public function watch($command, $interval = 500)
    {
        $count = 0;
        $activeTime = time();
        $maxTime = (int)$this->config['max_watch_time'];
        $intervalUs = $interval * 1000;

        //echo "watch command: $command, refresh interval: {$interval}ms\n";

        while (true) {
            $count++;
            $result = $this->command($command);

            if (0 === strpos($result, 'ERR')) {
                echo "$result\n";
                echo "error command: $command. ";
                break;
            }

            // clear screen before output
            echo "\033[2JThe {$count} times watch {$command} result(refresh interval: {$interval}ms):\n{$result}\n";

            if ($maxTime > 0 && time() - $activeTime >= $maxTime) {
                echo 'watch time end. ';
                break;
            }

            usleep($intervalUs);
        }

        echo "Quit\n";
    }