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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.
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.
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. |
|
Recent Tips » tcpdump » superblock » Further Notes on proc Read All Tips of the Trade |
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).
Juliet Kemp is a ServerWatch contributor.
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