wordpress - Exclude the_post_thumbnail from gallery shortcode -

i using code have simple gallery on page:

<?php echo do_shortcode('[gallery itemtag="ul" icontag="li" size="full" columns="0" link="file" ]'); ?> 

the problem end-user has upload image via media page before selecting image featured image.

i know solved adding featured image's id shortcode's exclude list, how id automatically?

function exclude_thumbnail_from_gallery($null, $attr) {     if (!$thumbnail_id = get_post_thumbnail_id())         return $null; // no point carrying on if no thumbnail id      // temporarily remove filter, otherwise endless loop!     remove_filter('post_gallery', 'exclude_thumbnail_from_gallery');      // pop in our excluded thumbnail     if (!isset($attr['exclude']) || empty($attr['exclude']))         $attr['exclude'] = array($thumbnail_id);     elseif (is_array($attr['exclude']))         $attr['exclude'][] = $thumbnail_id;      // manually invoke shortcode handler     $gallery = gallery_shortcode($attr);      // add filter     add_filter('post_gallery', 'exclude_thumbnail_from_gallery', 10, 2);      // return output calling instance of gallery_shortcode()     return $gallery; } add_filter('post_gallery', 'exclude_thumbnail_from_gallery', 10, 2);