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{{short description|Command-line interpreter for Unix operating system}}
[[File:Tcsh ejecutándose en escritorio Mac OSX.png|thumb|300px|right|tcsh and sh shell windows on a [[Mac OS X Leopard]]<ref name="Vleck" /> desktop]]
A '''Unix shell''' is a [[Command-line_interface#Command-line_interpreter|command-line interpreter]] or [[shell (computing)|shell]] that provides a command line [[user interface]] for [[Unix-like]] [[operating system]]s.  The shell is both an interactive [[command language]] and a [[scripting language]], and is used by the operating system to control the execution of the system using [[shell script]]s.<ref>{{cite news | url=https://archive.org/stream/byte-magazine-1983-10/1983_10_BYTE_08-10_UNIX#page/n187/mode/2up | title=The Unix Shell | work=BYTE | date=October 1983 | access-date=30 January 2015 | author=Bourne, Stephen R. | pages=187}}</ref>
Users typically interact with a Unix shell using a [[terminal emulator]]; however, direct operation via serial hardware connections or [[Secure Shell]] are common for server systems.  All Unix shells provide filename [[Wildcard character|wildcarding]], [[Pipeline (Unix)|piping]], [[here document]]s, [[command substitution]], [[Variable (programming)|variables]] and [[control flow|control structures]] for [[Conditional (programming)|condition-testing]] and [[iteration]].
==Concept==
Generally, a ''shell'' is a program that executes other programs in response to text commands. A sophisticated shell can also change the environment in which other programs execute by passing [[environment variable|named variables]], a parameter list, or an input source.
In Unix-like operating systems, users typically have many choices of command-line interpreters for interactive sessions. When a user [[Log in|logs into]] the system interactively, a shell program is automatically executed for the duration of the session. The type of shell, which may be customized for each user, is typically stored in the user's profile, for example in the local {{mono|[[Passwd (file)|passwd]]}} file or in a distributed configuration system such as [[Network Information Service|NIS]] or [[Lightweight Directory Access Protocol|LDAP]]; however, the user may execute any other available shell interactively.
On operating systems with a [[windowing system]], such as [[macOS]] and desktop [[Linux distribution]]s, some users may never use the shell directly. On Unix systems, the shell has historically been the implementation language of system startup scripts, including the program that starts a windowing system, configures networking, and many other essential functions.  However, some system vendors have replaced the traditional shell-based startup system ([[init]]) with different approaches, such as [[systemd]].
==Early shells==
The first Unix shell was the [[Thompson shell]], ''sh'', written by [[Ken Thompson (computer programmer)|Ken Thompson]] at [[Bell Labs]] and distributed with Versions 1 through 6 of Unix, from 1971 to 1975.<ref name="v6hist">{{ cite web|url=http://v6shell.org/history/ |title=V6 Thompson Shell Port - History |publisher=V6shell.org |access-date=2012-08-14 }}</ref> Though rudimentary by modern standards, it introduced many of the basic features common to all later Unix shells, including piping, simple control structures using <code>if</code> and <code>goto</code>, and filename wildcarding.  Though not in current use, it is still available as part of some [[Ancient UNIX]] systems.
It was modeled after the [[Multics]] shell, developed in 1965 by American software engineer [[Glenda Schroeder]]. Schroeder's Multics shell was itself modeled after the [[RUNCOM]] program [[Louis Pouzin]] showed to the Multics Team. The "rc" suffix on some Unix configuration files (for example, ".vimrc"), is a remnant of the RUNCOM ancestry of Unix shells.<ref name="Vleck">{{ cite web|author=Tom Van Vleck |url=http://www.multicians.org/unix.html |title=Unix and Multics |publisher=Multicians.org |date=1995-02-05 |access-date=2012-08-14 }}</ref><ref name="Pouzin">{{ cite web|author=Louis Pouzin |url=http://www.multicians.org/shell.html |title=The Origin of the Shell |publisher=Multicians.org |date=2000-11-25 |access-date=2012-08-14 }}</ref>
The [[PWB shell]] or Mashey shell, ''sh'', was an upward-compatible version of the Thompson shell, augmented by [[John Mashey]] and others and distributed with the [[PWB/UNIX|Programmer's Workbench UNIX]], circa 1975–1977. It focused on making shell programming practical, especially in large shared computing centers. It added shell variables (precursors of [[environment variable]]s, including the search path mechanism that evolved into $PATH), user-executable shell scripts, and interrupt-handling.  Control structures were extended from if/goto to if/then/else/endif, switch/breaksw/endsw, and while/end/break/continue.  As shell programming became widespread, these external commands were incorporated into the shell itself for performance.
