xmodmap

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Summary
A general overview of modifying keymaps and pointer mappings with xmodmap.
Related
Xorg
Extra Keyboard Keys
Extra Keyboard Keys in Xorg
Extra Keyboard Keys in Console

xmodmap is a utility for modifying keymaps and pointer button mappings in Xorg.

xmodmap is not directly related to XKB, it uses different (pre-XKB) ideas on how keycodes are processed within Xorg. Generally it is not recommended to use xmodmap, except maybe for the simplest tasks. See X KeyBoard extension if you have special demands on layout configuration.

Contents

Introduction

On a Linux system using Xorg, there are two types of keyboard values: keycodes and keysyms.

Keycode
The keycode is the numeric representation received by the Linux kernel when a keyboard key or a mouse button is pressed.
Keysym
The keysym is the value assigned to the keycode. For example, when you press the A key on the keyboard, it generates keycode 73. Keycode 73 is mapped to the keysym 0×61 which corresponds to the letter a in the ASCII table.
The keysyms are managed by Xorg in a table of keycodes defining the keycode-keysym relations which is called the keymap table. The command xmodmap can be used to show/modify that key table.

Installation

In order to use xmodmap, you need to install the xorg-xmodmap package from the official repositories.

Optionally, install also xkeycaps which provides a graphical front-end to xmodmap.

Keymap table

Print the current keymap table formatted into expressions: 

$ xmodmap -pke
keycode  57 = n N

Each keycode is followed by the keysym it is mapped to. The above example indicates that the keycode 57 is mapped to the lowercase n, while the uppercase N is mapped to keycode 57 plus Shift.

Each keysym column in the table corresponds to a particular combination of modifier keys:

  1. Key
  2. Shift+Key
  3. mode_switch+Key
  4. mode_switch+Shift+Key
  5. AltGr+Key
  6. AltGr+Shift+Key

Not all keysyms have to be set, but if you want to assign a latter keysym without assigning earlier ones, set them to NoSymbol.

You can check which keycode corresponds to a key on your keyboard with the xev utility, see Extra Keyboard Keys#In Xorg for details.

Tip: There are predefined descriptive keysyms for multimedia keys, e.g. XF86AudioMute or XF86Mail. These keysyms can be found in /usr/include/X11/XF86keysym.h. Many multimedia programs are designed to work with these keysyms out-of-the-box, without the need to configure any third-party application.

Custom table

You can create your own map and store it in a configuration file in your home directory (i.e. ~/.Xmodmap): 

xmodmap -pke > ~/.Xmodmap

Make the desired changes to ~/.Xmodmap and then test the new configuration with: 

xmodmap ~/.Xmodmap

Activate your custom table

If you are using GDM, XDM or KDM, there is no need to source your ~/.Xmodmap manually as these display managers source that file if it is present, whereas startx does not. Therefore, to activate your custom table when starting Xorg, add the following: 

~/.xinitrc
if [ -s ~/.Xmodmap ]; then
    xmodmap ~/.Xmodmap
fi

Alternatively, edit the global startup script /etc/X11/xinit/xinitrc.

Test changes

You can also make temporary changes for the current session. For example: 

xmodmap -e "keycode  46 = l L l L lstroke Lstroke lstroke"
xmodmap -e "keysym a = e E"

Special keys/signals

You can also also edit the keys: Shift, Ctrl, Alt and Super (there always exists a left and a right one (Alt_R=AltGr)).

For example this can be useful if your right Control key is not working like your left one but you would like it to.

At first you have to delete/clear the signals that should be edited. In the beginning of your ~/.Xmodmap: 

!clear Shift
!clear Lock
clear Control
!clear Mod1
!clear Mod2
!clear Mod3
clear Mod4
!clear Mod5
keycode   8 =
...

Remember, ! is a comment so only Control and Mod4 (Standard: Super_L Super_R) get cleared.

Write the new signals at the end of ~/.Xmodmap 

keycode 255 =
!add Shift   = Shift_L Shift_R
!add Lock    = Caps_Lock
add Control = Super_L Super_R
!add Mod1    = Alt_L Alt_R
!add Mod2    = Mode_switch
!add Mod3    =
add Mod4    = Control_L Control_R
!add Mod5    =

The Super keys have now been exchanged with the Ctrl keys.

If you get the following error message X Error of failed request: BadValue (integer parameter out of range for operation) it means the key you are trying to add is already in another modifier, so remove it using "remove MODIFIERNAME = KEYSYMNAME". Running xmodmap gives you a list of modifiers and keys that are assigned to them.

Reverse Scrolling

The natural scrolling feature available in OS X Lion (mimicking smartphone or tablet scrolling) can be replicated with xmodmap. Since the synaptics driver uses the buttons 4/5/6/7 for up/down/left/right scrolling, you simply need to swap the order of how the buttons are declared in ~/.Xmodmap.

Open ~/.Xmodmap and append the following line to the file: 

pointer = 1 2 3 5 4 7 6 8 9 10 11 12

Note how the 4 and 5 have been reversed.

Then update xmodmap: 

xmodmap ~/.Xmodmap

To return to regular scrolling simply reverse the order of the 4 and 5 or delete the line altogether. For more information check Peter Hutterer's post, Natural scrolling in the synaptics driver, or the Reverse scrolling direction ala Mac OS X Lion? forum thread.

See also

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