> there is a very clear baseline of functionality that works in every Bourne/POSIX descendant

Where is the best place to learn what this is? I'd love to make sure I'm writing portable shell scripts when I do have to.

There's also the issue that "shell scripts" often involve using binaries which you might not even realise are binaries (is "echo" a shell builtin or a binary? I forget) and which may differ from system to system. I've been bitten by grep issues before writing scripts across Ubuntu and OSX.