If you limit yourself to either a popular Linux distro or one of two Unixes/Unix-likes, then bash can be available out of the box. Just as C is not C++, shell i

If you limit yourself to either a popular Linux distro or one of two Unixes/Unix-likes, then bash can be available out of the box. Just as C is not C++, shell is not bash, so a shell script is what runs on (d)ash, busybox, toybox, ksh, bash, zsh, etc., without modifications. If it requires bash or zsh, then it's not a shell script but a bash/zsh/fish script. To name a popular non-Linux OS, take a fresh FreeBSD or OpenBSD install. No bash to be found, unless installed via ports and rightfully so. That said, I use bash myself all the time as an interactive shell but /bin/sh is not bash. sh (including bash and zsh) are terribly hard to write correct and resilient scripts in, and even rc is much saner to script in.