[HowTo] reboot / turn off your frozen computer: REISUB/REISUO

[jawanari]

@Fabby

First we have to unlock the SysRq key so:

• Add to your /etc/default/grub in the GRUB_CMDLINE_LINUX_DEFAULT parameter the sysrq_always_enabled=1 variable.
  XOR
• Execute echo kernel.sysrq=1 | sudo tee --append /etc/sysctl.d/99-sysctl.conf
  AND
• Execute sudo update-grub
  AND
• Reboot normally if you had to add one of these parameters and come back here to read the rest of the blurb…

If,
A = “Add to your /etc/default/grub . . .”
B = “Execute echo kernel.sysrq=1 | sudo tee --append /etc/sysctl.d/99-sysctl.conf”
C = “Execute sudo update-grub”
D = “Reboot normally”,
where do I have to put the parentheses in the ill-expressed statement of A XOR B AND C AND D? (Seriously asking).

Another part that I am not sure 100% understanding is in the key press sequences

@Fabby

• On full-size keyboards, it’s called by pressing Alt+SysRq
• On most laptop keyboards it’s called by pressing Fn+Alt+PrtSc
• On some keyboards, if none of the above Alt combinations work or the SysRq key is missing, please also try:
  • Alt+PrtSc
  • Alt Gr+PrtSc
  • Ctrl+Alt+PrtSc

Once you’ve located your SysRq key, please keep the Alt key pressed.

Now lightly tap these keys waiting between 1 second (fast, new machines) and 6 seconds (older or resource-starved machines¹) in-between keypresses : R E I S U B

am I supposed to press one of those Alt combinations and then release the keys other than the Alt, and with Alt key still pressed I should press the REISUB key sequences with the mentioned interval?