title: "No X11 DISPLAY variable" - what does it mean?
body: <p>I am trying to install a Java application on my Linux machine (Slackware). </p>
  
  <p>I have received the following error, and I do not understand it. </p>
  
  <p>Could you advise me how to approach the problem? Thank you.</p>
  
  <p>Here is what I get: (I see that some <strong>X11 DISPLAY</strong> variable needs to be set, but what value should I give it and how?)</p>
  
  <pre><code>~$ java -jar gate-5.0-beta1-build3048-installer.jar
  - ERROR -
  
  java.awt.HeadlessException:
  No X11 DISPLAY variable was set, but this program performed an operation which requires it.
  java.awt.HeadlessException:
  No X11 DISPLAY variable was set, but this program performed an operation which requires it.
  at java.awt.GraphicsEnvironment.checkHeadless(Graphic sEnvironment.java:159)
  at java.awt.Window.&lt;init&gt;(Window.java:407)
  at java.awt.Frame.&lt;init&gt;(Frame.java:402)
  at net.sourceforge.mlf.metouia.borders.MetouiaDotsBuf fer.&lt;init&gt;(MetouiaDotsBuffer.java:105)
  at net.sourceforge.mlf.metouia.borders.MetouiaDots.&lt;i nit&gt;(MetouiaDots.java:66)
  at net.sourceforge.mlf.metouia.borders.MetouiaToolBar Border.&lt;init&gt;(MetouiaToolBarBorder.java:49)
  at net.sourceforge.mlf.metouia.MetouiaLookAndFeel.ini tComponentDefaults(MetouiaLookAndFeel.java:241)
  at javax.swing.plaf.basic.BasicLookAndFeel.getDefault s(BasicLookAndFeel.java:130)
  at javax.swing.plaf.metal.MetalLookAndFeel.getDefault s(MetalLookAndFeel.java:1591)
  at javax.swing.UIManager.setLookAndFeel(UIManager.jav a:537)
  at javax.swing.UIManager.setLookAndFeel(UIManager.jav a:581)
  at com.izforge.izpack.installer.GUIInstaller.loadLook AndFeel(GUIInstaller.java:373)
  at com.izforge.izpack.installer.GUIInstaller.&lt;init&gt;(G UIInstaller.java:116)
  at sun.reflect.NativeConstructorAccessorImpl.newInsta nce0(Native Method)
  at sun.reflect.NativeConstructorAccessorImpl.newInsta nce(NativeConstructorAccessorImpl.java:39)
  at sun.reflect.DelegatingConstructorAccessorImpl.newI nstance(DelegatingConstructorAccessorImpl.java:27)
  at java.lang.reflect.Constructor.newInstance(Construc tor.java:513)
  at java.lang.Class.newInstance0(Class.java:355)
  at java.lang.Class.newInstance(Class.java:30
  at com.izforge.izpack.installer.Installer.main(Instal ler.java:62)
  </code></pre>