title: Renaming files from shell by removing letters
body: <p>I have a lot of jpg files with letters and numbers in their names. I want to remove all letters, for example <code>abc12d34efg.jpg</code> becomes <code>1234.jpg</code>. For the for loop I thought:</p>
  
  <pre><code> for i in *.jpg; do mv "$i" ...
  </code></pre>
  
  <p>but I can't find a command for what I want.</p>