title: use grep to return list of matching words in a line per file
body: <p>I have a list of files, and I want to look for some specific keywords in those files. The output should be a line for each file with matches, showing the words that we found just once. For example, if I have the following file test.txt</p>
  
  <pre><code>one,two,three
  four,five,six,
  seven,eight,nine
  </code></pre>
  
  <p>and i do a grep of the words  five and eight, it should return something like this:</p>
  
  <pre><code>test.txt:five,eight
  </code></pre>
  
  <p>I'm not interested in the lines, or the number of matches. I just want to know which words matched in each file. How can I do that?</p>