2010. 11. 29
Magic SysRq
Marc from our hosting provider pointed me to /proc/sysrq-trigger. Useful if something’s wedged, and you can’t get a box to reboot in the normal way.
This page has more details, as it doesn’t seem to be documented in a normal man page.
The capabilities should be documented in “man proc”, but (at least on my system) are not. The “sysrq” is mentioned, but not documented. You can find documentation in /usr/src/linux-2.4.29/Documentation/sysrq.txt if you have source installed. This is from that file:
'r' - Turns off keyboard raw mode and sets it to XLATE.
'k' - Secure Access Key (SAK) Kills all programs on the
current virtual console. NOTE: See important comments
below in SAK section.
'b' - Will immediately reboot the system without syncing
or unmounting your disks.
'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.
'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.)
'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 :-)
The key is also self-documenting: doing Alt-Sysrq-h (or any other unrecognized key) prints out a HELP message that briefly reminds you of the above functions. If you find that holding three keys is difficult, you do have some other choices. You can trigger it manually:
echo t > /proc/sysrq-trigger
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