Find out if a command exists on POSIX system

Question

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 ; th         


        

Answer 1

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).

Answer 2

POSIX does say, “If a command is not found, the exit status shall be 127.” So you could do

<command>
if [ "${?}" = 127 ]; then
   <handle not found>
fi

When writing shell scripts, it’s often permissible to require a bash shell (#!/bin/bash), because without arrays it’s pretty much impossible to handle arguments and/or filenames with spaces correctly. In that case, the bash builtin type -p is equivalent to which, and because it is builtin, it is portable.

Answer 3

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.