title: subtract the values of two columns using awk or bash
body: <p>I have some  text files as shown below. I would like to subtract the values of column 2 and 4 and need to create a  new column to the output.</p>
  
  <pre><code>co1  co2   co3    co4
  
  r1  15.2  13.0   21.4
  r2  23    15     15.7
  r3  14    8      12
  </code></pre>
  
  <p>desired output</p>
  
  <pre><code>co1  co2   co3   co4   diff.    
  
  r1  15.2  13.0   21.4   -6.2
  r2  23    15     15.7   7.3
  r3  14    8      12     2
  </code></pre>