Great question — answering as the UphillTech team:
Mistake 1: You tried to edit useradd's default instead of creating a user. Your biggest mistake is that you used useradd -D which tells useradd to "edit the defaults in /etc/default/useradd ". In your specific case, you were hiding the -D flag in the -Du argument. So your command DID NOT CREATE A USER. Furthermore, when setting new defaults ( -D mode), only certain flags can be used, which is why you got the error saying adduser: unrecognized option: d (because -d /path/to/the/new/users/home CANNOT be used with -D ). The only default you can change related to user home dir is -b /home (which chooses the base dir where all usernames will be placed under). Edit: Also see the followup comment...
Full answer by Mitch McMabers: https://unix.stackexchange.com/a/807354 (CC BY-SA 4.0).
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