FOSSwire has an interesting article explaining how to get back on your Linux box when it appears to be frozen without having to resort to the power button.
- Hold down the Alt and SysRq (Print Screen) keys.
- While holding those down, type the following in order. Nothing will appear to happen until the last letter is pressed: REISUB
- Watch your computer reboot magically.
“Raising Elephants Is So Utterly Bonkers” is a way of remembering the keystroke sequence.
The key combination consists of Alt, SysRq and another key, which controls the command issued:
- 0 through 9 sets the console log level, which controls the types of kernel messages that are output to the console.
- b immediately reboots the system, without unmounting partitions or syncing.
- c reboots kexec and outputs a crashdump.
- e sends the SIGTERM signal to all processes except init (PID 1).
- f calls oom_kill, which will kill a process that is consuming all available memory.
- h, or any key which is not bound to a command, outputs a terse help document to the console.
- i sends the SIGKILL signal to all processes except init.
- k kills all processes on the current virtual console (and thus can be used to kill X and svgalib programs, see below). This was originally designed to imitate a Secure Access Key.
- l sends the SIGKILL signal to all processes, including init.
- m outputs current memory information to the console.
- o shuts off the system.
- p outputs the current registers and flags to the console.
- r switches the keyboard from raw mode, the mode used by programs such as X11 and svgalib, to XLATE mode.
- s attempts to sync all mounted filesystems.
- t outputs a list of current tasks and their information to the console.
- u attempts to remount all mounted filesystems in read-only mode.
- v outputs Voyager SMP processor information.