Change linux screen brightness by a percentage

change_brightness.sh

#!/usr/bin/env bash

# Mimics the functionality of xbacklight by directly writing to a file.
# Must be run as a user with permission to write to the brightness file.
#
# usage:
# 
# sudo ./change_brightness.sh 10    # increase screen brightness by 10%
# sudo ./change_brightness.sh -10   # decrease screen brightness by 10%

BACKLIGHT_FILE=/sys/class/backlight/intel_backlight/brightness
PERCENTAGE_CHANGE=$1
# divide by one to round to an integer, since the file only accepts integer values
echo "((1 + 0.01 * $PERCENTAGE_CHANGE) * $(cat $BACKLIGHT_FILE))/1" | bc > $BACKLIGHT_FILE