title: How to find which line is missing in another file
body: <p>on Linux box 
  I have one file as below 
  <strong>A.txt</strong></p>
  
  <pre><code>1
  2
  3
  4
  </code></pre>
  
  <p>Second file as below
  <strong>B.txt</strong></p>
  
  <pre><code>1
  2
  3
  6
  </code></pre>
  
  <p>I want to know what is inside A.txt but not in  B.txt
  i.e. it should print value  4</p>
  
  <p>I want to do that on Linux.</p>