10.1 Shellology

There are several families of shells, most prominently the Bourne family and the C shell family which are deeply incompatible. If you want to write portable shell scripts, avoid members of the C shell family.

Below we describe some of the members of the Bourne shell family.

Ash

ash is often used on GNU/Linux and BSD systems as a light-weight Bourne-compatible shell. Ash 0.2 has some bugs that are fixed in the 0.3.x series, but portable shell scripts should workaround them, since version 0.2 is still shipped with many GNU/Linux distributions.

To be compatible with Ash 0.2:

• don't use `$?' after expanding empty or unset variables:

  foo= false $foo echo "Don't use it: $?"

• don't use command substitution within variable expansion:

  cat ${FOO=`bar`}

• beware that single builtin substitutions are not performed by a sub shell, hence their effect applies to the current shell! See section 10.5 Shell Substitutions, item "Command Substitution".

Bash

To detect whether you are running bash, test if BASH_VERSION is set. To disable its extensions and require POSIX compatibility, run `set -o posix'. See section `Bash POSIX Mode' in The GNU Bash Reference Manual, for details.

/usr/xpg4/bin/sh on Solaris

The POSIX-compliant Bourne shell on a Solaris system is /usr/xpg4/bin/sh and is part of an extra optional package. There is no extra charge for this package, but it is also not part of a minimal OS install and therefore some folks may not have it.

Zsh

To detect whether you are running zsh, test if ZSH_VERSION is set. By default zsh is not compatible with the Bourne shell: you have to run `emulate sh' and set NULLCMD to `:'. See section `Compatibility' in The Z Shell Manual, for details.

Zsh 3.0.8 is the native /bin/sh on Mac OS X 10.0.3.

The following discussion between Russ Allbery and Robert Lipe is worth reading:

Russ Allbery:

The GNU assumption that /bin/sh is the one and only shell leads to a permanent deadlock. Vendors don't want to break user's existant shell scripts, and there are some corner cases in the Bourne shell that are not completely compatible with a POSIX shell. Thus, vendors who have taken this route will never (OK..."never say never") replace the Bourne shell (as /bin/sh) with a POSIX shell.

Robert Lipe:

This is exactly the problem. While most (at least most System V's) do have a bourne shell that accepts shell functions most vendor /bin/sh programs are not the POSIX shell.

So while most modern systems do have a shell _somewhere_ that meets the POSIX standard, the challenge is to find it.