Change SwayWM wallpaper with a user systemd service

This post is inspired by this blog post with a few modifications.

This is my script.

#!/bin/bash
echo start wallpaper service
pkill swaybg
while :; do
	path=$(find '/home/rivers/Pictures/WallPaper' -type f | shuf -n1)
	echo "$path"
	swaybg -i "$path" --mode fill & 
	if [ $PID ]; then
		sleep 1
		kill $PID
	fi
	PID=$!

	#custom logic on wait time before changing wallpaper again
	sleep $(expr $RANDOM / 100 + 300 )
done

The pkill at the beginning of the script might be unncessary, but it's there just in case. I am not sure if a process can kill processes outside of its cgroup to be honest. The sleep 1 is necessary because the new swaybg process needs sometime to load the image file. If the old process is killed instantly there will still be a black screen in between. The 1 second interval may need to be larger if you are reading big files from a network mounted drive, for instance.

Initially I tried to use systemd timers to achieve this, but it does not work well with the requirement that you need to start a new process before killing the old one. Also systemd timers don't allow setting more complicated logic of delay times.

The tricky part is to make sure the unit starts after sway has fully started with all the environment variables set. The sway-session.target is provided by sway-systemd which is configured by default in Fedora.

So, to make this a systemd (user) unit that start automatically on your login, create this in ~/.config/systemd/user

[Unit]
Description=change wall paper
ConditionEnvironment=WAYLAND_DISPLAY
Requires=sway-session.target
After=sway-session.target

[Service]
Type=exec
ExecStart=%h/.config/systemd/user/changeWallpaper.sh
Restart=no

[Install]
WantedBy=sway-session.target

The ConditionEnvironment=WAYLAND_DISPLAY just provides a check to see if the unit dependency is set upcorrectly. Make sure systemd generates sway-session.target.wants in the folder when you enable the unit and not other targets if you had changed it. daemon-reload will not actually clear old wants folder and it can cause unexpected dependency chains.

If you are wondering why you can specify After and WantedBy to be the same target, see this answer

Multi-monitor

If you feel a bit fancy you can use systemd template to make each monitor update independently or using more sophiscated logic like separate sets of images. Natively sway doesn't support setting wallpaper per workspace unless you use external tools.