title: Extend Magento checkout CartController.php
body: <p>I need some help extending the magento CartController, I found many tutorials online, however without any result, this is what I got so far:</p>
  
  <blockquote>
    <p>config.xml</p>
  </blockquote>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
      &lt;config&gt;
          &lt;modules&gt;
              &lt;Juist_Overwrite&gt;
                  &lt;version&gt;0.1.0&lt;/version&gt;
              &lt;/Juist_Overwrite&gt;
          &lt;/modules&gt;
          &lt;frontend&gt;
              &lt;routers&gt;
                  &lt;Juist_Overwrite&gt;
                      &lt;use&gt;standard&lt;/use&gt;
                      &lt;args&gt;
                          &lt;module&gt;Juist_Overwrite&lt;/module&gt;
                          &lt;frontName&gt;overwrite&lt;/frontName&gt;
                      &lt;/args&gt;
                  &lt;/Juist_Overwrite&gt;
              &lt;/routers&gt;
          &lt;/frontend&gt;
          &lt;global&gt;
              &lt;routers&gt;
                  &lt;checkout&gt;
                      &lt;rewrite&gt;
                          &lt;cart&gt;
                              &lt;to&gt;overwrite/cart&lt;/to&gt;
                              &lt;override_actions&gt;true&lt;/override_actions&gt;
                              &lt;actions&gt;
                                  &lt;add&gt;
                                      &lt;to&gt;overwrite/cart/index&lt;/to&gt;
                                  &lt;/add&gt;
                              &lt;/actions&gt;
                          &lt;/cart&gt;
                      &lt;/rewrite&gt;
                  &lt;/checkout&gt;
              &lt;/routers&gt;
          &lt;/global&gt;
      &lt;/config&gt;
  </code></pre>
  
  <blockquote>
    <p>CartController.php</p>
  </blockquote>
  
  <pre><code> &lt;?php
  
      require_once 'Mage/Checkout/controllers/CartController.php';
  
      class Juist_Overwrite_CartController extends Mage_Checkout_CartController
      {
          public function addAction()
          {
              parent::addAction();
  
              echo 'Test';
          }
  
          public function indexAction()
          {
              parent::indexAction();
  
              echo 'Test';
          }
      }
  </code></pre>
  
  <blockquote>
    <p>See the result below, the test variable is printed as expected, however the
    content area stays white, none of the layout is loaded</p>
  </blockquote>
  
  <p><a href="https://i.stack.imgur.com/NWSP0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NWSP0.png" alt="See the result below, the test is printed as expected, however the content area stays white, none of the layout is loaded"></a></p>
  
  <p>How can I extend only the CartController.php?</p>