If you have ever suffered a kernel panic on a Linux system, only to reboot and find you need to fsck your harddrive, you will find these short cuts handy. I know I have needed something like this many times while messing around with Gentoo kernels and borked installs. If you have compiled your kernel with a yes to Magic SysRq key (CONFIG_MAGIC_SYSRQ) in the kernel .config file, you will have the ability to use these shortcuts.

By pressing Alt + SysReq + any of the keys below, you can perform any of the functions listed:

  • r - Turns off keyboard raw mode and sets it to XLATE.cc
  • k - Secure Access Key (SAK) Kills all programs on the current virtual console
  • b - Will immediately reboot the system without syncing or unmounting your disks
  • c - Will perform a kexec reboot in order to take a crashdump.
  • o - Will shut your system off (if configured and supported)
  • s - Will attempt to sync all mounted filesystems
  • u - Will attempt to remount all mounted filesystems read-only
  • p - Will dump the current registers and flags to your console
  • t - Will dump a list of current tasks and their information to your console
  • m - Will dump current memory info to your console
  • v - Dumps Voyager SMP processor info to your console
  • 0-9 - Sets the console log level, controlling which kernel messages will be printed to your console. (’0′, for example would make it so that only emergency messages like PANICs or OOPSes would make it to your console.)
  • f - Will call oom_kill to kill a memory hog process
  • e - Send a SIGTERM to all processes, except for init
  • i - Send a SIGKILL to all processes, except for init
  • l - Send a SIGKILL to all processes, INCLUDING init (Your system will be non-functional after this.)
  • h - Will display help ( actually any other key than those listed above will display help. but ‘h’ is easy to remember :-)

There are more suggestions in the official Linux kernel Documentation directory (that’s where this came from). See /usr/src/linux/Documentation/sysrq.txt for more information on this handy kernel feature.

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