Centos – Missing ‘which’ executable on CentOS

The one that gets output when you run which without -a is the one which will get executed. (and the second one with -a is preferred over the third one).

This doesn't take into account the shell's builtins, aliases, and functions which will run (from within the shell) before any other executable.

Therefore, it's better to use type instead.

executable=mysql

executable_path=$(command -v -- "$executable") &&
  dirname -- "$executable_path"

(don't use which).

Of course, that won't work if $executable is a shell builtin, function or alias. I'm not aware of any shell where mysql is a builtin. It won't be a function or alias unless you defined them earlier, but then you should know about it. An exception to that could be bash which supports exported functions.

$ bash -c 'command -v mysql'
/usr/bin/mysql
$ mysql='() { echo test;}' bash -c 'command -v mysql'
mysql