man foo
apropos foo
Info foo
Fun aside, I think -h/--help is nowadays the most widespread and accepted version in Linux/Unix-Land. A good CLI argument parser accepts both variants.
Nearly every tool does have a manpage (even a basic one) thanks to Debian’s manpage policy: “Each program, utility, and function should have an associated manual page included in the same package or a dependency.… If no manual page is available, this is considered as a bug and should be reported to the Debian Bug Tracking System.”
The rare times I encounter a command‐line tool that has no manpage, I’ll sometimes write one and contribute it upstream.
Of course some programs have a useless manpage that points to info. Thankfully that is pretty much limited to GNU tools which I’m lucky enough to not have to use very often.
shutdown -hThe most notable exception (to me) among modern tooling is the `aws` cli, which parses help as a subcommand. And if you try `--help` it just gives you an `Unkown option` response.
I'm mildly amused by the need to reserve a little bit of brain-space for the ways to ask cli tooling for help