All those rules add up to: "avoiding writing anything complicated or general purpose in the shell language that is intended to be used by any user over any set of files; stick to handling known input materials which are closely associated with the script."

I doubt it. set -e is POSIX, I think, but the rest are probably bash extensions.

I'll probably get flak for this, but for 99% of the scripts, at this point I'd just use bash. Linux has it as a default, as does Mac OS, anyone running BSDs can install it trivially, and Unixes probably have it as well since most of them have antiquated CLI environments and administrators usually install GNU utils to have a more human working environment.