Change random background from directory with persistence support.

waller.sh

#!/usr/bin/env bash

wallpapers_dir="$HOME/Pictures/Wallpapers"

get_random_file() {
	find "$1" -type f -print0 | xargs -0 shuf -e -n 1 -z
}

write_selected_wall() {
	echo "$1" >"$HOME/.current_wall"
}

write_selected_lock() {
	echo "$1" >"$HOME/.current_lock"
}

set_wallpaper() {
	if [ -n "$1" ]; then
		swaymsg output '*' bg "$1" fill
		write_selected_wall "$1"
	fi
}

set_lock() {
	swaylock --font "pango:Liberation Sans 10" -e -f -t -F -i "$1"
	write_selected_lock "$1"
}

loop() {
	while true; do
		wall=$(get_random_file "$1")
		set_wallpaper "$wall"
		sleep $((3600 * 2)) # 1 hour x 2
	done
}

if [ "$#" -ne 1 ]; then
	if [ -f "$HOME/.current_wall" ]; then
		set_wallpaper "$(cat "$HOME"/.current_wall)"
	else
		set_wallpaper "$(get_random_file "$wallpapers_dir")"
	fi
else
	if [ "$1" == "loop" ]; then
		loop "$wallpapers_dir"
		exit
	elif [ "$1" == "lock" ]; then
		if [ -f "$HOME/.current_lock" ]; then
			set_lock "$(cat "$HOME/.current_lock")"
		else
			set_lock "$(get_random_file "$wallpapers_dir")"
		fi
	elif [ -d "$1" ]; then
		if [ -n "$(ls -A "$1")" ]; then
			wall="$(get_random_file "$1")"
			if [ -n "$2" ] && [ "$2" = "path" ]; then
				exit 0
			elif [[ "$2" =~ ^[0-9]+$ ]]; then
				loop
			fi
			set_wallpaper "$wall"
		else
			echo "Dir is empty"
		fi
	elif [ -f "$1" ]; then
		set_wallpaper "$1"
	else
		echo "Path does not exist"
	fi
fi

Below first line loops at selected interval and second one configures swayidle and it's background.

If loop is not given in its first argument it gets random wallpaper and persists until state is deleted. So deleting ~/.current_wall or ~/.current_lock will cause a reset.

# config
exec_always '/path/to/file/waller.sh loop'

exec '/path/to/file/waller.sh lock'