How to run "xmodmap" command when a Bluetooth keyboard is reconnected.

1 what-to-do-to-run-xmodmap-cmd-on-bt-kbd-reconnecting.sh

# Check your Bluetooth keyboard MAC Address by
sudo bluetoothctl devices

# Prepare .xmodmap file.
# It crashes "xmodmap $HOME/.xmodmap" command when there is no Caps_Lock keysym.

# Prepare a script file of "reconnect.sh", and change the MAC address placeholder and some commands in it.

# Run this command.
sudo btmon | bash reconnect.sh
# Press Ctrl+C to stop it.

2 reconnect.sh

# Change <YOUR_MAC_ADDRESS> and add or remove the commands in the deepest "if" block as you like.

while read line; do
  if echo $line | grep -q 'Connect Complete'; then
    read line
    if echo $line | grep -q 'Status: Success'; then
      read line
      read line
      if echo $line | grep -q '<YOUR_MAC_ADDRESS>'; then
        while :; do
          sleep 1
          if xmodmap $HOME/.xmodmap; then
            xset r rate 170 130 # Remove this command if you don't need it.
            # And add any other commands as you like.
            break
          fi
        done
      fi
    fi
  fi
done

3 .xmodmap

remove Lock = Caps_Lock
keysym Caps_Lock = Control_L
add Control = Control_L

4 seeds-of-this-magic.txt

"btmon" command continues to output Bluetooth log messages.
When a keyboard is connected, there are some log messages like following:

...
> HCI Event: Connect Complete (0x03) plen 11                #96 [hci0] 20.027558
        Status: Success (0x00)
        Handle: 512
        Address: 01:23:45:AB:CD:EF (DARFON LIGHTING CORP)
        Link type: ACL (0x01)
        Encryption: Disabled (0x00)
...

But, it seems that the keyboard is not detected immediately.
I guess "xmodmap .xmodmap" command fails because Caps_Lock key doesn't exist yet.

So I make this script to wait a Caps_Lock key resurrection by "sleep 1" loop.

If the log message style is different on your environment, and so the script doesn't work well,
please fix "reconnect.sh" for your environment. I believe you can do it!