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Source code changes of the file "compress.1" between
ncompress-4.2.4.6.tar.gz and ncompress-5.0.tar.gz

About: (N)compress is an improved version of compress 4.1. Compress is a fast, simple LZW file compressor (but has a relatively low compression rate).

compress.1  (ncompress-4.2.4.6):compress.1  (ncompress-5.0)
COMPRESS(1) General Commands Manual COMPRESS(1) COMPRESS(1) General Commands Manual COMPRESS(1)
NAME NAME
compress, uncompress, zcat - compress and expand data compress, uncompress, zcat - compress and expand data
SYNOPSIS SYNOPSIS
compress [ -f ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ -- ] [ name ... compress [ -f ] [ -k ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ -- ] [ n
] ame ... ]
uncompress [ -f ] [ -v ] [ -c ] [ -V ] [ -- ] [ name ... ] uncompress [ -f ] [ -k ] [ -v ] [ -c ] [ -V ] [ -- ] [ name ... ]
zcat [ -V ] [ -- ] [ name ... ] zcat [ -V ] [ -- ] [ name ... ]
DESCRIPTION DESCRIPTION
Compress reduces the size of the named files using adaptive Lempel-Ziv coding. Whenever possible, each Compress reduces the size of the named files using adaptive Lempel-Ziv coding. Whenever possible, each
file is replaced by one with the extension .Z, while keeping the same own ership modes, access and modifi- file is replaced by one with the extension .Z, while keeping the same own ership modes, access and modifi-
cation times. If no files are specified, the standard input is compres sed to the standard output. Com- cation times. If no files are specified, the standard input is compres sed to the standard output. Com-
press will only attempt to compress regular files. In particular, it wil l ignore symbolic links. If a press will only attempt to compress regular files. In particular, it wil l ignore symbolic links. If a
file has multiple hard links, compress will refuse to compress it unless the -f flag is given. file has multiple hard links, compress will refuse to compress it unless the -f flag is given.
If -f is not given and compress is run in the foreground, the user is p rompted as to whether an existing If -f is not given and compress is run in the foreground, the user is p rompted as to whether an existing
file should be overwritten. file should be overwritten.
Compressed files can be restored to their original form using uncompress or zcat. Compressed files can be restored to their original form using uncompress or zcat.
uncompress takes a list of files on its command line and replaces each fi le whose name ends with .Z and uncompress takes a list of files on its command line and replaces each fi le whose name ends with .Z and
which begins with the correct magic number with an uncompressed file without the .Z. The uncompressed which begins with the correct magic number with an uncompressed file without the .Z. The uncompressed
file will have the mode, ownership and timestamps of the compressed file. file will have the mode, ownership and timestamps of the compressed file.
The -k option makes compress/uncompress keep the input files instead of a
utomatically removing them.
The -c option makes compress/uncompress write to the standard output; no files are changed. The -c option makes compress/uncompress write to the standard output; no files are changed.
zcat is identical to uncompress -c. zcat uncompresses either a list of f iles on the command line or its zcat is identical to uncompress -c. zcat uncompresses either a list of f iles on the command line or its
standard input and writes the uncompressed data on standard output. zca t will uncompress files that have standard input and writes the uncompressed data on standard output. zca t will uncompress files that have
the correct magic number whether they have a .Z suffix or not. the correct magic number whether they have a .Z suffix or not.
If the -r flag is specified, compress will operate recursively. If any of the file names specified on the If the -r flag is specified, compress will operate recursively. If any of the file names specified on the
command line are directories, compress will descend into the directory an d compress all the files it finds command line are directories, compress will descend into the directory an d compress all the files it finds
there. there. When compressing, any files already compressed will be ignored, a
nd when decompressing, any files
already decompressed will be ignored.
The -V flag tells each of these programs to print its version and patchle vel, along with any preprocessor The -V flag tells each of these programs to print its version and patchl evel, along with any preprocessor
flags specified during compilation, on stderr before doing any compressio n or uncompression. flags specified during compilation, on stderr before doing any compressio n or uncompression.
Compress uses the modified Lempel-Ziv algorithm popularized in "A Techniq ue for High Performance Data Com- Compress uses the modified Lempel-Ziv algorithm popularized in "A Techniq ue for High Performance Data Com-
pression", Terry A. Welch, IEEE Computer, vol. 17, no. 6 (June 1984), pp. pression", Terry A. Welch, IEEE Computer, vol. 17, no. 6 (June 1984), pp
8-19. Common substrings in the . 8-19. Common substrings in the
file are first replaced by 9-bit codes 257 and up. When code 512 is r file are first replaced by 9-bit codes 257 and up. When code 512 is reac
eached, the algorithm switches to hed, the algorithm switches to
10-bit codes and continues to use more bits until the limit specified by 10-bit codes and continues to use more bits until the limit specified b
the -b flag is reached (default y the -b flag is reached (default
16). Bits must be between 9 and 16. The default can be changed in the s ource to allow compress to be run 16). Bits must be between 9 and 16. The default can be changed in the s ource to allow compress to be run
on a smaller machine. on a smaller machine.
After the bits limit is attained, compress periodically checks the compre ssion ratio. If it is increas- After the bits limit is attained, compress periodically checks the comp ression ratio. If it is increas-
ing, compress continues to use the existing code dictionary. However, if the compression ratio decreases, ing, compress continues to use the existing code dictionary. However, if the compression ratio decreases,
