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The Almquist shell (also known as A Shell, ash and sh) was originally Kenneth Almquist's clone of the SVR4-variant of the Bourne shell; it is a fast, small, POSIX-compatible Unix shell designed to replace the Bourne shell in later BSD distributions. Originally it did not feature line editing[clarification needed] or command history mechanisms, because Almquist felt that such functionality should be moved into the terminal driver; however, current variants support it.
Derivatives of ash are installed as the default shell (/bin/sh) on FreeBSD, NetBSD, DragonFly BSD, MINIX and Android. Ash is also fairly popular in embedded Linux systems; its code was incorporated into the BusyBox catch-all executable often employed in this area.
Debian and derived Linux distributions such as Ubuntu ship a version of ash, known as dash (Debian Almquist shell), as the default /bin/sh, although Bash is the default login shell for interactive use. The reason for using dash is faster shell script execution, especially during startup of the operating system.
ash was first released via a posting to the
comp.sources.unix Usenet news group, approved and moderated by Rich Salz on May 30, 1989. It was described as "a reimplementation of the System V shell [with] most features of that shell, plus some additions."
ash (Kenneth Almquist's ash shell)
A lightweight (92K) Bourne compatible shell. Great for machines with low memory, but does not provide all the extras of shells like 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 and Ubuntu uses ash as its /bin/sh.
Ash has since been replaced on both Debian and Ubuntu. Dash became the replacement for ash in Debian and became the default /bin/sh in Debian Squeeze. Dash became the default /bin/sh in Ubuntu starting with the 6.10 release in October 2006. During the transition by Ubuntu, numerous scripts making use of Bash-specific functionality (but not declaring it) were discovered.
- "Android sh source".
- Almquist, Kenneth (May 30, 1989). Rich Salz, ed. "v19i001: A reimplementation of the System V shell, Part01/08". Usenet newsgroup, comp.sources.unix.
- Dash as /bin/sh
- Dash as /bin/sh in Ubuntu 6.10
- Ubuntu bug #61463: script that are using bash could be broken with the new symlink
- Ubuntu bug #141481: dash as #!/bin/sh introduces countless incompatibilities
- Peter Seebach (2008), Beginning Portable Shell Scripting: From Novice to Professional, Apress, ISBN 1-4302-1043-5