The Almquist Shell, a.k.a. ash, is a minimal implementation of an almost-POSIX shell, and also a replacement for the Bourne Shell. Ash has more features than the original Bourne Shell, but fewer features than most other shells; for example, it lacks arrays. For the most part, it only implements POSIX features, but it does have a few traditional BSD add-ons, such as
local variables in functions. Ash is the default shell,
/bin/sh, of some systems.
The Almquist Shell does not have any of the internationalization/localization or multi-byte character encoding support required by the POSIX standard, so it is not technically a POSIX-compliant shell. Nonetheless, since it lacks most non-POSIX features, it is helpful in validating that #!/bin/sh scripts are not dependent upon such features. In general, if a script works with Almquist Shell, it will probably also work with other popular Bourne-compatible shells, such as the Bourne Again SHell and Z Shell. It may not work with the Korn Shell, which uses typeset instead of local; unfortunately, while bash and zsh treat those commands as synonyms, ash in turn lacks typeset.
Almquist Shell filled the need for a free shell to replace Bourne Shell. Kenneth Almquist posted the first version of Ash to Usenet group comp.sources.unix at 30 May 1989. It was a clone of SVR3 Bourne Shell. BSD used Ash for
/bin/sh, added features from POSIX, and put a Berkeley copyright on this shell.
Ash has three major variants:
- Debian Almquist Shell (Dash), which adds support for echo -n and test -a/-o
- FreeBSD /bin/sh
- NetBSD /bin/sh
All three variants have similar features. Dash can run on GNU/Linux.
Ash is also the shell provided by BusyBox.