before @command{gawk} acquired the ability to split each character in a string into separate array elements. Thus, it makes repeated use of the @code{substr()}, @code{index()}, and @code{gsub()} built-in functions (@pxref{String Functions}). There are two functions. The first, @code{stranslate()}, takes three arguments: @table @code @item from A list of characters from which to translate @item to A list of characters to which to translate @item target The string on which to do the translation @end table Associative arrays make the translation part fairly easy. @code{t_ar} holds the ``to'' characters, indexed by the ``from'' characters. Then a simple loop goes through @code{from}, one character at a time. For each character in @code{from}, if the character appears in @code{target}, it is replaced with the corresponding @code{to} character. The @code{translate()} function calls @code{stranslate()}, using @code{$0} as the target. The main program sets two global variables, @code{FROM} and @code{TO}, from the command line, and then changes @code{ARGV} so that @command{awk} reads from the standard input. Finally, the processing rule simply calls @code{translate()} for each record: @cindex @code{translate.awk} program @example @c file eg/prog/translate.awk # translate.awk --- do tr-like stuff @c endfile @ignore @c file eg/prog/translate.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # August 1989 # February 2009 - bug fix @c endfile @end ignore @c file eg/prog/translate.awk # Bugs: does not handle things like tr A-Z a-z; it has # to be spelled out. However, if `to' is shorter than `from', # the last character in `to' is used for the rest of `from'. function stranslate(from, to, target, lf, lt, ltarget, t_ar, i, c, result) @{ lf = length(from) lt = length(to) ltarget = length(target) for (i = 1; i <= lt; i++) t_ar[substr(from, i, 1)] = substr(to, i, 1) if (lt < lf) for (; i <= lf; i++) t_ar[substr(from, i, 1)] = substr(to, lt, 1) for (i = 1; i <= ltarget; i++) @{ c = substr(target, i, 1) if (c in t_ar) c = t_ar[c] result = result c @} return result @} function translate(from, to) @{ return $0 = stranslate(from, to, $0) @} # main program BEGIN @{ @group if (ARGC < 3) @{ print "usage: translate from to" > "/dev/stderr" exit @} @end group FROM = ARGV[1] TO = ARGV[2] ARGC = 2 ARGV[1] = "-" @} @{ translate(FROM, TO) print @} @c endfile @end example It is possible to do character transliteration in a user-level function, but it is not necessarily efficient, and we (the @command{gawk} developers) started to consider adding a built-in function. However, shortly after writing this program, we learned that Brian Kernighan had added the @code{toupper()} and @code{tolower()} functions to his @command{awk} (@pxref{String Functions}). These functions handle the vast majority of the cases where character transliteration is necessary, and so we chose to simply add those functions to @command{gawk} as well and then leave well enough alone. An obvious improvement to this program would be to set up the @code{t_ar} array only once, in a @code{BEGIN} rule. However, this assumes that the ``from'' and ``to'' lists will never change throughout the lifetime of the program. Another obvious improvement is to enable the use of ranges, such as @samp{a-z}, as allowed by the @command{tr} utility. Look at the code for @file{cut.awk} (@pxref{Cut Program}) for inspiration. @node Labels Program @subsection Printing Mailing Labels @cindex printing @subentry mailing labels @cindex mailing labels, printing Here is a ``real-world''@footnote{``Real world'' is defined as ``a program actually used to get something done.''} program. This script reads lists of names and addresses and generates mailing labels. Each page of labels has 20 labels on it, two across and 10 down. The addresses are guaranteed to be no more than five lines of data. Each address is separated from the next by a blank line. The basic idea is to read 20 labels' worth of data. Each line of each label is stored in the @code{line} array. The single rule takes care of filling the @code{line} array and printing the page when 20 labels have been read. The @code{BEGIN} rule simply sets @code{RS} to the empty string, so that @command{awk} splits records at blank lines (@pxref{Records}). It sets @code{MAXLINES} to 100, because 100 is the maximum number of lines on the page @iftex (@math{20 @cdot 5 = 100}). @end iftex @ifnottex @ifnotdocbook (20 * 5 = 100). @end ifnotdocbook @end ifnottex @docbook (20 ⋅ 5 = 100). @end docbook Most of the work is done in the @code{printpage()} function. The label lines are stored sequentially in the @code{line} array. But they have to print horizontally: @code{line[1]} next to @code{line[6]}, @code{line[2]} next to @code{line[7]}, and so on. Two loops accomplish this. The outer loop, controlled by @code{i}, steps through every 10 lines of data; this is each row of labels. The inner loop, controlled by @code{j}, goes through the lines within the row. As @code{j} goes from 0 to 4, @samp{i+j} is the @code{j}th line in the row, and @samp{i+j+5} is the entry next to it. The output ends up looking something like this: @example line 1 line 6 line 2 line 7 line 3 line 8 line 4 line 9 line 5 line 10 @dots{} @end example @noindent The @code{printf} format string @samp{%-41s} left-aligns the data and prints it within a fixed-width field. As a final note, an extra blank line is printed at lines 21 and 61, to keep the output lined up on the labels. This is dependent on the particular brand of labels in use when the program was written. You will also note that there are two blank lines at the top and two blank lines at the bottom. The @code{END} rule arranges to flush the final page of labels; there may not have been an even multiple of 20 labels in the data: @cindex @code{labels.awk} program @example @c file eg/prog/labels.awk # labels.awk --- print mailing labels @c endfile @ignore @c file eg/prog/labels.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # June 1992 # December 2010, minor edits @c endfile @end ignore @c file eg/prog/labels.awk # Each label is 5 lines of data that may have blank lines. # The label sheets have 2 blank lines at the top and 2 at # the bottom. BEGIN @{ RS = "" ; MAXLINES = 100 @} function printpage( i, j) @{ if (Nlines <= 0) return printf "\n\n" # header for (i = 1; i <= Nlines; i += 10) @{ if (i == 21 || i == 61) print "" for (j = 0; j < 5; j++) @{ if (i + j > MAXLINES) break printf " %-41s %s\n", line[i+j], line[i+j+5] @} print "" @} printf "\n\n" # footer delete line @} # main rule @{ if (Count >= 20) @{ printpage() Count = 0 Nlines = 0 @} n = split($0, a, "\n") for (i = 1; i <= n; i++) line[++Nlines] = a[i] for (; i <= 5; i++) line[++Nlines] = "" Count++ @} END @{ printpage() @} @c endfile @end example @node Word Sorting @subsection Generating Word-Usage Counts @cindex words @subentry usage counts, generating When working with large amounts of text, it can be interesting to know how often different words appear. For example, an author may overuse certain words, in which case he or she might wish to find synonyms to substitute for words that appear too often. This @value{SUBSECTION} develops a program for counting words and presenting the frequency information in a useful format. At first glance, a program like this would seem to do the job: @example # wordfreq-first-try.awk --- print list of word frequencies @{ for (i = 1; i <= NF; i++) freq[$i]++ @} @group END @{ for (word in freq) printf "%s\t%d\n", word, freq[word] @} @end group @end example The program relies on @command{awk}'s default field-splitting mechanism to break each line up into ``words'' and uses an associative array named @code{freq}, indexed by each word, to count the number of times the word occurs. In the @code{END} rule, it prints the counts. This program has several problems that prevent it from being useful on real text files: @itemize @value{BULLET} @item The @command{awk} language considers upper- and lowercase characters to be distinct. Therefore, ``bartender'' and ``Bartender'' are not treated as the same word. This is undesirable, because words are capitalized if they begin sentences in normal text, and a frequency analyzer should not be sensitive to capitalization. @item Words are detected using the @command{awk} convention that fields are separated just by whitespace. Other characters in the input (except newlines) don't have any special meaning to @command{awk}. This means that punctuation characters count as part of words. @item The output does not come out in any useful order. You're more likely to be interested in which words occur most frequently or in having an alphabetized table of how frequently each word occurs. @end itemize @cindex @command{sort} utility The first problem can be solved by using @code{tolower()} to remove case distinctions. The second problem can be solved by using @code{gsub()} to remove punctuation characters. Finally, we solve the third problem by using the system @command{sort} utility to process the output of the @command{awk} script. Here is the new version of the program: @cindex @code{wordfreq.awk} program @example @c file eg/prog/wordfreq.awk # wordfreq.awk --- print list of word frequencies @{ $0 = tolower($0) # remove case distinctions # remove punctuation gsub(/[^[:alnum:]_[:blank:]]/, "", $0) for (i = 1; i <= NF; i++) freq[$i]++ @} @c endfile END @{ for (word in freq) printf "%s\t%d\n", word, freq[word] @} @end example The regexp @code{/[^[:alnum:]_[:blank:]]/} might have been written @code{/[[:punct:]]/}, but then underscores would also be removed, and we want to keep them. Assuming we have saved this program in a file named @file{wordfreq.awk}, and that the data is in @file{file1}, the following pipeline: @example awk -f wordfreq.awk file1 | sort -k 2nr @end example @noindent produces a table of the words appearing in @file{file1} in order of decreasing frequency. The @command{awk} program suitably massages the data and produces a word frequency table, which is not ordered. The @command{awk} script's output is then sorted by the @command{sort} utility and printed on the screen. The options given to @command{sort} specify a sort that uses the second field of each input line (skipping one field), that the sort keys should be treated as numeric quantities (otherwise @samp{15} would come before @samp{5}), and that the sorting should be done in descending (reverse) order. The @command{sort} could even be done from within the program, by changing the @code{END} action to: @example @c file eg/prog/wordfreq.awk END @{ sort = "sort -k 2nr" for (word in freq) printf "%s\t%d\n", word, freq[word] | sort close(sort) @} @c endfile @end example This way of sorting must be used on systems that do not have true pipes at the command-line (or batch-file) level. See the general operating system documentation for more information on how to use the @command{sort} program. @node History Sorting @subsection Removing Duplicates from Unsorted Text @cindex lines @subentry duplicate, removing The @command{uniq} program (@pxref{Uniq Program}) removes duplicate lines from @emph{sorted} data. Suppose, however, you need to remove duplicate lines from a @value{DF} but that you want to preserve the order the lines are in. A good example of this might be a shell history file. The history file keeps a copy of all the commands you have entered, and it is not unusual to repeat a command several times in a row. Occasionally you might want to compact the history by removing duplicate entries. Yet it is desirable to maintain the order of the original commands. This simple program does the job. It uses two arrays. The @code{data} array is indexed by the text of each line. For each line, @code{data[$0]} is incremented. If a particular line has not been seen before, then @code{data[$0]} is zero. In this case, the text of the line is stored in @code{lines[count]}. Each element of @code{lines} is a unique command, and the indices of @code{lines} indicate the order in which those lines are encountered. The @code{END} rule simply prints out the lines, in order: @cindex Rakitzis, Byron @cindex @code{histsort.awk} program @example @c file eg/prog/histsort.awk # histsort.awk --- compact a shell history file # Thanks to Byron Rakitzis for the general idea @c endfile @ignore @c file eg/prog/histsort.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 @c endfile @end ignore @c file eg/prog/histsort.awk @group @{ if (data[$0]++ == 0) lines[++count] = $0 @} @end group @group END @{ for (i = 1; i <= count; i++) print lines[i] @} @end group @c endfile @end example This program also provides a foundation for generating other useful information. For example, using the following @code{print} statement in the @code{END} rule indicates how often a particular command is used: @example print data[lines[i]], lines[i] @end example @noindent This works because @code{data[$0]} is incremented each time a line is seen. @c rick@openfortress.nl, Tue, 24 Dec 2019 13:43:06 +0100 Rick van Rein offers the following one-liner to do the same job of removing duplicates from unsorted text: @example awk '@{ if (! seen[$0]++) print @}' @end example This can be simplified even further, at the risk of becoming almost too obscure: @example awk '! seen[$0]++' @end example @noindent This version uses the expression as a pattern, relying on @command{awk}'s default action of printing the line when the pattern is true. @node Extract Program @subsection Extracting Programs from Texinfo Source Files @cindex Texinfo @subentry extracting programs from source files @cindex files @subentry Texinfo, extracting programs from @ifnotinfo Both this chapter and the previous chapter (@ref{Library Functions}) present a large number of @command{awk} programs. @end ifnotinfo @ifinfo The nodes @ref{Library Functions}, and @ref{Sample Programs}, are the top level nodes for a large number of @command{awk} programs. @end ifinfo If you want to experiment with these programs, it is tedious to type them in by hand. Here we present a program that can extract parts of a Texinfo input file into separate files. @cindex Texinfo This @value{DOCUMENT} is written in @uref{https://www.gnu.org/software/texinfo/, Texinfo}, the GNU Project's document formatting language. A single Texinfo source file can be used to produce both printed documentation, with @TeX{}, and online documentation. @ifnotinfo (Texinfo is fully documented in the book @cite{Texinfo---The GNU Documentation Format}, available from the Free Software Foundation, and also available @uref{https://www.gnu.org/software/texinfo/manual/texinfo/, online}.) @end ifnotinfo @ifinfo (The Texinfo language is described fully, starting with @inforef{Top, , Texinfo, texinfo,Texinfo---The GNU Documentation Format}.) @end ifinfo For our purposes, it is enough to know three things about Texinfo input files: @itemize @value{BULLET} @item The ``at'' symbol (@samp{@@}) is special in Texinfo, much as the backslash (@samp{\}) is in C or @command{awk}. Literal @samp{@@} symbols are represented in Texinfo source files as @samp{@@@@}. @item Comments start with either @samp{@@c} or @samp{@@comment}. The file-extraction program works by using special comments that start at the beginning of a line. @item Lines containing @samp{@@group} and @samp{@@end group} commands bracket example text that should not be split across a page boundary. (Unfortunately, @TeX{} isn't always smart enough to do things exactly right, so we have to give it some help.) @end itemize The following program, @file{extract.awk}, reads through a Texinfo source file and does two things, based on the special comments. Upon seeing @samp{@w{@@c system @dots{}}}, it runs a command, by extracting the command text from the control line and passing it on to the @code{system()} function (@pxref{I/O Functions}). Upon seeing @samp{@@c file @var{filename}}, each subsequent line is sent to the file @var{filename}, until @samp{@@c endfile} is encountered. The rules in @file{extract.awk} match either @samp{@@c} or @samp{@@comment} by letting the @samp{omment} part be optional. Lines containing @samp{@@group} and @samp{@@end group} are simply removed. @file{extract.awk} uses the @code{join()} library function (@pxref{Join Function}). The example programs in the online Texinfo source for @cite{@value{TITLE}} (@file{gawktexi.in}) have all been bracketed inside @samp{file} and @samp{endfile} lines. The @command{gawk} distribution uses a copy of @file{extract.awk} to extract the sample programs and install many of them in a standard directory where @command{gawk} can find them. The Texinfo file looks something like this: @example @dots{} This program has a @@code@{BEGIN@} rule that prints a nice message: @@example @@c file examples/messages.awk BEGIN @@@{ print "Don't panic!" @@@} @@c endfile @@end example It also prints some final advice: @@example @@c file examples/messages.awk END @@@{ print "Always avoid bored archaeologists!" @@@} @@c endfile @@end example @dots{} @end example @file{extract.awk} begins by setting @code{IGNORECASE} to one, so that mixed upper- and lowercase letters in the directives won't matter. The first rule handles calling @code{system()}, checking that a command is given (@code{NF} is at least three) and also checking that the command exits with a zero exit status, signifying OK: @cindex @code{extract.awk} program @example @c file eg/prog/extract.awk # extract.awk --- extract files and run programs from Texinfo files @c endfile @ignore @c file eg/prog/extract.