The magic SysRq key provides a way to instruct the kernel through the
/proc pseudo-filesystem. It must be enabled with the kernel during compile time with the
CONFIG_MAGIC_SYSRQ option and is standard for most distributions.
Suppose you have a system that you can SSH into but cannot reboot because the system disk is dying or the SATA bus is in some weird state and no IO is actually working.
# reboot bash: /sbin/reboot: Input/output error # shutdown -r now bash: /sbin/shutdown: Input/output error
Or perhaps the system disk is fine, but
sync() isn't returning which causes
reboot to hang even with the
To reboot the machine in such a state, enable sysrq and write 'b' to the sysrq-trigger.
## Enable sysrq # echo 1 > /proc/sys/kernel/sysrq ## Reboot the machine without sync immediately # echo b > /proc/sysrq-trigger