title: Find files that does not contain a string
body: <p>To find all the files that contain "foo" in current folder, I use:</p>
  
  <pre><code>grep -r "foo" .
  </code></pre>
  
  <p>To find all the files that contain "bar" in current folder, I use:</p>
  
  <pre><code>grep -r "bar" .
  </code></pre>
  
  <p>But how to find all files that does not contain 'foo' and 'bar'?</p>