File: autoconf.info,  Node: Shellology,  Next: Invoking the Shell,  Up: Portable Shell

11.1 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.  The
the Shell difference FAQ
(http://www.faqs.org/faqs/unix-faq/shell/shell-differences/) 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
     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:

        − don't use ‘$?’ after expanding empty or unset variables, or at
          the start of an ‘eval’:

               foo=
               false
               $foo
               echo "Do not use it: $?"
               false
               eval 'echo "Do not use it: $?"'

        − don't use command substitution within variable expansion:

               cat ${FOO=`bar`}

        − beware that single builtin substitutions are not performed by
          a subshell, hence their effect applies to the current shell!
          *Note Shell Substitutions::, item "Command Substitution".

Bash
     To detect whether you are running Bash, test whether ‘BASH_VERSION’
     is set.  To require Posix compatibility, run ‘set -o posix’.  *Note
     Bash Posix Mode: (bash)Bash POSIX Mode, for details.

Bash 2.05 and later
     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.

Ksh
     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 10, ‘/usr/xpg4/bin/sh’ is a Posix-compliant variant of
     ‘ksh88’ on Solaris 10 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.

Pdksh
     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 *note Shell Substitutions::.

Zsh
     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 ‘:’.  *Note
     Compatibility: (zsh)Compatibility, for details.

     The default Mac OS X ‘sh’ was originally Zsh; it was changed to
     Bash in Mac OS X 10.2.