understrap/inc/bootstrap-wp-gallery.php

<?php
//Based on Roots Sage Gallery: https://github.com/roots/sage/blob/5b9786b8ceecfe717db55666efe5bcf0c9e1801c/lib/gallery.php

// Remove built in shortcode
remove_shortcode('gallery', 'gallery_shortcode');

// Replace with custom shortcode
function shortcode_gallery($attr) {
  $post = get_post();
  static $instance = 0;
  $instance++;
  if (!empty($attr['ids'])) {
    if (empty($attr['orderby'])) {
      $attr['orderby'] = 'post__in';
    }
    $attr['include'] = $attr['ids'];
  }
  $output = apply_filters('post_gallery', '', $attr);
  if ($output != '') {
    return $output;
  }
  if (isset($attr['orderby'])) {
    $attr['orderby'] = sanitize_sql_orderby($attr['orderby']);
    if (!$attr['orderby']) {
      unset($attr['orderby']);
    }
  }
  extract(shortcode_atts([
    'order'      => 'ASC',
    'orderby'    => 'menu_order ID',
    'id'         => $post->ID,
    'itemtag'    => '',
    'icontag'    => '',
    'captiontag' => '',
    'columns'    => 3,
    'size'       => 'thumbnail',
    'include'    => '',
    'exclude'    => '',
    'link'       => ''
  ], $attr));
  $id = intval($id);
  $columns = (12 % $columns == 0) ? $columns : 3;
  $grid = sprintf('col-sm-%1$s col-lg-%1$s', 12 / $columns);
  if ($order === 'RAND') {
    $orderby = 'none';
  }
  if (!empty($include)) {
    $_attachments = get_posts(['include' => $include, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby]);
    $attachments = [];
    foreach ($_attachments as $key => $val) {
      $attachments[$val->ID] = $_attachments[$key];
    }
  } elseif (!empty($exclude)) {
    $attachments = get_children(['post_parent' => $id, 'exclude' => $exclude, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby]);
  } else {
    $attachments = get_children(['post_parent' => $id, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby]);
  }
  if (empty($attachments)) {
    return '';
  }
  if (is_feed()) {
    $output = "\n";
    foreach ($attachments as $att_id => $attachment) {
      $output .= wp_get_attachment_link($att_id, $size, true) . "\n";
    }
    return $output;
  }
  $unique = (get_query_var('page')) ? $instance . '-p' . get_query_var('page') : $instance;
  $output = '<div class="gallery gallery-' . $id . '-' . $unique . '">';
  $i = 0;
  foreach ($attachments as $id => $attachment) {
    switch($link) {
      case 'file':
        $image = wp_get_attachment_link($id, $size, false, false);
        break;
      case 'none':
        $image = wp_get_attachment_image($id, $size, false, ['class' => 'thumbnail img-thumbnail']);
        break;
      default:
        $image = wp_get_attachment_link($id, $size, true, false);
        break;
    }
    $output .= ($i % $columns == 0) ? '<div class="row gallery-row">' : '';
    $output .= '<div class="' . $grid .'">' . $image;
    if (trim($attachment->post_excerpt)) {
      $output .= '<div class="caption hidden">' . wptexturize($attachment->post_excerpt) . '</div>';
    }
    $output .= '</div>';
    $i++;
    $output .= ($i % $columns == 0) ? '</div>' : '';
  }
  $output .= ($i % $columns != 0 ) ? '</div>' : '';
  $output .= '</div>';
  return $output;
}
add_shortcode('gallery', 'shortcode_gallery');

/**
 * Add class="thumbnail img-thumbnail" to attachment items
 */
function attachment_link_class($html) {
  $html = str_replace('<a', '<a class="thumbnail img-thumbnail"', $html);
  return $html;
}
add_filter('wp_get_attachment_link', 'attachment_link_class', 10, 1);