Yesterday, I tried to update to the second public beta of macOS Sierra. For some reason, however, the update process stopped shortly before the completion, and since there was no progress even after many hours, I decided to shut down.
Everything works fine as far as the startup is concerned, but I can not login with my regular user account. While the password is recognized as correct, the display turns black when my data are supposed to be loaded, and I am again asked to enter my password.
Neither restarts nor safe boots were of help. I also reinstalled the first public beta via recovery mode completely, but the problem is still there, even in safe mode. Apparently, all of the disk data are still there as well, but I can simply not login due to the black screen. I saw that another user could solve the problem with the reinstallation, but in my case, the issue persists even after reinstalling the first public beta.
I tried to create a new admin user via Terminal to check if that account has the same issues. But the problem is that the sudo command, even whereis sudo and which sudo only gives me the line command not found after sudo, whereis and which, respectively.
echo $PATH gives me the response /usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin. That seems rather long; is it alright?
I would appreciate your help, be it regarding the login issue or enabling the sudo command.