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{{short description|Command-line interpreter for Unix operating system}}
[[File:TcshAndShScreenCaptureCropped.png|thumb|300px|right|tcsh and sh shell windows on an [[OS X]] desktop]]
[[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>
⚫
A '''Unix shell''' is a [[command-line interpreter]] or [[shell (computing)|shell]] that provides a traditional [[Unix-like]] command line [[user interface]]. Users direct the operation of the [[computer]] by entering commands as text for a [[command line interpreter]] to execute, or by creating text scripts of one or more such commands. Users typically interact with a Unix shell using a [[terminal emulator]], however, direct operation via serial hardware connections, or networking session, 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]].

⚫
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==
==Concept==
The most generic sense of the term ''shell'' means any program that users employ to type commands. A shell hides the details of the underlying operating system and manages the technical details of the operating system [[Kernel (computer science)|kernel]] interface, which is the lowest-level, or "inner-most" component of most operating systems.
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 in]] to 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 <tt>[[Passwd (file)|passwd]]</tt> 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.
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]].
⚫
The Unix shell is both an interactive [[Shell (computing)|command language]] as well as a [[scripting programming language]], and is used by the operating system as the facility to control ([[shell script]]) the execution of the system.<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 | accessdate=30 January 2015 | author=Bourne, Stephen R. | pages=187}}</ref> Shells created for other [[operating system]]s often provide similar functionality.

⚫
On hosts with a [[windowing system]], like [[OS&nbsp;X]], 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==
==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 |date= |accessdate=2012-08-14 }}</ref> Though rudimentary by modern standards, it introduced many of the basic features common to all later Unix shells, including [[Pipeline (Unix)|piping]], simple [[control flow|control structures]] using if and goto, and filename [[Wildcard character|wildcarding]]. Though not in current use, it is still available as part of some [[Ancient UNIX Systems]].
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, 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 |accessdate=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 |accessdate=2012-08-14 }}</ref>
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 variables, 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.
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|authorlink=Nikolai Bezroukov|publisher=Softpanorama|date=2015-08-13|accessdate=2016-08-21}}</ref>
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/http://www.softpanorama.org/People/Shell_giants/introduction.shtml|archive-date=2022-06-08|url-status=dead}}</ref>


===Bourne shell===
===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.
{{main article|Bourne shell}}
⚫
** [[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]]
⚫
The Bourne shell, ''sh'', was a complete rewrite by [[Stephen R. Bourne|Stephen Bourne]] at Bell Labs.<ref>{{cite web|url=http://www.computerworld.com.au/article/279011/-z_programming_languages_bourne_shell_sh|title=Bourne shell, or sh|date=2009-03-05|publisher=[[Computerworld]]|work=Interview with [[Stephen R. Bourne|Steve Bourne]]|accessdate=2016-08-21}}</ref> Distributed as the shell for UNIX Version 7 in 1979, it introduced the rest of the basic features considered common to all the 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''|publisher=|accessdate=20 September 2014}}</ref> Traditionally, the Bourne shell program name is <tt>sh</tt> and its path in the Unix file system hierarchy is <tt>/bin/sh</tt>. 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:
⚫
* [[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>
⚫
* [[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.
⚫
* [[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]] and [[Mac OS X]] systems.
⚫
* [[Debian Almquist shell]] (dash): a modern replacement for ash in [[Debian]] and [[Ubuntu (operating system)|Ubuntu]]
⚫
* [[Korn shell]] (ksh): written by [[David Korn (computer scientist)|David Korn]] based on the Bourne shell sources<ref>
{{citation
{{citation
| title = ksh - An Extensible High Level Language
| title = ksh - An Extensible High Level Language
Line 40: Line 39:
| url = https://www.usenix.org/legacy/publications/library/proceedings/vhll/full_papers/korn.ksh.a
| 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.
| 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.
| accessdate = February 5, 2015
| access-date = February 5, 2015
}}</ref> while working at [[Bell Labs]]
}}</ref> while working at [[Bell Labs]]
* [[pdksh|Public domain Korn shell]] (pdksh)
* [[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]]
** [[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]]
* [[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]].
⚫
* [[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 implentation of a Bourne shell.


The [[POSIX]] standard specifies its standard shell as a strict subset of the [[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 (<tt>$</tt>).
The [[POSIX]] standard specifies its standard shell as a strict subset of the [[KornShell|Korn shell]], an enhanced version of the Bourne shell. From a user's perspective the Bourne shell was immediately recognized when active by its characteristic default command line prompt character, the dollar sign ({{mono|$}}).


