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<TITLE>The Linux keyboard and console HOWTO: Ctrl-Alt-Del and other special key combinations</TITLE>
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ctrl-alt-del!action taken by
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If you press Ctrl-Alt-Del (or whatever key was assigned the keysym Boot by
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loadkeys) then either the machine reboots immediately (without sync), or
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init is sent a SIGINT. The former behaviour is the default. The default
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can be changed by root, using the system call reboot(), see ctrlaltdel(8).
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Some init's change the default. What happens when init gets SIGINT depends
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on the version of init used - often it will be determined by the pf entry in
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/etc/inittab (which means that you can run an arbitrary program
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in this case).
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In the current kernel Ctrl-AltGr-Del is no longer by default assigned to Boot.
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keyboard!special key combinations
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key combinations, special
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Name Default binding
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-------------------------------
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Show_Memory Shift-Scrollock
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Show_Registers AltGr-ScrollLock
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Show_State Ctrl-ScrollLock
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Console_n Alt-Fn and Ctrl-Alt-Fn (1 <= n <= 12)
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Console_{n+12} AltGr-Fn (1 <= n <= 12)
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Incr_Console Alt-RightArrow
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Decr_Console Alt-LeftArrow
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Last_Console Alt[Gr]-PrintScreen
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Scroll_Backward Shift-PageUp
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Scroll_Forward Shift-PageDown
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Caps_On (CapsLock is a toggle; this key sets)
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Compose Ctrl-.
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keyboard!key combinations for X
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key combinations, X
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X!key combinations for
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Ctrl-Alt-Fn Switch to VT n
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Ctrl-Alt-KP+ Next mode
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Ctrl-Alt-KP- Previous mode
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Ctrl-Alt-Backspace Kill X
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On some motherboards, Ctrl-Alt-KP- and Ctrl-Alt-KP+ will be equivalent to
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pressing the Turbo button. That is, both will produce the scancodes
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1d 38 4a ca b8 9d and 1d 38 4e ce b8 9d, and both will switch between
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Turbo (>= 25MHz) and non-Turbo (8 or 12 MHz).
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(Often these key combinations only function this way when enabled
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by jumpers on the motherboard.)
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Perry F Nguyen (pfnguyen@netcom22.netcom.com) writes:
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AMI BIOS has a feature that locks up the keyboard and flashes the LED's
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if the Ctrl-Alt-Backspace combination is pressed while a BIOS password
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is enabled, until the CMOS/BIOS password is typed in.
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On some SiS based motherboards the combination Ctrl-Alt-Backspace
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will cause a power off, or puts the machine in power save mode.
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(Reported for SiS 630 and for SiS645DX.)
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key combinations!dosemu
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dosemu!key combinations for
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Ctrl-Alt-Fn Switch to VT n (from version 0.50; earlier Alt-Fn)
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Ctrl-Alt-PgDn Kill dosemu (when in RAW keyboard mode)
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(and many other combinations - see the dosemu documentation)
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keyboard!composing symbols with
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symbols!composing with keyboard
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One symbol may be constructed using several keystrokes.
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LeftAlt-press, followed by a decimal number typed on the keypad, followed
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by LeftAlt-release, yields the symbol with code given by this number.
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(In Unicode mode this same mechanism, but then with 4 hexadecimal digits,
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may be used to define a Unicode symbol.)
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A dead diacritic followed by a symbol, yields that symbol adorned with
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that diacritic. If the combination is undefined, both keys are taken
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separately.
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Which keys are dead diacritics is user-settable; none is by default.
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Five (since 2.0.25 six) dead diacritics can be defined (using loadkeys(1)):
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dead_grave, dead_acute, dead_circumflex, dead_tilde, dead_diaeresis
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(and dead_cedilla).
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Precisely what this adorning means is also user-settable:
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dead-diacritic, symbol is equivalent to Compose + diacritic + symbol.
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Compose followed by two symbols yields a combination symbol. These
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combinations are user-settable. Today there are 68 combinations
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defined by default; you can see them by saying "dumpkeys | grep compose".
