Ubuntu – Write application indicator with no icon

I think that the need here is actually more determining of the python-appindicator library is present. If it is present, it will support all the fallback cases that you need. It will handle XFCE, KDE and older GNOME appropriately. Good example of how to do it in this answer.

The appindicator library will use DBus to check if the application indicator rendering process is available. This will be the case on Unity, or if the indicator-applet is running. If it is available it will use that, if not, it will fallback to using a GtkStatusIcon with the same menu.

Unfortunately, I believe you'd have to keep both code paths if you want to handle the case of the library not being available. Though, we'd be happy to help get the library in other distros :)

The Application Indicator Service has an internal API that will allow grabbing a list of all the indicators, which includes their DBus addresses. You can do that using this command line:

gdbus call --session --dest com.canonical.indicator.application --object-path /com/canonical/indicator/application/service --method com.canonical.indicator.application.service.GetApplications

You'll get back something not super useful, as it's in a psuedo-JSON format. If you write a script you can pull it into a parser and work with it. If you're curious about doing it just once, just read through the text. You're looking for a field that is like this: :1.XXX That is the DBus address for the indicator.

Once you have the address for the indicator you can ask DBus what the PID is for that indicator. You request the PID of :1.123 like this:

gdbus call --session --dest org.freedesktop.DBus --object-path / --method org.freedesktop.DBus.GetConnectionUnixProcessID :1.123

After you now have the PID you can ask /proc for information on the process. For example if you wanted to know the command use to launch process number 1 you could do:

cat /proc/1/cmndline

Good luck!