#!/bin/bash -e

STEPS=16
DIR="$(printf '%s\n' /sys/class/backlight/* | head -n1)"

CUR="$(cat "$DIR/brightness")"
MAX=$(cat "$DIR/max_brightness")

case "${1:0:1}" in
    '')      echo "$CUR"; exit ;;
    '+'|'-') CUR="$(((CUR / STEPS + $1) * STEPS))" ;;
    *)       CUR="$1" ;;
esac

[ "$CUR" -lt 0 ] && CUR=0
[ "$CUR" -gt "$MAX" ] && CUR="$MAX"

echo "$CUR" > "$DIR/brightness"