title: Delete empty lines using sed
body: <p>I am trying to delete empty lines using sed:</p>
  
  <pre><code>sed '/^$/d'
  </code></pre>
  
  <p>but I have no luck with it.</p>
  
  <p>For example, I have these lines:</p>
  
  <pre><code>xxxxxx
  
  
  yyyyyy
  
  
  zzzzzz
  </code></pre>
  
  <p>and I want it to be like:</p>
  
  <pre><code>xxxxxx
  yyyyyy
  zzzzzz
  </code></pre>
  
  <p>What should be the code for this?</p>