13.2.2 POSIX shells

Several packages provide a POSIX shell in Debian:

• dash (Sarge)

  • Priority: optional

  • Installed-Size: 176

  • Smallest and fastest by far – best for initial boot

• ash (Woody)

  • Priority: optional

  • Installed-Size: 180

  • Smaller and much faster – good for initial boot

• bash

  • Essential: yes

  • Priority: required

  • Installed-Size: 580

  • Larger and featureful – many extensions implemented

• pdksh

  • Priority: optional

  • Installed-Size: 408

  • Complete AT&T ksh look-alike

If you are writing a shell script for portability, it is best to write it as a POSIX shell script. Use /bin/sh linked to ash (or dash) to test its POSIX compliance. Avoid writing scripts with "bashisms" or the "zshisms". For example, avoid:

• if [ foo == bar ] ; then ...

• diff -u file.c{.orig,}

• mkdir /foo{bar,baz}

The description for the shell in this document applies only for the POSIX type shells and thus does not apply for the csh type shells including tcsh.