90-backlight.rules

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"

backlight.sh

#!/bin/bash

# script to adjust backlight brightness on thinkpad x250
# usage:
# ./backlight inc
# ./backlight dec

read max < /sys/class/backlight/intel_backlight/max_brightness
read cur < /sys/class/backlight/intel_backlight/actual_brightness

steps=10
per_step=$((max / steps))
current_step=$((cur / per_step))

if [ "$1" == "inc" ]
then
	next_step=$((current_step + 1))
	if [ $next_step -gt $steps ]
	then
		next_step=$steps
	fi
elif [ "$1" == "dec" ]
then
	next_step=$((current_step - 1))
	if [ $next_step -lt "0" ]
	then
		next_step="0"
	fi
fi

brightness=$((next_step * per_step))

notify-send -t 250 Brightness "${next_step}/${steps}"

echo $brightness > /sys/class/backlight/intel_backlight/brightness