Node:Shellology, Next:Here-Documents, Up:Portable Shell
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.
ashis 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:
$?after expanding empty or unset variables:
foo= false $foo echo "Don't use it: $?"
bash, test if
BASH_VERSIONis set. To disable its extensions and require POSIX compatibility, run
set -o posix. See Bash POSIX Mode, for details.
bashuse a different format for the output of the
setbuiltin, designed to make evaluating this output easier. However, this output is not compatible with earlier versions of
bash(or with many other shells, probably). So if you use
bash2.05 or higher to execute
configure, you'll need to use
bash2.05 for all other build tasks as well.
/usr/xpg4/bin/shand 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, test if
ZSH_VERSIONis set. By default
zshis not compatible with the Bourne shell: you have to run
emulate shand set
:. See Compatibility, 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:
The GNU assumption that
/bin/shis 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.
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/shprograms 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.