Screenshot of a sample Bash session, taken on Gentoo Linux.

A Unix shell, also called "the command line", provides the traditional user interface for the Unix operating system and for Unix-like systems. Users direct the operation of the computer by entering command input as text for a shell to execute.

The most generic sense of the term shell means any program that users use to type commands. Since in the Unix operating system users can select which shell they want to use (which program should execute when they login), many shells have been developed. It is called a "shell" because it hides the details of the underlying operating system behind the shell's interface (in contrast with the "kernel", which refers to the lowest-level, or 'inner-most' component of an operating system). Similarly, graphical user interfaces for Unix, such as GNOME, KDE, and Xfce can be 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, such as 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 at least one shell compatible with 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. POSIX specifies its standard shell as a strict subset of the Korn shell.

Other types of shells

The Unix shell was unusual when first created. Since it is both an interactive command language and the language used to script the system, it is a scripting programming language. Many shells created for other operating systems offer rough equivalents to Unix shell functionality.

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

On MS-DOS, OS/2, and Windows, equivalents to Unix system scripts are called batch files, and have either a ".bat" or ".cmd" extension. A newer CLI - Windows PowerShell, will replace the existing NT command line, cmd.exe; it has many features derived from Unix shells, though it uses a somewhat different syntax.

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

Due to the recent movement in favor of open source, most Unix shells have at least one version that is open source.

Shell categories

Unix shells can be broadly divided into four categories: Bourne-like, C Shell-like, nontraditional, and historical.

Bourne shell compatible

• Bourne shell (sh) -- Written by Steve Bourne, while at Bell Labs. First distributed with Version 7 Unix, circa 1978, and enhanced over the years.
  • Almquist shell (ash) -- Written as a BSD-licensed replacement for the Bourne Shell; often used in resource-constrained environments. The sh of FreeBSD, NetBSD (and their derivatives) are based on ash that has been enhanced to be POSIX conformant for the occasion.
  • Bourne-Again shell (bash) -- Written as part of the GNU project to provide a superset of Bourne Shell functionality.
  • Debian Almquist shell (dash) -- Dash is a modern replacement for ash in Debian and is expected to be the default /bin/sh for Debian Lenny.
  • Korn shell (ksh) -- Written by David Korn, while at Bell Labs.
  • Z shell (zsh) -- considered as the most complete (read: the most features) shell: it is the closest thing that exists to a superset of sh, ash, bash, csh, ksh, and tcsh.

C shell compatible

• C shell (csh) Written by Bill Joy, while at the University of California, Berkeley. First distributed with BSD, circa 1979.
  • TENEX C shell (tcsh)

Other or exotic

• fish, friendly interactive shell, first released in 2005.
• mudsh, an "intelligent" game-like shell that operates like a MUD.
• zoidberg, a modular Perl shell written, configured, and operated entirely in Perl.
• rc, the default shell on Plan 9 from Bell Labs and Version 10 Unix written by Tom Duff. Ports have been made to Inferno and Unix-like operating systems.
  • es shell (es) A functional programming rc-compatible shell written in the mid-1990s.
• scsh (Scheme Shell)

A list of various shells may be found at www.freebsd.org.

Configuration files for shells

Shells read configuration files on multiple circumstances which differ depending on the shell. This table shows to configuration files for popular shells:

Explanation:

• "no" means a file is not read by a shell at all.
• "yes" means a file is always read by a shell.
• "login" means a file is read if the shell is a login shell.
• "n/login" means a file is read if the shell is not a login shell.
• "int." means a file is read if the shell is interactive.
• "i.login" means a file is read if the shell is an interactive login shell.

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. Considered very rudimentary by modern standards and not used on current systems, though available as part of some Ancient UNIX Systems.
• PWB shell or Mashey 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.

Non-Unix shells

Within the Microsoft Windows suite of operating systems the analogous program is command.com, or cmd.exe for Windows NT-based operating systems, and Windows PowerShell introduced with Windows Server 2008.

Further reading

• — a history of the various shells, and the uses of and responsibilities of a shell on Unix

See also

• Shell
• Comparison of command shells
• Shell script
• List of Unix programs
• Shell account

External links

• Unix Shells - csh, ksh, bash, zsh, ...
• shell encyclopedia - history, types, definitions, technical information and resources
• Linux Shell Scripting Tutorial - A Beginner's handbook

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