Almquist shell (also known as A Shell, ash and sh) is a lightweight

Unix shell 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 systems. The shell is both an interactive command languag ...

originally written by Kenneth Almquist in the late 1980s. Initially a clone of the System V.4 variant of the Bourne shell, it replaced the original Bourne shell in the BSD versions of Unix released in the early 1990s.

History

ash was first released via a posting to the

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news group, approved and moderated by Rich Salz on 30 May 1989. It was described as "a reimplementation of the System V shell ithmost features of that shell, plus some additions". Fast, small, and virtually compatible with the

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standard's specification of the Unix shell, ash did not provide line editing or command history mechanisms, because Almquist felt that such functionality should be moved into the terminal driver. However, modern variants support it. The following is extracted from the ash package information from Slackware v14: Myriad forks have been produced from the original ash release. These derivatives of ash are installed as the default shell (/bin/sh) on FreeBSD, NetBSD, DragonFly BSD, MINIX, and in some Linux distributions. MINIX 3.2 used the original ash version, whose test feature differed from POSIX. That version of the shell was replaced in MINIX 3.3. Android used ash until Android 4.0, at which point it switched to mksh.

Dash

In 1997 Herbert Xu ported ash from NetBSD to 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. Like its predecessor, Dash implements support for neither

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nor multi-byte character encoding (both required in POSIX). Line editing and history support based on GNU Readline is optional ().

Adoption in Debian and Ubuntu

Because of its slimness, Ubuntu decided to adopt Dash as the default /bin/sh in October 2006 with version 6.10. The reason for using Dash is faster shell script execution, especially during startup of the operating system, compared to previous versions of Debian and Ubuntu that used Bash for this purpose, although Bash is still the default login shell for interactive use. A result of the shift is that many shell scripts were found making use of Bash-specific functionalities ("bashisms") without properly declaring it in the shebang line. The problem was first spotted in Ubuntu and the maintainers decided to make all the scripts comply with the

standard. The changes were later upstreamed to Debian, which eventually adopted Dash as its default too in Debian 6 (Squeeze), released in February 2011. 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 (, , ). A similar transition has happened in Slackware Linux, although its version of is only partially based on Dash.

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 features which are enabled in modern distributions like Alpine Linux, Tiny Core Linux and Linux-based router firmware such as

OpenWrt OpenWrt (from ''open wireless router'') is an open-source project for embedded operating systems based on Linux kernel, Linux, primarily used on Embedded system, embedded devices to Router (computing), route network traffic. The main components ...

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and DD-WRT.

References

External links