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # May 1993 # Revised September 2000 @c endfile @end ignore @c file eg/prog/extract.awk BEGIN @{ IGNORECASE = 1 @} /^@@c(omment)?[ \t]+system/ @{ if (NF < 3) @{ e = ("extract: " FILENAME ":" FNR) e = (e ": badly formed `system' line") print e > "/dev/stderr" next @} $1 = "" $2 = "" stat = system($0) if (stat != 0) @{ e = ("extract: " FILENAME ":" FNR) e = (e ": warning: system returned " stat) print e > "/dev/stderr" @} @} @c endfile @end example @noindent The variable @code{e} is used so that the rule fits nicely on the @value{PAGE}. The second rule handles moving data into files. It verifies that a @value{FN} is given in the directive. If the file named is not the current file, then the current file is closed. Keeping the current file open until a new file is encountered allows the use of the @samp{>} redirection for printing the contents, keeping open-file management simple. The @code{for} loop does the work. It reads lines using @code{getline} (@pxref{Getline}). For an unexpected end-of-file, it calls the @code{@w{unexpected_eof()}} function. If the line is an ``endfile'' line, then it breaks out of the loop. If the line is an @samp{@@group} or @samp{@@end group} line, then it ignores it and goes on to the next line. Similarly, comments within examples are also ignored. Most of the work is in the following few lines. If the line has no @samp{@@} symbols, the program can print it directly. Otherwise, each leading @samp{@@} must be stripped off. To remove the @samp{@@} symbols, the line is split into separate elements of the array @code{a}, using the @code{split()} function (@pxref{String Functions}). The @samp{@@} symbol is used as the separator character. Each element of @code{a} that is empty indicates two successive @samp{@@} symbols in the original line. For each two empty elements (@samp{@@@@} in the original file), we have to add a single @samp{@@} symbol back in. When the processing of the array is finished, @code{join()} is called with the value of @code{SUBSEP} (@pxref{Multidimensional}), to rejoin the pieces back into a single line. That line is then printed to the output file: @example @c file eg/prog/extract.awk /^@@c(omment)?[ \t]+file/ @{ if (NF != 3) @{ e = ("extract: " FILENAME ":" FNR ": badly formed `file' line") print e > "/dev/stderr" next @} if ($3 != curfile) @{ if (curfile != "") filelist[curfile] = 1 # save to close later curfile = $3 @} for (;;) @{ if ((getline line) <= 0) unexpected_eof() if (line ~ /^@@c(omment)?[ \t]+endfile/) break else if (line ~ /^@@(end[ \t]+)?group/) continue else if (line ~ /^@@c(omment+)?[ \t]+/) continue if (index(line, "@@") == 0) @{ print line > curfile continue @} n = split(line, a, "@@") # if a[1] == "", means leading @@, # don't add one back in. for (i = 2; i <= n; i++) @{ if (a[i] == "") @{ # was an @@@@ a[i] = "@@" if (a[i+1] == "") i++ @} @} @group print join(a, 1, n, SUBSEP) > curfile @} @} @end group @c endfile @end example An important thing to note is the use of the @samp{>} redirection. Output done with @samp{>} only opens the file once; it stays open and subsequent output is appended to the file (@pxref{Redirection}). This makes it easy to mix program text and explanatory prose for the same sample source file (as has been done here!) without any hassle. The file is only closed when a new @value{DF} name is encountered or at the end of the input file. When a new @value{FN} is encountered, instead of closing the file, the program saves the name of the current file in @code{filelist}. This makes it possible to interleave the code for more than one file in the Texinfo input file. (Previous versions of this program @emph{did} close the file. But because of the @samp{>} redirection, a file whose parts were not all one after the other ended up getting clobbered.) An @code{END} rule then closes all the open files when processing is finished: @example @c file eg/prog/extract.awk @group END @{ close(curfile) # close the last one for (f in filelist) # close all the rest close(f) @} @end group @c endfile @end example Finally, the function @code{@w{unexpected_eof()}} prints an appropriate error message and then exits: @example @c file eg/prog/extract.awk @group function unexpected_eof() @{ printf("extract: %s:%d: unexpected EOF or error\n", FILENAME, FNR) > "/dev/stderr" exit 1 @} @end group @c endfile @end example @node Simple Sed @subsection A Simple Stream Editor @cindex @command{sed} utility @cindex stream editors The @command{sed} utility is a @dfn{stream editor}, a program that reads a stream of data, makes changes to it, and passes it on. It is often used to make global changes to a large file or to a stream of data generated by a pipeline of commands. Although @command{sed} is a complicated program in its own right, its most common use is to perform global substitutions in the middle of a pipeline: @example @var{command1} < orig.data | sed 's/old/new/g' | @var{command2} > result @end example Here, @samp{s/old/new/g} tells @command{sed} to look for the regexp @samp{old} on each input line and globally replace it with the text @samp{new} (i.e., all the occurrences on a line). This is similar to @command{awk}'s @code{gsub()} function (@pxref{String Functions}). The following program, @file{awksed.awk}, accepts at least two command-line arguments: the pattern to look for and the text to replace it with. Any additional arguments are treated as @value{DF} names to process. If none are provided, the standard input is used: @cindex Brennan, Michael @cindex @command{awksed.awk} program @c @cindex simple stream editor @c @cindex stream editor, simple @example @c file eg/prog/awksed.awk # awksed.awk --- do s/foo/bar/g using just print # Thanks to Michael Brennan for the idea @c endfile @ignore @c file eg/prog/awksed.awk # # Arnold Robbins, arnold@@skeeve.com, Public Domain # August 1995 @c endfile @end ignore @c file eg/prog/awksed.awk function usage() @{ print "usage: awksed pat repl [files...]" > "/dev/stderr" exit 1 @} @group BEGIN @{ # validate arguments if (ARGC < 3) usage() @end group RS = ARGV[1] ORS = ARGV[2] # don't use arguments as files ARGV[1] = ARGV[2] = "" @} @group # look ma, no hands! @{ if (RT == "") printf "%s", $0 else print @} @end group @c endfile @end example The program relies on @command{gawk}'s ability to have @code{RS} be a regexp, as well as on the setting of @code{RT} to the actual text that terminates the record (@pxref{Records}). The idea is to have @code{RS} be the pattern to look for. @command{gawk} automatically sets @code{$0} to the text between matches of the pattern. This is text that we want to keep, unmodified. Then, by setting @code{ORS} to the replacement text, a simple @code{print} statement outputs the text we want to keep, followed by the replacement text. There is one wrinkle to this scheme, which is what to do if the last record doesn't end with text that matches @code{RS}. Using a @code{print} statement unconditionally prints the replacement text, which is not correct. However, if the file did not end in text that matches @code{RS}, @code{RT} is set to the null string. In this case, we can print @code{$0} using @code{printf} (@pxref{Printf}). The @code{BEGIN} rule handles the setup, checking for the right number of arguments and calling @code{usage()} if there is a problem. Then it sets @code{RS} and @code{ORS} from the command-line arguments and sets @code{ARGV[1]} and @code{ARGV[2]} to the null string, so that they are not treated as @value{FN}s (@pxref{ARGC and ARGV}). The @code{usage()} function prints an error message and exits. Finally, the single rule handles the printing scheme outlined earlier, using @code{print} or @code{printf} as appropriate, depending upon the value of @code{RT}. @node Igawk Program @subsection An Easy Way to Use Library Functions @cindex libraries of @command{awk} functions @subentry example program for using @cindex functions @subentry library @subentry example program for using In @ref{Include Files}, we saw how @command{gawk} provides a built-in file-inclusion capability. However, this is a @command{gawk} extension. This @value{SECTION} provides the motivation for making file inclusion available for standard @command{awk}, and shows how to do it using a combination of shell and @command{awk} programming. Using library functions in @command{awk} can be very beneficial. It encourages code reuse and the writing of general functions. Programs are smaller and therefore clearer. However, using library functions is only easy when writing @command{awk} programs; it is painful when running them, requiring multiple @option{-f} options. If @command{gawk} is unavailable, then so too is the @env{AWKPATH} environment variable and the ability to put @command{awk} functions into a library directory (@pxref{Options}). It would be nice to be able to write programs in the following manner: @example # library functions @@include getopt.awk @@include join.awk @dots{} # main program BEGIN @{ while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1) @dots{} @dots{} @} @end example The following program, @file{igawk.sh}, provides this service. It simulates @command{gawk}'s searching of the @env{AWKPATH} variable and also allows @dfn{nested} includes (i.e., a file that is included with @code{@@include} can contain further @code{@@include} statements). @command{igawk} makes an effort to only include files once, so that nested includes don't accidentally include a library function twice. @command{igawk} should behave just like @command{gawk} externally. This means it should accept all of @command{gawk}'s command-line arguments, including the ability to have multiple source files specified via @option{-f} and the ability to mix command-line and library source files. The program is written using the POSIX Shell (@command{sh}) command language.@footnote{Fully explaining the @command{sh} language is beyond the scope of this book. We provide some minimal explanations, but see a good shell programming book if you wish to understand things in more depth.} It works as follows: @enumerate @item Loop through the arguments, saving anything that doesn't represent @command{awk} source code for later, when the expanded program is run. @item For any arguments that do represent @command{awk} text, put the arguments into a shell variable that will be expanded. There are two cases: @enumerate a @item Literal text, provided with @option{-e} or @option{--source}. This text is just appended directly. @item Source @value{FN}s, provided with @option{-f}. We use a neat trick and append @samp{@@include @var{filename}} to the shell variable's contents. Because the file-inclusion program works the way @command{gawk} does, this gets the text of the file included in the program at the correct point. @end enumerate @item Run an @command{awk} program (naturally) over the shell variable's contents to expand @code{@@include} statements. The expanded program is placed in a second shell variable. @item Run the expanded program with @command{gawk} and any other original command-line arguments that the user supplied (such as the @value{DF} names). @end enumerate This program uses shell variables extensively: for storing command-line arguments and the text of the @command{awk} program that will expand the user's program, for the user's original program, and for the expanded program. Doing so removes some potential problems that might arise were we to use temporary files instead, at the cost of making the script somewhat more complicated. The initial part of the program turns on shell tracing if the first argument is @samp{debug}. The next part loops through all the command-line arguments. There are several cases of interest: @c @asis for docbook @table @asis @item @option{--} This ends the arguments to @command{igawk}. Anything else should be passed on to the user's @command{awk} program without being evaluated. @item @option{-W} This indicates that the next option is specific to @command{gawk}. To make argument processing easier, the @option{-W} is appended to the front of the remaining arguments and the loop continues. (This is an @command{sh} programming trick. Don't worry about it if you are not familiar with @command{sh}.) @item @option{-v}, @option{-F} These are saved and passed on to @command{gawk}. @item @option{-f}, @option{--file}, @option{--file=}, @option{-Wfile=} The @value{FN} is appended to the shell variable @code{program} with an @code{@@include} statement. The @command{expr} utility is used to remove the leading option part of the argument (e.g., @samp{--file=}). (Typical @command{sh} usage would be to use the @command{echo} and @command{sed} utilities to do this work. Unfortunately, some versions of @command{echo} evaluate escape sequences in their arguments, possibly mangling the program text. Using @command{expr} avoids this problem.) @item @option{--source}, @option{--source=}, @option{-Wsource=} The source text is appended to @code{program}. @item @option{--version}, @option{-Wversion} @command{igawk} prints its version number, runs @samp{gawk --version} to get the @command{gawk} version information, and then exits. @end table If none of the @option{-f}, @option{--file}, @option{-Wfile}, @option{--source}, or @option{-Wsource} arguments are supplied, then the first nonoption argument should be the @command{awk} program. If there are no command-line arguments left, @command{igawk} prints an error message and exits. Otherwise, the first argument is appended to @code{program}. In any case, after the arguments have been processed, the shell variable @code{program} contains the complete text of the original @command{awk} program. The program is as follows: @cindex @code{igawk.sh} program @example @c file eg/prog/igawk.sh #! /bin/sh # igawk --- like gawk but do @@include processing @c endfile @ignore @c file eg/prog/igawk.sh # # Arnold Robbins, arnold@@skeeve.com, Public Domain # July 1993 # December 2010, minor edits @c endfile @end ignore @c file eg/prog/igawk.sh if [ "$1" = debug ] then set -x shift fi # A literal newline, so that program text is formatted correctly n=' ' # Initialize variables to empty program= opts= while [ $# -ne 0 ] # loop over arguments do case $1 in --) shift break ;; -W) shift # The $@{x?'message here'@} construct prints a # diagnostic if $x is the null string set -- -W"$@{@@?'missing operand'@}" continue ;; -[vF]) opts="$opts $1 '$@{2?'missing operand'@}'" shift ;; -[vF]*) opts="$opts '$1'" ;; -f) program="$program$n@@include $@{2?'missing operand'@}" shift ;; -f*) f=$(expr "$1" : '-f\(.*\)') program="$program$n@@include $f" ;; -[W-]file=*) f=$(expr "$1" : '-.file=\(.*\)') program="$program$n@@include $f" ;; -[W-]file) program="$program$n@@include $@{2?'missing operand'@}" shift ;; -[W-]source=*) t=$(expr "$1" : '-.source=\(.*\)') program="$program$n$t" ;; -[W-]source) program="$program$n$@{2?'missing operand'@}" shift ;; -[W-]version) echo igawk: version 3.0 1>&2 gawk --version exit 0 ;; -[W-]*) opts="$opts '$1'" ;; *) break ;; esac shift done if [ -z "$program" ] then program=$@{1?'missing program'@} shift fi # At this point, `program' has the program. @c endfile @end example The @command{awk} program to process @code{@@include} directives is stored in the shell variable @code{expand_prog}. Doing this keeps the shell script readable. The @command{awk} program reads through the user's program, one line at a time, using @code{getline} (@pxref{Getline}). The input @value{FN}s and @code{@@include} statements are managed using a stack. As each @code{@@include} is encountered, the current @value{FN} is ``pushed'' onto the stack and the file named in the @code{@@include} directive becomes the current @value{FN}. As each file is finished, the stack is ``popped,'' and the previous input file becomes the current input file again. The process is started by making the original file the first one on the stack. The @code{pathto()} function does the work of finding the full path to a file. It simulates @command{gawk}'s behavior when searching the @env{AWKPATH} environment variable (@pxref{AWKPATH Variable}). If a @value{FN} has a @samp{/} in it, no path search is done. Similarly, if the @value{FN} is @code{"-"}, then that string is used as-is. Otherwise, the @value{FN} is concatenated with the name of each directory in the path, and an attempt is made to open the generated @value{FN}. The only way to test if a file can be read in @command{awk} is to go ahead and try to read it with @code{getline}; this is what @code{pathto()} does.