Find out if a command exists on POSIX system

command -v is a POSIX specified command that does what which does.

It is defined to to return >0 when the command is not found or an error occurs.

You could read the stdout/stderr of "which" into a variable or an array (using backticks) rather than checking for an exit code.

If the system does not have a "which" or "where" command, you could also grab the contents of the $PATH variable, then loop over all the directories and search for the given executable. That's essentially what which does (although it might use some caching/optimization of $PATH results).