Next: Invoking the Shell, Up: Portable Shell Programming [Contents][Index]
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.
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 work around them, since version 0.2 is still shipped with many GNU/Linux distributions.
To be compatible with Ash 0.2:
eval
:
foo= false $foo echo "Do not use it: $?" false eval 'echo "Do not use it: $?"'
cat ${FOO=`bar`}
To detect whether you are running Bash, test whether
BASH_VERSION
is set. To require
Posix compatibility, run ‘set -o posix’. See Bash Posix Mode in The GNU Bash Reference
Manual, for details.
Versions 2.05 and later of Bash use a different format for the
output of the set
builtin, designed to make evaluating its
output easier. However, this output is not compatible with earlier
versions of Bash (or with many other shells, probably). So if
you use Bash 2.05 or higher to execute configure
,
you’ll need to use Bash 2.05 for all other build tasks as well.
The Korn shell is compatible with the Bourne family and it mostly
conforms to Posix. It has two major variants commonly
called ‘ksh88’ and ‘ksh93’, named after the years of initial
release. It is usually called ksh
, but is called sh
on some hosts if you set your path appropriately.
On Solaris 11, /bin/sh
and /usr/bin/ksh
are both
‘ksh93’. On Solaris 10 and earlier, /bin/sh
is a
pre-Posix Bourne shell and the Korn shell is found elsewhere:
/usr/bin/ksh
is ‘ksh88’ on Solaris 2.0 through 10,
/usr/xpg4/bin/sh
is a Posix-compliant variant of
‘ksh88’ on Solaris 9 and later,
and /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.
A public-domain clone of the Korn shell called pdksh
is widely
available: it has most of the ‘ksh88’ features along with a few of
its own. It usually sets 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.
To detect whether you are running zsh
, test whether
ZSH_VERSION
is 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 NULLCMD
to ‘:’. See Compatibility in The Z Shell Manual, for details.
The default Mac OS X sh
was originally Zsh; it was changed to
Bash in Mac OS X 10.2.
Next: Invoking the Shell, Up: Portable Shell Programming [Contents][Index]