Dash

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Dash (Debian Almquist shell) is a modern POSIX-compliant implementation of /bin/sh (sh, Bourne shell).

Dash is not Bash compatible, but Bash tries to be mostly compatible with POSIX, and thus Dash.

Dash shines in:

  • Speed of execution. Roughly 4x times faster than Bash and others.
  • Very limited resources (disk space, RAM or CPU). As minimalistic as possible - much smaller (134.1 kB vs 6.5 MB installed, 13 kSLOC vs 176 kSLOC) than Bash and others.
  • Security. Dash is a long-established, tiny project with simple and long-established functionality; one that is still very much alive, and with many active developers. Thus, Dash has a much smaller attack surface, while still having many eyes on its code.
  • If classic /bin/sh needed only.

Installation

Install the dash package from the official repositories or dash-static-muslAUR from the AUR.

Use Dash as /bin/sh

Most POSIX compliant scripts specify /bin/sh at the first line of the script, which means it will run /bin/sh as the shell, which by default in Arch is a symlink to /bin/bash.

You can re-symlink /bin/sh to /bin/dash, which can improve system performance, but first you must verify that none of the scripts that are not explicitly #!/bin/bash require any of Bash's features and that all /bin/sh scripts are safely POSIX compliant.

Identifying bashisms

Features of bash that are not included in Dash ('bashisms') will not work without being explicitly pointed to /bin/bash. The following instructions will allow you to find any scripts that may need modification.

Install checkbashisms.

Common places to check

  • Installed scripts with a #!/bin/sh shebang:
$ find /usr/bin/ -type f -perm -o=r -print0 | xargs -0 gawk '/^#!.*( |[/])sh/{printf "%s\0", FILENAME} {nextfile}' | xargs -0 checkbashisms
  • pacman -Qlq can be used to list all pacman-installed files.

Relinking /bin/sh

Once you have verified that it will not break any functionality, it should be safe to relink /bin/sh. To do so use the following command: 

# ln -sfT dash /usr/bin/sh

Updates of Bash will overwrite /bin/sh with the default symlink. To prevent this, use the following pacman hook, which will relink /bin/sh after every affected update: 

[Trigger]
Type = Package
Operation = Install
Operation = Upgrade
Target = bash

[Action]
Description = Re-pointing /bin/sh symlink to dash...
When = PostTransaction
Exec = /usr/bin/ln -sfT dash /usr/bin/sh
Depends = dash

This is provided by dashbinshAUR.

See also