compress discards the table of substrings and rebuilds it from scratch. This allows the algorithm to compress discards the table of substrings and rebuilds it from scrat ch. This allows the algorithm to
adapt to the next "block" of the file. adapt to the next "block" of the file.
Note that the -b flag is omitted for uncompress, since the bits paramete Note that the -b flag is omitted for uncompress, since the bits parameter
r specified during compression is specified during compression is
encoded within the output, along with a magic number to ensure that neith encoded within the output, along with a magic number to ensure that nei
er decompression of random data ther decompression of random data
nor recompression of compressed data is attempted. nor recompression of compressed data is attempted.
The amount of compression obtained depends on the size of the input, the The amount of compression obtained depends on the size of the input, the
number of bits per code, and the number of bits per code, and the
distribution of common substrings. Typically, text such as source code o distribution of common substrings. Typically, text such as source code
r English is reduced by 50-60%. or English is reduced by 50-60%.
Compression is generally much better than that achieved by Huffman codi Compression is generally much better than that achieved by Huffman coding
ng (as used in pack), or adaptive (as used in pack), or adaptive
Huffman coding (compact), and takes less time to compute. Huffman coding (compact), and takes less time to compute.
Under the -v option, a message is printed yielding the percentage of redu ction for each file compressed. Under the -v option, a message is printed yielding the percentage of redu ction for each file compressed.
-- may be used to halt option parsing and force all remaining arguments t o be treated as paths. -- may be used to halt option parsing and force all remaining arguments t o be treated as paths.
DIAGNOSTICS DIAGNOSTICS
Exit status is normally 0; if the last file is larger after (attempted) c ompression, the status is 2; if Exit status is normally 0; if the last file is larger after (attempted) compression, the status is 2; if
an error occurs, exit status is 1. an error occurs, exit status is 1.
Usage: compress [-dfvcVr] [-b maxbits] [file ...] Usage: compress [-dfvcVr] [-b maxbits] [file ...]
Invalid options were specified on the command line. Invalid options were specified on the command line.
Missing maxbits Missing maxbits
Maxbits must follow -b. Maxbits must follow -b.
file: not in compressed format file: not in compressed format
The file specified to uncompress has not been compressed. The file specified to uncompress has not been compressed.
file: compressed with xx bits, can only handle yy bits file: compressed with xx bits, can only handle yy bits
File was compressed by a program that could deal with more bi ts than the compress code on this File was compressed by a program that could deal with more bits t han the compress code on this
machine. Recompress the file with smaller bits. machine. Recompress the file with smaller bits.
file: already has .Z suffix -- no change file: already has .Z suffix -- no change
The file is assumed to be already compressed. Rename the file an d try again. The file is assumed to be already compressed. Rename the file an d try again.
file: filename too long to tack on .Z file: filename too long to tack on .Z
The file cannot be compressed because its name is longer than 1 2 characters. Rename and try The file cannot be compressed because its name is longer th an 12 characters. Rename and try
again. This message does not occur on BSD systems. again. This message does not occur on BSD systems.
file already exists; do you wish to overwrite (y or n)? file already exists; do you wish to overwrite (y or n)?
Respond "y" if you want the output file to be replaced; "n" if no t. Respond "y" if you want the output file to be replaced; "n" if no t.
uncompress: corrupt input uncompress: corrupt input
A SIGSEGV violation was detected which usually means that the inp ut file has been corrupted. A SIGSEGV violation was detected which usually means that the inp ut file has been corrupted.
Compression: xx.xx% Compression: xx.xx%
Percentage of the input saved by compression. (Relevant only for -v.) Percentage of the input saved by compression. (Relevant only for -v.)
-- not a regular file or directory: ignored -- not a regular file or directory: ignored
When the input file is not a regular file or directory, (e. g. a symbolic link, socket, FIFO, When the input file is not a regular file or directory, (e.g. a symbolic link, socket, FIFO,
device file), it is left unaltered. device file), it is left unaltered.
-- has xx other links: unchanged -- has xx other links: unchanged
The input file has links; it is left unchanged. See ln(1) for mo re information. Use the -f flag The input file has links; it is left unchanged. See ln(1) for more information. Use the -f flag
to force compression of multiply-linked files. to force compression of multiply-linked files.
-- file unchanged -- file unchanged
No savings is achieved by compression. The input remains virgin. No savings is achieved by compression. The input remains virgin.
BUGS BUGS
Although compressed files are compatible between machines with large mem Although compressed files are compatible between machines with large memo
ory, -b12 should be used for file ry, -b12 should be used for file
transfer to architectures with a small process data space (64KB or less, transfer to architectures with a small process data space (64KB or
as exhibited by the DEC PDP less, as exhibited by the DEC PDP
series, the Intel 80286, etc.) series, the Intel 80286, etc.)
Invoking compress with a -r flag will occasionally cause it to produce sp
urious error warnings of the form
"<filename>.Z already has .Z suffix - ignored"
These warnings can be ignored. See the comments in compress42.c:compdir
() in the source distribution for
an explanation.
SEE ALSO SEE ALSO
pack(1), compact(1) pack(1), compact(1)
local COMPRESS(1) local COMPRESS(1)
 End of changes. 16 change blocks. 
41 lines changed or deleted 37 lines changed or added

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