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The Ctrl-Alt-Backspace key combination currently "zaps" (hard-restarts) the X server, and thus loses any unsaved data in applications, etc. This key combination is also largely undocumented, so users (probably ex-Windows users) may press this key combination without expecting data loss. This spec proposes to follow upstream's lead and disable this key combination by default in order to prevent this usability issue from occurring in normal installs.

Release Note

The key combination of Ctrl-Alt-Backspace, which previously immediately exited X (and thus reset the graphics system back to the login screen), has been changed to disabled by default. It can be re-enabled by setting the DontZap xorg.conf option to False or by using the AltGr-SysReq-K key combination instead. This should help reduce the frequency of unexpected data loss and activity interruption for users unaware of its function that strike it accidentally.


The key combination of Ctrl-Alt-Backspace terminates the user's current graphical shell, which can cause data loss if invoked accidentally. For some keyboards, backspace is in a location that can lead to accidentally hitting Ctrl-Alt-Backspace when using other Ctrl-Alt- key combinations. This can cause unexpected data loss and workflow interruption. It could also be seen as a form of local denial of service attack when people have access to the keyboard.

Upstream has recently disabled this key combination by default, for many of the same reasons above. It makes sense for Ubuntu to follow their lead on this change.

Historically, it has been one mechanism for users to exit/restart X. Of course, there are other ways to restart. For "normal" logouts, the logout button on the panel or the ctrl-alt-delete combo is the preferred mechanism, as unlike ctrl-alt-backspace they are designed to shut down apps nicely and give the user several options on how to logout (hibernate, suspend, etc.) Ctrl-alt-delete includes a timeout mechanism so can work even if the display is not visible.

Another use case for ctrl-alt-backspace is to restart a locked up X session. In many of these situations, ctrl-alt-delete will work as well. In some situations where ctrl-alt-delete won't work, neither will ctrl-alt-backspace, and so the power button will be the only recourse. For many non-technical users, the power button will be the most obvious mechanism anyway.

Some users will not prefer this change. For these users, the DontZap option is available in xorg.conf to re-enable it.

Use Cases

  • Brian is a relatively new Ubuntu user working on a large image file in the GIMP, but also has Blender open on a different workspace which he used to render the image. Brian wants to close Blender as he is finished with it and needs to free up some memory for the GIMP. He remembers in Windows he could bring up an application list via Ctrl-Alt-Delete, but that doesn't work so he tries pressing Ctrl-Alt-Backspace. The key combination does nothing so he decides to open the System Monitor from the System > Administration menu instead. Once Blender is closed he continues to work on his image in the GIMP, saving it when he is finished.

  • Elisee presses ctrl + alt + left or right to change from one workspace to another, and realizes that he wants to erase what he was writing on his IM program just before switching workspace, and accidentally hits backspace before releasing Ctrl and Alt. Thankfully, this didn't result in X exiting as it used to. He breaths a sigh of relief.
  • Joel's computer interface has frozen due to a graphics driver problem. He still has daemons running which need to remain open, so he cannot reboot. He remembered that Ctrl-Alt-Backspace would restart X, and is surprised that it doesn't restart immediately. Instead, he switches to a terminal console and restarts the gdm process and is back in business. After reading up on the change, he decides to re-enable DontZap in his xorg.conf to make things work the way he's accustomed to in the future.


This spec assumes that users do not use the Ctrl-Alt-Backspace functionality regularly, and that those who do will be able to change it back on in xorg.conf.


The DontZap option is changed to be True by default.


1. We simply merge the upstream xserver codebase, with no Ubuntu changes required.

2. We search through our documentation for any instances where we are using ctrl+alt+backspace. We shouldn't be recommending this anywhere.

3. As part of the Xorg.conf Options Editor specification, GUI tools will be implemented that permit re-enabling this. Tools must be available for both GNOME and KDE.

Test/Demo Plan

To test the implementation the keypress should be held, and X should not restart.


Previous discussion has been taken into account with this version of the spec, and is archived at XorgCtrlAltBackspace/Discussion.

See also:


For people coming here because they are missing the nice functionality of being able to kill the X server in case nothing else works anymore in the GUI, it might be interesting to know that x can now be killed with "AltGr" - "SysReq" - K (AltGr is the right Alt key. SysReq also labeled "Print, or "Druck" in Germany)! Found here.


XorgCtrlAltBackspace (last edited 2010-06-12 04:57:24 by dhcp198-158)