title: add line to a file ONLY if it is not in file already
body: <p>I want to add the following line: </p>
  
  <pre><code>nohup java -jar /mnt/fusion/nfs/labStats/LabInfoAutoLog.jar &gt; /dev/null &amp;
  </code></pre>
  
  <p>to the end of the file <code>/etc/rc.d/rc.local</code> if it does not already exist. </p>
  
  <p>How can I do that from <em>linux</em> command line? I assume <code>grep</code> or <code>sed</code> would work, but I am not familiar enough with either to get it to work. Right now I use <code>echo</code>, but that just keeps adding it over and over again.</p>