===C shell===
===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}}
{{main|C shell}}

The [[C shell]], ''csh'', was written by [[Bill Joy]] while a graduate student at [[University of California, Berkeley]] and widely distributed with [[Berkeley Software Distribution|BSD Unix]].<ref>Harley Hahn, [http://unix.harley.com/instructors/timeline.html Harley Hahn's Guide to Unix and Linux].</ref> The language, including the control structures and the expression grammar, was modeled on C. The C shell also introduced a large number of 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 csh. Though the C shell's interactive features have been copied in most other current shells, the language itself 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|publisher=|accessdate=20 September 2014}}</ref>
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==
==Configuration files==
Shells read configuration files on multiple circumstances that differ depending on the shell. These files usually contain commands for the particular shell and are executed when loaded; they are usually used to set important variables used to find executables, like [[$PATH]], and others that control the behavior and appearance of the shell. The table in this section shows the configuration files for popular shells.
Shells read configuration files in various circumstances. These files usually contain commands for the shell and are executed when loaded; they are usually used to set important variables used to find executables, like [[$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/http://www.unixnote.com/2010/05/different-unix-shell.html|archive-date=2016-04-03|url-status=dead}}</ref>
<ref>{{cite web|url=http://www.unixnote.com/2010/05/different-unix-shell.html|title=Different UNIX Shells|publisher=unixnote.com|date=2010|accessdate=2016-08-21}}</ref>


{| class="wikitable floatleft" style="text-align: center; margin-right: 2em;"
{| class="wikitable floatleft sortable" style="text-align: center; margin-right: 2em;"
|-
|-
! Configuration file
⚫
||
![[Bourne shell|sh]]
!sh
!ksh
![[KornShell|ksh]]
!csh
![[C shell|csh]]
!tcsh
![[tcsh]]
![[Bash (Unix shell)|bash]]
!bash
!zsh
![[Z shell|zsh]]
|-
|-
|style="text-align:left;"|<tt>/etc/.login</tt>
|style="text-align:left;"|{{mono|/etc/.login}}
|
|
|
|
Line 75: Line 73:
|
|
|-
|-
|style="text-align:left;"|<tt>/etc/csh.cshrc</tt>
|style="text-align:left;"|{{mono|/etc/csh.cshrc}}
|
|
|
|
Line 83: Line 81:
|
|
|-
|-
|style="text-align:left;"|<tt>/etc/csh.login</tt>
|style="text-align:left;"|{{mono|/etc/csh.login}}
|
|
|
|
Line 91: Line 89:
|
|
|-
|-
|style="text-align:left;"|<tt>~/.tcshrc</tt>
|style="text-align:left;"|{{mono|~/.tcshrc}}
|
|
|
|
Line 99: Line 97:
|
|
|-
|-
|style="text-align:left;"|<tt>~/.cshrc</tt>
|style="text-align:left;"|{{mono|~/.cshrc}}
|
|
|
|
|yes
|yes
|yes{{Efn|only if <tt>~/.tcshrc</tt> not found}}
|yes{{Efn|only if {{mono|~/.tcshrc}} not found}}
|
|
|
|
|-
|-
|style="text-align:left;"|<tt>~/etc/ksh.kshrc</tt>
|style="text-align:left;"|{{mono|/etc/ksh.kshrc}}
|
|
|int.
|int.
Line 115: Line 113:
|
|
|-
|-
|style="text-align:left;"|<tt>/etc/sh.shrc</tt>
|style="text-align:left;"|{{mono|/etc/sh.shrc}}
|int.{{Efn|Newer versions of the Bourne Shell only}}
|int.{{Efn|Newer versions of the Bourne Shell only}}
|
|
Line 123: Line 121:
|
|
|-
|-
|style="text-align:left;"|<tt>$ENV (typically ~/.kshrc)</tt><ref>SCO Unix Group, [http://unix.harley.com/instructors/timeline.html SCO Unixware 7 documentation, 22 Apr 2004, retrieved 18 Oct 2012].</ref>
|style="text-align:left;"|{{mono|$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|accessdate=15 June 2015}}</ref>}}{{Efn|$ENV is $HOME/.shrc in newer versions of the Bourne Shell}}
|int.{{Efn|Available on systems that support the "User Portability Utilities option"; value of the variable must be an ''absolute'' path, and it is ignored "if the user's real and effective user IDs or real and effective group IDs are different."<ref>{{cite web|url=http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_03|title=Shell Command Language|work=opengroup.org|access-date=15 June 2015}}</ref>}}{{Efn|{{mono|$ENV}} is {{mono|$HOME/.shrc}} in newer versions of the Bourne Shell}}
|int.
|int.
|
|
|
|
|int.{{Efn|Same behavior as <tt>sh</tt>, but only if invoked as <tt>sh</tt> (bash 2+) or, since bash 4.2, also if invoked ''explicitly'' in POSIX compatibility mode (with options <tt>--posix</tt> or <tt>-o posix</tt>).<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|accessdate=15 June 2015}}</ref>}}
|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;"|<tt>~/.login</tt>
|style="text-align:left;"|{{mono|~/.login}}
|
|
|
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Line 139: Line 137:
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|-
|-
|style="text-align:left;"|<tt>~/.logout</tt>
|style="text-align:left;"|{{mono|~/.logout}}
|
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Line 147: Line 145:
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|-
|-
|style="text-align:left;"|<tt>/etc/profile</tt>
|style="text-align:left;"|{{mono|/etc/profile}}
|login
|login
|login
|login
Line 155: Line 153:
|login{{Efn|name="zsh profiles"|Only in sh/ksh compatibility mode (when invoked as bash, sh, ksh)}}
|login{{Efn|name="zsh profiles"|Only in sh/ksh compatibility mode (when invoked as bash, sh, ksh)}}
|-
|-
|style="text-align:left;"|<tt>~/.profile</tt>
|style="text-align:left;"|{{mono|~/.profile}}
|login
|login
|login
|login
|
|
|
|
|login{{Efn|name="bash profiles"|in fact, the first readable of <tt>~/.bash_profile</tt>, <tt>~/.bash_login</tt> and <tt>~/.profile</tt>; and only <tt>~/.profile</tt> if invoked as <tt>sh</tt> or, as of at least Bash 4.2, if invoked ''explicitly'' in POSIX compatibility mode (with options <tt>--posix</tt> or <tt>-o posix</tt>)}}
|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"}}
|login{{Efn|name="zsh profiles"}}
|-
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|style="text-align:left;"|<tt>~/.bash_profile</tt>
|style="text-align:left;"|{{mono|~/.bash_profile}}
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|style="text-align:left;"|<tt>~/.bash_login</tt>
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|style="text-align:left;"|<tt>~/.bash_logout</tt>
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|style="text-align:left;"|<tt>~/.bashrc</tt>
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|int.
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|style="text-align:left;"|<tt>~/.zlogin</tt>
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⚫
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{{Clear}}{{Notelist}}
{{Clear}}{{Notelist}}


