Shell
The shell is a program that interprets commands and acts as an intermediary between the user and the inner workings of the Unix system. Providing a command-line interface (i.e., the shell prompt or command prompt), the shell is analogous to DOS and serves a purpose similar to graphical interfaces like Windows, Mac OS, Mac OS X, and the X Window System.
On most Unix systems, there are several shells available. For the average user, they offer similar functionality, but each has different syntax and capabilities. Most shells fall within one of two classes: those descended from the Bourne shell (i.e., sh), which first appeared in Version 7 Unix, and those that arose from the C Shell (i.e., csh), which made its debut in BSD. Nearly every Unix system has these two shells installed, but may also have several others, including:
Korn shell (ksh)
Class: sh
Description: An extension of the Bourne shell with several features adapted from the C shell; the POSIX shell is based on the Korn shell
TC shell (tcsh)
Class: csh
Description: A revision of the C shell with substantially expanded capabilities; the default shell in modern BSD implementations, including Mac OS X/Darwin
Bourne-again shell (bash)
Class: sh
Description: An extension of the Bourne shell, but with unique features; part of the GNU project and the default shell for Linux
You may find out what options you have for your login shell (i.e., your default shell) by looking at the file /etc/shells.
Most shells double as interpreted programming languages. To automate tasks, you may write scripts containing built-in shell and Unix commands. When you execute a script, the shell interprets these commands just as if you had entered them from the command-line prompt. Compared to compiled programs, shell scripts are slow, but easy to write and debug.
- Note: In general, shells of the Bourne shell class are better for scripting than those derived from the C shell.
To Unix, the shell is nothing more than another program. For this reason, any program can be designated a login shell in /etc/shells. For example, some Emacs users pride themselves on never needing a traditional shell prompt.
