title: Failed to start redis.service: Unit redis-server.service is masked
body: <p>I Installed Redis Server on ubuntu 16.04. but when I try to start the redis service using</p>
  <pre><code>$ sudo systemctl start redis
  </code></pre>
  <p>I  receive message:</p>
  <pre><code>Failed to start redis.service: Unit redis-server.service is masked.
  </code></pre>
  <p>I don't have any idea about this error.</p>