Backlight brightness control for MacBook Air 11" on Gentoo using Intel driver. (Adapted from https://wiki.gentoo.org/wiki/Apple_Macbook_Pro_Retina_(early_2013)#Display_backlight)

bl-brightness

#!/bin/bash
bldb='/sys/class/backlight/intel_backlight/brightness'
step=113
current=`cat $bldb`
new=$current
if [ "$1" == "up" ];then
   new=$(($current + $step))
elif [ "$1" == "down" ];then
   new=$(($current - $step))
fi
if [ $new -le 0 ];then
   new=0
fi
echo $new > $bldb
current=`cat $bldb`

Not sure if it's a good idea, but can chgrp wheel that file and and chmod g+w to allow users in wheel group to write to it. Make sure to chmod g+x bl-brightness