i want to useradd a user that doesn't show up in the login window, is using the -r (system user) flag a good idea for that? is there another option that i can't find?

Depending on which display manager you use you could always just use a theme that requires typing a username rather than selecting from a list. I know there are a few SDDM themes like that.

The answer will depend on which desktop environment or login manager you're using.

System users are not the right solution. The use-case for such accounts is when you want certain background services to be linked to a non-human account. Eg: Serving web requests from an http user account that only has access to nginx and the /var/www directory. By default, users created in this way don't even have a home directory.

i'm using kde with sddm i believe

on another pc i actually do do that, but i was wondering if there was another way

In that case, it looks like you need to edit sddm.conf and set the HideUsers= field. Source.

I did some quick research and SDDM can do what you want by editing the sddm.conf file. You just have to add the username to HideUsers in the [Users] section.

ah i haven't thought of that
thank you!

thank you!

is it normal that the sddm.conf is empty? it's supposed to be /etc/sddm.conf right?

Your system may be reading the default config from /usr/lib/sddm/sddm.conf.d/default.conf, in which case the config file in /etc will probably be empty. You should not edit the contents of a default config.

The recommended location for sddm.conf is within the /etc/sddm.conf.d/ directory, so also check for files there.