Magic Tricks With the Sysreq Key

Tip of the Trade: Remember the Sysreq key? It's in the same spot it's always been. Get reacquainted with it, and rediscover all the useful things it can do.


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Posted December 10, 2008

Juliet Kemp

Juliet Kemp

Juliet Kemp
The SysReq key is hidden up the top right of your keyboard, and looks a bit like a relic from a former era of computing. (Which, indeed, it is.) But there's a bunch of things you can still do with it that may come in handy. Note that this is documented only on i386 on Linux, and you need your kernel to have been compiled with the "Magic SysRq Key" option. Find out if it has been by looking at /proc/sys/kernel/sysrq — if it exists and contains the value 1, all possible requests are allowed.

Press Alt-Sysreq, then one of the following letters:

r Unraw: Restores the keyboard after an X crash or similar.
0 Changes console loglevel to 0 and so reduces error messages.
k System attention key: Kills all processes on the current virtual console.
e Terminate: Kills all processes except init on the current terminal.
i Kill: Kills all processes except init, everywhere.
s Sync: Attempts to sync all mounted filesystems. Outputs OK and Done when it's managed. This can reduce the chances of needing to run fsck at a later stage so it can be useful if you're having disk problems.
u Umount: Attempts to remount all mounted filesystems read-only.
b Reboot: immediately reboots the system without syncing or unmounting disks. Not a good idea unless in extremis! This may lead to data loss.
p Dumps current registers and flags to the console.
m Dumps current memory info to the console.

If this doesn't work, check the documentation at /usr/share/doc/linux-doc-2.6.18/Documentation/sysrq.txt.gz for some suggestions (including checking the keycode being sent).

This article was first published on ServerWatch.com.

Tags: Linux, data, IT, data loss

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