linux "magic sysrq key" cheat sheet

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10:43am EDT, 18 Apr 2004

Most people don't know that Linux has a "magic sysrq key" that lets you get out of certain Really Bad Almost-Crash situations, or at least shut down a little more gracefully than hitting the power switch. There are a dozen or so key combinations you can use to sync discs, force a reboot, get keyboard control back when X crashes, etc. Your Linux distributor may have disabled them for security or idiot-proofing and you have to be at the physical console, since they are handled at the kernel level. That said, they work in Mandrake by default and I'm tired of looking for a cheat sheet and seeing "They are dangerous! Don't use them! But here are a few I know offhand…"

Naturally, if you do any of these things and you get screwed, don't come running to me. This is all at your own risk.

The first thing you need to know is that the "Magic SysRq Key" is actually two keys held down at the same time: Alt and SysRq (otherwise known as the Print Screen key.) Hold them both down at the same time as though it were something like Control and Alt, and press one of the following keys to have something dangerous (or possibly data-saving) happen:

b - Force an immediate, graceless reboot.
e - Kill (gracefully) all running processes except init.
i - Kill (-9) all running processes except init.
k - Kill all processes running in the current virtual terminal.
l - Kill all running processes INCLUDING init.
o - Attempt to shut computer off (as in power down.)
r - Turn off raw keys mode (i.e. recover from crashed X.)
s - Sync all filesystems.
u - Attempt to remount root filesystem as read-only.

The following are only useful from a text console:

0-9 - Set kernel logging to the specified level.
m - Dump memory usage info to console.
p - Dump register info to console.
t - Dump info on running tasks to console.
Any other key - dump a terse summary of these actions to the console.