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Shell Scripting - Introduction

A Unix shell, also called "the command line", provides the traditional user interface for the Unix operating system. Users direct the operation of the computer by entering command input as text for a shell to execute. Within the Microsoft Windows suite of operating systems the analogous program is command.com, or cmd.exe for Windows NT-based operating systems.

The most generic sense of the term shell means any program that users use to type commands; it is called a "shell" because it hides the details of the underlying operating system behind the shell's interface (contrast "kernel", which refers to the lowest-level, or 'inner-most' component of an operating system). Similarly, graphical user interfaces for Unix, such as GNOME and KDE, are sometimes called visual shells or graphical shells. By itself, the term shell is usually associated with the command line. In Unix, any program can be the user's shell; users who want to use a different syntax for typing commands can specify a different program as their shell.

Such a file is known as a shell script file. The language used in that file is called shell script language. Like other programming languages it has variables and flow control statements (e.g. if-then-else, while, for, goto).

In Unix there are several shells that can be used, the C shell ( csh and its extension, the T C shell tcsh), the Bourne Shell (sh and its extensions the Bourne Again Shell bash and the highly programmable Korn shell ksh) being the more commonly used.

Note that you can run any shell simply by typing its name. For example, if I am now running csh and wish to switch to ksh, I simply type ksh, and a Korn shell will start up for me. All my commands from that point on will be read and processed by the Korn shell (though when I eventually want to log off, exiting the Korn shell will still leave me in the C shell, so I will have to exit from it too).

For the purposes of this guide we will be concentrating on the Bourne Again SHell (bash)

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