Shell Programming:Introduction

* The UNIX operating system provides a flexible set of simple tools to perform a wide variety of system-management, text-processing, and general-purpose tasks. These simple tools can be used in very powerful ways by tying them together programmatically, using "shell scripts" or "shell programs".

The UNIX "shell" itself is a user-interface program that accepts commands from the user and executes them. It can also accept the same commands written as a list in a file, along with various other statements that the shell can interpret to provide input, output, decision-making, looping, variable storage, option specification, and so on. This file is a shell program.

Shell programs are, like any other programming language, useful for some things but not for others. They are excellent for system-management tasks but not for general-purpose programming of any sophistication. Shell programs, though generally simple to write, are also tricky to debug and slow in operation.

There are three versions of the UNIX shell: the original "Bourne shell (sh)", the "C shell (csh)" that was derived from it, and the "Korn shell (ksh)" that is in predominant use. The Bourne shell is in popular use as the freeware "Bourne-again shell" AKA "bash".