How does that work? It doesn't look like ! is a command or shell builtin, so I guess it's an argument to if.
No, it's part of the shell. Do "man bash" or "man sh", then search with "/\!". From man bash: "If the reserved word ! precedes a pipeline, the exit status of that pipeline is the logical negation of the exit status".
Amazing! I've been writing shell scripts since the 90s and now I still learn something new. Thank you!