title: Find lines from a file which are not present in another file
body: <p>I have two files (let's say <code>a.txt</code> and <code>b.txt</code>), both of which has a list of names. I have already run <code>sort</code> on both the files.</p>
  
  <p>Now I want to find lines from <code>a.txt</code> which are not present in <code>b.txt</code>. </p>
  
  <p>(I spent lot of time to find the answer for this question, so documenting it for future reference)</p>