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키보드에는 캡쳐할 때 흔히 쓰는 Print Screen 키가 있는데, 그 키에는 (현재는 잘 쓰이지 않지만)또다른 기능이 있다.

SysRq (System Request)

아래는 SysRq 키에 대해서 설명한 내용이다.

magic 이라고 표현 할 만큼 멋진 기능을 가지고 있다.

영어지만 시스템 관리자라면 충분히 이해할 만한 쉬운 수준이므로 (귀찮아서) 해석은 생략한다.

Hack 75. Safe Reboot Of Linux Using Magic SysRq Key

The magic SysRq key is a key combination in the Linux kernel which allows the user to perform various low level commands regardless of the system’s state.

It is often used to recover from freezes, or to reboot a computer without corrupting the filesystem. The key combination consists of Alt+SysRq+commandkey. In many systems the SysRq key is the printscreen key.

First, you need to enable the SysRq key, as shown below.

echo "1" > /proc/sys/kernel/sysrq

List of SysRq Command Keys

Following are the command keys available for Alt+SysRq+commandkey.

o ‘k’ – Kills all the process running on the current virtual console.
o ’s’ – This will attempt to sync all the mounted file system.
o ‘b’ - Immediately reboot the system, without unmounting partitions or syncing.
o ‘e’ – Sends SIGTERM to all process except init.
o ‘m’ – Output current memory information to the console.
o ‘i’ - Send the SIGKILL signal to all processes except init
o ‘r’ - Switch the keyboard from raw mode (the mode used by programs such as X11), to XLATE mode.
o ’s’ – sync all mounted file system.
o ‘t’ - Output a list of current tasks and their information to the console.
o ‘u’ - Remount all mounted filesystems in readonly mode.
o ‘o’ – Shutdown the system immediately.
o ‘p’ – Print the current registers and flags to the console.
o ‘0-9′ - Sets the console log level, controlling which kernel messages will be printed to your console.
o ‘f’ - Will call oom_kill to kill process which takes more memory.
o ‘h’ – Used to display the help. But any other keys than the above listed will print help.

We can also do this by echoing the keys to the /proc/sysrq-trigger file. For example, to re-boot a system you can perform the following.

echo "b" > /proc/sysrq-trigger

Perform a Safe reboot of Linux using Magic SysRq Key

To perform a safe reboot of a Linux computer which hangs up, do the following. This will avoid the fsck during the next re-booting. i.e Press Alt+SysRq+letter highlighted below.

o unRaw (take control of keyboard back from X11,
o tErminate (send SIGTERM to all processes, allowing them to terminate gracefully),
o kIll (send SIGILL to all processes, forcing them to terminate immediately),
o Sync (flush data to disk),
o Unmount (remount all filesystems read-only),
o reBoot.

// source – Linux 101 Hacks by Ramesh Natarnajan

// references - http://en.wikipedia.org/wiki/Magic_SysRq_key