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I had a device running Ubuntu 12.04 LTS have its screen freeze. Generally I have simply jumped onto an available tty, i.e. Ctrl-Alt-F1 and executed the command /etc/init.d/lightdm restart or service lightdm restart. This however kills everything I have open. How do I avoid this?

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    You can't achieve this. It is lightdm which displays/manages windows in x system. – Apple II Mar 23 '13 at 1:46
  • This will also kill whatever you have open, but once upon a time ctrl-alt-backspace would restart X. That's easier than switching to a TTY, logging in, and asking X to restart itself. I'm not sure if it still works though. – user162316 Mar 23 '13 at 3:46
  • @Douglas B. Staple - It doesn't work with Ubuntu 12.04 LTS. Not sure whether it still works with other distributions or whether this is specific to Unity. – PeanutsMonkey Mar 23 '13 at 7:11
  • There may be some way to replace the window manager while running. (It's possible with the GNOME-Shell at least, or with the i3 window manager so there could be some way). – FSMaxB Mar 24 '13 at 8:51
  • @FSMaxB - Could you please elaborate on how this can be achieved using the Gnome-Shell or i3 Window Manager? – PeanutsMonkey Mar 26 '13 at 17:59
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DISCLAIMER: According to Brenden's comment, this might be outdated and not work anymore with newer versions of Ubuntu. Also please be prepared that you might lose your open windows.

Now I got it for Unity (I just replaced my GNOME shell with unity without logging out or closing windows):

  1. Go to tty1 [Ctrl]+[Alt]+[F1]
  2. Type in DISPLAY=:0 so that programs know on what port to find the X-Server
  3. export DISPLAY
  4. unity --replace or gnome-shell --replace (you can also replace GNOME-Shell by unity or Unity by the GNOME-Shell)

This only works if not the X-Server or GNOME is frozen but only Unity or the GNOME-Shell.

How it works: The communication between programs and the X-Server for displaying their UI is working over a network socket. This normally is at "localhost:0" short ":0". By setting the DISPLAY variable, the programs know where to find the X-Server to communicate with it, so programs are displayed on the X-Server on tty7 even if the program is running on tty1.

The content of windows is managed by the X-Server whereas the position and borders of the windows is managed by the window manager integrated in unity or the GNOM-shell so there is a way to replace only them without restarting the applications on the X-Server.

PS: I'm just describing what I understand it to be, so please edit and correct me if I'm wrong in some points.

EDIT: For the GNOME-Shell, there is another way to restart it. After changing to tty simply type in pkill -HUP gnome-shell, I don't know if there's something similar for Unity.

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  • That worked like a miracle. In fact it saved me today. Thanks so much. I would be so grateful if you could elaborate what each of the steps do bar step 1. I don't quite understand what is happening under the hood. – PeanutsMonkey Apr 1 '13 at 4:56
  • Thanks for answer! Note unity --help says: "--replace Run unity /!\ This is for compatibility with other desktop interfaces and acts the same as running unity without --replace" – Tom Jul 13 '15 at 19:59
  • Also I found I had to run unity via sudo to save my windows. – Tom Jul 13 '15 at 20:00
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    --replace is deprecated and does nothing and this solution will restart your session meaning you lose all of your windows :( – Brenden Aug 5 '15 at 15:23
  • Still works in 2018 – user000001 Apr 29 '18 at 14:51

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