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.
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-.
Ctrl-Alt-Fn Switch to VT n Ctrl-Alt-KP+ Next mode Ctrl-Alt-KP- Previous mode Ctrl-Alt-Backspace Kill XOn 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.
On some SiS based motherboards the combination Ctrl-Alt-Backspace will cause a power off, or puts the machine in power save mode. (Reported for SiS 630 and for SiS645DX.)
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)
One symbol may be constructed using several keystrokes.
Note that there are at least four such composition mechanisms:
/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.
:digraphs.
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
A good keyboard accurately reports multiple key presses. Especially for people using a keyboard as Braille input device, where they have to be able to press up to six keys simultaneously, this is important. Many modern keyboards fail here due to sloppy design and testing - they misreport or fail in other ways when four or more keys are pressed simultaneously, sometimes already when two non-modifier keys are pressed simultaneously. This affects Linux users a bit: the SysRq key is not a modifier key, and people report problems using Alt-SysRq-X for various letters or digits X.
Long ago a similar problem (`ghosting') was caused by the design of the scan matrix: when three keys were pressed a fourth was also seen. That problem was solved by the use of diodes. Today it is the interface logic that is deficient.