Dash as /bin/sh

Dash is a minimalist POSIX-compliant shell. It can be much faster than Bash, and takes up less memory when in use. 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 increase 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.

This article is for those who want to help dash become the new /bin/sh.

From 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
• All install scripts must be sh-compatible (or we need a way of executing non-sh scripts, there is a bug open for that)
• Any daemon/init script is probably written for /bin/bash as our rc.conf and other initscripts files use bash arrays, so those need to be pointed correctly

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 checkbashisms^AUR from the AUR.

Common places to check

• Installed scripts with a #!/bin/sh shebang:

$ find {,/usr}/bin -type f \
    -exec grep -q -- '^#!/bin/sh' {} \; \
    -exec checkbashisms -f -p {} +

• Package install scripts:

$ find /var/lib/pacman/local -mindepth 2 -type f -name install \
    -exec checkbashisms -f -p {} + 2>&1 |\
    grep -v -e '^you may get strange results' \
            -e 'does not appear to have a #! interpreter line;$'

• Initscripts and daemons:

$ find \
    /etc/rc.{conf,d,local{,.shutdown},multi,shutdown,single,sysinit} -type f \
    -exec checkbashisms -f -p {} + 2>&1 |\
    grep -v -e '^you may get strange results' \
            -e 'does not appear to have a #! interpreter line;$'

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 (as root):

 ln -sfT /bin/dash /bin/sh