The Linux Keyboard 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 dead diacritics can be defined (using loadkeys(1)): dead_grave, dead_acute, dead_circumflex, dead_tilde, dead_diaeresis. 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 ˆC as SControl, C and Ctrl-Alt-BackSpace as SControl, SAlt, BackSpace.

Note that there are at least three 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.
The emacs mechanism obtained by loading "iso-insert.el".

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 the list of compose combinations is fixed. Linux and emacs are flexible. The three default lists are somewhat similar, but the details are different.