Shells in UNIX

Shells in UNIX

The command interpreter in MPE is built-in: with UNIX it's just another program. As a consequence, UNIX has many competing command interpreters which are collectively known as shells.

The most common three shells are

sh, the Bourne shell, whose default prompt is $; it is the oldest, is on every UNIX system, has no job control, is the default for root, and is used for writing command files.
csh, the C shell, whose default prompt is %; it is from Berkeley, and has lots of tricky features.
ksh, the Korn shell, which is compatible with sh but which also has many csh features, among them command history editing. It also offers reliability.

Your startup shell is specified by a program name as the last field in your /etc/passwd entry. For example, /bin/csh means run csh in the /bin directory.

One confusing thing for MPE users is that some commands, such as printenv, are built into a specific shell, while others such as rm and cp are just programs in the /bin or /usr/bin directories. Commands that are implemented as external programs are the same in all shells, but require some extra system overhead to create a separate process. Built-in commands usually do not have their own man pages, so you have to do man ksh and read the description of the Korn shell. For the benefit of MPE users who are new to UNIX we have created a command cross-reference showing the commands in each shell that correspond to each MPE command.