title: Copy data inside one field XML
body: <p>I have some lines like:</p>
  
  <pre><code>&lt;channel update="i" site="merge-xmltv" site_id="" xmltv_id="Rai 1"&gt;Rai 1&lt;/channel&gt;
  &lt;channel update="i" site="merge-xmltv" site_id="" xmltv_id="Rai 1 +2HD"&gt;Rai 1 +2HD&lt;/channel&gt;
  &lt;channel update="i" site="merge-xmltv" site_id="" xmltv_id="Rai 1 +1HD"&gt;Rai 1 +1HD&lt;/channel&gt;
  </code></pre>
  
  <p>I need copy the value of <code>xmltv_id</code> into <code>site_id</code> like this:</p>
  
  <pre><code>&lt;channel update="i" site="merge-xmltv" site_id="Rai 1" xmltv_id="Rai 1"&gt;Rai 1&lt;/channel&gt;
  &lt;channel update="i" site="merge-xmltv" site_id="Rai 1 +2HD" xmltv_id="Rai 1 +2HD"&gt;Rai 1 +2HD&lt;/channel&gt;
  &lt;channel update="i" site="merge-xmltv" site_id="Rai 1 +1HD" xmltv_id="Rai 1 +1HD"&gt;Rai 1 +1HD&lt;/channel&gt;
  </code></pre>
  
  <p>I was thinking to use SED but I am in the trouble.</p>
  
  <p>Thank you in someone want help me!</p>