这是我的代码，我想知道它的短代码是什么？是吗[login_form] 还是我们需要添加更多内容？具有[login_form] 结果没有输出

function pippin_login_form() {

if(!is_user_logged_in()) {

    global $pippin_load_css;

    // set this to true so the CSS is loaded
    $pippin_load_css = true;

    $output = pippin_login_form_fields();
} else {
    // could show some logged in user info here
    // $output = \'user info here\';
}
return $output;
}
add_shortcode(\'login_form\', \'pippin_login_form\');