Ubuntu + i3-wm laptop display brightness control - a helper script that does not require sudo/root privileges

brightness.sh

#!/usr/bin/env bash

set -eu

# Helper script for controlling laptop screen brightness WITHOUT sudo/root privileges.
# 
# Tested on Lenovo Thinkpad x240 running Ubuntu 18.04.
#
# Author: Markus H (MawKKe) markus@mawkke.fi


# --------------------------------------------------------------------
# Install instructions: 
# 1) Add this file somewhere in your PATH and mark it excutable (chmod +x <...>)
# 2) Add the following in your i3-wm config:
#
#   bindsym XF86MonBrightnessUp   exec brightness.sh inc 
#   bindsym XF86MonBrightnessDown exec brightness.sh dec 
#
# 3) Reload i3 (Mod-shift-r or something) 
#
# ...and now your laptop keys should control the backlight!
#
# You may notice that this script does not require root privileges,
# unlike 'xbacklight' or all the crappy "echo $num > /sys/class.." solutions do :)
#
# --------------------------------------------------------------------

# all changes happen via this PolicyKit helper program
# Unfortunately it does not support relative incrementation.
# Solution found at https://forum.xfce.org/viewtopic.php?id=11956
cmd="pkexec /usr/lib/gnome-settings-daemon/gsd-backlight-helper"

# NOTE these may change depending on hardware.
# Run $cmd --get-max-brightness to get the maxv value; here we
# hardcode them to avoid reading the value on every keypress..
# The lower limit (minv) is unlikely to change; replace maxv with value on your system.
minv=1
maxv=851

amount_default=100

if [ $# -lt 1 ]; then
    echo "Usage: $0 inc|dec|get [<delta:integer>]"
    echo "By default changes brightness value by ${amount_default}. Override by specifying 'delta'."
    echo "Current brightness limits: min=${minv} maxv=${maxv}"
    exit 1
fi

direction=$1

# user value or 100
amount=${2:-${amount_default}}

if ! [[ "$amount" =~ ^[0-9]+$ ]]; then
    echo "Error: argument to inc/dec argument must be an integer"
    exit 2
fi


set_value(){
    val=$1
    if [ $val -gt $maxv ]; then
        val=$maxv
    fi
    if [ $val -lt $minv ]; then
        val=1
    fi

    echo "Setting brightness to $val"
    $cmd --set-brightness $val
}

current_value=$($cmd --get-brightness)

case $direction in
    get)
        echo "Current value is: $current_value"
        ;;
    inc)
        set_value $(($current_value + $amount))
        ;;
    dec)
        set_value $(($current_value - $amount))
        ;;
    *)
        echo "Argument not understood. Allowed: 'inc', 'dec', 'get'"
        exit 2
    ;;
esac