title: How to get IzPack tocheck for an existing version of the software?
body: <p>I have an application for which we use <a href="http://izpack.org" rel="nofollow noreferrer">IzPack</a> to create the installer. The installer is working fine in its current state, however, I need to add functionality that enables it to <strong>check</strong> whether an <strong>existing version</strong> of the software has been installed.</p>
  
  <p>I understand IzPack supports this out of the box using its <a href="http://izpack.org/documentation/panels.html#checkedhellopanel" rel="nofollow noreferrer"><code>CheckedHelloPanel</code></a>, unfortunately that only works for Windows, as it appears to depend on the windows registry.</p>
  
  <p>Is there a way to configure IzPack such that it is able to <strong>detect existing installations</strong>?</p>
  
  <p>I need to be able to detect whether there is one and merely display a message notifying the user... Bonus points for how to give the user the option to trigger the uninstaller of the existing installation.</p>
  
  <hr>
  
  <ul>
  <li>Assume that the software will only be installed using the new installer</li>
  <li>IzPack only: please do not suggest alternatives, as we cannot change that now</li>
  <li>If you suggest using <a href="http://izpack.org/documentation/advanced-features.html#validators-for-packs" rel="nofollow noreferrer"><code>&lt;validator&gt;</code></a>, please include a code sample of the validator class, because I have considered this, but don't know where to start</li>
  </ul>