#!/bin/bash -e

STEPS=16
DIR="$(find /sys/class/backlight/* -print -quit)"

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

if [ "$CUR" -lt 0 ]; then
    CUR=0
elif [ "$CUR" -gt "$MAX" ]; then
    CUR="$MAX"
fi

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