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Then there are `Sticky' modifier keys (since 1.3.33). For example,
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one can type Ctrl-C as SControl, C and Ctrl-Alt-BackSpace as SControl,
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SAlt, BackSpace.
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Note that there are at least four such composition mechanisms:
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The Linux keyboard driver mechanism, used in conjunction with loadkeys.
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The X mechanism - see X386keybd(1), later XFree86kbd(1).
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Under X11R6: edit /usr/X11R6/lib/X11/locale/iso8859-1/Compose.See also Andrew D. Balsa's comments at
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http://wauug.erols.com/~balsa/linux/deadkeys/index.html.
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The emacs mechanism obtained by loading "iso-insert.el" or
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calling `iso-accents-mode'.
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The vim mechanism: insert a composed symbol by pressing Ctrl-K
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followed by two symbols. A list of the possible combinations is
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obtained by the command :digraphs.
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For X the order of the two symbols is arbitrary: both Compose-,-c and
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Compose-c-, yield a c-cedilla; for Linux and emacs only the former sequence
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works by default. For X and vim the list of compose combinations is fixed.
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Linux and emacs are flexible.
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The default lists are somewhat similar, but the details are different.
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SysRq key
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In case your kernel was compiled with CONFIG_MAGIC_SYSRQ enabled
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(a feature that is present since Linux 2.1.43)
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there is a single key (defined in <linux/keyboard.h>)
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to which special system functions are attached, regardless of the
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current keyboard mode. For the PC architecture this special key is,
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naturally, the Alt+SysRq key, and any of the two Alt keys will work.
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(Note that if CONFIG_MAGIC_SYSRQ was not enabled, the default action
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of this key is to return to the previous console.)
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If you press this key, do not release it, and hit another key,
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a corresponding action is performed. The action is performed whether
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anybody is logged in or not, is root or not. For the details, see
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drivers/char/sysrq.c. Since this feature is meant only
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for kernel hackers, that should suffice. Still, let me add a few
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remarks.
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For the key r the keyboard mode is reset to K_XLATE.
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For the key k a SAK and console reset is done.
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For the key b the machine is rebooted immediately.
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(See, not something you want to have enabled on a production machine.)
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For the key o the power is turned off (when the machine is capable of that).
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For the key s an emergency sync is scheduled.
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For the key u an emergency read-only remount is scheduled.
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For the keys p,t,m various information is shown
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(namely the same information also shown for RAlt,RCtrl,RShift+ScrollLock).
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For the keys e,i,l all processes get a SIG_TERM or SIG_KILL, respectively;
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for l even the init process is killed.
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Digits set the log level. Anything else prints a short summary:
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SysRq: unRaw saK Boot Off Sync Unmount showPc showTasks showMem
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loglevel0-8 tErm kIll killalL.
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Note: These are very dangerous actions! And they do not use your
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keymap - indeed, are meant for emergency cases where the state
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of your keymap, or even of the entire kernel, is uncertain.
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If you use a dvorak keyboard - bad luck! Most other people will
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be able to survive: the dangerous letters A,M,Q,W,Y,Z that are
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differently placed on English, French and German keyboards, are
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not used for actions. (But if your finger slips and you hit L
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instead of K - bye bye to your system.)
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In Linux 2.3.13 the possibility to enable/disable SysRq was added.
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echo 0 > /proc/sys/kernel/sysrq
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will disable it (if the kernel was compiled with CONFIG_MAGIC_SYSRQ).
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A good keyboard accurately reports multiple key presses.
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Especially for people using a keyboard as Braille input device,
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where they have to be able to press up to six keys simultaneously,
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this is important. Many modern keyboards fail here due to sloppy
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design and testing - they misreport or fail in other ways when
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four or more keys are pressed simultaneously, sometimes already
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when two non-modifier keys are pressed simultaneously.
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This affects Linux users a bit: the SysRq key is not a modifier key,
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and people report problems using Alt-SysRq-X for various
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letters or digits X.
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Long ago a similar problem (`ghosting') was caused by the design
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of the scan matrix: when three keys were pressed a fourth was
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also seen. That problem was solved by the use of diodes.
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Today it is the interface logic that is deficient.
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