title: Addition of values from two files shell scripting
body: <p>I have 2 files with data in below format.
  File1.txt</p>
  
  <pre><code>        abc , 234
          bcd , 457
          xyz , 123
  </code></pre>
  
  <p>File2.txt</p>
  
  <pre><code>        abc , 23
          bcd , 458
  </code></pre>
  
  <p>Now, I am looking for following out :-</p>
  
  <pre><code>       abc , 211 ( diff of file1 and file2)
         bcd, -1
  </code></pre>
  
  <p>and </p>
  
  <pre><code>sum of file1 values present in file2    =  691 (abc count + bcd count in file 1 )
  
  sum of file1 values present in file2    =  481 (abc count + bcd count in file 2 ) 
  </code></pre>
  
  <p>I know using grep -f we can merge these two files but how to do addition and substraction I am not sure. </p>
  
  <p>Any help will be much appriciated.</p>
  
  <p>Regards.</p>