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. The the Shell difference FAQ includes a small history of Posix shells, and a comparison between several of them.
Below we describe some of the members of the Bourne shell family.
To be compatible with Ash 0.2:
foo= false $foo echo "Do not use it: $?" false eval 'echo "Do not use it: $?"'
BASH_VERSIONis set. To require Posix compatibility, run ‘set -o posix’. See Bash Posix Mode, for details.
Solaris systems have three variants: /usr/bin/ksh is ‘ksh88’; it is standard on Solaris 2.0 and later. /usr/xpg4/bin/sh is a Posix-compliant variant of ‘ksh88’; it is standard on Solaris 9 and later. /usr/dt/bin/dtksh is ‘ksh93’. Variants that are not standard may be parts of optional packages. There is no extra charge for these packages, but they are not part of a minimal OS install and therefore some installations may not have it.
Starting with Tru64 Version 4.0, the Korn shell /usr/bin/ksh
is also available as /usr/bin/posix/sh. If the environment
variable BIN_SH is set to
xpg4, subsidiary invocations of
the standard shell conform to Posix.
KSH_VERSION, except if invoked as /bin/sh on OpenBSD, and similarly to Bash you can require Posix compatibility by running ‘set -o posix’. Unfortunately, with pdksh 5.2.14 (the latest stable version as of January 2007) Posix mode is buggy and causes pdksh to depart from Posix in at least one respect, see Shell Substitutions.
ZSH_VERSIONis set. By default zsh is not compatible with the Bourne shell: you must execute ‘emulate sh’, and for zsh versions before 3.1.6-dev-18 you must also set
NULLCMDto ‘:’. See Compatibility, for details.
The default Mac OS X sh was originally Zsh; it was changed to Bash in Mac OS X 10.2.