Mirror Question: https://unix.stackexchange.com/questions/217300/where-how-is-user-group-information-stored-in-ubuntu. (I’ll remove one of them after I got the answer) Two places possible: /etc/group and /etc/passwd. If I use command: adduser [username] [groupname], then the user would be added to the group, and the file /etc/group would then be updated. However, the file /etc/passwd is not updated. if I check which group I belongs to, via groups