It's a cultural thing. GNU is intended to replace Unix, so interoperability is (at best) not a priority. Same as Microsoft's “embrace, extend, extinguish”, whic

It's a cultural thing. GNU is intended to replace Unix, so interoperability is (at best) not a priority. Same as Microsoft's “embrace, extend, extinguish”, which shows how important a good slogan is if you want your ambitions recognized. A POSIX (or older Bourne-descended) shell can be distinguished from ‘bash --posix’ in a script with a fragment like “date&>F”, because Bash authors either didn't realize or didn't care that the sequence ‘&>’ already had meaning.