This is useful for trying to recover from an unresponsive system. However, by default, Ubuntu disables the key to gain raw keyboard access.\
Sometimes part of the issue is that X/Wayland display servers have crashed due to bad graphics drivers and keyboard input isn't processed anymore, whereby changing to another console, e.g. ctrl + alt + F1 doesn't work. Since the default disables keyboard control, alt + sysrq + r, can't help salvage the situation. Hence the suggestion to at least enable system request raw keyboard control when trying to recover a system.
Ubuntu 16.04 LTS default value:
cat /proc/sys/kernel/sysrq
176Sysrq bit encoded values:
| Bit | Bit value | Decimal | Function | Default | Wanted |
|---|---|---|---|---|---|
| 0000 0000 | 0 | disable sysrq completely | |||
| 1 | 0000 0001 | 1 | enable all functions of sysrq | ||
| 2 | 0000 0010 | 2 | enable control of console logging level | y | |
| 3 | 0000 0100 | 4 | enable control of keyboard (SAK, unraw) | y | |
| 4 | 0000 1000 | 8 | enable debugging dumps of processes etc. | ||
| 5 | 0001 0000 | 16 | enable sync command | y | |
| 6 | 0010 0000 | 32 | enable remount read-only | y | |
| 7 | 0100 0000 | 64 | enable signalling of processes (term, kill, oom-kill) | y | |
| 8 | 1000 0000 | 128 | allow reboot/poweroff | y | |
| 9 | 0001 0000 0000 | 256 | allow nicing of all RT tasks |
176 default decoded = 1011 0000 = 128 + 32 + 16. Notice 3rd bit (decimal 4) isn't included.
The default was set via /etc/sysctl.d/10-magic-sysrq.conf
Note, some functions like being able to dump memory to a console (m sysrq key) could expose sensitive data, so it's a low-risk local access issue.
Swith to a console, login and try invoke sysreq help via alt + sysrq 8 and then alt + sysrq + h. Something like SysRq: HELP should emit to the console if it's working.
Run showkey -k while attempting the sysreq.
Gnome shell uses this shorcut to take a screenshot of the currently active window and save it to a PNG file in pictures. Rather remap or disble this shortcut to avoid conflicts with SysRq, given alt + printscreen is the
Also, `ctrl+alt+print screen
I chose to have some extra features enabled:
sudo bash -c "echo 'kernel.sysrq = 246' > /etc/sysctl.d/98-magic-sysrq-cust.conf"246 = 11110110 which added keyboard control, signalling and debugging dumps.
The sysrq key is typically mapped to the print-screen key and alt + sysrq (print-screen) should invoke it. One alt + sysrq is pressed in combination, they can be released before pressing the system command key that follows.
When pressing ther e i s u b keys, take a fair time to pause between the e and i to allow all processes time to gracefully exit where possible:
r: Turns off keyboard raw mode and sets it to XLATE.e: Send a SIGTERM to all processes, except for init.i: Send a SIGKILL to all processes, except for init.s: Will attempt to sync all mounted filesystems.u: Will attempt to remount all mounted filesystems read-only.b: Will immediately reboot the system without syncing or unmounting your disks.
"Reboot Even If System Utterly Broken" ("BUSIER" backwards).
If after requesting raw access via r doesn't seem to allow switching consoles, as per kernel docs, k might help:
useful when you want to exit a program that will not let you switch consoles. (For example, X or a svgalib program.)
The description for k:
Secure Access Key (SAK) Kills all programs on the current virtual console.
Another key to try if X/Wayland/GPU drivers have caused a non-responsive is v:
Forcefully restores framebuffer console.
In some cases a full reboot via r e i s u b might be unnecessary if alt + sysrq and r allows switching and logging into another console (e.g. ctrl + alt + F1). Commands can be run to kill and restart display managers or use systemd to try reboot instead of sending SIGTERM to every process at once.
Also note, keys 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.)
alt + sysrq and 8 was recommend because many distros boot in quiet mode not showing kernel console output.