sysrq

sysrq ― Activates the System Request Key, if this value is set to anything other than zero ( 0), the default.

The System Request Key allows immediate input to the kernel through simple key combinations. For example, the System Request Key can be used to immediately shut down or restart a system, sync all mounted file systems, or dump important information to the console. To initiate a System Request Key, type Alt- SysRq- <system request code> . Replace <system request code> with one of the following system request codes:

• r ― Disables raw mode for the keyboard and sets it to XLATE (a limited keyboard mode which does not recognize modifiers such as Alt, Ctrl, or Shift for all keys).
• k ― Kills all processes active in a virtual console. Also called Secure Access Key ( SAK), it is often used to verify that the login prompt is spawned from init and not a trojan copy designed to capture usernames and passwords.
• b ― Reboots the kernel without first unmounting file systems or syncing disks attached to the system.
• c ― Crashes the system without first unmounting file systems or syncing disks attached to the system.
• o ― Shuts off the system.
• s ― Attempts to sync disks attached to the system.
• u ― Attempts to unmount and remount all file systems as read-only.
• p ― Outputs all flags and registers to the console.
• t ― Outputs a list of processes to the console.
• m ― Outputs memory statistics to the console.
• 0 through 9 ― Sets the log level for the console.
• e ― Kills all processes except init using SIGTERM.
• i ― Kills all processes except init using SIGKILL.
• l ― Kills all processes using SIGKILL (including init). The system is unusable after issuing this System Request Key code.
• h ― Displays help text.