Dash - ArchWiki

# Dash

From ArchWiki

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 (disc space, RAM or CPU). As minimalistic - much smaller (134.1 kB vs 6.5 MB installed, 13 kSLOC vs 176 kSLOC) then 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 dash 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 aren't explicitly `#!/bin/bash` scripts are safely POSIX compliant and do not require any of Bash's features.

### Identifying bashisms

Features of bash that aren't 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 checkbashismsAUR from the AUR.

#### Common places to check

- Installed scripts with a `#!/bin/sh` shebang:

    $ gawk '/^#!.*( |[/])sh/{printf "%s\0", FILENAME} {nextfile}' /usr/bin/* | xargs -0 checkbashisms

- `pacman -Qlq` can be used to list all pacman-installed files.

### Relinking /bin/sh

Once you have verified that it won't 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

## See also

- http://article.gmane.org/gmane.linux.arch.devel/11418:
- https://mailman.archlinux.org/pipermail/arch-dev-public/2007-November/003053.html
- https://launchpad.net/ubuntu/+spec/dash-as-bin-sh
- https://wiki.ubuntu.com/DashAsBinSh

Category:

- Command shells