Find out if a command exists on POSIX system

I want to be able to tell if a command exists on any POSIX system from a shell script.

On Linux, I can do the following:

if which <command>; then
   ...snip...
fi

However, Solaris and MacOS which do not give an exit failure code when the command does not exist, they just print an error message to STDOUT.

Also, I recently discovered that the which command itself is not POSIX (see http://pubs.opengroup.org/onlinepubs/9699919799/idx/utilities.html)

Any ideas?

Solution 1:

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.

Solution 2:

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).