linux: set brightness using bash for later assign a key bind

brightness.sh

#!/bin/bash
# Author: Sebastian Treu
# Date: 2018.06.01

#
# You will need root access for using this script. The best thing you can do
# is to add this script to the sudoers file (or just don't use it :)
#
usage() {
    echo "Usage:"
    echo -e "\t$0 { [+|-]threshold }"
    echo
    echo "Increase, decrease or set the brightness"
    echo
    echo "Example 1: Increase brightness +100 without exceeding max brightness value: ./brightness +100"
    echo "Example 2: Set brightness to 100: ./brightness 100"
    echo "Example 3: Decrease brightness by default increase threshold: ./brightness -"
}

# threshold argument must be passed
if [ $# -ne 1 ]; then
    usage
    exit 1
fi

# valid threshold argument is: (+|-)?[0-9]+
# i.e. +100, -30, 91, -, +
if ! $(echo ${1} | grep -q "^\(+\|-\)\?[0-9]*$"); then
    usage
    exit 1
fi

# Default threshold used when invoked "./brightness +"
DEFAULT_THRESHOLD=10
# Tune this path
BACKLIGHT_PATH=/sys/class/backlight/intel_backlight
BACKLIGHT_MAX_BRIGHTNESS_PATH=${BACKLIGHT_PATH}/max_brightness
BACKLIGHT_BRIGHTNESS_PATH=${BACKLIGHT_PATH}/brightness

# get the maximum brightness in order to not exceed that value
if [ -r ${BACKLIGHT_MAX_BRIGHTNESS_PATH} ]; then
    MAX_BRIGHTNESS=$(cat ${BACKLIGHT_PATH}/max_brightness)
else
    echo "Could not get MAX_BRIGHTNESS from ${BACKLIGHT_MAX_BRIGHTNESS_PATH}."
    exit 1
fi

if [ -w ${BACKLIGHT_BRIGHTNESS_PATH} ]; then
    case $1 in
        +*|-*)
            # this will strip of the minus or plus sign
            OPERATION=${1:0:1}
            # this will strip of the threshold value
            THRESHOLD=${1:1}
            # this will add or subtract the threshold and will use the default threshold if none supplied
            VALUE=$(($(cat ${BACKLIGHT_PATH}/brightness) ${OPERATION} ${THRESHOLD:-${DEFAULT_THRESHOLD}}))
            ;;
        *)
            # no plus nor minus sign, set the threshold argument as the value
            VALUE=${1}
            ;;
    esac
    # dont allow to exceed the max brightness
    if [ ${VALUE} -ge ${MAX_BRIGHTNESS} ]; then
        VALUE=${MAX_BRIGHTNESS}
    fi
    # set the brightness :)
    echo ${VALUE} > ${BACKLIGHT_BRIGHTNESS_PATH}
else
    echo "Could not set brightness."
    exit 1
fi