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Using AltSysRq : The last resort before hard reboot on Linux February 14, 2011

Posted by Tournas Dimitrios in Linux.

Linux is without doubt one of the most stable operating system .  I always hack kernel parameters via the terminal ( just for learning purposes) . But sometimes , it can go unresponsive , and the most obvious reaction is the well known ” Ctl + Alt + Del ” combination .
Using system requests can be useful to diagnose the problem , they are activated by special key-combinations . By default system requests are disabled (for security reasons) , but  passing the proper parameter to the kernel , they can be enabled . When activated , anyone at the console can execute these system requests without being logged into the system and without entering additional authentication information . So , this “state ” should only be enabled to diagnose problems with the system and when the box is in a secure location or monitored by physical presence of an administrator . To enable system request functionality , just pass the value “1” (one) to the kernel :
echo 1 > /proc/sys/kernel/sysrq   or
sysctl  -w   kernel.sysrq=1

Passing values to the /proc/sys virtual filesystem with the echo command takes effect immediately ,but  temporarily until the next reboot  of the computer, read my previous article .

As the system request functionality is enabled , use the key combination “Alt- SysRq -hotkey ” .Most modern keyboards have the SysRq key labeled , others are using the PringScreen key for this function . Alternatively go to console mode “Ctrl -Alt -F1 ” , “Ctrl -Alt -F2 ” ….

The table below list the available<hotkey>  keys for executing system requests :

Hot keys
-h Display all hot-keys for reference
-e Terminate all proccesses except init with a SIGTERM signal
-l (el) Kill all processes including init (this will log-out ) like Ctrl -Alt-Backspase
-o Shut down now
-t Display current task list to the console
-m Display current memory information
0-9 Set what type of messages will be displayed on the console . If set to 0 only kernel panic . The higher the number the more messages are printed
Perform a Kexec reboot so a crash dump can be taken 

The packages Kdump  and kexec must be installed for this functionality

-u Remount all file systems as RO if posible
-s Sync filesystems . This should be used before reboot to minimize data loss
-k Kill all programs on  the current virtual console

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