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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 the Korn shell and Bash
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.  Some old systems have quite small limits on
the length of the `#!' line too, for instance 32 bytes (not including
the newline) on SunOS 4.

   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;
see Note: Limitations of Usual Tools.

Shellology

A zoology of shells

Here-Documents

Quirks and tricks

File Descriptors

FDs and redirections

File System Conventions

File- and pathnames

Shell Substitutions

Variable and command expansions

Assignments

Varying side effects of assignments

Special Shell Variables

Variables you should not change

Limitations of Builtins

Portable use of not so portable /bin/sh

Limitations of Usual Tools

Portable use of portable tools

Limitations of Make

Portable Makefiles