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

[Skaperen]

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.

how does one execute a .desktop file from a script? is executing what the Exec= line has good enough?

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

no.

[Skaperen]

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

sounds like what your looking for

i cannot determine from that page if my script would be run in place of some other script. if so, that would probably make it necessary to add stuff to my script, such as actually running the DE. if so, how to run it? in the background? use exec (or its python equivalent) to replace my process? retain certain environment variables?

[halogen2]

how does one execute a .desktop file from a script? is executing what the Exec= line has good enough?

What I had in mind was the reverse: have the .desktop file execute a wrapper script that calls your script. Then yes, have the wrapper script execute what the Exec= line has.

probably make it necessary to add stuff to my script, such as actually running the DE. if so, how to run it? ... use exec (or its python equivalent) to replace my process?

This, I think.

If you try my implementation idea, it wouldn't necessarily be your main script though, you could write the wrapper script in bash and just use exec from there after your main script exits.