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I have the situation that my Ubuntu session is frozen such that I cannot move my mouse or type. I managed to stop all heavy tasks by accessing a tty with Ctrl+Alt+F2, so technically it should recover to a usable state.

However, the screen is still frozen, so I suspect it would be a good idea to force compiz to restart.

It is safe to kill compiz and then restart it without any of my applications/terminals to be killed?

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If it's frozen anyway, you could always try. (worst advice ever probably XD) – Dr_Bunsen Jan 7 '13 at 18:40
@Dr_Bunsen indeed: worst advice ever :D – stefan Jan 7 '13 at 18:40
up vote 4 down vote accepted

The way I see it Compiz is just a Windows manager so doesn't interact with your running applications other than providing a graphical structure within which to run them as it is a window manager.

I know its not the most reliable source but citing Wikipedia Window Management:

A window manager is system software that controls the placement and appearance of windows within a windowing system in a graphical user interface.

And more importantly:

Elements usually associated with window managers allow the user to open, close, minimize, maximize, move, resize, and keep track of running windows, including window decorators.

This last phrase explains that the options you have with windows managers running are to close, minimize etc. The way I see it, by restarting Compiz you won't tell your current applications to do anything, you haven't given them a specific command by clicking on any of the window manager icons so the applications have no reason to act differently than if you left your PC for a few seconds. You'll just have them idle until they are put back into a window manager (Compiz in your case).

So yes it is safe to kill and restart compiz

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perfect, thank you for assuring me on that. In the end, it actually did turn out to be necessary since it suddenly restored after three times of hibernating. But it's good to know that it would actually work. – stefan Jan 7 '13 at 22:13

It is perfectly fine to kill (-9) compiz. It will just restart compiz, and all your applications will go to the first work space. No applications will be affected and if you experience problems like the window borders disappearing you can usually change stuff in CCSM.

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