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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
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:
foo= false $foo echo "Don't use it: $?" |
cat ${FOO=`bar`} |
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
/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
, 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.
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