BIOS - EFI HD Boot Sector Boot Loader(GRUB2)

Kernel
Initramfs (add driver kernel)
  Systemd

systemd systemctl poweroff systemctl reboot systemctl hibernate

legacy

poweroff shutdown reboot

echo b > /proc/sysrq-trigger

• https://matoski.com/article/emergency-reboot-shutdown-linux-sysrq/

ls /etc/default/grub

grub2-mkconfig -o /boot/grub/grub.cfg grub2-mkconfig -o /boot/efi/EFI/xxx/grub/grub.cfg

cat /boot/grub/grub.cfg

systemd.unit=rescue.target systemd.unit=emergency.target