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.

Implementation

The UNIX Shell may be implemented by any of a wide variety of programs. Notable programs used as shells include the Bourne SHell, the Korn SHell, and the Bourne Again SHell.

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.