But the most widely distributed and influential of the early Unix shells were the [[Bourne shell]] and the [[C shell]]. Both shells have been used as the coding base and model for many derivative and work-alike shells with extended feature sets.<ref>{{cite web|url=http://www.softpanorama.org/People/Shell_giants/introduction.shtml|title=Introduction to the Unix shell history|author=Nikolai Bezroukov|author-link=Nikolai Bezroukov|publisher=Softpanorama|date=2015-08-13|access-date=2016-08-21|archive-url=https://web.archive.org/web/20220608181527/www.softpanorama.org/People/Shell_giants/introduction.shtml|archive-date=2022-06-08|url-status=dead}}</ref>
===Bourne shell===
The [[Bourne shell]], ''sh'', was a new Unix shell by [[Stephen R. Bourne|Stephen Bourne]] at Bell Labs.<ref>{{cite interview|url=https://www2.computerworld.com.au/article/279011/a-z_programming_languages_bourne_shell_sh/|title=The A-Z of Programming Languages: Bourne shell, or sh|first=Stephen|last=Bourne|subject-link=Stephen R. Bourne|interviewer=Howard Dahdah|date=2009-03-05|publisher=[[Computerworld]]|access-date=2022-08-16}}</ref> Distributed as the shell for UNIX Version 7 in 1979, it introduced the rest of the basic features considered common to all the later Unix shells, including [[here document]]s, [[command substitution]], more generic [[Variable (programming)|variables]] and more extensive builtin [[control flow|control structures]].  The language, including the use of a reversed keyword to mark the end of a block, was influenced by [[ALGOL 68]].<ref>{{cite web|url=http://groups.google.com/group/comp.lang.misc/msg/d58db4799c33e093?hl=en&dmode=source|title=''Re: Late Bloomers Revisited''|access-date=20 September 2014}}</ref>  Traditionally, the Bourne shell program name is {{mono|sh}} and its path in the Unix file system hierarchy is {{mono|/bin/sh}}.  But a number of compatible work-alikes are also available with various improvements and additional features.  On many systems, sh may be a [[symbolic link]] or [[hard link]] to one of these alternatives:
* [[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.
** [[Busybox]]: a set of Unix utilities for small and embedded systems, which includes 2 shells: ash, a derivative of the Almquist shell; and hush, an independent implementation of a Bourne shell.
** [[Debian Almquist shell]] (dash): a modern replacement for ash in [[Debian]] and [[Ubuntu (operating system)|Ubuntu]]
* [[Bash (Unix shell)|Bourne-Again shell]] (bash): written as part of the [[GNU Project]] to provide a superset of Bourne Shell functionality. This shell can be found installed and is the default interactive shell for users on most [[Linux]] systems.
* [[KornShell]] (ksh): written by [[David Korn (computer scientist)|David Korn]] based on the Bourne shell sources<ref>
{{citation
| title = ksh - An Extensible High Level Language
| last = Korn
| first = David G.
| publisher = USENIX Association
| journal = Proceedings of the USENIX 1994 Very High Level Languages Symposium
| date = October 26, 1994
| url = https://www.usenix.org/legacy/publications/library/proceedings/vhll/full_papers/korn.ksh.a
| quote = Instead of inventing a new script language, we built a form entry system by modifying the Bourne shell, adding built-in commands as necessary.
| access-date = February 5, 2015
}}</ref> while working at [[Bell Labs]]
* [[pdksh|Public domain Korn shell]] (pdksh)
** [[mksh|MirBSD Korn shell]] (mksh): a descendant of the [[OpenBSD]] /bin/ksh and pdksh, developed as part of [[MirOS BSD]]
* [[Z shell]] (zsh): a relatively modern shell that is [[backward compatible]] with [[bash (Unix shell)|bash]]. It's the default shell in [[Kali Linux]] since 2020.4 and [[macOS]] since 10.15 [[macOS Catalina|Catalina]].
The [[POSIX]] standard specifies its standard shell as a strict subset of the [[KornShell|Korn shell]], an enhanced version of the Bourne shell. From a user's perspective the Bourne shell was immediately recognized when active by its characteristic default command line prompt character, the dollar sign ({{mono|$}}).
