Linux: emergency reboot or shutdown with magic commands

The post describes how most Linux distributions try to stop daemons and unmount storage cleanly during reboot or shutdown, but a badly behaving remote server can make a normal reboot impossible. In that situation, SysRq magic commands let you force a reboot or shut the machine down directly from /proc/sysrq-trigger.

Reading notes

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• Most Linux distributions use scripts to stop daemons and unmount storage volumes gracefully during reboot or shutdown.
• If a remote server is misbehaving, a plain reboot command may fail before SSH stops running.
• Writing 1 to /proc/sys/kernel/sysrq enables SysRq access.
• echo b > /proc/sysrq-trigger forces an immediate reboot with no graceful shutdown, no filesystem sync, and possible filesystem damage.
• The post mentions a safer reboot sequence from Wikipedia using the mnemonic “Reboot Even If System Utterly Broken”.
• The safer sequence is unRaw, tErminate, kIll, Sync, Unmount, reBoot.
• The author says they cannot vouch for that sequence actually working and adds an update warning that it is a very bad idea with ReiserFS.
• echo o > /proc/sysrq-trigger shuts the machine down entirely.
• SysRq can be kept enabled all the time with kernel.sysrq = 1 in sysctl.conf.