ss_hook.php

<?php
// Exec
// call: do_action('ss_cmd', 'cmd1');

add_action('ss_cmd', function($cmd) {
    switch($cmd) {
        case 'cmd1':
            return shell_exec('cmd1');
        break;
        case 'cmd2':
            return shell_exec('cmd2');
        break;
    }
    return false;
});

// FIFO file
// call: do_action('ss_cmd', 'cmd1');
add_action('ss_cmd', function($cmd) {
    $path = WP_CONTENT_DIR.'/ss_cmd.txt';
    switch($cmd) {
        case 'cmd1':
            return file_put_contents($path, "cmd1\n", FILE_APPEND|LOCK_EX);
        break;
        case 'cmd2':
            return file_put_contents($path, "cmd2\n", FILE_APPEND|LOCK_EX);
        break;
    }
    return false;
});