(autoconf.info)Portable Shell

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5.5 Portable Shell Programming
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When writing your own checks, there are some shell script programming
techniques you should avoid in order to make your code portable.  The
Bourne shell and upward-compatible shells like Bash and the Korn shell
have evolved over the years, but to prevent trouble, do not take
advantage of features that were added after UNIX version 7, circa 1977.
You should not use shell functions, aliases, negated character classes,
or other features that are not found in all Bourne-compatible shells;
restrict yourself to the lowest common denominator.  Even 'unset' is not
supported by all shells!  Also, include a space after the exclamation
point in interpreter specifications, like this:
     #! /usr/bin/perl
   If you omit the space before the path, then 4.2BSD based systems
(such as Sequent DYNIX) will ignore the line, because they interpret '#!
/' as a 4-byte magic number.

   The set of external programs you should run in a 'configure' script
is fairly small.  Note: Utilities in Makefiles,
 for the list.  This restriction allows users to start out
with a fairly small set of programs and build the rest, avoiding too
many interdependencies between packages.

   Some of these external utilities have a portable subset of features,
as well; for example, don't rely on 'ln' having a '-f' option or 'cat'
having any options.  'sed' scripts should not contain comments or use
branch labels longer than 8 characters.  Don't use 'grep -s' to suppress
output, because 'grep -s' on System V does not suppress output, only
error messages.  Instead, redirect the standard output and standard
error (in case the file doesn't exist) of 'grep' to '/dev/null'.  Check
the exit status of 'grep' to determine whether it found a match.