Editing Almquist shell

{{Short description|Lightweight Unix shell}}
{{Infobox software
| name                   = Almquist shell
| logo                   =
| screenshot             =
| caption                =
| developer              = Kenneth Almquist
| released               = {{Start date and age|1989|05|30}}
| latest release version =
| latest release date    =
| programming language   = [[C (programming language)|C]]
| operating system       = [[Unix-like]]
| platform               = [[Cross-platform]]
| genre                  = [[Unix shell]]
| license                =
| website                = 
}}
'''Almquist shell''' (also known as '''A Shell''', '''ash''' and '''sh''') is a lightweight [[Unix shell]] originally written by [[Kenneth Almquist]] in the late 1980s. Initially a clone of the [[SVR4|System V.4]] variant of the [[Bourne shell]], it replaced the original Bourne shell in the [[Berkeley Software Distribution|BSD]] versions of Unix released in the early 1990s.

==History==
ash was first released via a posting to the {{mono|[[comp.* hierarchy|comp]].sources.unix}} [[Usenet]] news group, approved and moderated by [[Rich Salz]] on 30 May 1989. It was described as "a reimplementation of the System V shell [with] most features of that shell, plus some additions".<ref>{{Cite web| title=v19i001: A reimplementation of the System V shell, Part01/08 | url=https://groups.google.com/d/topic/comp.sources.unix/A6cnyKX-Gq4/discussion | date=May 30, 1989 | editor=Rich Salz | editor-link=Rich Salz | author=Almquist, Kenneth | publisher=[[Usenet]] newsgroup, comp.sources.unix}}</ref>

Fast, small, and virtually compatible{{citation needed|date=February 2016}} with the [[POSIX]] standard's specification of the Unix shell, ash did not provide [[Line editor|line editing]] or [[command history]] mechanisms, because Almquist felt that such functionality should be moved into the [[Computer terminal|terminal]] [[Device driver|driver]]. However, modern variants support it.

The following is extracted from the ash package information from [[Slackware]] v14:

{{Blockquote|<p>ash (Kenneth Almquist's ash shell)</p><p>A lightweight (92K) Bourne compatible shell. Great for machines with low memory, but does not provide all the extras of shells like [[Bash (Unix shell)|bash]], [[tcsh]], and [[zsh]]. Runs most shell scripts compatible with the Bourne shell. Note that under [[Linux]], most scripts seem to use at least some bash-specific syntax. The Slackware setup scripts are a notable exception, since ash is the shell used on the install disks. [[NetBSD]] uses ash as its /bin/sh.</p>}}

Myriad forks have been produced from the original ash release.<ref name=vars>{{cite web |last1=Mascheck |first1=Sven |title=Ash (Almquist Shell) Variants
 |url=https://www.in-ulm.de/~mascheck/various/ash/ |website=www.in-ulm.de}}</ref> These derivatives of ash are installed as the default shell (<code>/bin/sh</code>) on [[FreeBSD]], [[NetBSD]], [[DragonFly BSD]], [[MINIX]], and in some [[Linux distribution]]s. MINIX 3.2 used the original ash version, whose [[test (Unix)|test]] feature differed from POSIX.<ref>{{cite web |url=https://invisible-island.net/autoconf/portability-test.html |title=TEST versus Portability |author=Thomas E. Dickey |year=2015 |access-date=March 1, 2020}}</ref>  That version of the shell was replaced in MINIX 3.3. Android used ash until [[Android Ice Cream Sandwich|Android 4.0]], at which point it switched to [[KornShell|mksh]].<ref>{{cite web
| url=https://android.googlesource.com/platform/system/core/+/master/shell_and_utilities/README.md
| title=Android's shell and utilities
| author=Elliott Hughes
| date=2018-06-20
| access-date=2020-02-29}}</ref>

=={{anchor|dash}}Dash ==
{{Infobox software
| name                   = Debian Almquist shell (DASH)
| logo                   =
| screenshot             =
| caption                =
| developer              = Herbert Xu
| released               = {{Start date and age|1997|07|15}}
| latest release version = 0.5.12
| latest release date    ={{Start date and age|2022|12|11}}
| programming language   = [[C (programming language)|C]]
| operating system       = [[Linux]], [[Android (operating system)|Android]]
| platform               = 
| genre                  = [[Unix shell]]
| license                = [[BSD licenses|3-clause BSD license]] with mksignames under [[GNU GPL]]<ref>{{cite web |last1=Xu |first1=Herbert |title=COPYING |url=https://git.kernel.org/pub/scm/utils/dash/dash.git/tree/COPYING?h=v0.5.12 |website=The Linux Kernel Archives |access-date=23 December 2023}}</ref>
| website                = {{URL|gondor.apana.org.au/~herbert/dash/}}
| repo                   = {{URL|git.kernel.org/pub/scm/utils/dash/dash.git}}
}}
In 1997 Herbert Xu ported <code>ash</code> from NetBSD to [[Debian|Debian Linux]]. In September 2002, with release 0.4.1, this port was renamed to '''Dash''' ('''Debian Almquist shell'''). Xu's main priorities are POSIX conformance and slim implementation.<ref name=vars/>

Like its predecessor, Dash implements support for neither [[internationalization and localization]] nor [[Variable-width encoding|multi-byte character encoding]] (both required in POSIX).{{Citation needed|date=March 2023}} Line editing and history support based on [[GNU Readline]] is optional ({{code|--with-libedit}}).

