title: Error: Can't open display: (null) when using Xclip to copy ssh public key
body: <p>I’m following in <a href="https://help.github.com/articles/generating-ssh-keys#platform-linux" rel="noreferrer">Generating SSH Keys</a>, it says</p>
  <blockquote>
  <p><code>sudo apt-get install xclip</code></p>
  </blockquote>
  <p>Downloads and installs xclip. If you don't have <code>apt-get</code>, you might need to use another installer (like <code>yum</code>)</p>
  <blockquote>
  <p><code>xclip -sel clip &lt; ~/.ssh/id_rsa.pub</code></p>
  </blockquote>
  <p>Copies the contents of the id_rsa.pub file to your clipboard</p>
  <p>But after I run<code>xclip -sel clip &lt; ~/.ssh/id_rsa.pub</code> I get <code>Error: Can't open display: (null)</code>
  What is the problem? I googled around but found nothing about it</p>