Au contraire, shell script in a (real) POSIX environment is one of the most portable things... It's interpreted and there's always some kind of shell on any Unix system, unlike other interpreted languages. That's why lots of 3rd party vendors use shell scripts to package their setup tools.
Yes the tools behave differently on different systems, but 99.9% of the time there's very basic syntax that is respected across all versions of the tools. Stick to BSD4.4 C Shell syntax (http://docs.freebsd.org/44doc/usd/04.csh/paper.html), don't rely on regular expressions in grep, and stick to the very basics of sed and awk and you can go pretty far in Unix.
Any of the following commands which existed on some archaic version of Unix will still exist and their limited but useful functionality can be extended in lots of ways, and is pretty darn portable.
https://secure.wikimedia.org/wikipedia/en/wiki/List_of_Unix_...
Yes the tools behave differently on different systems, but 99.9% of the time there's very basic syntax that is respected across all versions of the tools. Stick to BSD4.4 C Shell syntax (http://docs.freebsd.org/44doc/usd/04.csh/paper.html), don't rely on regular expressions in grep, and stick to the very basics of sed and awk and you can go pretty far in Unix.
Any of the following commands which existed on some archaic version of Unix will still exist and their limited but useful functionality can be extended in lots of ways, and is pretty darn portable. https://secure.wikimedia.org/wikipedia/en/wiki/List_of_Unix_...