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(autoconf.info)Shellology

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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.

   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 workaround
     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:

               foo=
               false
               $foo
               echo "Don't use it: $?"

        - don't use command substitution within variable expansion:

               cat ${FOO=`bar`}

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

Bash
     To detect whether you are running `bash', test if `BASH_VERSION'
     is set.  To disable its extensions and 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 this
     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.

`/usr/xpg4/bin/sh' on Solaris
     The POSIX-compliant Bourne shell on a Solaris system is
     `/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
     To detect whether you are running `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 `:'. Note:
     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:

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.