Some bash functions to manage screen brightness when your fn+<key> does not work.

brightness_control_i3wm.sh

function max() {
    if [ $1 -gt $2 ]
    then
        echo $1
    else
        echo $2
    fi
}

function min() {
    if [ $1 -lt $2 ]
    then
        echo $1
    else
        echo $2
    fi
}

function decbright() {
    brightness=$(cat /sys/class/backlight/intel_backlight/brightness)
    let new=$brightness-300
    let brightness=$(max $new 0)
    echo $brightness | sudo tee /sys/class/backlight/intel_backlight/brightness
}


function incbright() {
    max_brightness=$(cat /sys/class/backlight/intel_backlight/max_brightness)
    brightness=$(cat /sys/class/backlight/intel_backlight/brightness)
    let new=$brightness+300
    let brightness=$(min $new $max_brightness)
    echo $max_brightness
    echo $brightness | sudo tee /sys/class/backlight/intel_backlight/brightness
}