title: Deleting lines from one file which are in another file
body: <p>I have a file <code>f1</code>:</p>
  
  <pre><code>line1
  line2
  line3
  line4
  ..
  ..
  </code></pre>
  
  <p>I want to delete all the lines which are in another file <code>f2</code>:</p>
  
  <pre><code>line2
  line8
  ..
  ..
  </code></pre>
  
  <p>I tried something with <code>cat</code> and <code>sed</code>, which wasn't even close to what I intended. How can I do this?</p>