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

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

if [ "${?}" = 127 ]; then
   
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.