What program starts dbus-daemon?

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http://askubuntu.com – What program starts the per user session dbus-daemon process in GNOME 2 and GNOME 3 (presumably via dbus-launch)? I would like to know because I would like to add a directory to the list of directories that dbus-daemon checks for .service files. (Obviously if it's a sh program, "sh" isn't the answer I'm looking for ;-) How could I have answered this question for myself efficiently? What documentation is there on desktop and session startup? (HowTos)