#!/bin/bash
max_brightness=$(cat /sys/class/backlight/intel_backlight/max_brightness)
brightness=$(cat /sys/class/backlight/intel_backlight/brightness)
step=$(( max_brightness / 8 ))
inc=2
low_delta=$(( brightness - step ))
high_delta=$(( brightness + step ))

if { [ "$1" = "down" ] && [ $low_delta -ge $step ]; }  ||  { [ "$1" = "up" ] && [ $high_delta -le "$max_brightness" ]; }
then
    for i in $(seq $step); do
        case $1 in
        'up')
            brightnessctl s +$inc 1> /dev/null
        ;;
        'down')
            brightnessctl s $inc- 1> /dev/null
        ;;
        esac
    done
else
    echo "Brightness too high or low"
fi