#!/bin/sh
set -eu

BRIGHTNESS_DIFF="$1"

HANDLER=/sys/class/backlight/"$BACKLIGHT_DEVICE"

OLD_BRIGHTNESS="$(cat "$HANDLER"/brightness)"
MAX_BRIGHTNESS="$(cat "$HANDLER"/max_brightness)"
SUM=$((OLD_BRIGHTNESS + BRIGHTNESS_DIFF))
NEW_BRIGHTNESS="$(clamp -- "$SUM" 0 "$MAX_BRIGHTNESS")"
echo "$NEW_BRIGHTNESS" > "$HANDLER"/brightness || sudo chmod 666 "$HANDLER"/brightness