How to put some keyboard layout aside from layout switching ring?

Three useful commands:

1. Get the current keyboard layout, from this topic :

setxkbmap -print | grep xkb_symbols | awk -F"+" '{print $2}'

Returns layout(variant) where variant is optional

2. Know if the current keyboard layout is layout(variant):

 setxkbmap -print | grep "layout(variant)"

Or without "(variant)".

Returns nothing is not.

3. Set the keyboard layout to a specified value:

setxkbmap layout variant

Where variant is optional.

(see man setxkbmap for more informations)

What you can do:

1. Create a simple executable (chmod +x file) script that changes the keyboard layout on the fly, depending on the current one. For instance, if we deal with the first point:

#!/bin/bash

if setxkbmap -print | grep "us"
then
  setxkbmap ru
else
  setxkbmap us
fi

2. With your keyboard shortcuts manager, assign the Shift-CapsLock keys to this script.

Good luck!

What you exactly want is not possible as it would require statefull process (to know the state previous of switching to by layout).

However, along with the ISO_Prev_Group and ISO_Next_Group symbols allowing to decrement/increment in the layouts ring, there is also those two: ISO_First_Group and ISO_Last_Group. With this ring: us,by,ru you can always access any of the three layouts in a single toggle.

For example, define Shift-CapsLock to send ISO_Last_Group when in us layout, ISO_First_Group when in ru or by; define Ctrl-RightShift to send ISO_Next_Group when in us ISO_Prev_Group when in ru and ISO_Next_Group (or Last) when in by.

The difference with what you want is that you have to manually choose (with different shortcuts) the layout you want to go, when in by.

Also, if you need by layout for just a few belarussian specific cyrillic letters; another option would be to use a modified ru layout, with extra symbols in some keys, accessible with an AltGr or similar key, like in most latin keyboards.

As in the by file there are only 3 different keys from ru it seems that would be the right approach. In such cas I would stack the layouts as: us,ru,by

Use ISO_Next_Group in "us", ISO_Prev_Group in "ru" for Shift-Capslock, and in "ru" define a key with a latching ISO_Next_Group.

To do that: Create a ~/.xkb/keymap/mykbd where you put the output of setxkbmap, it will be your base keyboard definition; eg:

setxkbmap "us,ru,by" ; setxkbmap -print > ~/.xkb/keymap/mykbd

then create a file ~/.xkb/symbols/mysymbols with:

partial modifier_keys
xkb_symbols "shift_caps_us_ru_by" {
  key <CAPS> {
    symbols[Group1] = [  Caps_Lock, ISO_Next_Group   ],
    symbols[Group2] = [  Caps_Lock, ISO_Prev_Group   ],
    symbols[Group3] = [  Caps_Lock, ISO_First_Group  ]
  };
  key <RALT> {
    symbols[Group2] = [ ISO_Group_Latch ]
  };
};

edit the ~/.xkb/keymap/mykbd file, and change the xkb_symbols line to add +mysymbols(shift_caps_us_ru_by)

finally, you can load it with xkbcomp -I$HOME/.xkb ~/.xkb/keymap/mykbd and now Shift-CapsLock switch between latin (us) and cyrillic (ru) layouts; and while in cyrillic, you can access belarusian-specific letters holding AltGr (right Alt) key.

All right, so what I was really wanted was switching layouts in MRU (most recently used) order. And guess what, GNOME's default layout switcher does precisely that (make sure no checkboxes in "Tweaks" → "Keyboard & Mouse" → "Additional Layout Options" → "Keys to change layout" are activated!). So currently I have 4 layouts in a ring (Russian, Belarusian, Ukrainian, English) and switching between most recent ones is one SuperSpace away. So that solves the issue for me and then some.

I highly recommend using sxhkd (simple X hotkey daemon).Its configuration should not be challenging at all. Follow the steps presented in the archwiki tutorial and you'll get it up and running in no time.

For the sake of simplicity, sxhkd gives you the power to map/bind input events (like pressing a key down...) to actions (like changing the keyboard layout back and forth...)

I, personally, switch between 3 different layouts on the fly. The mappings I use :

super + k; {u,f,r}
  setxkbmap {us,fr,ru}

super is a special key (called modifier), you can use other modifier keys like Ctrl or Alt etc (it's all documented)

So when I hold press my modifier key (super the windows key in my case) and k then release both and press one of the three keys u f r it runs the command setxkbmap with one of the three arguments supplied depending on the key pressed (order is respected). Let's say I hit superkthenf I get setxkbmap fr the french layout

ps: if you bind, by accident, a combination of keys that is already associated with an action, you might get indesirable/unexpected results (most likely the late bindings will shadow the early ones). But keep in mind that everyting has always a fix. So dont be afraid to experiment.