The shell is an interface to the operating system just like the graphic user interface (GUI) is. Typically, the shell acts as a command interpreter; it takes each command and passes it to the operating system. It then displays the results of this operation on your screen.

The most common shells are

To identify which shell you are currently running, you may type in your current shell echo $shell

Historically, the shell came before the GUI, however the latter has become more popular since it allows in several cases easier usage of a computer. In reality, the shell is well-suited in specific types of usage and extra functionality that it may provide can make it quite productive.

This document describes the tcshrc project, a project that demonstrates exactly this extra functionality of the tcsh shell and makes it more accesible to the end user.