A Unix shell, also called "the command line", provides the traditional user interface for the Unix operating system. Users direct the operation of the computer by entering command input as text for a shell to execute. Within the Microsoft Windows suite of operating systems the analogous program is command.com, or cmd.exe for Windows NT-based operating systems.

The most generic sense of the term shell means any program that users use to type commands; it is called a "shell" because it hides the details of the underlying operating system behind the shell's interface (contrast "kernel", which refers to the lowest-level, or 'inner-most' component of an operating system). Similarly, graphical user interfaces for Unix, such as GNOME and KDE, are sometimes called visual shells or graphical shells. By itself, the term shell is usually associated with the command line. In Unix, any program can be the user's shell; users who want to use a different syntax for typing commands can specify a different program as their shell.

The term shell also refers to a particular program, namely the Bourne shell, sh. The Bourne shell was the shell used in early versions of Unix and became a de facto standard; every Unix-like system has the equivalent of the Bourne shell. The Bourne shell program is located in the UNIX file hierarchy at /bin/sh. On some systems, such as BSD, /bin/sh is a Bourne shell or equivalent, but on other systems such as Linux, /bin/sh is likely to be a link to a compatible, but more feature-rich shell, such as Bash. POSIX specifies the standard shell as a strict subset of the Korn shell.

The Unix shell is unusual since it is in both an interactive command language and the language used to script the system; it is a scripting programming language. This is unlike the Microsoft Windows command interpreters.

On systems using a windowing system, some users may never use the shell directly, though on Unix systems, the shell is still the implementation language of system startup scripts, including the program that starts the windowing system, the program that dials into the Internet, and many other essential functions.

On Windows systems, the command interpreter is never used except when the user invokes it directly. Much functionality that was available in earlier versions of the command environment has been removed from modern versions of Windows. Equivalents to Unix system scripts are distributed in binary form.

Many regular users of a UNIX system still find a modern command line shell much more convenient for many tasks than any GUI application.

Unix shells

• Almquist shell (ash)
• Bourne shell (sh) Written by Steve Bourne, while at Bell Labs. First distributed with Version 7 UNIX, circa 1978.
• Bourne-Again shell (bash)
• C shell (csh) Written by Bill Joy, while at the University of California, Berkeley. First distributed with BSD, circa 1979.
• scsh (Scheme Shell)
• TENEX C shell (tcsh)
• Korn shell (ksh) Written by David Korn, while at Bell Labs.
• rc shell (rc) The Plan 9 shell by Tom Duff while at Bell Labs, later backported to Unix and other Operating Systems.
• Z shell (zsh)

Historic

• Thompson shell (sh) The first Unix shell, written by Ken Thompson at Bell Labs. Distributed with Versions 1 through 6 of Unix, from 1971 to 1975.
• PWB shell (sh) A version of the Thompson shell, augmented by John Mashey and others, while at Bell Labs. Distributed with the Programmer's Workbench UNIX, circa 1976.

See also

• Shebang
• Shell script
• List of Unix programs
• Shell account

External links

• Unix Shells - csh, ksh, bash, zsh, ... by Christopher Browne