The magic SysRq key is a key combination understood by the Linux kernel, 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 a computer without corrupting the filesystem. Its effect is similar to the computer's hardware reset button (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. 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 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 keyboard layout; for example, on the Dvorak keyboard layout, 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 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