Sunday, February 1, 2009

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.

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.
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.
Ctrl+Alt+Esc usually do the thrick for me ;)

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