Where is the behavior of dead keys on Ubuntu keyboard layouts defined?

Question

I'm on Ubuntu, and using a keyboard layout (specifically US with AltGr dead keys) which as the name implies includes some dead keys. Some dead key combinations are unexpected but very useful, such as superscript and subscript numbers using the ^ and ˇ dead keys followed by the appropriate number, respectively.

In case there are more useful combinations that I don't know about, I want to find a list of all dead key combinations for this keyboard layout, but I have been unable to do so. I know from searching that the keyboard layout is defined in usr/share/X11/xkb/symbols/, but this references the dead keys by names/keysyms such as dead_grave, dead_tilde, and so on. These keysyms are apparently defined in /usr/include/X11/keysymdef.h (or they were before, I can't find the file on my computer). Looking at a version of that file online, the dead keys are only assigned a code, not defined with a list of combinations.

My question is, where are the dead keys defined?

Answer 1

Not sure it's the layout, I think it's the composition behaviour, probably affected by locale.

At any rate, you can have a look here for combinations:

https://cgit.freedesktop.org/xorg/lib/libX11/plain/nls/en_US.UTF-8/Compose.pre

Answer 2

It’s been quite a while since the question was asked, but just in case others come across this page…

The default compose sequences provided by Ubuntu are defined in your system locale’s Compose file that is inside your locale's folder. E.g. /usr/share/X11/locale/en_US.UTF-8/Compose for the en_US.UTF-8 locale.

You need root privileges to modify this file. Alternatively, you can override the default sequences by saving this Compose file as .XCompose in your home directory (~/.XCompose) and then editing it. Or even starting from your own empty ~/.XCompose. This way you don’t modify system files.

To activate your changes in .XCompose, you obviously need to refresh your config with something like logging off/on or restarting your window manager.