title: replacing one word in one line in a file
body: <p>I understand I can use sed to replace a word in a file with a different word. but what if I have a word that might appear several times in a file and I only want to change it in a specific line, in the end of it. 
  lets say I have </p>
  
  <pre><code>hello unix world olga unix olga unix 
  hello unix 2013 world
  </code></pre>
  
  <p>assuming I know the number of the line, how do I replace the second "unix" in the first line and leave others unchanged?
  all expressions as well as the line number are saved in variables</p>