How to start a php script from cli within wordpress

I wrote a PHP script, which was meant to be a WP-Cron-cronjob and which uses wordpress specific functions. Due to some restrictions in its runtime enviroment, I need to start this script from the command line with /usr/bin/php -q longThing.php instead of as a WP-Cron event. How can I ensure that all the wordpress core functions are callable in my script?

Answer

Xaedes solution works quite well:

<?php
    if( php_sapi_name() !== 'cli' ) {
        die("Meant to be run from command line");
    }

    function find_wordpress_base_path() {
        $dir = dirname(__FILE__);
        do {
            //it is possible to check for other files here
            if( file_exists($dir."/wp-config.php") ) {
                return $dir;
            }
        } while( $dir = realpath("$dir/..") );
        return null;
    }

    define( 'BASE_PATH', find_wordpress_base_path()."/" );
    define('WP_USE_THEMES', false);
    global $wp, $wp_query, $wp_the_query, $wp_rewrite, $wp_did_header;
    require(BASE_PATH . 'wp-load.php');