Linux kernel: everything is a file
Create user-related files: wd save
Type less /etc/passwd at the command line
The first line 
: (each line represents a user's information, divided into 7 segments with: as a separator)
The root user, x represents a password, which is a placeholder. Otherwise, the user information is leaked, which means that the user's permissions are leaked.
Delete the x to indicate that there is no password, log in directly with the user name
0 means user id == uid is the first person to come to this operating system, which means that this is equal to 0, indicating that there is user authority
0-500 are executive-level, and after 500, ordinary users can use it
0 means group id == gid, which group is marked in
root represents the description of the user
/root represents root's home directory
/bin/bash The first executable file logged by the user, as long as it is bash under bin, it proves that the user can log in to the operating system
User exits twice before exiting
Second line 
:
bin user
sbin/nologin inoperable 
input successfully exits automatically
So /bin/bash can be changed to sbin/nologin to achieve the function that users cannot log in
uid：
If you want to modify it, enter the directory file
After deleting the uid, there is an empty space there
The password is actually placed under etc/shadow
Enter the command to view
Another point is that the root user is limited by uid
guide
less /etc/group
The group corresponds to tom 500; shizhengwen 501; . .