A hashbang is not defined by posix, so using it without args isn't "posix compatible". From https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap0

A hashbang is not defined by posix, so using it without args isn't "posix compatible". From https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V... > If the first line of a file of shell commands starts with the characters "#!", the results are unspecified. There is no more specification for "#!/usr/bash" than "#!/usr/bin/jq -jf" The exec page provides even fewer words about how to interpret shebangs if you thought perhaps I was linking to the wrong portion of the posix spec