3.3. Shell Flavors

Many different Unix shells are available. This quick reference describes the three most popular shells:

• The GNU Project's Bash (Bourne-Again SHell), arguably the most popular shell in use today. It is a superset of the original Bourne shell , including command-line editing and many features first implemented in the Korn shell.

• The Korn shell, a superset of the original Bourne shell that lets you edit the command line. There are two commonly available versions of the Korn shell, distinguished by the year they were released, and referred to in this book as ksh88 and ksh93 respectively.

• The "Tenex" C shell, an enhanced version of the original BSD C shell, which uses C-like syntax and is more convenient for the interactive user than the original Bourne shell.

The original Bourne shell is available as /bin/sh on commercial Unix systems, and if invoked as sh, Bash will do its best to emulate the original Bourne shell's behavior. However, it is rare today to find the original Bourne shell being used interactively as a login shell; other shells that provide better interactive features and the Bourne shell's programming language, such as Bash and ksh, are more popular. However, when writing shell scripts, most people are careful to restrict themselves to just those features of the Bourne shell.

The /etc/passwd file determines which shell takes effect during your interactive Unix session. When you log in, the system checks your entry in /etc/passwd. The last field of each entry names a program to run as the default shell.^[*] For example:

^[*] On Solaris or other networked Unix systems, this information may come from NIS or NIS+. Usually, your system administrator will handle this for you; just don't be surprised if your login name doesn't appear in /etc/passwd.

If the program name is:	Your shell is the:
/bin/sh	Bourne shell
/bin/bash	The Bash shell
/bin/ksh	Korn shell
/usr/dt/bin/dtksh	The Desktop Korn shell , a version of ksh93 (Solaris only)
/bin/csh	C shell or Tenex C shell (system dependent)
/bin/tcsh	Tenex C shell

You can change to another shell by typing the program name at the command line. For example, to change from the Bourne shell to the Korn shell, type:

     $ exec ksh

3.3.1. Which Shell Do I Want?

If you are new to Unix, picking a shell may be a bewildering question. Before ksh was commonly available, the general advice was to use csh for interactive use (because it supported job control and had other features that made it a better interactive shell than the Bourne shell), but to use the Bourne shell for scripting (because it is a more powerful programming language, and more universally available).

Today, a wide variety of Bourne-compatible shells are available that all support job control and some sort of command history and command-line editing. Of these, Bash is arguably the most popular, and it is a good choice. If Bash is not available but the Korn shell is, you should use the Korn shell. In any case, source code for both shells (as well as others) are easily available from the Internet.