there is no ~/.xsession file - what selects the DE to run?

[Skaperen]

there is no ~/.xsession file - what selects the DE to run? i have a script that back traces process parentage/ancestry but it goes from terminal to init (PID 1). so everything was backgrounded and i can't see what started what without stracing everything opendm does.

[halogen2]

The display manager. You choose it in the display manager greeter before you log in. For Xubuntu at least some of the options are stored in /usr/share/xsessions/ I don't know if that's the only place though.

[Skaperen]

what chooses to start Xfce in Xubuntu ... or ... what designates that the user is an Xubuntu/Xfxe (or whatever DM) user? is it something in lightdm?

what i want to do is run some additional code (in a Python script or an ELF binary ... that i could build statically linked if necessary, so PATH is not needed, yet) for every user at login time. running this AS that user with no privilege to become root is OK. but needs to, at least, know which user (it will switch real and effective user to be that user, if it is root). this code is intended to do some cleanup before anything (else) starts.

[GhX6GZMB]

I run Lubuntu myself with SDDM as display manager, but I imagine the setups are pretty much alike.

The items for the environment choices during login are in /usr/share/xsessions

As far as I can tell, the default or last chosen environment is defined in etc/sddm.conf, in your case probably /etc/lightdm.conf or /etc/lightdm/lightdm.conf

https://wiki.ubuntu.com/LightDM

[halogen2]

what chooses to start Xfce in Xubuntu ... or ... what designates that the user is an Xubuntu/Xfxe (or whatever DM) user? is it something in lightdm?

Yes. I wonder if that is stored in /var/cache/lightdm ?

what i want to do is run some additional code (in a Python script or an ELF binary ... that i could build statically linked if necessary, so PATH is not needed, yet) for every user at login time.

Can you try editing the file in /usr/share/xsessions (or copying one and creating a new one) to somehow run your Python script before running the existing user session command?

[deadflowr]

User info like Xsession the user has set is stored in /var/lib/AccountsService/users/Username.

[GhX6GZMB]

That one's empty on my machine.

[Skaperen]

if something(s) hold what DE the user has chosen then i need the map of DE to /what/to/run. either i fudge the map or stub the files to run. i need my script to run ASAP after user credentials are authenticated for a login. i will need to know or find out which DE the user is expecting (iff they chose a different one at login then i need to know or find out what that is).

[halogen2]

If I were doing this I would edit (or better, if possible, copy somewhere it would override the original and edit the copy) the /usr/share/xsessions/*.desktop files to point to a wrapper script that calls your script with the DE name as an argument, and after your script is done exec the command that was originally in said .desktop file. I'd have your script keep its own track of what DEs the user logs into.

Unless you also need this for non-GUI/tty sessions?

[dddman]

An xprofile file, ~/.xprofile and /etc/xprofile, allows you to execute commands at the beginning of the X user session - before the window manager is started.

https://wiki.archlinux.org/title/Xprofile

sounds like what your looking for