===C shell===
The [[C shell]], ''csh'', was modeled on the C programming language, including the control structures and the expression grammar. It was written by [[Bill Joy]] as a graduate student at [[University of California, Berkeley]], and was widely distributed with [[Berkeley Software Distribution|BSD Unix]].<ref>Harley Hahn, [https://www.harley.com/unix-book/book/chapters/h.html Harley Hahn's Guide to Unix and Linux: Unix/Linux Timeline].</ref>{{better source|date=September 2023}}
The C shell also introduced many features for interactive work, including the [[C shell#History|history]] and [[C shell#Editing operators|editing]] mechanisms, [[C shell#Aliases|aliases]], [[C shell#Directory stack|directory stacks]], [[C shell#Tilde notation|tilde notation]], [[C shell#Cdpath|cdpath]], [[C shell#Job control|job control]] and [[C shell#Path hashing|path hashing]].  On many systems, csh may be a [[symbolic link]] or [[hard link]] to [[TENEX C shell]] (tcsh), an improved version of Joy's original version.  Although the interactive features of csh have been copied to most other shells, the language structure has not been widely copied.  The only work-alike is [[Hamilton C shell]], written by Nicole Hamilton, first distributed on [[OS/2]] in 1988 and on [[Windows]] since 1992.<ref>{{cite web|url=http://hamiltonlabs.com/ReleaseNotes.htm|title=Hamilton C shell for Windows Release Notes 4.0|access-date=20 September 2014}}</ref>
==Configuration files==
Shells read configuration files in various circumstances. These files usually contain commands for the shell and are executed when loaded; they are usually used to set important variables used to find executables, like [[$PATH]], and others that control the behavior and appearance of the shell. The table in this section shows the configuration files for popular shells.<ref>{{cite web|url=http://www.unixnote.com/2010/05/different-unix-shell.html|title=Different UNIX Shells|publisher=unixnote.com|date=2010|access-date=2016-08-21|archive-url=https://web.archive.org/web/20160403120601/www.unixnote.com/2010/05/different-unix-shell.html|archive-date=2016-04-03|url-status=dead}}</ref>
{| class="wikitable floatleft sortable" style="text-align: center; margin-right: 2em;"
|-
! Configuration file
![[Bourne shell|sh]]
![[KornShell|ksh]]
![[C shell|csh]]
![[tcsh]]
![[Bash (Unix shell)|bash]]
![[Z shell|zsh]]
|-
|style="text-align:left;"|{{mono|/etc/.login}}
|
|
|login
|login
|
|
|-
|style="text-align:left;"|{{mono|/etc/csh.cshrc}}
|
|
|yes
|yes
|
|
|-
|style="text-align:left;"|{{mono|/etc/csh.login}}
|
|
|login
|login
|
|
|-
|style="text-align:left;"|{{mono|~/.tcshrc}}
|
|
|
|yes
|
|
|-
|style="text-align:left;"|{{mono|~/.cshrc}}
|
|
|yes
|yes{{Efn|only if {{mono|~/.tcshrc}} not found}}
|
|
|-
|style="text-align:left;"|{{mono|/etc/ksh.kshrc}}
|
|int.
|
|
|
|
|-
|style="text-align:left;"|{{mono|/etc/sh.shrc}}
|int.{{Efn|Newer versions of the Bourne Shell only}}
|
|
|
|
|
|-
|style="text-align:left;"|{{mono|$ENV}} (typically {{mono|~/.kshrc}})<ref>SCO Unix Group, [http://unix.harley.com/instructors/timeline.html SCO Unixware 7 documentation, 22 Apr 2004, retrieved 18 Oct 2012].</ref>
|int.{{Efn|Available on systems that support the "User Portability Utilities option"; value of the variable must be an ''absolute'' path, and it is ignored "if the user's real and effective user IDs or real and effective group IDs are different."<ref>{{cite web|url=http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_03|title=Shell Command Language|work=opengroup.org|access-date=15 June 2015}}</ref>}}{{Efn|{{mono|$ENV}} is {{mono|$HOME/.shrc}} in newer versions of the Bourne Shell}}
|int.
