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This was tested on 13.10+, type the following on the terminal:

sudo dpkg-reconfigure keyboard-configuration

You will see the keyboard configuration. Press ENTER 5 times to skip all keyboard related options except for the Kill X part. When you get to the Kill X option which looks like this:

Select YES and press enter. Wait a bit while everything configures and after that you should have the ability to CTRL+ALT+BCKSPC right on the current session. After saving any work you have pending, try it yourself.

Perfect to solve any visual issues or annoying apps.

For 13.10 and newer:

If you are looking for a solution which is independent of Gnome/KDE/Fluxbox or any Desktop Environment or Window Manager, try the following X Window System command. I need this because my Unity desktop is not loading (but X works fine), therefore there is no logout button.

From https://wiki.ubuntu.com/X/Config/DontZap#Using_the_command_line

Using the command line

You can type the following command to enable Zapping immediately.

setxkbmap -option terminate:ctrl_alt_bksp

If you're happy with the new
behaviour you can add that command to your ~/.xinitrc in order to make
the change permanent.

Also, according to the Ubuntu 10.04 LTS (Lucid Lynx), this has replaced the old DontZap feature from older versions of the X Window System.

In addition, the Ctrl+Alt+Backspace option is now configured as an X
keymap (XKB) option, replacing the X server "DontZap" option and
allowing per-user configuration of this setting.

This is explained further in the Xorg X11R7.5 Release Notes which, among other details, say:

Users who wish to have this functionality available by default may

enable it via the XKB configuration option “terminate:ctrl_alt_bksp”.

You can also use dconf-editor. This option will make the setting persistent across sessions.

sudo apt-get install dconf-editor

After starting the dconf-editor, navigate to org >> gnome >> desktop >> input-sources

Add the options that you need in xkb-options. The option strings are surrounded by single quotes and separated by commas. Be careful not to delete the brackets on the ends.

To enable ctrl+alt+backspace to kill the X-session, add 'terminate:ctrl_alt_bksp'

You can use this method to enter most of the traditional xkb options that are no longer available in System Settings >> Text Entry. The exceptions are the settings for switching the keyboard layouts, which currently do not work because of a bug.

For a list of the options and the syntax, use man 7 xkeyboard-config in a terminal.

To run the commands equivalent to using dconf-editor from a terminal, you use

gsettings set org.gnome.desktop.input-sources xkb-options "['terminate:ctrl_alt_bksp']"

Note the single quotes around the value, the square brackets around that, the double quotes around the whole thing. In other words, just like in dconf-editor.

Note that this will delete your other settings in xkb-options, if you have any.

You can get the existing settings with

gsettings get org.gnome.desktop.input-sources xkb-options

If you don't change these values very often, it might be simplest to combine existing settings with the new one by hand and the run gsettings set.

It can also be done all at once on the command line. I've put it in a short script here to make it easier to read.

#!/bin/bash

options=$(echo $(gsettings get org.gnome.desktop.input-sources xkb-options)|sed 's/]//g')", 'terminate:ctrl_alt_bksp']"

gsettings set org.gnome.desktop.input-sources xkb-options "$options"

The 2nd line gets the current values and concatenates the terminate... option. The sed command strips the right square bracket from the current values.

Note that while the argument string to the gsettings get command in the terminal needs double quotes around the value

"['val', 'val2']"

the argument string when run in a script should not have the double quotes.

['val', 'val2']

It was changed to Alt+Prt Scr+k.

I can't remember exactly when it was but it was supposed to stop accidental usage.

The idea being those who know they need it will be able to find the new key combination easily enough.

And I think someone at the time suggested it would be easier to remember as it is alternative screen kill. I am not sure that is the case.

The functionality is still there but under a different key combination.

(This is for Gnome Ubuntu, but I suppose it is the same for Unity --- please someone confirm this).

You should install gnome-tweak-tool (if not installed already) and run it. You will find the option you are looking for under "Typing":

If you feel more confortable with dconf-editor (or you do not want to install gnome-tweak-tool), you can open dconf-editor, navigate to org.gnome.desktop.input-sources, and then add the word "terminate:ctrl_alt_bksp" to the xkb-options property.

Note that the other properties in the list will be probably different for your keyboard.

You can still get back Ctrl+ Alt+Backspace shortcut to restart X by opening a terminal and copy paste the following:

sudo dpkg-reconfigure keyboard-configuration

This didn't appear to change anything at all and didn't activate the key combination.

In Kubuntu (KDE, plasma-framework version 5.23 on Kubuntu 16.04 Xenial) I found the option under K > System Settings > Input Devices > Keyboard > Advanced. Open the menu item "key sequence to kill X server" and check the only option "Ctrl + Alt + Backspace", apply.

Using strace I can see that this adds a line to /home/$USER/.config/kxkbrc which reads: Options=terminate:ctrl_alt_bksp.

The setxkbmap rule did work though and places a line reading terminate:ctrl_alt_bksp = +terminate(ctrl_alt_bksp) in the /usr/share/X11/xkb/rules/evdev file.