From Rosetta Code

The UNIX Shell is a component of terminal-based UNIX-derived systems which offers both a command-line interface for running system commands, as well as programming interface for intelligently automating tasks which use system commands.

[edit] Implementation

There are many UNIX Shells and most of them can be categorized into two families. For purposes of the Rosette Code, all examples are in Bourne-compatible syntax. The other family of shells, with a markedly different syntax, are csh and it's tcsh (Tenex C Shell) "clone." Common Bourne compatible shells include the original Bourne Shell (/bin/sh on most versions of UNIX), the GNU Bourne Again SHell (bash --- which is linked to /bin/sh on many distributions of Linux, making it their default shell), the Korn Shell (ksh), the Public Domain Korn SHell (pdksh), the Almquist SHell (ash) and the Debian Almquist SHell (dash) and the Z SHell (zsh).

Main article: UNIX Shell Implementations

[edit] Language

While UNIX Shells vary in the programming languages they support, such languages carry a minimum set of features. Each language allows the programmer to execute system commands as though he were typing the commands himself, and each language allows for a header line which specifies which shell implementation is used to interpret the script.

This one tells the operating system to use the Bourne Shell:

#!/bin/sh

This line tells the operating system to use the Bourne Again SHell:

#!/bin/bash

And this one tells the operating system to use the Korn Shell:

#!/bin/ksh

Each header line consists of a hash, a bang, and the path to the interpreter binary.