|
|
|int.{{Efn|Same behavior as {{mono|sh}}, but only if invoked as {{mono|sh}} (bash 2+) or, since bash 4.2, also if invoked ''explicitly'' in POSIX compatibility mode (with options {{mono|--posix}} or {{mono|-o posix}}).<ref>{{cite web|url=https://www.gnu.org/software/bash/manual/html_node/Bash-Startup-Files.html|title=Bash Reference Manual: Bash Startup Files|work=gnu.org|access-date=15 June 2015}}</ref>}}
|
|-
|style="text-align:left;"|{{mono|~/.login}}
|
|
|login
|login
|
|
|-
|style="text-align:left;"|{{mono|~/.logout}}
|
|
|login
|login
|
|
|-
|style="text-align:left;"|{{mono|/etc/profile}}
|login
|login
|
|
|login
|login{{Efn|name="zsh profiles"|Only in sh/ksh compatibility mode (when invoked as bash, sh, ksh)}}
|-
|style="text-align:left;"|{{mono|~/.profile}}
|login
|login
|
|
|login{{Efn|name="bash profiles"|The first readable file in order of {{mono|~/.bash_profile}}, {{mono|~/.bash_login}} and {{mono|~/.profile}}; and only {{mono|~/.profile}} if invoked as {{mono|sh}} or, as of at least Bash 4.2, if invoked ''explicitly'' in POSIX compatibility mode (with options {{mono|--posix}} or {{mono|-o posix}})}}
|login{{Efn|name="zsh profiles"}}
|-
|style="text-align:left;"|{{mono|~/.bash_profile}}
|
|
|
|
|login{{Efn|name="bash profiles"}}
|
|-
|style="text-align:left;"|{{mono|~/.bash_login}}
|
|
|
|
|login{{Efn|name="bash profiles"}}
|
|-
|style="text-align:left;"|{{mono|~/.bash_logout}}
|
|
|
|
|login
|
|-
|style="text-align:left;"|{{mono|~/.bashrc}}
|
|
|
|
|int.+n/login
|
|-
|style="text-align:left;"|{{mono|/etc/zshenv}}
|
|
|
|
|
|yes
|-
|style="text-align:left;"|{{mono|/etc/zprofile}}
|
|
|
|
|
|login
|-
|style="text-align:left;"|{{mono|/etc/zshrc}}
|
|
|
|
|
|int.
|-
|style="text-align:left;"|{{mono|/etc/zlogin}}
|
|
|
|
|
|login
|-
|style="text-align:left;"|{{mono|/etc/zlogout}}
|
|
|
|
|
|login
|-
|style="text-align:left;"|{{mono|~/.zshenv}}
|
|
|
|
|
|yes
|-
|style="text-align:left;"|{{mono|~/.zprofile}}
|
|
|
|
|
|login
|-
|style="text-align:left;"|{{mono|~/.zshrc}}
|
|
|
|
|
|int.
|-
|style="text-align:left;"|{{mono|~/.zlogin}}
|
|
|
|
|
|login
|-
|style="text-align:left;"|{{mono|~/.zlogout}}
|
|
|
|
|
|login
|}
Explanation:
* blank means a file is not read by a shell at all.
* "yes" means a file is always read by a shell upon startup.
* "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.
{{Clear}}{{Notelist}}
==Other shells==
Variations on the Unix shell concept that don't derive from Bourne shell or C shell include the following:<ref>{{cite web|url=http://www.freebsd.org/ports/shells.html |title=FreeBSD Ports: Shells |publisher=Freebsd.org |date=2014-03-30 |access-date=2014-04-05|archive-url=https://web.archive.org/web/20210112142623/www.freebsd.org/ports/shells.html|archive-date=2021-01-12|url-status=dead}}</ref>
* [[es (Unix shell)|es]] – A [[functional programming]] rc-compatible shell written in the mid-1990s.
* [[Friendly interactive shell]] (fish) – First released in 2005.
* [[PowerShell]] – An [[object-oriented]] shell developed originally for Windows OS and now available to macOS and Linux.
* [[Qshell]] – A shell on the [[IBM i]] operating system based on [[POSIX]] and [[X/Open]] standards.
* [[rc shell|rc]] – The default shell on [[Plan 9 from Bell Labs]] and [[Version 10 Unix]] written by [[Tom Duff]]. Ports have been made to various [[Unix-like]] operating systems.
* [[scsh]] – A [[Scheme (programming language)|Scheme]] Shell.
* [[wish (Unix shell)|wish]] – A windowing shell for [[Tcl/Tk]].
==See also==
{{Columns-list|
* [[Comparison of command shells]]
* [[List of POSIX commands]]
* [[Read–eval–print loop]]
* [[Restricted shell]]
* [[Shell (computing)]]
* [[Shell account]]
* [[Shell script]]
* [[Shell shoveling]]
}}
== References ==
{{Reflist}}
{{-}}
{{Unix shells}}
{{Unix}}
[[Category:System administration]]
[[Category:Unix shells| ]]