How can I know what keys will invoke the magic SysRq key? Provided I am in front of a Linux machine which is not frozen (yet). I need something easier than trying all common combinations of magic SysRq key to discover if it's working.

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The magic SysRq key itself is AltSysRq or AltPrint Screen on PCs, which is in turn combined with a third key (letter), indicating some specific action.

The short version is that, at a console, AltSysRqSpace will display the available shortcuts (thanks to Josip Rodin for pointing that out).

The magic SysRq key, if enabled, is controlled by /proc/sys/kernel/sysrq; you can determine if it's enabled and what functions are allowed by running

cat /proc/sys/kernel/sysrq

If this complains that there's no such file or directory, then magic SysRq isn't enabled at all. Otherwise it will show a number which determines the available functions:

  • 0: disable SysRq completely
  • 1: enable all functions
  • any other value is a bitmask of allowed functions:
    • 2: control console logging level
    • 4: control keyboard
    • 8: process debug dumping
    • 16: sync
    • 32: read-only remounts
    • 64: process signalling
    • 128: reboot/power-off
    • 256: nicing of all RT tasks

If the system is operational as far as having a working root shell, you can also invoke a magic SysRq function by writing its letter to /proc/sysrq-trigger; e.g.

echo s > /proc/sysrq-trigger

will perform an emergency sync.

This is all detailed in the kernel documentation.

  • This is a nice answer; however, what about non-standard keyboards that don't have either a 'SysRq' or 'Print Screen' key? What would happen if I were to unplug a keyboard that does have a SysRq key and plug in one that doesn't? Ref. new question I have asked here.
    – Time4Tea
    Dec 19, 2021 at 0:06

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