The magic SysRq key is a key combination understood by the

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, which allows the user to perform various low-level commands regardless of the system's state. It is often used to recover from freezes, or to

reboot In computing, rebooting is the process by which a running computer system is restarted, either intentionally or unintentionally. Reboots can be either a cold reboot (alternatively known as a hard reboot) in which the power to the system is physi ...

a computer without corrupting the

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. Its effect is similar to the computer's hardware

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(or power switch) but with many more options and much more control. This key combination provides access to powerful features for software development and disaster recovery. In this sense, it can be considered a form of

escape sequence In computer science, an escape sequence is a combination of characters that has a meaning other than the literal characters contained therein; it is marked by one or more preceding (and possibly terminating) characters. Examples * In C and man ...

. Principal among the offered commands are means to forcibly unmount file systems, kill processes, recover keyboard state, and write unwritten data to disk. With respect to these tasks, this feature serves as a tool of last resort. The magic SysRq key cannot work under certain conditions, such as a

kernel panic A kernel panic (sometimes abbreviated as KP) is a safety measure taken by an operating system's kernel upon detecting an internal fatal error in which either it is unable to safely recover or continuing to run the system would have a higher ...

or a hardware failure preventing the kernel from running properly.

Commands

The key combination consists of and another key, which controls the command issued. may be released before pressing the command key, as long as remains held down. The combinations always assume the

QWERTY QWERTY () is a keyboard layout for Latin-script alphabets. The name comes from the order of the first six Computer keyboard keys#Types, keys on the top left letter row of the keyboard ( ). The QWERTY design is based on a layout created f ...

keyboard layout; for example, on the

Dvorak keyboard layout Dvorak is a keyboard layout for English patented in 1936 by August Dvorak and his brother-in-law, William Dealey, as a faster and more ergonomic alternative to the QWERTY layout (the ''de facto'' standard keyboard layout). Dvorak proponent ...

, the combination to shut down the system uses the key instead of . Furthermore, some keyboards may not provide a separate key. In this case, a separate key should be present. On some devices, notably laptops, the key may need to be pressed to use the magic key, e.g. on Thinkpad Carbon X1 the is activated by pressing simultaneously, then releasing and while still holding
see here
. On a ChromeOS device press . SysRq functionalities can also be accessed without a keyboard. See below. Note that some commands may be disabled out of the box as specified in the

bitmask In computer science, a mask or bitmask is data that is used for bitwise operations, particularly in a bit field. Using a mask, multiple bits in a byte, nibble, word, etc. can be set either on or off, or inverted from on to off (or vice versa) in ...

value in /proc/sys/kernel/sysrq. Output of the on a x86_64 kernel: sysrq: HELP : loglevel(0-9) reboot(b) crash(c) terminate-all-tasks(e) memory-full-oom-kill(f) kill-all-tasks(i) thaw-filesystems(j) sak(k) show-backtrace-all-active-cpus(l) show-memory-usage(m) nice-all-RT-tasks(n) poweroff(o) show-registers(p) show-all-timers(q) unraw(r) sync(s) show-task-states(t) unmount(u) force-fb(v) show-blocked-tasks(w) dump-ftrace-buffer(z)

Uses

Before the advent of journaled filesystems a common use of the magic SysRq key was to perform a safe reboot of a Linux computer which has otherwise locked up (abbr. REISUB), which avoided a risk of filesystem corruption. With modern filesystems, this practice is not encouraged, offering no upsides over straight reoot, though the default value of ''kernel.sysrq'' in such distributions as Ubuntu and Debian remains 176 and 438 respectively. Another past use was to kill a frozen

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program, as the

X Window System The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems. X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting wit ...

used to have complete control over graphical mode and input devices. On distributions that do not include a command, the key command may sometimes be able to force a return to a text console. (Use , , , ... , , where is the highest number of text consoles set up by the distribution. would normally be used to reenter GUI mode on a system on which the X server has not crashed.) There are also distributions where + (, ... ,) opens text consoles, reenter GUI mode, reenter GUI mode on the login page, in order to log in another user.

Configuration

The feature is controlled both by a

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-time option in the kernel configuration, , and a

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kernel parameter, . On newer kernels (since 2.6.12), it is possible to have more fine-grained control over how the magic SysRq key can be used. On these machines, the number written to can be 0, 1, or a number greater than 1 which is a bitmask indicating which features to allow. On Ubuntu this is set at boot time to the value defined in .

Other ways to invoke Magic SysRq

While the magic SysRq key was originally implemented as part of the kernel's keyboard handler for debugging, the functionality has been also exposed via the proc filesystem and is commonly used to provide extended management capabilities to headless and remote systems. From user space programs (such as a command line shell), SysRq may be accessed by writing to (e.g., ). Many embedded systems have no attached keyboard, but instead use a serial console for text input/output to the running system. It is possible to invoke a Magic SysRq feature over a serial console by sending a serial break signal, followed by the desired key. The method of sending a break is dependent on the terminal program or hardware used to connect to the serial console. A

option needs to be set to enable this function. The Linux daemons and provide a method of accessing SysRq features over a TCP connection after authenticating with a plain-text password. The daemon will invoke pre-configured SysRq triggers when system load average exceeds a certain threshold. The

Xen Xen (pronounced ) is a type-1 hypervisor, providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently. It was originally developed by the University of Cambridge Computer Laboratory an ...

hypervisor has functionality to send magic commands to hosted domains via its command. Additionally, a SysRq command can be invoked from a Xen paravirtual console by sending a break sequence followed by the desired key.

Chromebook A Chromebook (sometimes stylized in lowercase as chromebook) is a laptop or tablet running the Linux-based ChromeOS as its operating system. Initially designed to heavily rely on web applications for tasks using the Google Chrome browser, Chromeb ...

s have a keyboard but no dedicated SysRq key. They use () instead, however some keys have a different function. IBM

Power Systems An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The e ...

servers can invoke the Magic SysRq feature using followed by the desired key from the

Hardware Management Console {{Refimprove, date=October 2020 Hardware Management Console (HMC) is a Physical / Virtual appliance used to manage IBM Systems including IBM AS/400, IBM System p, IBM System z, and IBM Power Systems. HMC supports command line (ssh) as well as web ...

. IBM mainframe partitions can invoke the Magic SysRq feature using followed by the desired key on 3270 or HMC console.

References

External links