Most of the shell utilities described by posix have standard flags, and then gnu/bsd extra flags. If you use the standard ones (and use the "posix mode" flags

Most of the shell utilities described by posix have standard flags, and then gnu/bsd extra flags. If you use the standard ones (and use the "posix mode" flags when available) you're mostly ok. Also, a shell script can have logic to handle different tools available (either different flavours of the same tool or even different tools that do similar things). If the basic syntax it uses (or the shebang) are bash specific then you need bash to run it.