Shell script to adjust backlight by absolute or relative values

light.sh

#!/bin/sh
backlight=$(find /sys/class/backlight -type l -print -quit)
if [ -z "$backlight" ]; then
    echo "Backlight not found"
    exit 1
fi
max_brightness=$(cat "$backlight/max_brightness")
current_brightness=$(cat "$backlight/brightness")
case "$1" in
    +[0-9]*%)
        percentage="${1#"+"}"
        percentage="${percentage%"%"}"
        change=$(((current_brightness * percentage + 50) / 100))
        if [ $percentage -ne 0 ] && [ $change -eq 0 ]; then
            change=1
        fi
        new_brightness=$((current_brightness + change))
    ;;
    -[0-9]*%)
        percentage="${1#"-"}"
        percentage="${percentage%"%"}"
        change=$(((current_brightness * percentage + 50) / 100))
        if [ $percentage -ne 0 ] && [ $change -eq 0 ]; then
            change=1
        fi
        new_brightness=$((current_brightness - change))
    ;;
    +[0-9]*)
        change="${1#"+"}"
        new_brightness=$((current_brightness + change))
    ;;
    -[0-9]*)
        change="${1#"-"}"
        new_brightness=$((current_brightness - change))
    ;;
    [0-9]*)
        new_brightness="$1"
    ;;
    "")
        echo Usage:
        echo "   light.sh <ABSVAL>       - set absolute value (0-255)"
        echo "   light.sh [+-]<ABSDELTA> - add/subtract from absolute value"
        echo "   light.sh [+-]<REL>%     - change relative to current level"
        exit 0
    ;;
    *)
        echo "Invalid backlight change (either absolute value, +/-delta, or +/-relative%)"
        exit 1
    ;;
esac

if [ $new_brightness -gt $max_brightness ]; then
    new_brightness=$max_brightness
elif [ $new_brightness -lt 0 ]; then
    new_brightness=0
fi
echo $new_brightness > "$backlight/brightness"