==Exotic shells==
==Other shells==
Other, more exotic variations on the Unix shell concept 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 |accessdate=2014-04-05}}</ref>
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/http://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.
* [[es (Unix shell)|es]] – A [[functional programming]] rc-compatible shell written in the mid-1990s.
* [[Friendly interactive shell]] (fish): First released in 2005.
* [[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.
⚫
* [[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.
* [[Qshell]] – A shell on the [[IBM i]] operating system based on [[POSIX]] and [[X/Open]] standards.
⚫
* [[scsh]], a [[Scheme (programming language)|Scheme]] Shell.
⚫
* [[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.
⚫
* [[wish (Unix shell)|wish]]: A windowing shell for [[Tcl/Tk]].
⚫
* [[scsh]] – A [[Scheme (programming language)|Scheme]] Shell.
[[name (Unix shell)|wish]]: mounika for [[Tcl/Tk]].
⚫
* [[wish (Unix shell)|wish]] – A windowing shell for [[Tcl/Tk]].


==See also==
==See also==
{{Columns-list|
{{Div col||25em}}
* [[Comparison of command shells]]
* [[Comparison of command shells]]
* [[List of Unix programs]]
* [[List of POSIX commands]]
* [[Read–eval–print loop]]
* [[Restricted shell]]
* [[Restricted shell]]
* [[Shell (computing)]]
* [[Shell (computing)]]
Line 295: Line 303:
* [[Shell script]]
* [[Shell script]]
* [[Shell shoveling]]
* [[Shell shoveling]]
}}
{{Div col end}}


== References ==
== References ==
{{Reflist|30em}}
{{Reflist}}


{{-}}
{{Unix shells}}
{{Unix shells}}
{{Unix}}


[[Category:System administration]]
[[Category:System administration]]

Latest revision as of 20:23, 22 December 2024

tcsh and sh shell windows on a Mac OS X Leopard[1] desktop

A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. 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 scripts.[2]

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 wildcarding, piping, here documents, command substitution, variables and control structures for condition-testing and iteration.

Concept

[edit]

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 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 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 passwd file or in a distributed configuration system such as NIS or LDAP; however, the user may execute any other available shell interactively.

