Script to use xbacklight and prevent full black screen

light.sh

#!/bin/sh

val=$(xbacklight -get)

increase_light() {
    # Try to be a bit progressive when in low brightness
    if [ $val -le 20 ] ; then
        percent=2
    else
        percent=5
    fi

    new_value=$(($val + $percent))

    if [ $new_value -ge 100 ] ; then
        exit 0
    fi

    xbacklight -inc $percent
}

decrease_light() {
    if [ $val -ge 50 ] ; then
        percent=10
    elif [ $val -ge 30 ] ; then
        percent=5
    else
        percent=2
    fi

    new_value=$(($val - $percent))
    if [ $new_value -le 1 ] ; then
        exit 0
    fi

    xbacklight -dec $percent
}

if [ "$1" = "inc" ] ; then
    increase_light
else
    decrease_light
fi

# Use:
# `light.sh inc` or `light.sh dec`