There are currently many ways of starting software when a user session starts.

This is an attempt to collect a list of pointers to piece the big picture together. It's partial and some parts might be imprecise or incorrect, but it's a start, and I'm happy to keep it updated if I receive corrections.

x11-common

man xsession

• Started by the display manager for example, /usr/share/lightdm/lightdm.conf.d/01_debian.conf or /etc/gdm3/Xsession
• Debian specific
• Runs scripts in /etc/X11/Xsession.d/
• /etc/X11/Xsession.d/40x11-common_xsessionrc sources ~/.xsessionrc which can do little more than set env vars, because it is run at the beginning of X session startup
• At the end, it starts the session manager (gnome-session, xfce4-session, and so on)

systemd --user

• https://wiki.archlinux.org/index.php/Systemd/User
• Started by pam_systemd, so it might not have a DISPLAY variable set in the environment yet
• Manages units in:
  • /usr/lib/systemd/user/ where units provided by installed packages belong.
  • ~/.local/share/systemd/user/ where units of packages that have been installed in the home directory belong.
  • /etc/systemd/user/ where system-wide user units are placed by the system administrator.
  • ~/.config/systemd/user/ where the users put their own units.
• A trick to start a systemd user unit when the X session has been set up and the DISPLAY variable is available, is to call systemctl start from a .desktop autostart file.

dbus activation

• https://dbus.freedesktop.org/doc/system-activation.txt
• A user process making a dbus request can trigger starting a server program
• For systems debugging, is there a way of monitoring what services are getting dbus activated?

X session manager

• https://en.wikipedia.org/wiki/X_session_manager
• Run by x11-common's Xsession.d
• Runs freedesktop autostart .desktop files
• Runs Desktop Environment specific software

xdg autostart

• https://specifications.freedesktop.org/autostart-spec/autostart-spec-latest.html
• Run by the session manager
• If /etc/xdg/autostart/foo.desktop and ~/.config/autostart/foo.desktop exist then only the file ~/.config/autostart/foo.desktop will be used because ~/.config/autostart/ is more important than /etc/xdg/autostart/
• Is there an ordering or is it all in parallel?

Other startup notes

~/.Xauthority

To connect to an X server, a client needs to send a token from ~/.Xauthority, which proves that they can read the user's provate data.

~/.Xauthority contains a token generated by display manager and communicated to X at startup.

To view its contents, use xauth -i -f ~/.Xauthority list