In WordPress loops, show the excerpt, the content before <!--MORE-->, or something else, but avoid showing the full post content.

wordpress_listing_excerpt_or_more.php

<?php
/**
 * Let's say you want to support BOTH post excerpts and use of the <!--more--> tag
 * to truncate post content in The Loop.
 * 
 * And let's also say that you definitely do not want to show the full content of the post 
 * if either of those are absent. Maybe you're displaying posts in little Pinterest-y 
 * cards so the text/excerpt can't be too long.
 * 
 * Let's say this is your priority:
 *   1. If there's an excerpt, use that. It's hand-crafted(tm)!
 *   2. If there's a <!--more--> tag, use that. At least someone put thought into where to split things.
 *   3. If neither of those are there, show nothing (or maybe just a link to the post). You don't want to rely on 
 *      some sort of auto-truncation because truncation is not a content stra
 * 
 * Some issues that come up:
 *   - <!--more--> tags don't work everywhere you might think. If you have a "static" home page (Settings -> Reading) 
 *     with a Loop of posts, the_content() shows full content instead of just the pre-<!--more--> truncated content.
 *   - If you use the_content() as a fallback for excerpts, you might get the full text if there's no excerpt.
 *       if ( get_the_excerpt() ) { the_excerpt(); } else { the_content(); // could be the whole text }
 *   - If you're on a page where <!--more--> DOES work, the_content() and get_the_content() return 
 *     the TRUNCATED content. So you can't do strstr( '<!--more-->', the_content() ) to detect it.
 * 
 * So here's all the tricks I've got to prevent that.
 * 
 * Use this within a Loop (obvs)
 */

// This makes sure <!--more--> works on pages where it might not, like a Static Front Page
// If you're going this way, make sure you don't use this code everywhere. It'll truncate posts if you do.
// http://codex.wordpress.org/Customizing_the_Read_More#How_to_use_Read_More_in_Pages
global $more; 
$original_more = $more; 
$more = 0;

if ( trim(get_the_excerpt()) ) :
  // priority 1: if there's an excerpt, use that
  the_excerpt();
elseif ( FALSE !== strstr( get_the_content('nonsensenonsense'), 'nonsensenonsense') ) :
 /**
  * here's where I get too clever for my own good:
  * (get_)the_content() will be truncated now at <!--more-->. The actual "<!--more-->" string is gone.
  * SO you can't just strstr(<!--more-->, get_the_content()) to detect IF there's a more tag.
  * BUT, the first argument of (get_)the_content() is the text content for the More link.
  * So I can put some nonsense text there, and strstr for that nonsense text.
  * BOOM: you've detected if there's a valid <!--more--> tag there.
  * If it's there, we can now show the (truncated) content
  */

 the_content('',FALSE); // you can set that first parameter to whatever you want the more link text to be.

else: 
  ?>

  // whatever you want the fallback to be. let's say just a link to the post
  <a href="<?php the_permalink(); ?>">Read the whole post&hellip;</a>

<?php
endif;


// After this and after you close The Loop, you may have to reset the global $more like follows:
// global $more; 
// global $original_more; 
// $more = $original_more;

?>