Note on managing UID and gid for CNU/Linux Users

① What a user can do depends on his/her UID and GID.

② File permissions involve three types of users:
● Owner
● Group
● Other

③ There are three files for user management:
●/Etc/passwd: there are 7 fields, the number 3 is uid, And the number 4 is GID, which can be accessed by any user
●/Etc/shadow: contains nine fields, stores the encrypted password of the user in the passwd file, and only the root user can view the password.
●/Etc/group

④ How useradd works is to modify the three files mentioned in ③ and create a home directory.

⑤ Add a user name to the group to which the user belongs.