protected function getUrlToBeCrawled()
    {
        if (! is_null($this->option('url'))) {
            return $this->option('url');
        }

        if (config('laravel-link-checker.url') != '') {
            return config('laravel-link-checker.url');
        }

        if (config('app.url') != '') {
            return config('app.url');
        }

        throw new Exception('could not determine which url to be crawled.');
    }