The key combination consists of Alt, SysRq and another key, which controls the command issued (as shown in the table below). Some keyboards may not provide a separate SysRq key. In this case, a separate "PrintScrn" key should be present. Under graphical environments (such as GNOME or KDE) 'Alt'+'PrintScrn/SysRq'+key combination generally only leads to a screenshot being dumped. To avoid this Print Screen feature the magic SysRq combination should include the Ctrl, becoming 'Ctrl'+'Alt'+'SysRq'+key. For the same purposes the AltGr key, if present, can be used in place of the Alt key. On some laptops SysRq is accessible only by pressing 'Fn'. In this case the combination is a bit trickier: hold 'Alt', hold 'Fn', hold 'SysRq', release 'Fn', press key.
|Set the console log level, which controls the types of kernel messages that are output to the console||0 through 9||0 through 9||0 through 9
(without using shift)
|0 through 9|
|Immediately reboot the system, without unmounting or syncing filesystems||b||x||b||b|
|Perform a system crash. A crashdump will be taken if it is configured.||c||j||c||c|
|Display all currently held Locks (CONFIG_LOCKDEP kernel option is required)||d||e||d||s|
|Send the SIGTERM signal to all processes except init (PID 1)||e||.||e||f|
|Call oom_kill, which kills a process to alleviate an OOM condition||f||u||f||t|
|When using Kernel Mode Setting, provides emergency support for switching back to the kernel's framebuffer console If the in-kernel debugger 'kdb' is present, enter the debugger.||g||i||g||d|
|Output a terse help document to the console
Any key which is not bound to a command should also perform this action
|Send the SIGKILL signal to all processes except init||i||c||i||u|
|Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.||j||h||j||n|
|Kill all processes on the current virtual console (Can be used to kill X and svgalib programs, see below)
This was originally designed to imitate a secure attention key
|Shows a stack backtrace for all active CPUs.||l||n||l||i|
|Output current memory information to the console||m||m||,||m|
|Reset the nice level of all high-priority and real-time tasks||n||b||n||k|
|Shut off the system||o||r||o||y|
|Output the current registers and flags to the console||p||l||p||;|
|Display all active high-resolution timers and clock sources.||q||'||a||q|
|Switch the keyboard from raw mode, the mode used by programs such as X11 and svgalib, to XLATE mode||r||p||r||p|
|Sync all mounted filesystems||s||o||s||r|
|Output a list of current tasks and their information to the console||t||y||t||g|
|Remount all mounted filesystems in read-only mode||u||g||u||l|
|Forcefully restores framebuffer console, except for ARM processors, where this key causes ETM buffer dump||v||k||v||v|
|Display list of blocked (D state) tasks||w||,||z||w|
|Used by xmon interface on PPC/PowerPC platforms.||x||q||x||x|
|Show global CPU registers (SPARC-64 specific)||y||f||y||j|
|Dump the ftrace buffer||z||;||w||z|
A common use of the magic SysRq key is to perform a safe reboot of a Linux computer which has otherwise locked up. This can prevent a fsck being required on reboot and gives some programs a chance to save emergency backups of unsaved work. The QWERTY mnemonic: "Reboot Even If System Utterly Broken" or simply the word "BUSIER" read backwards, are often used to remember the following Sysrq-keys sequence:
unRaw (take control of keyboard back from X), tErminate (send SIGTERM to all processes, allowing them to terminate gracefully), kIll (send SIGKILL to all processes, forcing them to terminate immediately), Sync (flush data to disk), Unmount (remount all filesystems read-only), reBoot.
- Hold down the Alt and SysRq (Print Screen) keys.
- While holding those down, type the following keys in order, several seconds apart: REISUB
- Computer should reboot.
In practice, each command may require a few seconds to complete, especially if feedback is unavailable from the screen due to a freeze or display corruption.
When magic SysRq keys are used to kill a frozen graphical program, the program has no chance to restore text mode. This can make everything unreadable. The commands
textmode (part of SVGAlib) and
reset can restore text mode and make the console readable again.
On distributions that do not include a
textmode executable, the key command 'Ctrl'+'Alt'+'F1' may sometimes to force a return to a text console. (Use 'F1', 'F2', 'F3', ..., 'F(n)', where 'n' is the highest number of text consoles set up by the distribution. 'Ctrl'+'Alt'+ 'F(n+1)' would normally be used to reenter GUI mode on a system on which the X server has not crashed.)