On operating systems with a windowing system, such as macOS and desktop Linux distributions, 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

[edit]

The first Unix shell was the Thompson shell, sh, written by Ken Thompson at Bell Labs and distributed with Versions 1 through 6 of Unix, from 1971 to 1975.[3] Though rudimentary by modern standards, it introduced many of the basic features common to all later Unix shells, including piping, simple control structures using if and goto, 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.[1][4]

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 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 variables, 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.[5]

Bourne shell

[edit]

The Bourne shell, sh, was a new Unix shell by Stephen Bourne at Bell Labs.[6] 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 documents, command substitution, more generic variables and more extensive builtin control structures. The language, including the use of a reversed keyword to mark the end of a block, was influenced by ALGOL 68.[7] Traditionally, the Bourne shell program name is sh and its path in the Unix file system hierarchy is /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:

The POSIX standard specifies its standard shell as a strict subset of the 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 ($).

C shell

[edit]

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 BSD Unix.[9][better source needed]

The C shell also introduced many features for interactive work, including the history and editing mechanisms, aliases, directory stacks, tilde notation, cdpath, job control and 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.[10]

Configuration files

[edit]

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.[11]

Configuration file sh ksh csh tcsh bash zsh
/etc/.login login login
/etc/csh.cshrc yes yes
/etc/csh.login login login
~/.tcshrc yes
~/.cshrc yes yes[a]
/etc/ksh.kshrc int.
/etc/sh.shrc int.[b]
$ENV (typically ~/.kshrc)[12] int.[c][d] int. int.[e]
~/.login login login
~/.logout login login
/etc/profile login login login login[f]
~/.profile login login login[g] login[f]
~/.bash_profile login[g]
~/.bash_login login[g]
~/.bash_logout login
~/.bashrc int.+n/login
/etc/zshenv yes
/etc/zprofile login
/etc/zshrc int.
/etc/zlogin login
/etc/zlogout login
~/.zshenv yes
~/.zprofile login
~/.zshrc int.
~/.zlogin login
~/.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.
  1. ^ only if ~/.tcshrc not found
  2. ^ Newer versions of the Bourne Shell only
  3. ^ 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."[13]
  4. ^ $ENV is $HOME/.shrc in newer versions of the Bourne Shell
  5. ^ Same behavior as sh, but only if invoked as sh (bash 2+) or, since bash 4.2, also if invoked explicitly in POSIX compatibility mode (with options --posix or -o posix).[14]
  6. ^ a b Only in sh/ksh compatibility mode (when invoked as bash, sh, ksh)
  7. ^ a b c The first readable file in order of ~/.bash_profile, ~/.bash_login and ~/.profile; and only ~/.profile if invoked as sh or, as of at least Bash 4.2, if invoked explicitly in POSIX compatibility mode (with options --posix or -o posix)

Other shells

[edit]

Variations on the Unix shell concept that don't derive from Bourne shell or C shell include the following:[15]

See also

[edit]

References

[edit]
  1. ^ a b Tom Van Vleck (1995-02-05). "Unix and Multics". Multicians.org. Retrieved 2012-08-14.
  2. ^ Bourne, Stephen R. (October 1983). "The Unix Shell". BYTE. p. 187. Retrieved 30 January 2015.
  3. ^ "V6 Thompson Shell Port - History". V6shell.org. Retrieved 2012-08-14.
  4. ^ Louis Pouzin (2000-11-25). "The Origin of the Shell". Multicians.org. Retrieved 2012-08-14.
  5. ^ Nikolai Bezroukov (2015-08-13). "Introduction to the Unix shell history". Softpanorama. Archived from the original on 2022-06-08. Retrieved 2016-08-21.
  6. ^ Bourne, Stephen (2009-03-05). "The A-Z of Programming Languages: Bourne shell, or sh" (Interview). Interviewed by Howard Dahdah. Computerworld. Retrieved 2022-08-16.
  7. ^ "Re: Late Bloomers Revisited". Retrieved 20 September 2014.
  8. ^ Korn, David G. (October 26, 1994), "ksh - An Extensible High Level Language", Proceedings of the USENIX 1994 Very High Level Languages Symposium, USENIX Association, retrieved February 5, 2015, Instead of inventing a new script language, we built a form entry system by modifying the Bourne shell, adding built-in commands as necessary.
  9. ^ Harley Hahn, Harley Hahn's Guide to Unix and Linux: Unix/Linux Timeline.
  10. ^ "Hamilton C shell for Windows Release Notes 4.0". Retrieved 20 September 2014.
  11. ^ "Different UNIX Shells". unixnote.com. 2010. Archived from the original on 2016-04-03. Retrieved 2016-08-21.
  12. ^ SCO Unix Group, SCO Unixware 7 documentation, 22 Apr 2004, retrieved 18 Oct 2012.
  13. ^ "Shell Command Language". opengroup.org. Retrieved 15 June 2015.
  14. ^ "Bash Reference Manual: Bash Startup Files". gnu.org. Retrieved 15 June 2015.
  15. ^ "FreeBSD Ports: Shells". Freebsd.org. 2014-03-30. Archived from the original on 2021-01-12. Retrieved 2014-04-05.
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