sysreq stuff: second update

From: Adam (
Date: Tue Mar 14 2000 - 19:48:46 EST

Hello, here's second update to sysrq.txt file.
I have incorporated comments I have received

        I have incorporated comments about SAK not being
        true c2-style SAK thing.

On a side note, I was thinking to change sysrq stuff so that
sak is separate. You would use :


to activate it. The values you could echo-in would be :

        0 - SAK disabled

        1 - SAK works as it is now,
                  * lets me logout someone even if they X locked screen,
                  * lets me make sure console was not trojanized.

        2 - c2-style SAK. Should not kill X, work only
                  if at console
                  .. not implemented right now.

The sysrq files attached below:


                      MAGIC SYSRQ KEY DOCUMENTATION v1.31
                        [Mon Mar 13 21:45:48 EST 2000]

* What is the magic SysRQ key?
It is a 'magical' key combo you can hit which kernel will respond to
regardless of whatever else it is doing, unless it is completely locked up.

* How do I enable the magic SysRQ key?
You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
configuring the kernel. This option is only available in 2.1.x or later
kernels. Once you boot the new kernel, you need to enable it manually
using following command:

         echo "1" > /proc/sys/kernel/sysrq

* How do I use the magic SysRQ key?
On x86 - You press the key combo 'ALT-SysRQ-<command key>'. Note - Some
           (older?) may not have a key labeled 'SysRQ'. The 'SysRQ' key is
           also known as the 'Print Screen' key.

On SPARC - You press 'ALT-STOP-<command key>', I believe.

On other - If you know of the key combos for other architectures, please
           let me know so I can add them to this section.

* What are the 'command' keys?
'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

'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 :-)

* Okay, so what can I use them for?
Well, un'R'aw is very handy when your X server or a svgalib program crashes.

sa'K' (Secure Access Key) is usefull when you want to be sure there are no
trojan program is running at console and which could grab your password
when you would try to login. It will kill all programs on given console
and thus letting you make sure that the login prompt you see is actually
the one from init, not some trojan program.
IMPORTANT:In its true form it is not a true SAK like the one in :IMPORTANT
IMPORTATN:c2 compliant systems, and it should be mistook as such. :IMPORTANT
        It seems other find it usefull as (System Attention Key) which is
useful when you want to exit a program that will not let you switch consoles.
(For example, X or a svgalib program.)

re'B'oot is good when you're unable to shut down. But you should also 'S'ync
and 'U'mount first.

'S'ync is great when your system is locked up, it allows you to sync your
disks and will certainly lessen the chance of data loss and fscking. Note
that the sync hasn't taken place until you see the "OK" and "Done" appear
on the screen. (If the kernel is really in strife, you may not ever get the
OK or Done message...)

'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
Again, the unmount (remount read-only) hasn't taken place until you see the
"OK" and "Done" message appear on the screen.

The loglevel'0'-'9' is useful when your console is being flooded with
kernel messages you do not want to see. Setting '0' will prevent all but
the most urgent kernel messages from reaching your console. (They will
still be logged if syslogd/klogd are alive, though.)

t'E'rm and k'I'll are useful if you have some sort of runaway process you
are unable to kill any other way, especially if it's spawning other

* Sometimes SysRQ seems to get 'stuck' after using it, what can I do?
That happens to me, also. I've found that tapping shift, alt, and control
on both sides of the keyboard, and hitting an invalid sysrq sequence again
will fix the problem. (ie, something like alt-sysrq-z). Switching to another
virtual console (ALT+Fn) and then back again should also help.

* I hit SysRQ, but nothing seems to happen, what's wrong?
There are some keyboards which do not support 'SysRQ', you can try running
'showkey -s' and pressing SysRQ or alt-SysRQ to see if it generates any
0x54 codes. If it doesn't, you may define the magic sysrq sequence to a
different key. Find the keycode with showkey, and change the define of
'#define SYSRQ_KEY 0x54' in [/usr/src/linux/]include/asm/keyboard.h to
the keycode of the key you wish to use, then recompile. Oh, and by the way,
you exit 'showkey' by not typing anything for ten seconds.

* I have more questions, who can I ask?
You may feel free to send email to, and I will
respond as soon as possible.

* Credits
Written by Mydraal <>
Updated by Adam Sulmicki <>

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