Script to quickly change the brightness value on Linux

brightness.sh

#!/bin/sh

low_brightness_clamp=200  # you should probably change this depending on the brightness range of your driver
dev=$1
prm=$2

sys="/sys/class/backlight/$dev"

if [ ! -e "$sys" ]; then
  >&2 echo "device not found"
  exit 1
fi

[ -n "$prm" ] && [ "$prm" -eq "$prm" ] 2>/dev/null
if [ $? -ne 0 ]; then
  >&2 echo "invalid input"
  exit 2
fi

max_brightness=$(cat "$sys/max_brightness")
brightness=$(cat "$sys/brightness")

new_brightness=$(($brightness + $prm))

if [ $new_brightness -gt $max_brightness ]; then
  new_brightness=$max_brightness
fi

if [ $new_brightness -lt $low_brightness_clamp ]; then
  new_brightness=$low_brightness_clamp
fi

echo $new_brightness > "$sys/brightness"

udev

ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness"
ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"

You'd probably want to put the script in /usr/bin/brightness, and the udev rules in /etc/udev/rules.d/50-backlight.rules.

Then you should add your user to the video group with
usermod -aG [username]

Restart your PC to have the udev rules applied, then use as e.g.:

brightness intel_backlight +500
brightness intel_backlight -500

(you can find out what to put in the place of "intel_backlight" by looking in /sys/class/backlight/)