Keyboard input

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Many keyboards include some "special keys" (also called hotkeys, such as HP Quickplay), which are supposed to execute an application or print special characters (not included in the standard national keymaps). The lack of specification for these extra keys makes it impossible for the kernel to know what to do with them and that is why we need to map the keys to actions. There are 3 ways of doing that:

  • The most portable way using low level tools, such as acpid. Not all keys are supported, but configuration in uniform way is possible for keyboard keys, power adapter connection and even headphone jack (un)plugging events.
  • The universal way using Xorg utilities (and eventually your desktop environment tools)
  • The quicker way using a third-party program to do everything in GUI, such as the Gnome Control Center or Keytouch

Before starting you need to learn some vocabulary:

A scancode is the lowest identification number for a key. If a key does not have a scancode then we cannot do anything because it means that the kernel does not see it.

A keycode is the second level of identification for a key, a keycode corresponds to a function.

A symbol is the third level of identification for a key, it is the way Xorg refers to keys.

Step 1: Check for keycodes

Most of your keys should already have a keycode, or at least a scancode. Keys without a scancode are not recognized by the kernel.

Using xev

Use the graphical X program "xev" (without having to switch to a console environment). Install the xev program:

# pacman -S xorg-xev

With the following line you can start xev and directly grep the important parts:

$ xev | grep -A2 --line-buffered '^KeyRelease' | sed -n '/keycode /s/^.*keycode \([0-9]*\).* (.*, \(.*\)).*$/\1 \2/p'

In the example below I pressed the "a", "r", "c" and "h" keys and two of the media keys on my Dell keyboard. This gives me the following output:

38 a
27 r
54 c
43 h
153 NoSymbol
144 NoSymbol

This means that the "a", "r", "c" and "h" keys have the keycodes 38, 27, 54 and 43 and are properly bound while the media keys with the keycodes 153 and 144 have no function yet, which is indicated by "NoSymbol". If you press a key and nothing appears in the terminal, this means that the kernel does not see that key or that it is not mapped.

Using showkey

The universal way to know if a key has a keycode is to use the kernel showkey program. showkey waits for a key to be pressed and if none is during 10 seconds it quits, note that this is the only way to exit the program. To execute showkey you need to be in a real console, it means not in a graphical environment so switch using Ctrl+Alt+F1.

# showkey

and try to push your hotkeys. If a keycode appears the key is mapped, if not it can mean either that the kernel does not see the key or that the key is not mapped.

2.6 kernels

According to the keymap man page:

Note: In 2.6 kernels raw mode, or scancode mode, is not very raw at all. Scan codes are first translated to key codes, and when scancodes are desired the key codes are translated back...there is no guarantee at all that the final result corresponds to what the keyboard hardware did send. To change behavior back to the old raw mode, add the parameter atkbd.softraw=0 to your kernel while booting. This can be removed for later boots when the old raw functionality is not required.

This is relevant if the keymaps obtained from showkey and the ones set by setkeycodes differ from the ones obtained by xev in X. Keep this in mind when translating the keymaps into keysyms using xmodmap (See Extra Keyboard Keys in Xorg).

If all your keys have a keycode you can go directly to the second step. If not keep reading below:

Check for scancodes

If a key does not have a keycode you can know if it has a scancode by looking at the kernel log using the dmesg command:

$ dmesg|tail -5

If when you press the key something like that appears:

atkbd.c: Unknown key pressed (translated set 2, code 0xf1 on isa0060/serio0).
atkbd.c: Use 'setkeycodes e071 <keycode>' to make it known.

then your key has a scancode which can be mapped to a keycode. See Map scancodes to keycodes.

If nothing new appears in dmesg then your key does not have a scancode, which means that it is not recognized by the kernel and cannot be used.

Step 2: Map keycodes

In Console

See the dedicated article: Extra Keyboard Keys in Console.

When in console, hotkeys can be used to print sequences of characters, including escape sequences. Thus, printing the sequence of characters constituting a command followed by the escape sequence for a new will execute the command.

In Xorg

See the dedicated article: Extra Keyboard Keys in Xorg.


Asus M series

In order to have control over the light sensor and the multimedia keys on your Asus machine, you should use the following command:

# echo 0 > /sys/devices/platform/asus-laptop

To have it run on boot:

echo 0 > /sys/devices/platform/asus-laptop/ls_switch

Note that this may work for other Asus notebook models. See the detailed article: lineak.