Save ways to exit with system freeze

In Linux, when your entire system freezes, there are plenty of safe ways to exit and get back on track immediately. Alt + SysRq + (a selection of other keys) will do the magic trick. Note: 'SysRq' key is equivalent to the 'Print Screen' key.

Alt + SysR + K
Kill all processes (including X), which are running on the currently active virtual console.

Alt + SysRq + E
Send the TERM signal to all running processes except init, asking them to exit.

Alt + SysRq + I
Send the KILL signal to all running processes except init.

Alt + SysRq + L
Send the KILL signal to all processes, including init.

Alt + SysRq + S
Run an emergency sync (cache write) on all mounted filesystems. This can prevent data loss.

Alt + SysRq + U
Remount all mounted filesystems as read-only. This has the same effect as the sync combination above, but with one important benefit: if the operation is successful, fsck won't have to check all filesystems after a computer hardware reset.

Alt + SysRq + R
Turn off keyboard raw mode. This can be useful when your X session hangs. After issueing this command you may be able to use .

Alt + SysRq + B
Reboot immediately without syncing or unmounting your disks. Using this, you will likely end up with filesystem errors, so this is not highly recommended.

Alt + SysRq + O
Shut the system off right away.

If all these ‘Alt + SysRq’ keyboard combo are hard to remember, you can press Alt + SysRq + H to display a helpful list of the shortcuts above.

http://www.junauza.com/2009/01/linux-keyboard-shortcuts-to-exit-safely.html

Useful comments:
The first thing you should try in something hangs, is just Alt-SysRq-K (or Ctrl-Shift-Backspace while in a graphic X environment, which is a little less brutal). But mostly plain patience will do the trick.

A great way to use these keys is holding down Alt-SysRq while slowly typing REISUB (BUSIER backwards). This will reboot your system in a safe way (just look at the explanations above).

Also note: Alt-SysRq-H only helps you out in the command line.
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If you're having the screenshot (screen capture) try this; it worked for me in Ubuntu:
add this line to the end of /etc/sysctl.conf

kernel.sysrq = 1

then reboot.

For it to work properly, you need to press alt+sysrq simultaneously, then press your exit code (eg k) while holding the other keys down.
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Ctrl+Alt+Esc usually do the thrick for me ;)

Essential skills: best keyboard shortcuts Gnome

1. Switch to the next/previous workspace

If you make use of the workspace very frequently, you can easily switch between different workspaces by pressing Ctrl + Alt + Left/Right Arrow. The Left key brings you to the previous workspace while the Right key brings you to the next adjacent. If you have enabled Compiz, you can even get it to show all the workspace by pressing Super + E on the keyboard.

2. Move the current window to another workspace

By pressing Shift+ Ctrl + Alt + Left/Right Arrow, you can easily move your current window to another workspace in the specified direction. This keyboard shortcut works very well with the one mentioned above. If you have the habit of opening many applications/windows when doing your work, but don’t like to see your desktop and menubar cluttered with all the application windows, you can use this shortcut key to move your applications to another workspace and get your desktop organized.

3. Show the desktop

Ctrl + Alt + D enables you to quickly minimize all windows and give focus to the desktop. When all windows are minimized, this shortcut can also maximize all the windows to their previous state.

4. Keyboard shortcut for the mouse right-click

In most applications, you can always right-click on the mouse to access the options menu. On the keyboard, you can simply press Shift + F10 to achieve this ‘right-click‘ effect

5. Restart session and recover from crashes

There are very few instances where Ubuntu will crash totally. But if it does, you can press Ctrl + Alt + Backspace to restart the session, and 90% of the time, it will recover from the crashes.

6. Lock the screen quickly

If you need to leave your workstation for a while, you can quickly lock up your screen by pressing Ctrl + Alt + L and prevent unauthorized access by others.

7. Switch between windows in the reverse direction

Alt + Tab is a common shortcut key that allow you to switch between open windows. But do you know that by including the ‘Shift‘ button, you can reverse the windows switching direction? This is useful when you press Alt + Tab too fast and passed the window that you want to switch to. Simply press down the ‘Shift‘ button to go back to the previous window in the switch cycle.

8. Move windows with arrow keys

Press Alt+F7 to activate the Move window function and use any arrows key (up, down, left, right) to move the window around the screen.

9. Show hidden files

Most of the time, you won’t need to view the hidden files in your home folder, but in the event that you need to, you can press Ctrl + H inside the Nautilus (the file manager for Ubuntu) to show all hidden files.

10. Show file properties without right-clicking the mouse

The conventional way to view a file/folder properties is to right-click the mouse and select ‘Properties‘. Now you can just press Alt + Enter to get the Properties window to appear.

source: http://www.makeuseof.com/tag/10-useful-ubuntu-keyboard-shortcuts-that-you-might-not-know-of/