Or just change manually the id after installing the package:
https://www.cyberciti.biz/faq/linux-change-user-group-uid-gid-for-all-owned-files/
As setting a fixed UID in the preinstall script could, potentially, encounter a conflict for someone as the id has been already taken (very unlikely, but still posible, for example there is another package in AUR that creates a user with the ID 999 too), and as setting an UID < 1000 is not needed by almost anyone, I prefer that those that wants it to be < 1000 change it manually after the install, which is just a few commands. Also, if you want this procedure to be automatic for production (installing guest user for a lot of machines and also setting UID < 1000) then you can just compile the package, and edit the systemd-guest-user.install file to just set manually there the UID in the useradd command, as you said
Pinned Comments
akrai commented on 2018-08-01 14:49 (UTC) (edited on 2019-11-04 16:11 (UTC) by akrai)
Installing the package gets your system ready to use the guest user, which is basically a normal user with a tmpfs home folder (which gets erased after every reboot, not after a logout), but please notice two things:
You probably want to add the guest user to some system groups you may consider in order to be able to use some resources of your computer, the same way as any other human user
The script copies any file placed in the /etc/skel/ folder to the guest home after every reboot, intended mainly for copying config files you may consider useful to copy in every new guest session, for example if you need special graphical or network config files placed in order to get a properly working guest user. Any suggestion is welcomed