@footnote{On some very old versions of @command{awk}, the test @samp{getline junk < t} can loop forever if the file exists but is empty.} If the file can be read, it is closed and the @value{FN} is returned: @ignore An alternative way to test for the file's existence would be to call @samp{system("test -r " t)}, which uses the @command{test} utility to see if the file exists and is readable. The disadvantage to this method is that it requires creating an extra process and can thus be slightly slower. @end ignore @example @c file eg/prog/igawk.sh expand_prog=' function pathto(file, i, t, junk) @{ if (index(file, "/") != 0) return file if (file == "-") return file for (i = 1; i <= ndirs; i++) @{ t = (pathlist[i] "/" file) @group if ((getline junk < t) > 0) @{ # found it close(t) return t @} @end group @} return "" @} @c endfile @end example The main program is contained inside one @code{BEGIN} rule. The first thing it does is set up the @code{pathlist} array that @code{pathto()} uses. After splitting the path on @samp{:}, null elements are replaced with @code{"."}, which represents the current directory: @example @c file eg/prog/igawk.sh BEGIN @{ path = ENVIRON["AWKPATH"] ndirs = split(path, pathlist, ":") for (i = 1; i <= ndirs; i++) @{ if (pathlist[i] == "") pathlist[i] = "." @} @c endfile @end example The stack is initialized with @code{ARGV[1]}, which will be @code{"/dev/stdin"}. The main loop comes next. Input lines are read in succession. Lines that do not start with @code{@@include} are printed verbatim. If the line does start with @code{@@include}, the @value{FN} is in @code{$2}. @code{pathto()} is called to generate the full path. If it cannot, then the program prints an error message and continues. The next thing to check is if the file is included already. The @code{processed} array is indexed by the full @value{FN} of each included file and it tracks this information for us. If the file is seen again, a warning message is printed. Otherwise, the new @value{FN} is pushed onto the stack and processing continues. Finally, when @code{getline} encounters the end of the input file, the file is closed and the stack is popped. When @code{stackptr} is less than zero, the program is done: @example @c file eg/prog/igawk.sh stackptr = 0 input[stackptr] = ARGV[1] # ARGV[1] is first file for (; stackptr >= 0; stackptr--) @{ while ((getline < input[stackptr]) > 0) @{ if (tolower($1) != "@@include") @{ print continue @} fpath = pathto($2) if (fpath == "") @{ printf("igawk: %s:%d: cannot find %s\n", input[stackptr], FNR, $2) > "/dev/stderr" continue @} if (! (fpath in processed)) @{ processed[fpath] = input[stackptr] input[++stackptr] = fpath # push onto stack @} else print $2, "included in", input[stackptr], "already included in", processed[fpath] > "/dev/stderr" @} close(input[stackptr]) @} @}' # close quote ends `expand_prog' variable processed_program=$(gawk -- "$expand_prog" /dev/stdin << EOF $program EOF ) @c endfile @end example The shell construct @samp{@var{command} << @var{marker}} is called a @dfn{here document}. Everything in the shell script up to the @var{marker} is fed to @var{command} as input. The shell processes the contents of the here document for variable and command substitution (and possibly other things as well, depending upon the shell). The shell construct @samp{$(@dots{})} is called @dfn{command substitution}. The output of the command inside the parentheses is substituted into the command line. Because the result is used in a variable assignment, it is saved as a single string, even if the results contain whitespace. The expanded program is saved in the variable @code{processed_program}. It's done in these steps: @enumerate @item Run @command{gawk} with the @code{@@include}-processing program (the value of the @code{expand_prog} shell variable) reading standard input. @item Standard input is the contents of the user's program, from the shell variable @code{program}. Feed its contents to @command{gawk} via a here document. @item Save the results of this processing in the shell variable @code{processed_program} by using command substitution. @end enumerate The last step is to call @command{gawk} with the expanded program, along with the original options and command-line arguments that the user supplied: @example @c file eg/prog/igawk.sh eval gawk $opts -- '"$processed_program"' '"$@@"' @c endfile @end example The @command{eval} command is a shell construct that reruns the shell's parsing process. This keeps things properly quoted. This version of @command{igawk} represents the fifth version of this program. There are four key simplifications that make the program work better: @itemize @value{BULLET} @item Using @code{@@include} even for the files named with @option{-f} makes building the initial collected @command{awk} program much simpler; all the @code{@@include} processing can be done once. @item Not trying to save the line read with @code{getline} in the @code{pathto()} function when testing for the file's accessibility for use with the main program simplifies things considerably. @item Using a @code{getline} loop in the @code{BEGIN} rule does it all in one place. It is not necessary to call out to a separate loop for processing nested @code{@@include} statements. @item Instead of saving the expanded program in a temporary file, putting it in a shell variable avoids some potential security problems. This has the disadvantage that the script relies upon more features of the @command{sh} language, making it harder to follow for those who aren't familiar with @command{sh}. @end itemize Also, this program illustrates that it is often worthwhile to combine @command{sh} and @command{awk} programming together. You can usually accomplish quite a lot, without having to resort to low-level programming in C or C++, and it is frequently easier to do certain kinds of string and argument manipulation using the shell than it is in @command{awk}. Finally, @command{igawk} shows that it is not always necessary to add new features to a program; they can often be layered on top.@footnote{@command{gawk} does @code{@@include} processing itself in order to support the use of @command{awk} programs as Web CGI scripts.} @node Anagram Program @subsection Finding Anagrams from a Dictionary @cindex anagrams, finding An interesting programming challenge is to search for @dfn{anagrams} in a word list (such as @file{/usr/share/dict/words} on many GNU/Linux systems). One word is an anagram of another if both words contain the same letters (e.g., ``babbling'' and ``blabbing''). Column 2, Problem C, of Jon Bentley's @cite{Programming Pearls}, Second Edition, presents an elegant algorithm. The idea is to give words that are anagrams a common signature, sort all the words together by their signatures, and then print them. Dr.@: Bentley observes that taking the letters in each word and sorting them produces those common signatures. The following program uses arrays of arrays to bring together words with the same signature and array sorting to print the words in sorted order: @cindex @code{anagram.awk} program @example @c file eg/prog/anagram.awk # anagram.awk --- An implementation of the anagram-finding algorithm # from Jon Bentley's "Programming Pearls," 2nd edition. # Addison Wesley, 2000, ISBN 0-201-65788-0. # Column 2, Problem C, section 2.8, pp 18-20. @c endfile @ignore @c file eg/prog/anagram.awk # # This program requires gawk 4.0 or newer. # Required gawk-specific features: # - True multidimensional arrays # - split() with "" as separator splits out individual characters # - asort() and asorti() functions # # See https://savannah.gnu.org/projects/gawk. # # Arnold Robbins # arnold@@skeeve.com # Public Domain # January, 2011 @c endfile @end ignore @c file eg/prog/anagram.awk /'s$/ @{ next @} # Skip possessives @c endfile @end example The program starts with a header, and then a rule to skip possessives in the dictionary file. The next rule builds up the data structure. The first dimension of the array is indexed by the signature; the second dimension is the word itself: @example @c file