##Writing Units for X11 Applications

Writing daemon units is easy, it makes sense to write units for mpd, gpg-agent, etc. But what if you want every possible thing to be handled by systemd --user? You can also write units that work with X11 applications, such as your web browser, Steam, GIMP and anything else.

Multiple instances of the same service

Sometimes you want to have two of the same unit running at the same time. If your unit is called name.service and you try to run systemctl --user start name.service while name.service is already started, you will run into issues. One way to get around this is to name your unit name@.service. This way you can specify a variable when starting, stopping, or enabling the unit. Take a look at the dwb unit that I have written:

[Unit]
Description=lightweight web browser for display %i
After=i3wm.target

[Service]
Environment=DISPLAY=%i
ExecStart=/usr/bin/dwb

[Install]
WantedBy=mystuff.target

Note that in the description it has this little %i in there. When you start or enable a systemd unit, it will replace %i with the string between the @ and the .service part of your unit. So if I start dwb using systemctl --user start dwb@:0.service then it will substitue :0 wherever there is a %i, so it will look like

[Unit]
Description=lightweight web browser for display :0
After=i3wm.target

[Service]
Environment=DISPLAY=:0
ExecStart=/usr/bin/dwb

[Install]
WantedBy=mystuff.target

to systemd. I made the %i correspond to the DISPLAY variable, but you can put %i anywhere you want. For tmux I have it specify which target session it will start, so if I start tmux@wgiokas.service and tmux@not-me.service they will not confilict and can be tracked separately.

Some notes

X11 does some weird thing with the DISPLAY variable, and for most units that run or require an X server you will have to add a line reading Environment=DISPLAY=<your display or %i> I personally try to use the %i as often as possible because it allows me to change the display without having to change the unit itself.