If GNU system is not responding (for whatever reason which could be analysed through the system logs), there are different combinations of keys which allow taking control of the system back. It is the magic keys, a combination of keys which are usually activated in all distributions.

The system petition function is usually assigned to the Print Screen key.

Alt+PrintScreen+:

  • r: it takes control of the keyboard in an X system. Many times it is the X system itself which causes the system freeze. After Alt+PrintScreen+r, the combination Ctrl+Alt+Backspace can be tried to log out of X.
  • s: writes data to the hard disk cache.
  • e: sends the SIGTERM signal to all processes but init.
  • i: sends the SIGKILL signal to all processes but init.
  • u: remounts the whole filesystem in read-only mode, to ensure a safe reboot.
  • b: reboots the system.

These are the classical well-known combinations, but there are other magic combinations which can be useful in other situations:

  • 0 - 9: adjusts the kernel log level, controlling the messages which will be shown directly to console.
  • f: call the Out Of Memory Killer, which closes the current processes which are consuming all the available memory.
  • h: shows help.
  • k: closes all the processes in the current session.
  • l: send the SIGKILL to all processes, init included.
  • m: dumps the information from memory in the console.
  • o: shuts down the system via ACPI, in older systems using APM.
  • p: dumps logs information to console.
  • q: dumps timers information to console.
  • t: dumps tasks information to console.
  • w: dumps all uninterrupted processes information to console.