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.
Perform a Safe reboot of Linux
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),
- Reboot.
To perform a safe reboot of a Linux Xen Virtual Server which hangs up, do the following. This will avoid the fsck during the next re-booting.
Run the below command in Xen Dom0.
#xm sysrq <domainid> s
#xm sysrq <domainid> u
#xm sysrq <domainid> b
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