=== Adoption in Debian and Ubuntu ===
{{anchor|Slimness}}Because of its slimness, [[Ubuntu (operating system)|Ubuntu]] decided to adopt Dash as the default <code>/bin/sh</code><ref name="Debian">{{cite web
| url=https://wiki.debian.org/Shell
| title=Non-interactive Shell
| website=[[Debian]] Wiki
| date=2020-01-13
| access-date=2020-02-29}}</ref><ref name="Ubuntu">{{cite web
| url=https://wiki.ubuntu.com/DashAsBinSh
| title=Dash as /bin/sh
| website=[[Ubuntu (operating system)|Ubuntu]] Wiki
| date=2017-12-16
| access-date=2020-02-29}}</ref> in October 2006 with version 6.10. The reason for using Dash is faster [[shell script]] execution,<ref>{{cite book
| author=Neal Krawetz
| title=Ubuntu: Powerful Hacks and Customizations
| year=2011
| publisher=[[John Wiley & Sons]]
| pages=178
| url=https://books.google.com/books?id=h_zclqESvu8C&pg=PT178
| isbn=9781118080382}}</ref> especially during startup of the operating system, compared to previous versions of Debian and Ubuntu that used Bash for this purpose, although [[Bash (Unix shell)|Bash]] is still the default login shell for interactive use.<ref>{{cite book
| author1=Christopher Negus
| author2=Francois Caen
| title=Ubuntu Linux Toolbox
| publisher=John Wiley & Sons
| year=2011
| url=https://books.google.com/books?id=L_IXUtw_w-AC&pg=PA49
| pages=49
| isbn=9781118079140}}</ref>

A result of the shift is that many [[shell script]]s were found making use of [[Bash (Unix shell)|Bash]]-specific functionalities ("bashisms") without properly declaring it in the [[shebang (Unix)|shebang]] line.<ref>{{cite web
| url=https://bugs.launchpad.net/ubuntu/+source/dash/+bug/61463
| title=Script that are using bash could be broken with the new symlink
| author=Egil Hasting
| date=2006-09-20
| publisher=[[Launchpad (website)|Launchpad]]
| access-date=2020-02-29}}</ref><ref>{{cite web
| url=https://bugs.launchpad.net/ubuntu/+source/dash/+bug/141481
| title=dash as #!/bin/sh introduces countless incompatibilities
| author=comotion
| date=2007-09-21
| publisher=Launchpad
| access-date=2020-02-29}}</ref> The problem was first spotted in Ubuntu and the maintainers decided to make all the scripts comply with the [[POSIX]] standard. The changes were later upstreamed to Debian, which eventually adopted Dash as its default {{code|/bin/sh}} too in Debian 6 (Squeeze), released in February 2011.<ref name="Debian"/> As a result, Debian policy was amended to allow script developers to assume a largely POSIX-compliant shell, save for the extensions merged into Dash for convenience ({{code|local}}, {{code|echo -n}}, {{code|test -a / -o}}).<ref name=deb>{{cite web |title=10. Files |url=https://www.debian.org/doc/debian-policy/ch-files.html#scripts |website=Debian Policy Manual v4.5.0.2}}</ref><ref>{{man|1|checkbashisms|ManKier}}</ref> A similar transition has happened in Slackware Linux, although its version of {{code|ash}} is only partially based on Dash.<ref name=vars/>

===Embedded Linux===
Ash (mainly the Dash fork) is also fairly popular in [[embedded Linux]] systems. Dash version 0.3.8-5 was incorporated into [[BusyBox]], the catch-all executable often employed in this area. Modern BusyBox versions support additional [[Bash (Unix shell)|Bash]] features which are enabled in modern distributions like [[Alpine Linux]], [[Tiny Core Linux]] and Linux-based [[Router (computing)|router]] firmware such as [[OpenWrt]], [[Tomato (firmware)|Tomato]] and [[DD-WRT]].

==See also==
* [[Comparison of command shells]]

== References ==
{{Reflist}}

==External links==
* {{Official website}}
* {{man|1|dash|Linux}}
* {{man|1|dash|Debian}}
* {{man|1|sh|FreeBSD}}
* {{man|1|sh|NetBSD}}

{{Unix shells}}

[[Category:1989 software]]
[[Category:Cross-platform software]]
[[Category:Scripting languages]]
[[Category:Text-oriented programming languages]]
[[Category:Unix shells]]