How do I print a specific meta tag attribute value in html.tpl.php?
create a preprocess_html function in template.php
function porto_sub_preprocess_html(&amp;$vars){
  if (drupal_is_front_page()) {
    if (isset($vars['page']) &amp;&amp; isset($vars['page']['content']) &amp;&amp; isset($vars['page']['content']['metatags'])) {
      $meta_tags = $vars['page']['content']['metatags'];
      if (isset($meta_tags['global:frontpage']) &amp;&amp; isset($meta_tags['global:frontpage']['description'])) {
        $meta_desc_list = $meta_tags['global:frontpage']['description'];
        if (isset($meta_desc_list['#attached']) &amp;&amp; isset($meta_desc_list['#attached']['drupal_add_html_head'])) {
          if (isset($meta_desc_list['#attached']['drupal_add_html_head'][0]) &amp;&amp; isset($meta_desc_list['#attached']['drupal_add_html_head'][0][0])) {
            if ($meta_desc_list['#attached']['drupal_add_html_head'][0][0]['#value']) {
              $vars['front_page_meta_desc'] = $meta_desc_list['#attached']['drupal_add_html_head'][0][0]['#value'];
            }
          }
        }
      }
    }
  }
  drupal_add_http_header('Cache-Tag','mktg',true);



 
2. This defined meta tag is accessible from html.tpl.php
 if (!isset($front_page_meta_desc)) {
       $front_page_meta_desc = '';
     }

In the schema JSONLD, you need to call the variable.
&quot;description&quot;: &quot;' . $front_page_meta_desc . '&quot;,