Help with creating Systemd Timers to Automate Window Theme Changes

Question

I am trying to get Ubuntu 20.04 LTS ("Focal Fossa") to automatically switch to the "light" Window Theme at 0600 (6AM) every morning, and the "dark" Window Theme at 1800 (6PM) every night.

The following Terminal command can be used to change to the "light" Window Theme:

gsettings set org.gnome.desktop.interface gtk-theme Yaru-light

The following Terminal command can be used to change to the "dark" Window Theme:

gsettings set org.gnome.desktop.interface gtk-theme Yaru-dark

But as noted above, I would like to automate this process.

The first suggestion presented to me was via a cron job, however this has repeatedly proved unsuccessful, so another user suggested a more "modern" approach, via Systemd "timers"... Unfortunately, I am unfamiliar with Systemd and the process of creating timers, so I have be learning as I go, without success to date.

At this time, I have six files in my "home" folder:

• dark.service
• dark.timer
• dark.sh
• light.service
• light.timer
• light.sh

The contents of "dark.service" is:

[Unit]
Description=Automatically change the "Window Theme" to "dark" in the evening.

[Service]
ExecStart=/home/gregory/dark.sh

[Install]
WantedBy=dark.sh

The contents of dark.timer is:

[Unit]
Description=Automatically change the "Window Theme" to "dark" in the evening.

[Timer]
OnCalendar=*-*-* 18:00:00
Persistent=true

[Install]
WantedBy=dark.service

The contents of "dark.sh" is:

export DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
gsettings set org.gnome.desktop.interface gtk-theme Yaru-dark

The contents of "light.service" is:

[Unit]
Description=Automatically change the "Window Theme" to "light" in the morning.

[Service]
ExecStart=/home/gregory/light.sh

[Install]
WantedBy=light.sh

The contents of "light.timer" is:

[Unit]
Description=Automatically change the "Window Theme" to "light" in the morning.

[Timer]
OnCalendar=*-*-* 06:00:00
Persistent=true

[Install]
WantedBy=light.service

The contents of "light.sh" is:

export DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
gsettings set org.gnome.desktop.interface gtk-theme Yaru-light

I used "Startup Application Preferences" (gnome-session-properties) to run "light.timer" and "dark.timer" on login.

Based on the advice I have been given elsewhere and what I have read online, I think creating "timers" via Systemd is probably the right approach to achieve what I want (automatic switching between "light" and "dark" Window Themes, based on the time of the day)... I just need a little help getting things working, as Systemd, timers and scripts are a whole new world to me.

Answer 1

(Late edit of WantedBy=default.service to WantedBy=default.target)

This may be a long journey, but here are some pointers. All of the WantedBy entries are wrong. They cannot depend on a shell script. A timer unit x.timer is always associated with a service unit x.service. If you were to start the timers, they will start their service at the given time. But you would need to do this at each login. So instead you need to enable the timers, and have systemd start them automatically. To do this you need to have the timers WantedBy= a target that is fulfilled at login for users. Typically, this is default.target. Though there are many system targets, there are very few user targets.

Since your scripts are very short, you might like to include them in the service units, to keep things smaller. The service and timer files have to be in ~/config/systemd/user/. You need to create this directory if it does not exist. Eg: dark.service:

[Unit]
Description=Automatically change the "Window Theme" to "dark" in the evening.
[Service]
Environment=DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
ExecStart=/usr/bin/gsettings set org.gnome.desktop.interface gtk-theme Yaru-dark

dark.timer:

[Unit]
Description=Automatically change the "Window Theme" to "dark" in the evening.
[Timer]
OnCalendar=*-*-* 18:00:00
Persistent=true
[Install]
WantedBy=default.target

light.service:

[Unit]
Description=Automatically change the "Window Theme" to "light" in the morning.
[Service]
Environment=DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
ExecStart=/usr/bin/gsettings set org.gnome.desktop.interface gtk-theme Yaru-light

light.timer:

[Unit]
Description=Automatically change the "Window Theme" to "light" in the morning.
[Timer]
OnCalendar=*-*-* 06:00:00
Persistent=true
[Install]
WantedBy=default.target

Whenever you change the content of these files, or add or remove files from this directory, you need to notify systemd by giving the command:

systemctl --user daemon-reload

I'm not sure what gnome "Startup Application Preferences" does. The systemd method to start using these units is to do, once only, not as root:

systemctl --user enable dark.timer light.timer

Now, whenever you login, these units will be started, and stopped when you logout.

---

While developing the units you may want to test them immediately, without needing to login/logout. You can start them explicitly with:

systemctl --user start dark.timer light.timer

and you can stop them with

systemctl --user stop dark.timer light.timer

You need to stop them before you can start them again. You can combine stop and start with

systemctl --user restart dark.timer light.timer

You can check their status with

systemctl --user list-timers

which should say when they will next trigger, and what service they will run.

It would be nice to have a tutorial on systemd user units, but I haven't found anything suitable. They mostly talk about system units. You can glean some basics from this large redhat document, but it is for administrators, so a little too detailed, but it would help to understand that some of it is in common for user units.