The magic SysRq key is a key combination understood by 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. To be able to use this functionality the CONFIG_MAGIC_SYSRQ option has to be enabled at kernel compile time.
PurposeMuch like Sun Microsystems's Open Firmware (OpenBoot), this key combination provides access to powerful tools for software development and disaster recovery. In this sense, it can be considered a form of escape sequence. Principal among the offered commands are means to forcibly unmount file systems, kill processes, recover keyboard state, and write unwritten data to disk. With respect to these tasks, this feature serves as a tool of last resort. Magic commandsThe key combination consists of Alt, SysRq and another key, which controls the command issued (as shown in the table below). Users with a keyboard layout other than QWERTY have to remember that their layout becomes QWERTY when they use one of these combinations. For example, on a Dvorak keyboard, the key below '9' and '0' counts as an 'o', not as an 'r', so it shuts the system down instead of switching the keyboard to raw mode. Furthermore, some keyboards may not provide a separate SysRq key. In this case, a separate "Print" key should be present. Under graphical environments (such as Gnome or KDE) 'Alt'+'PrintScrn/SysRq'+key combination generally only leads to a screenshot being dumped. To avoid this Print Screen feature the magic SysRq combination should include the Ctrl, becoming 'Ctrl'+'Alt'+'SysRq'+key. For the same purposes the AltGr key, if present, can be used in place of the Alt key. The magic SysRq can also be accessed from the serial console. Read more: Wikipedia
PurposeMuch like Sun Microsystems's Open Firmware (OpenBoot), this key combination provides access to powerful tools for software development and disaster recovery. In this sense, it can be considered a form of escape sequence. Principal among the offered commands are means to forcibly unmount file systems, kill processes, recover keyboard state, and write unwritten data to disk. With respect to these tasks, this feature serves as a tool of last resort. Magic commandsThe key combination consists of Alt, SysRq and another key, which controls the command issued (as shown in the table below). Users with a keyboard layout other than QWERTY have to remember that their layout becomes QWERTY when they use one of these combinations. For example, on a Dvorak keyboard, the key below '9' and '0' counts as an 'o', not as an 'r', so it shuts the system down instead of switching the keyboard to raw mode. Furthermore, some keyboards may not provide a separate SysRq key. In this case, a separate "Print" key should be present. Under graphical environments (such as Gnome or KDE) 'Alt'+'PrintScrn/SysRq'+key combination generally only leads to a screenshot being dumped. To avoid this Print Screen feature the magic SysRq combination should include the Ctrl, becoming 'Ctrl'+'Alt'+'SysRq'+key. For the same purposes the AltGr key, if present, can be used in place of the Alt key. The magic SysRq can also be accessed from the serial console. Read more: Wikipedia
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