eg/prog/anagram.awk @{ key = word2key($1) # Build signature data[key][$1] = $1 # Store word with signature @} @c endfile @end example The @code{word2key()} function creates the signature. It splits the word apart into individual letters, sorts the letters, and then joins them back together: @example @c file eg/prog/anagram.awk # word2key --- split word apart into letters, sort, and join back together function word2key(word, a, i, n, result) @{ n = split(word, a, "") asort(a) for (i = 1; i <= n; i++) result = result a[i] return result @} @c endfile @end example Finally, the @code{END} rule traverses the array and prints out the anagram lists. It sends the output to the system @command{sort} command because otherwise the anagrams would appear in arbitrary order: @example @c file eg/prog/anagram.awk END @{ sort = "sort" for (key in data) @{ # Sort words with same key nwords = asorti(data[key], words) if (nwords == 1) continue # And print. Minor glitch: trailing space at end of each line for (j = 1; j <= nwords; j++) printf("%s ", words[j]) | sort print "" | sort @} close(sort) @} @c endfile @end example Here is some partial output when the program is run: @example $ @kbd{gawk -f anagram.awk /usr/share/dict/words | grep '^b'} @dots{} babbled blabbed babbler blabber brabble babblers blabbers brabbles babbling blabbing babbly blabby babel bable babels beslab babery yabber @dots{} @end example @node Signature Program @subsection And Now for Something Completely Different @cindex signature program @cindex Brini, Davide The following program was written by Davide Brini @c (@email{dave_br@@gmx.com}) and is published on @uref{http://backreference.org/2011/02/03/obfuscated-awk/, his website}. It serves as his signature in the Usenet group @code{comp.lang.awk}. He supplies the following copyright terms: @quotation Copyright @copyright{} 2008 Davide Brini Copying and distribution of the code published in this page, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. @end quotation Here is the program: @example @group awk 'BEGIN@{O="~"~"~";o="=="=="==";o+=+o;x=O""O;while(X++<=x+o+o)c=c"%c"; printf c,(x-O)*(x-O),x*(x-o)-o,x*(x-O)+x-O-o,+x*(x-O)-x+o,X*(o*o+O)+x-O, X*(X-x)-o*o,(x+X)*o*o+o,x*(X-x)-O-O,x-O+(O+o+X+x)*(o+O),X*X-X*(x-O)-x+O, O+X*(o*(o+O)+O),+x+O+X*o,x*(x-o),(o+X+x)*o*o-(x-O-O),O+(X-x)*(X+O),x-O@}' @end group @end example @cindex Johansen, Chris We leave it to you to determine what the program does. (If you are truly desperate to understand it, see Chris Johansen's explanation, which is embedded in the Texinfo source file for this @value{DOCUMENT}.) @ignore To: "Arnold Robbins" Date: Sat, 20 Aug 2011 13:50:46 -0400 Subject: The GNU Awk User's Guide, Section 13.3.11 From: "Chris Johansen" Message-ID: Arnold, you don't know me, but we have a tenuous connection. My wife is Barbara A. Field, FAIA, GIT '65 (B. Arch.). I have had a couple of paper copies of "Effective Awk Programming" for years, and now I'm going through a Kindle version of "The GNU Awk User's Guide" again. When I got to section 13.3.11, I reformatted and lightly commented Davide Brin's signature script to understand its workings. It occurs to me that this might have pedagogical value as an example (although imperfect) of the value of whitespace and comments, and a starting point for that discussion. It certainly helped _me_ understand what's going on. You are welcome to it, as-is or modified (subject to Davide's constraints, of course, which I think I have met). If I were to include it in a future edition, I would put it at some distance from section 13.3.11, say, as a note or an appendix, so as not to be a "spoiler" to the puzzle. Best regards, -- Chris Johansen {johansen at main dot nc dot us} . . . collapsing the probability wave function, sending ripples of certainty through the space-time continuum. #! /usr/bin/gawk -f # From "13.3.11 And Now For Something Completely Different" # https://www.gnu.org/software/gawk/manual/html_node/Signature-Program.html#Signature-Program # Copyright @copyright{} 2008 Davide Brini # Copying and distribution of the code published in this page, with # or without modification, are permitted in any medium without # royalty provided the copyright notice and this notice are preserved. BEGIN { O = "~" ~ "~"; # 1 o = "==" == "=="; # 1 o += +o; # 2 x = O "" O; # 11 while ( X++ <= x + o + o ) c = c "%c"; # O is 1 # o is 2 # x is 11 # X is 17 # c is "%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c" printf c, ( x - O )*( x - O), # 100 d x*( x - o ) - o, # 97 a x*( x - O ) + x - O - o, # 118 v +x*( x - O ) - x + o, # 101 e X*( o*o + O ) + x - O, # 95 _ X*( X - x ) - o*o, # 98 b ( x + X )*o*o + o, # 114 r x*( X - x ) - O - O, # 64 @ x - O + ( O + o + X + x )*( o + O ), # 103 g X*X - X*( x - O ) - x + O, # 109 m O + X*( o*( o + O ) + O ), # 120 x +x + O + X*o, # 46 . x*( x - o), # 99 c ( o + X + x )*o*o - ( x - O - O ), # 111 0 O + ( X - x )*( X + O ), # 109 m x - O # 10 \n } @end ignore @node Programs Summary @section Summary @itemize @value{BULLET} @item The programs provided in this @value{CHAPTER} continue on the theme that reading programs is an excellent way to learn Good Programming. @item Using @samp{#!} to make @command{awk} programs directly runnable makes them easier to use. Otherwise, invoke the program using @samp{awk -f @dots{}}. @item Reimplementing standard POSIX programs in @command{awk} is a pleasant exercise; @command{awk}'s expressive power lets you write such programs in relatively few lines of code, yet they are functionally complete and usable. @item One of standard @command{awk}'s weaknesses is working with individual characters. The ability to use @code{split()} with the empty string as the separator can considerably simplify such tasks. @item The examples here demonstrate the usefulness of the library functions from @ref{Library Functions} for a number of real (if small) programs. @item Besides reinventing POSIX wheels, other programs solved a selection of interesting problems, such as finding duplicate words in text, printing mailing labels, and finding anagrams. @end itemize @c EXCLUDE START @node Programs Exercises @section Exercises @enumerate @item Rewrite @file{cut.awk} (@pxref{Cut Program}) using @code{split()} with @code{""} as the separator. @item In @ref{Egrep Program}, we mentioned that @samp{egrep -i} could be simulated in versions of @command{awk} without @code{IGNORECASE} by using @code{tolower()} on the line and the pattern. In a footnote there, we also mentioned that this solution has a bug: the translated line is output, and not the original one. Fix this problem. @c Exercise: Fix this, w/array and new line as key to original line @item The POSIX version of @command{id} takes options that control which information is printed. Modify the @command{awk} version (@pxref{Id Program}) to accept the same arguments and perform in the same way. @item The @code{split.awk} program (@pxref{Split Program}) assumes that letters are contiguous in the character set, which isn't true for EBCDIC systems. Fix this problem. (Hint: Consider a different way to work through the alphabet, without relying on @code{ord()} and @code{chr()}.) @item In @file{uniq.awk} (@pxref{Uniq Program}, the logic for choosing which lines to print represents a @dfn{state machine}, which is ``a device that can be in one of a set number of stable conditions depending on its previous condition and on the present values of its inputs.''@footnote{This is the definition returned from entering @code{define: state machine} into Google.