Xkill is part of the X11 utilities pre-installed in Ubuntu and a tool for terminating misbehaving X clients or unresponsive programs. You can easily add a shortcut key to launch xkill with the steps below:
– Go to System > Preferences > Keyboard Shortcuts.
– Click the Add button to create a custom shortcut.
– Enter xkill to both the Name and Command boxes and click the Apply button.
– Click on Disabled at the xkill row in the Keyboard Shortcuts window (Disabled is then changed to New shortcut…).
– Press a new key combination, e.g. Ctrl+Alt+X (New shortcut… is then changed to Ctrl+Alt+X).
– Click the Close button.
Xkill is ready for use. Press the above key combination to turn the cursor to an X-sign, move the X-sign and drop it into a program interface to terminate the unresponsive program, or cancel the X-sign with a right-click.