private function getShell()
    {
        if (null !== self::$shell) {
            return self::$shell;
        }

        self::$shell = false;

        if (file_exists('/usr/bin/env')) {
            // handle other OSs with bash/zsh/ksh/csh if available to hide the answer
            $test = "/usr/bin/env %s -c 'echo OK' 2> /dev/null";
            foreach (array('bash', 'zsh', 'ksh', 'csh') as $sh) {
                if ('OK' === rtrim(shell_exec(sprintf($test, $sh)))) {
                    self::$shell = $sh;
                    break;
                }
            }
        }

        return self::$shell;
    }