Note on managing UID and GID for CNU/Linux Users

Note on managing UID and GID for CNU/Linux users ① what a user can do depends on UID and GID. ② file permissions involve three types of users: ● owner ● group ● other ③ manage three User Files: ●/etc/passwd: there are 7 fields, and UID No. 4 is the GID, which can be accessed by any user ●/etc/shadow: there are nine fields, the password encrypted by the corresponding user in the passwd file is saved, and only the root user has the right to view ●/etc/group ④ the working principle of useradd is actually modifying 3 mentioned 3 file, and create a home directory ⑤ groups and user name to display the group to which the user belongs