Frequently Asked Question

What is POSIX sh, and why does it matter?

POSIX is a family of standards (IEEE 1003) that codifies what a Unix-like system must offer, from system calls up to shell behaviour. POSIX sh is the chapter that specifies the shell language: which builtins must exist, how variables expand, how quoting works, how if, for, and while are written. Any shell that conforms can run any POSIX shell script.

Why it matters: portability. A script written in pure POSIX sh will run on Debian's dash, on a busybox-based router, on macOS, on FreeBSD, and on any Linux system, without surprises. Once you start using bash-specific features ("bashisms") like [[ ... ]], arrays, <<<, or ${var,,}, the script will fail on dash and on any minimal shell. System scripts on Debian and Ubuntu enforce this strictly, that's why /bin/sh points to dash. For your own scripts the practical rule is: if you start it with #!/bin/sh, stick to POSIX; if you want bash features, start it with #!/bin/bash.