title: Filter list of files to those that exist
body: <p>How do I filter a list of files to the ones that exist?</p>
  
  <p>For example,</p>
  
  <pre><code>echo 'a.txt
  does/not.exist
  b.txt' | &lt;???&gt;
  </code></pre>
  
  <p>would print</p>
  
  <pre><code>a.txt
  b.txt 
  </code></pre>