Photo courtesy of KCIvey
This is a guest post written by Lakshmanan G.
If you are working on kernel development, or device drivers, or running a code that could cause kernel panic, SysRq key will be very valuable. 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.
- ‘k’ – Kills all the process running on the current virtual console.
- ‘s’ – This will attempt to sync all the mounted file system.
- ‘b’ – Immediately reboot the system, without unmounting partitions or syncing.
- ‘e’ – Sends SIGTERM to all process except init.
- ‘m’ – Output current memory information to the console.
- ‘i’ – Send the SIGKILL signal to all processes except init
- ‘r’ – Switch the keyboard from raw mode (the mode used by programs such as X11), to XLATE mode.
- ‘s’ – sync all mounted file system.
- ‘t’ – Output a list of current tasks and their information to the console.
- ‘u’ – Remount all mounted filesystems in readonly mode.
- ‘o’ – Shutdown the system immediately.
- ‘p’ – Print the current registers and flags to the console.
- ‘0-9’ – Sets the console log level, controlling which kernel messages will be printed to your console.
- ‘f’ – Will call oom_kill to kill process which takes more memory.
- ‘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.
- unRaw (take control of keyboard back from X11,
- tErminate (send SIGTERM to all processes, allowing them to terminate gracefully),
- kIll (send SIGILL to all processes, forcing them to terminate immediately),
- Sync (flush data to disk),
- Unmount (remount all filesystems read-only),
This article was written by Lakshmanan G. He is working in bk Systems (p) Ltd, and interested in contributing to the open source. The Geek Stuff welcomes your tips and guest articles.
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Now when a system hangs, no need to worry much about the crashes…
What is SysRq means ??
Thanks for your appreciation.
System Request is the abbreviation for SysRq.
How can I fake a kernel panic, in order to test that the settings I’ve made work correctly when a real kernel panic comes along?
You have a slight error in you list of keys to hit for a reboot. The “i” should be bolded in “kill”, not the “k”.
An easy way to remember is the mnemonic “Raising Elephants Is So Utterly Boring”
I think (may be wrong also), please correct.
1st enable the sysrq to 1 in proc/sys/kernel
2nd press al+sysrq+R (hold for sometime)
then press these buttons one by one (hold for some time each button) s e i u b
HI. Could you please explain the sequence once again. do we need to press the keys in small or capital letters?
“‘h’ – Used to display the help. But any other keys than the above listed will print help.”
I don’t think that ‘c’ key will print help.. 🙂