title: Writing shell script to scan a list of folders
body: <p>I have a file <code>folders.txt</code></p>
  
  <pre><code>one
  two
  three
  four
  ...
  </code></pre>
  
  <p>that has a list of folder names. [<code>one</code>, <code>two</code>, <code>three</code> and <code>four</code> are names of folders].</p>
  
  <p>Each of these folders has a number of files of different types (different extensions). I want a list of all the files in all the folders of one particular extension, say <code>.txt</code>.</p>
  
  <p><strong><em>How should my shell script look like?</em></strong></p>