Fish is not a POSIX shell and does not try to be one. In contrast, bash, zsh, and others are supersets of the original sh and backwards-compatible. Fish is not.

Fish is not a POSIX shell and does not try to be one. In contrast, bash, zsh, and others are supersets of the original sh and backwards-compatible. Fish is not. For example the exit code of the previous command, instead of POSIX $?, you can get it as $status.