} Brian Kernighan suggests that ``an alternative approach to state machines is to just read the input into an array, then use indexing. It's almost always easier code, and for most inputs where you would use this, just as fast.'' Rewrite the logic to follow this suggestion. @item Why can't the @file{wc.awk} program (@pxref{Wc Program}) just use the value of @code{FNR} in @code{endfile()}? Hint: Examine the code in @ref{Filetrans Function}. @ignore @command{wc} can't just use the value of @code{FNR} in @code{endfile()}. If you examine the code in @ref{Filetrans Function}, you will see that @code{FNR} has already been reset by the time @code{endfile()} is called. @end ignore @item Manipulation of individual characters in the @command{translate} program (@pxref{Translate Program}) is painful using standard @command{awk} functions. Given that @command{gawk} can split strings into individual characters using @code{""} as the separator, how might you use this feature to simplify the program? @item The @file{extract.awk} program (@pxref{Extract Program}) was written before @command{gawk} had the @code{gensub()} function. Use it to simplify the code. @item Compare the performance of the @file{awksed.awk} program (@pxref{Simple Sed}) with the more straightforward: @example BEGIN @{ pat = ARGV[1] repl = ARGV[2] ARGV[1] = ARGV[2] = "" @} @{ gsub(pat, repl); print @} @end example @item What are the advantages and disadvantages of @file{awksed.awk} versus the real @command{sed} utility? @ignore Advantage: egrep regexps speed (?) Disadvantage: no & in replacement text Others? @end ignore @item In @ref{Igawk Program}, we mentioned that not trying to save the line read with @code{getline} in the @code{pathto()} function when testing for the file's accessibility for use with the main program simplifies things considerably. What problem does this engender though? @c answer, reading from "-" or /dev/stdin @cindex search paths @cindex search paths @subentry for source files @cindex source files, search path for @cindex files @subentry source, search path for @cindex directories @subentry searching @subentry for source files @item As an additional example of the idea that it is not always necessary to add new features to a program, consider the idea of having two files in a directory in the search path: @table @file @item default.awk This file contains a set of default library functions, such as @code{getopt()} and @code{assert()}. @item site.awk This file contains library functions that are specific to a site or installation; i.e., locally developed functions. Having a separate file allows @file{default.awk} to change with new @command{gawk} releases, without requiring the system administrator to update it each time by adding the local functions. @end table One user @c Karl Berry, karl@ileaf.com, 10/95 suggested that @command{gawk} be modified to automatically read these files upon startup. Instead, it would be very simple to modify @command{igawk} to do this. Since @command{igawk} can process nested @code{@@include} directives, @file{default.awk} could simply contain @code{@@include} statements for the desired library functions. Make this change. @item Modify @file{anagram.awk} (@pxref{Anagram Program}), to avoid the use of the external @command{sort} utility. @end enumerate @c EXCLUDE END @ifnotinfo @part @value{PART3}Moving Beyond Standard @command{awk} with @command{gawk} @end ifnotinfo @ifdocbook Part III focuses on features specific to @command{gawk}. It contains the following chapters: @itemize @value{BULLET} @item @ref{Namespaces} @item @ref{Advanced Features} @item @ref{Internationalization} @item @ref{Debugger} @item @ref{Arbitrary Precision Arithmetic} @item @ref{Dynamic Extensions} @end itemize @end ifdocbook @node Advanced Features @chapter Advanced Features of @command{gawk} @cindex @command{gawk} @subentry features @subentry advanced @cindex advanced features @subentry @command{gawk} @ignore Contributed by: Peter Langston Found in Steve English's "signature" line: "Write documentation as if whoever reads it is a violent psychopath who knows where you live." @end ignore @cindex Langston, Peter @cindex English, Steve @quotation @i{Write documentation as if whoever reads it is a violent psychopath who knows where you live.} @author Steve English, as quoted by Peter Langston @end quotation This @value{CHAPTER} discusses advanced features in @command{gawk}. It's a bit of a ``grab bag'' of items that are otherwise unrelated to each other. First, we look at a command-line option that allows @command{gawk} to recognize nondecimal numbers in input data, not just in @command{awk} programs. Then, @command{gawk}'s special features for sorting arrays are presented. Next, two-way I/O, discussed briefly in earlier parts of this @value{DOCUMENT}, is described in full detail, along with the basics of TCP/IP networking. Finally, we see how @command{gawk} can @dfn{profile} an @command{awk} program, making it possible to tune it for performance. @c FULLXREF ON Additional advanced features are discussed in separate @value{CHAPTER}s of their own: @itemize @value{BULLET} @item @ref{Internationalization}, discusses how to internationalize your @command{awk} programs, so that they can speak multiple national languages. @item @ref{Debugger}, describes @command{gawk}'s built-in command-line debugger for debugging @command{awk} programs. @item @ref{Arbitrary Precision Arithmetic}, describes how you can use @command{gawk} to perform arbitrary-precision arithmetic. @item @ref{Dynamic Extensions}, discusses the ability to dynamically add new built-in functions to @command{gawk}. @end itemize @c FULLXREF OFF @menu * Nondecimal Data:: Allowing nondecimal input data. * Array Sorting:: Facilities for controlling array traversal and sorting arrays. * Two-way I/O:: Two-way communications with another process. * TCP/IP Networking:: Using @command{gawk} for network programming. * Profiling:: Profiling your @command{awk} programs. * Advanced Features Summary:: Summary of advanced features. @end menu @node Nondecimal Data @section Allowing Nondecimal Input Data @cindex @option{--non-decimal-data} option @cindex advanced features @subentry nondecimal input data @cindex input @subentry data, nondecimal @cindex constants @subentry nondecimal If you run @command{gawk} with the @option{--non-decimal-data} option, you can have nondecimal values in your input data: @example $ @kbd{echo 0123 123 0x123 |} > @kbd{gawk --non-decimal-data '@{ printf "%d, %d, %d\n", $1, $2, $3 @}'} @print{} 83, 123, 291 @end example For this feature to work, write your program so that @command{gawk} treats your data as numeric: @example $ @kbd{echo 0123 123 0x123 | gawk '@{ print $1, $2, $3 @}'} @print{} 0123 123 0x123 @end example @noindent The @code{print} statement treats its expressions as strings. Although the fields can act as numbers when necessary, they are still strings, so @code{print} does not try to treat them numerically. You need to add zero to a field to force it to be treated as a number. For example: @example $ @kbd{echo 0123 123 0x123 | gawk --non-decimal-data '} > @kbd{@{ print $1, $2, $3} > @kbd{print $1 + 0, $2 + 0, $3 + 0 @}'} @print{} 0123 123 0x123 @print{} 83 123 291 @end example Because it is common to have decimal data with leading zeros, and because using this facility could lead to surprising results, the default is to leave it disabled. If you want it, you must explicitly request it. @cindex programming conventions @subentry @code{--non-decimal-data} option @cindex @option{--non-decimal-data} option @subentry @code{strtonum()} function and @cindex @code{strtonum()} function (@command{gawk}) @subentry @code{--non-decimal-data} option and @quotation CAUTION @emph{Use of this option is not recommended.} It can break old programs very badly. Instead, use the @code{strtonum()} function to convert your data (@pxref{String Functions}). This makes your programs easier to write and easier to read, and leads to less surprising results. This option may disappear in a future version of @command{gawk}. @end quotation @node Array Sorting @section Controlling Array Traversal and Array Sorting @command{gawk} lets you control the order in which a @samp{for (@var{indx} in @var{array})} loop traverses an array. In addition, two built-in functions, @code{asort()} and @code{asorti()}, let you sort arrays based on the array values and indices, respectively. These two functions also provide control over the sorting criteria used to order the elements during sorting. @menu * Controlling Array Traversal:: How to use PROCINFO["sorted_in"]. * Array Sorting Functions:: How to use @code{asort()} and @code{asorti()}. @end menu @node Controlling Array Traversal @subsection Controlling Array Traversal By default, the order in which a @samp{for (@var{indx} in @var{array})} loop scans an array is not defined; it is generally based upon the internal implementation of arrays inside @command{awk}. Often, though, it is desirable to be able to loop over the elements in a particular order that you, the programmer, choose. @command{gawk} lets you do this. @ref{Controlling Scanning} describes how you can assign special, predefined values to @code{PROCINFO["sorted_in"]} in order to control the order in which @command{gawk} traverses an array during a @code{for} loop. In addition, the value of @code{PROCINFO["sorted_in"]} can be a function name.