qmk_firmware_custom/keyboards/ergodox/keymaps/algernon/tools/layer-notify

#!/bin/sh
HL="${HID_LISTEN:-$HOME/src/ext/hid_listen/hid_listen}"

sudo "${HL}" | grep --line-buffered LAYER: | \
(while read line; do
     case $line in
         LAYER:*)
             layer="$(echo $(echo $line | cut -d: -f2-))"
             notify-send -i mark-location-symbolic "Switched to layer: $layer"
             ;;
     esac
 done)