Right control key on keyboard as i3 modifier in Ubuntu 20.04

For some historical, complicated reasons, I am using the key labeled “Ctrl“─right of the space bar─as the modifier for i3. But on a different device and installation, I struggled heavily to do that via xmodmap changes applied when i3 reads the config. I found many reports on the internet where people having tried a similar thing and failed, without any obvious solution.

What I want is that the “right Ctrl” physical key on my keyboard is configured as i3 modifier.

xmodmap -pm initially shows:

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x42)
control     Control_L (0x25), Control_R (0x69)
mod1        Alt_L (0x40),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3
mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce),  Hyper_L (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Mode_switch (0xcb)

mod3 is unused, so I can use xmodmap to map mod3 to Control_R:

xmodmap -e "clear control"
xmodmap -e "add control = Control_L"
xmodmap -e "clear mod3"
xmodmap -e "add mod3 = Control_R"

xmodmap -pm now shows:

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x42)
control     Control_L (0x25)
mod1        Alt_L (0x40),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3        Control_R (0x69)
mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce),  Hyper_L (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Mode_switch (0xcb)

The only problem is that when I cannot get xmodmap changes to persist when I login (using GDM), and worse, the changes get lost when I e.g. switch my monitor between devices (and the keyboard gets unplugged and plugged i n again). These are the points where a lot of reports are found on the Internet. So the above path is not an attractive option.

What I ultimately resorted to: editing /usr/share/X11/xkb/symbols/pc. This might break during updates. But hey, I’ll have the recipe here on how to fix it.

The original excerpt from /usr/share/X11/xkb/symbols/pc showed Control_R mapped to the Control modifier:

// Beginning of modifier mappings.
modifier_map Shift  { Shift_L, Shift_R };
modifier_map Lock   { Caps_Lock };
modifier_map Control{ Control_L, Control_R };
modifier_map Mod2   { Num_Lock };
modifier_map Mod4   { Super_L, Super_R };

After my changes, the excerpt from /usr/share/X11/xkb/symbols/pc now shows Control_R mapped to Mod3:

// Beginning of modifier mappings.
modifier_map Shift  { Shift_L, Shift_R };
modifier_map Lock   { Caps_Lock };
modifier_map Control{ Control_L };
modifier_map Mod2   { Num_Lock };
modifier_map Mod3   { Control_R };
modifier_map Mod4   { Super_L, Super_R };

Now, the right Ctrl key is always mapped to Mod3, regardless of whether an i3 session is running or not. As long as the keyboard layout is built upon /usr/share/X11/xkb/symbols/pc.

This is what I have in my .config/i3/config, telling i3 to use Mod3 as identifier:

set $mod Mod3
exec --no-startup-id i3-sensible-terminal
exec --no-startup-id google-chrome

Finally, so that I can always fiddle with configuration if needed, I have the following in my .config/i3/config:

exec --no-startup-id i3-sensible-terminal
exec --no-startup-id google-chrome

This starts a browser and a terminal at the beginning of the i3 session. Things can go wrong after all.