Adjust external monitor brightness

#!/usr/bin/env bash

# check if external monitor connected
xrandr -q | grep -q "DisplayPort-4 connected" || exit 0

(
  # Wait for lock on /var/lock/adjust_external_monitors_brightness.exclusivelock (fd 200) for 20 seconds
  flock -x -w 20 200 || exit 1

  # main screen brightness (from 0 to 255)
  main=$(cat /sys/class/backlight/amdgpu_bl0/brightness)

  # external monitor
  l27_max=100 # max brightness
  l27_0=40 # equivalent brightness of main for l27 min brightness
  l27_100=255 # equivalent brightness of main for l27 max brightness

  # equivalent brightness for the external monitor
  l27=$(( 100 * ( (main > l27_0 ? main : l27_0) - l27_0 ) / (l27_100 - l27_0) ))

  # try to set it until success
  until ddccontrol -r 0x10 -w $l27 dev:/dev/i2c-11 | grep -q "Writing 0x10";
  do
    sleep 1;
  done

) 200>/var/lock/adjust_external_monitors_brightness.exclusivelock