title: Insert entry into crontab unless it already exists (as one-liner if possible)
body: <p>What's the preferred method to insert an entry into /etc/crontab unless it exists, preferably using a one-liner?</p>
  
  <p>Here's my example entry I wish to place into /etc/crontab unless it already exists in there.</p>
  
  <pre><code>*/1 *  *  *  * some_user python /mount/share/script.py
  </code></pre>
  
  <p>I'm on CentOS 6.6 and so far I have this:</p>
  
  <pre><code>if grep "*/1 *  *  *  * some_user python /mount/share/script.py" /etc/crontab; then echo "Entry already in crontab"; else echo "*/1 *  *  *  * some_user python /mount/share/script.py" &gt;&gt; /etc/crontab; fi 
  </code></pre>