Sets random wallpaper every 30 minutes using feh(A lightweight and powerful image viewer).

auto-wall

#!/usr/bin/bash
img=(`find ~/pictures/wallpapers/ -name '*' -exec file {} \; | grep -o -P '^.+: \w+ image' | cut -d':' -f1`)
while true
do
   feh --bg-scale "${img[$RANDOM % ${#img[@]} ]}"
sleep 30m
done

Single line using shuf

shuf -e -n1 ~/Pictures/nordic-wallpapers/wallpapers/* | xargs feh --bg-scale