The Linux keyboard and console HOWTO: Ctrl-Alt-Del and other special key combinations

8. Ctrl-Alt-Del and other special key combinations

8.1 Ctrl-Alt-Del (Boot)

If you press Ctrl-Alt-Del (or whatever key was assigned the keysym Boot by loadkeys) then either the machine reboots immediately (without sync), or init is sent a SIGINT. The former behaviour is the default. The default can be changed by root, using the system call reboot(), see ctrlaltdel(8). Some init's change the default. What happens when init gets SIGINT depends on the version of init used - often it will be determined by the pf entry in /etc/inittab (which means that you can run an arbitrary program in this case). In the current kernel Ctrl-AltGr-Del is no longer by default assigned to Boot.

8.2 Other combinations

Name            Default binding
-------------------------------
Show_Memory     Shift-Scrollock
Show_Registers  AltGr-ScrollLock
Show_State      Ctrl-ScrollLock
Console_n       Alt-Fn and Ctrl-Alt-Fn  (1 <= n <= 12)
Console_{n+12}  AltGr-Fn                (1 <= n <= 12)
Incr_Console    Alt-RightArrow
Decr_Console    Alt-LeftArrow
Last_Console    Alt[Gr]-PrintScreen
Scroll_Backward Shift-PageUp
Scroll_Forward  Shift-PageDown
Caps_On                                 (CapsLock is a toggle; this key sets)
Compose         Ctrl-.

8.3 X Combinations

Ctrl-Alt-Fn     Switch to VT n
Ctrl-Alt-KP+    Next mode
Ctrl-Alt-KP-    Previous mode
Ctrl-Alt-Backspace      Kill X

On some motherboards, Ctrl-Alt-KP- and Ctrl-Alt-KP+ will be equivalent to pressing the Turbo button. That is, both will produce the scancodes 1d 38 4a ca b8 9d and 1d 38 4e ce b8 9d, and both will switch between Turbo (>= 25MHz) and non-Turbo (8 or 12 MHz). (Often these key combinations only function this way when enabled by jumpers on the motherboard.)

Perry F Nguyen (pfnguyen@netcom22.netcom.com) writes: AMI BIOS has a feature that locks up the keyboard and flashes the LED's if the Ctrl-Alt-Backspace combination is pressed while a BIOS password is enabled, until the CMOS/BIOS password is typed in.

8.4 Dosemu Combinations

Ctrl-Alt-Fn     Switch to VT n (from version 0.50; earlier Alt-Fn)
Ctrl-Alt-PgDn   Kill dosemu (when in RAW keyboard mode)
(and many other combinations - see the dosemu documentation)

8.5 Composing symbols

One symbol may be constructed using several keystrokes.

LeftAlt-press, followed by a decimal number typed on the keypad, followed by LeftAlt-release, yields the symbol with code given by this number. (In Unicode mode this same mechanism, but then with 4 hexadecimal digits, may be used to define a Unicode symbol.)
A dead diacritic followed by a symbol, yields that symbol adorned with that diacritic. If the combination is undefined, both keys are taken separately. Which keys are dead diacritics is user-settable; none is by default. Five (since 2.0.25 six) dead diacritics can be defined (using loadkeys(1)): dead_grave, dead_acute, dead_circumflex, dead_tilde, dead_diaeresis (and dead_cedilla). Precisely what this adorning means is also user-settable: dead-diacritic, symbol is equivalent to Compose + diacritic + symbol.
Compose followed by two symbols yields a combination symbol. These combinations are user-settable. Today there are 68 combinations defined by default; you can see them by saying "dumpkeys | grep compose".
Then there are `Sticky' modifier keys (since 1.3.33). For example, one can type Ctrl-C as SControl, C and Ctrl-Alt-BackSpace as SControl, SAlt, BackSpace.

Note that there are at least four such composition mechanisms:

The Linux keyboard driver mechanism, used in conjunction with loadkeys.
The X mechanism - see X386keybd(1), later XFree86kbd(1). Under X11R6: edit /usr/X11R6/lib/X11/locale/iso8859-1/Compose.
See also Andrew D. Balsa's comments at http://wauug.erols.com/~balsa/linux/deadkeys/index.html.
The emacs mechanism obtained by loading "iso-insert.el" or calling `iso-accents-mode'.
The vim mechanism: insert a composed symbol by pressing Ctrl-K followed by two symbols. A list of the possible combinations is obtained by the command :digraphs.

For X the order of the two symbols is arbitrary: both Compose-,-c and Compose-c-, yield a c-cedilla; for Linux and emacs only the former sequence works by default. For X and vim the list of compose combinations is fixed. Linux and emacs are flexible. The default lists are somewhat similar, but the details are different.

8.6 The SysRq key

In case your kernel was compiled with CONFIG_MAGIC_SYSRQ enabled (a feature that is present since Linux 2.1.43) there is a single key (defined in <linux/keyboard.h>) to which special system functions are attached, regardless of the current keyboard mode. For the PC architecture this special key is, naturally, the Alt+SysRq key, and any of the two Alt keys will work. (Note that if CONFIG_MAGIC_SYSRQ was not enabled, the default action of this key is to return to the previous console.)

If you press this key, do not release it, and hit another key, a corresponding action is performed. The action is performed whether anybody is logged in or not, is root or not. For the details, see drivers/char/sysrq.c. Since this feature is meant only for kernel hackers, that should suffice. Still, let me add a few remarks.

For the key r the keyboard mode is reset to K_XLATE. For the key k a SAK and console reset is done. For the key b the machine is rebooted immediately. (See, not something you want to have enabled on a production machine.) For the key o the power is turned off (when the machine is capable of that). For the key s an emergency sync is scheduled. For the key u an emergency read-only remount is scheduled. For the keys p,t,m various information is shown (namely the same information also shown for RAlt,RCtrl,RShift+ScrollLock). For the keys e,i,l all processes get a SIG_TERM or SIG_KILL, respectively; for l even the init process is killed. Digits set the log level. Anything else prints a short summary: SysRq: unRaw saK Boot Off Sync Unmount showPc showTasks showMem loglevel0-8 tErm kIll killalL.

Note: These are very dangerous actions! And they do not use your keymap - indeed, are meant for emergency cases where the state of your keymap, or even of the entire kernel, is uncertain. If you use a dvorak keyboard - bad luck! Most other people will be able to survive: the dangerous letters A,M,Q,W,Y,Z that are differently placed on English, French and German keyboards, are not used for actions. (But if your finger slips and you hit L instead of K - bye bye to your system.)

In Linux 2.3.13 the possibility to enable/disable SysRq was added.


        echo 0 > /proc/sys/kernel/sysrq

will disable it (if the kernel was compiled with CONFIG_MAGIC_SYSRQ).

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