title: XMLStarlet - replace a part of attribute
body: <p>I am using XMLStartlet for a quick deploy cmd (Windows) script for my app, and I am changing the configuration xml file.</p>
  
  <p>Manipulation of the whole nodes/attributes works just perfect, but i need to replace a part of an attribute with a specific value, for example:</p>
  
  <pre><code>&lt;list&gt;
      &lt;address id="a1"&gt;
          &lt;data url="http://localhost:8000/a1.html" /&gt;
      &lt;/address&gt;
      &lt;address id="a2"&gt;
          &lt;data url="http://localhost:8000/a2.html" /&gt;
      &lt;/address&gt;
  &lt;/list&gt;
  </code></pre>
  
  <p>I need to change the port part of a <code>/list/address/data/@url</code> to get:</p>
  
  <pre><code>&lt;list&gt;
      &lt;address id="a1"&gt;
          &lt;data url="http://localhost:8001/a1.html" /&gt;
      &lt;/address&gt;
      &lt;address id="a2"&gt;
          &lt;data url="http://localhost:8001/a2.html" /&gt;
      &lt;/address&gt;
  &lt;/list&gt;
  </code></pre>
  
  <p>Any help with a suitable xmlstarlet command would be much appreciated. I don't want to mix sed into my script.</p>