Yes you canEven in Debian you can not add a user if you want.
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I have never seen any Linux, Bsd, or Unix, that root could not add users to the system, but have seen some that do not automatically set up a non root user account, in other words , the base install is just "root" as the default user,..but even on those, the install documentation recommends setting up a "non administrative" user, immediately, it is generally one of the first things one needs to do after the initial install. Most , installers now a days, do include the option to setup a normal user account, but one does not have to do that.
Just saying,..
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I apologize, worded that badly, maybe "break" or "broken" is not the right way to say it, but what I meant, for example, a wrong or bad configuration, or something, there have been several threads where the user could not get sudo to work, and in at least 1 , we had to explain how to boot as single user, and set a root password , so the user can log in as root, and start repairing the problem.How would sudo break? Or any other pacgkage. If using stable Debian, there might not be any other reason than wrong configuration, and that can be fixed.
Even in this thread, when the OP started, in order to run the usermod command, they needed to use 'su', to modify how they had sudo set up.
Yes, I agree normally using "sudo", instead of all ways logging in as root, is better, etc,... just saying it is a good idea to set a root password, and have that setup correctly as well, just in case for some reason sudo does not work.
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to remove the previous edit.