- Unix shell
A Unix shell, is a command line shell that provides the traditional
user interfacefor the Unix operating systemand for Unix-likesystems. Users direct the operation of the computerby entering command input as text for a command line interpreterto 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 systembehind 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 Xfcecan 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. POSIXspecifies its standard shell as a strict subset of the Korn shell.
Other types of shells
The Unix shell was unusual when it was 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
Due to the recent movement in favor of
open source, most Unix shells have at least one version that is open source.
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 POSIXconformant for the occasion.
Bourne-Again shell(bash) -- Written as part of the GNUproject to provide a superset of Bourne Shell functionality.
Debian Almquist shell(dash) -- Dash is a modern replacement for ash in Debianand 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
*zoidberg, a modular Perl shell written, configured, and operated entirely in
*rc, the default shell on
Plan 9 from Bell Labsand Version 10 Unixwritten by Tom Duff. Ports have been made to Inferno and Unix-likeoperating systems.
es shell(es) A functional programmingrc-compatible shell written in the mid-1990s.
A list of various shells may be found at [http://www.freebsd.org/ports/shells.html 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:
* "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.
Thompson shell(sh) -- The first Unix shell, written by Ken Thompson at Bell Labs. Distributed with Versions 1 through 6 of Unix, from 1971to 1975. Considered very rudimentary by modern standards and not used on current systems, though available as part of some Ancient UNIX Systems.
PWB shellor Mashey shell (sh) -- A version of the Thompson shell, augmented by John Masheyand others, while at Bell Labs. Distributed with the Programmer's Workbench UNIX, circa 1976.
Microsoft Windowssuite of operating systems the analogous programs are command.com, or cmd.exefor Windows NT-based operating systems, and Windows PowerShellintroduced with Windows Server 2008.
*cite book|title=Unix Shells by Example|author=Ellie Quigley|date=2001|publisher=Prentice Hall PTR|id=ISBN 013066538X|chapter=Introduction to UNIX shells — a history of the various shells, and the uses of and responsibilities of a shell on Unix
Comparison of command shells
List of Unix programs
* [http://www.shell.ir shell encyclopedia - history, types, definitions, technical information and resources]
* [http://www.freeos.com/guides/lsst/ Linux Shell Scripting Tutorial - A Beginner's handbook]
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