In most Linux systems, your default shell is the bash shell. To find out what your current login shell is, type the following command:

$ echo $SHELL
/bin/bash

In this example, it’s the bash shell. There are many other shells, and you can activate a different one by simply typing the new shell’s command (ksh, tcsh, csh, sh, bash, and so forth) from the current shell. For example, to change temporarily to the C shell, type the following command:

$ csh

You might want to choose a different shell to use because:

• You are used to using UNIX System V systems (often ksh by default) or Sun Microsystems and other Berkeley UNIX–based distributions (frequently csh by default), and you are more comfortable using default shells from those environments.

• You want to run shell scripts that were created for a particular shell environment, and you need to run the shell for which they were made so you can test or use those scripts.

• You might simply prefer features in one shell over those in another. For example, a member of my Linux Users Group prefers ksh over bash because he doesn’t like the way aliases are always set up with bash.

Although most Linux users have a preference for one shell or another, when you know how to use one shell, you can quickly learn any of the others by occasionally referring to the shell’s man page (for example, type man bash). Most people use bash just because they don’t have a particular reason for using a different shell.

Using bash (and Earlier sh) Shells
As mentioned earlier, the name bash is an acronym for Bourne Again Shell, acknowledging the roots of bash coming from the Bourne shell (sh command) created by Steve Bourne at AT&T Bell Labs. Brian Fox of the Free Software Foundation created bash, under the auspices of the GNU Project. Development was later taken over by Chet Ramey at Case Western Reserve University.

Bash includes features originally developed for sh and ksh shells in early UNIX systems, as well as some csh features. Expect bash to be the default shell in whatever Linux system you are using, with the exception of some specialized Linux systems (such as those run on embedded devices or run from a floppy disk) that may require a smaller shell that needs less memory and entails fewer features.

Bash can be run in various compatibility modes so that it behaves like different shells. It can be run to behave as a Bourne shell (bash +B) or as a POSIX-compliant shell (type bash --posix), for example, enabling it to read configuration files that are specific to those shells and run initialization shell scripts written directly for those shells, with a greater chance of success.

Using tcsh (and Earlier csh) Shells
The tcsh shell is the open source version of the C shell (csh). The csh shell was created by Bill Joy and used with most Berkeley UNIX systems (such as those produced by Sun Microsystems) as the default shell. Features from the TENEX and TOPS-20 operating systems (used on PDP-11s in the 1970s) that are included in this shell are responsible for the T in tcsh. Many features of the original csh shell, such as command-line editing and its history mechanism, are included in tcsh as well as in other shells. While you can run both csh and tcsh on most Linux systems, both commands actually point to the same executable file. In other words, starting csh actually runs the tcsh shell in csh compatibility mode.

Using ash
The ash shell is a lightweight version of the Berkeley UNIX sh shell. It doesn’t include many of the sh shell’s basic features, and is missing such features as command histories. Kenneth Almquist created the ash shell. The ash shell is a good shell for embedded systems that have fewer system resources available. The ash shell is about one-seventh the size of bash (about 100K versus 712K for bash). Because of cheaper memory prices these days, however, many embedded and small bootable Linux systems have enough space to include the full bash shell.

Using ksh
The ksh shell was created by David Korn at AT&T Bell Labs and is the successor to the sh shell. It became the default and most commonly used shell with UNIX System V systems. The open source version of ksh was originally available in many rpm-based systems (such as Fedora and Red Hat Enterprise Linux) as part of the pdksh package. Now, however, David Korn has released the original ksh shell as open source, so you can look for it as part of a ksh software package in most Linux systems (see www.kornshell.com).

Using zsh
The zsh shell is another clone of the sh shell. It is POSIX-compliant (as is bash), but includes some different features, such as spell checking and a different approach to command editing. The first Mac OS X systems used zsh as the default shell, although now bash is used by default.