@footnote{This is why the predefined sorting orders start with an @samp{@@} character, which cannot be part of an identifier.} This lets you traverse an array based on any custom criterion. The array elements are ordered according to the return value of this function. The comparison function should be defined with at least four arguments: @example function comp_func(i1, v1, i2, v2) @{ @var{compare elements 1 and 2 in some fashion} @var{return < 0; 0; or > 0} @} @end example Here, @code{i1} and @code{i2} are the indices, and @code{v1} and @code{v2} are the corresponding values of the two elements being compared. Either @code{v1} or @code{v2}, or both, can be arrays if the array being traversed contains subarrays as values. (@xref{Arrays of Arrays} for more information about subarrays.) The three possible return values are interpreted as follows: @table @code @item comp_func(i1, v1, i2, v2) < 0 Index @code{i1} comes before index @code{i2} during loop traversal. @item comp_func(i1, v1, i2, v2) == 0 Indices @code{i1} and @code{i2} come together, but the relative order with respect to each other is undefined. @item comp_func(i1, v1, i2, v2) > 0 Index @code{i1} comes after index @code{i2} during loop traversal. @end table Our first comparison function can be used to scan an array in numerical order of the indices: @example @group function cmp_num_idx(i1, v1, i2, v2) @{ # numerical index comparison, ascending order return (i1 - i2) @} @end group @end example Our second function traverses an array based on the string order of the element values rather than by indices: @example function cmp_str_val(i1, v1, i2, v2) @{ # string value comparison, ascending order v1 = v1 "" v2 = v2 "" if (v1 < v2) return -1 return (v1 != v2) @} @end example The third comparison function makes all numbers, and numeric strings without any leading or trailing spaces, come out first during loop traversal: @example function cmp_num_str_val(i1, v1, i2, v2, n1, n2) @{ # numbers before string value comparison, ascending order n1 = v1 + 0 n2 = v2 + 0 if (n1 == v1) return (n2 == v2) ? (n1 - n2) : -1 else if (n2 == v2) return 1 return (v1 < v2) ? -1 : (v1 != v2) @} @end example Here is a main program to demonstrate how @command{gawk} behaves using each of the previous functions: @example BEGIN @{ data["one"] = 10 data["two"] = 20 data[10] = "one" data[100] = 100 data[20] = "two" f[1] = "cmp_num_idx" f[2] = "cmp_str_val" f[3] = "cmp_num_str_val" for (i = 1; i <= 3; i++) @{ printf("Sort function: %s\n", f[i]) PROCINFO["sorted_in"] = f[i] for (j in data) printf("\tdata[%s] = %s\n", j, data[j]) print "" @} @} @end example Here are the results when the program is run: @example $ @kbd{gawk -f compdemo.awk} @print{} Sort function: cmp_num_idx @ii{Sort by numeric index} @print{} data[two] = 20 @print{} data[one] = 10 @ii{Both strings are numerically zero} @print{} data[10] = one @print{} data[20] = two @print{} data[100] = 100 @print{} @print{} Sort function: cmp_str_val @ii{Sort by element values as strings} @print{} data[one] = 10 @print{} data[100] = 100 @ii{String 100 is less than string 20} @print{} data[two] = 20 @print{} data[10] = one @print{} data[20] = two @print{} @print{} Sort function: cmp_num_str_val @ii{Sort all numeric values before all strings} @print{} data[one] = 10 @print{} data[two] = 20 @print{} data[100] = 100 @print{} data[10] = one @print{} data[20] = two @end example Consider sorting the entries of a GNU/Linux system password file according to login name. The following program sorts records by a specific field position and can be used for this purpose: @example # passwd-sort.awk --- simple program to sort by field position # field position is specified by the global variable POS function cmp_field(i1, v1, i2, v2) @{ # comparison by value, as string, and ascending order return v1[POS] < v2[POS] ? -1 : (v1[POS] != v2[POS]) @} @{ for (i = 1; i <= NF; i++) a[NR][i] = $i @} @group END @{ PROCINFO["sorted_in"] = "cmp_field" @end group if (POS < 1 || POS > NF) POS = 1 for (i in a) @{ for (j = 1; j <= NF; j++) printf("%s%c", a[i][j], j < NF ? ":" : "") print "" @} @} @end example The first field in each entry of the password file is the user's login name, and the fields are separated by colons. Each record defines a subarray, with each field as an element in the subarray. Running the program produces the following output: @example $ @kbd{gawk -v POS=1 -F: -f sort.awk /etc/passwd} @print{} adm:x:3:4:adm:/var/adm:/sbin/nologin @print{} apache:x:48:48:Apache:/var/www:/sbin/nologin @print{} avahi:x:70:70:Avahi daemon:/:/sbin/nologin @dots{} @end example The comparison should normally always return the same value when given a specific pair of array elements as its arguments. If inconsistent results are returned, then the order is undefined. This behavior can be exploited to introduce random order into otherwise seemingly ordered data: @example function cmp_randomize(i1, v1, i2, v2) @{ # random order (caution: this may never terminate!) return (2 - 4 * rand()) @} @end example As already mentioned, the order of the indices is arbitrary if two elements compare equal. This is usually not a problem, but letting the tied elements come out in arbitrary order can be an issue, especially when comparing item values. The partial ordering of the equal elements may change the next time the array is traversed, if other elements are added to or removed from the array. One way to resolve ties when comparing elements with otherwise equal values is to include the indices in the comparison rules. Note that doing this may make the loop traversal less efficient, so consider it only if necessary. The following comparison functions force a deterministic order, and are based on the fact that the (string) indices of two elements are never equal: @example function cmp_numeric(i1, v1, i2, v2) @{ # numerical value (and index) comparison, descending order return (v1 != v2) ? (v2 - v1) : (i2 - i1) @} @group function cmp_string(i1, v1, i2, v2) @{ # string value (and index) comparison, descending order v1 = v1 i1 v2 = v2 i2 return (v1 > v2) ? -1 : (v1 != v2) @} @end group @end example @c Avoid using the term ``stable'' when describing the unpredictable behavior @c if two items compare equal. Usually, the goal of a "stable algorithm" @c is to maintain the original order of the items, which is a meaningless @c concept for a list constructed from a hash. A custom comparison function can often simplify ordered loop traversal, and the sky is really the limit when it comes to designing such a function. When string comparisons are made during a sort, either for element values where one or both aren't numbers, or for element indices handled as strings, the value of @code{IGNORECASE} (@pxref{Built-in Variables}) controls whether the comparisons treat corresponding upper- and lowercase letters as equivalent or distinct. Another point to keep in mind is that in the case of subarrays, the element values can themselves be arrays; a production comparison function should use the @code{isarray()} function (@pxref{Type Functions}) to check for this, and choose a defined sorting order for subarrays. @cindex POSIX mode All sorting based on @code{PROCINFO["sorted_in"]} is disabled in POSIX mode, because the @code{PROCINFO} array is not special in that case. As a side note, sorting the array indices before traversing the array has been reported to add a 15% to 20% overhead to the execution time of @command{awk} programs. For this reason, sorted array traversal is not the default. @c The @command{gawk} @c maintainers believe that only the people who wish to use a @c feature should have to pay for it. @node Array Sorting Functions @subsection Sorting Array Values and Indices with @command{gawk} @cindex arrays @subentry sorting @subentry @code{asort()} function (@command{gawk}) @cindex arrays @subentry sorting @subentry @code{asorti()} function (@command{gawk}) @cindexgawkfunc{asort} @cindex @code{asort()} function (@command{gawk}) @subentry arrays, sorting @cindex @code{asort()} function (@command{gawk}) @subentry side effects @cindexgawkfunc{asorti} @cindex @code{asorti()} function (@command{gawk}) @subentry arrays, sorting @cindex @code{asorti()} function (@command{gawk}) @subentry side effects @cindex sort function, arrays, sorting In most @command{awk} implementations, sorting an array requires writing a @code{sort()} function. This c