#!/bin/bash
BACKLIGHT="/sys/class/backlight/intel_backlight/brightness"

declare -i MAXVAL
declare -i MINVAL
MAXVAL=1388
MINVAL=0

if [ "$#" -ne 1 ]
then
        echo "Usage: ./brightness.sh <+-number>"
        exit 0
fi

declare -i amount
amount=$1

declare -i current
current=$(cat $BACKLIGHT)

declare -i added
added=amount+current

if (( added > MAXVAL ))
then
        added=MAXVAL
elif (( added < MINVAL ))
then
        added=MINVAL
fi
echo "$current -> $added to $BACKLIGHT"
echo "$added" > $BACKLIGHT