Shell Programming and Scripting

I would like to write unix shell scripts which are strictly POSIX compliant, and do not use any specific functionality that ties them down to one particular shell.

What is the best way to do this?

Any script or program that can be run to determine strict POSIX compliance?

Or should I make sure that all my scripts work with dash
Debian Almquist shell - Wikipedia, the free encyclopedia
which I understand offers minimal functionality above POSIX?

And if dash is the best route, does anyone know how to install it underneath cygwin?

well. this is a huge problem you are traing to tacle here ....
the best aprouch is the one that the people of bigbrother did (the system monitor)
they use a config script that source alot of things.
their main scripts source that oconfiguration and work on.
so you ony have to make sure the config script is kinda good.

for example, in during the isntalationm, if you tell the script you are on linux, it creates a var in the config called AWK that points to gawk
in their scripts, for all platforms, you use $AWK regardless

Using the --posix flag in bash would seem like a simple and straightforward first step.

If you really want to write UNIX shell scripts which are strictly POSIX compliant, the first question you need to answer is which version of the relevant POSIX standards do which to conform to. The original versions were POSIX.2-1992 (Shell and Utilities) and POSIX.2a-1992 (Interactive shell and Utilities). The latest version is IEEE Std 1003.1, 2004 (POSIX.1-2004) which includes POSIX.2 and various amendments.

There are various test suites available (VSC/PCTS) but they generally test the shell for conformance to a specific version of a standard rather than test that a script is strictly POSIX conformant.