Comparison of Shells

A shell is the program that interprets what you type on the command line and decides what to do with it. A shell can also be invoked in a non-interactive way, for example to execute a pre-typed list of commands contained in a text file (a "shell script"). Think of a shell as the equivalent of the DOS "command.com" (command-line interpreter) and the shell script files as the equivalent of the DOS batch files (*.bat). In comparison with their DOS cousins, the Linux shell and scripting are on steroids.

There are several shells available on the Linux system (if you installed them): bash ("Bourne Again" shell), sh (Bourne shell, standard on many UNIX systems), csh (C shell, with a syntax akin to the "c" programming language, available on most UNIX systems), pdksh (public domain Korn shell), tcsh (tiny C shell, often used on small systems), sash (stand-alone shell, could be used when libraries are not available), ash, zsh, and perhaps a couple more.

The default shell on my system (and most probably on yours too) is bash , which is an excellent and standard shell, and I really cannot see a reason why a newbie like myself would want to change it. bash is fully backwards-compatible with the Bourne shell (the most popular shell on UNIX) and incorporates many enhancements and best features from other shells. From a newbie perspective, the different shells are included with Linux for historical reasons and backwards-compatibility of shell scripts that may require a particular shell to run. [Some shells may be useful if you write programs targeted for specialized "embedded" devices, that might run a "tiny" shell.]

You can determine the shell you are running using:

echo $SHELL

If you wanted to try another shell, type, for example:

tcsh

which will start the tiny c shell. When done, type

exit

which will return you to the previous shell (using exit on your first shell will log you out). You can find out how many shells you stacked on each other by displaying the "shell level" environmental variable:

echo $SHLVL

In the above command, the "$" means "expand the value of a shell environment variable", "SHLVL" is the variable name, and "echo" is a command that prints things.

The shell for each user is specified as the last field in the password file /etc/passwd . If you really wanted to change it, edit (as root) this file and replace the "/bin/bash" with the shell of your choice.