title: Getting custom field value to functions.php and displaying in on the shortcode
body: <p>Maybe someone would be able to help me with the issue I have as I'm stuck with no ideas.</p>
  
  <p>I have a shortcode on my site that is responsible for displaying photosets directly from Flickr (via external plugin).</p>
  
  <p>The code generated by the plugin is the following:
  [justified_image_grid preset=c1 flickr_user=USERID flickr_photoset=PHOTOSETID]</p>
  
  <p>My blog posts displays various photosets from Flickr. I'd like to avoid having to edit shortcode each and every time to update the shortcode code with the proper photoset ID so I decided to use custom field (Key = FlicktPhotoset, Value = Photoset ID) and add function to functions.php that would create my shortcode which would include original shortcode with the value from custom field.</p>
  
  <p>Code in functions.php is the following:</p>
  
  <pre><code>function flickr_shortcode() {
      echo do_shortcode('[justified_image_grid preset=c1 flickr_user=USERNAME flickr_photoset=PHOTOID]');
  }
  function flickr_shortcodes_init() {
      add_shortcode('flickr', 'flickr_shortcode');
  }
  add_action('init', 'flickr_shortcodes_init');
  </code></pre>
  
  <p>What I'm stuck at is how to pass shortcode value into this code to automatically fetch PHOTOID from the custom field value.</p>