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Preface

    Linux The system is a multi-user and multi task time-sharing operating system , Any user who wants to use system resources , You must first apply to the system administrator for an account , Then enter the system as this account .
	
	 On the one hand, the user's account can help the system administrator to track the users who use the system , And control their access to system resources ; On the other hand, it can help users organize files , And provide security protection for users .
	
	 Each user account has a unique user name and password .
	
	 After the user enters the correct user name and password at login , You can access the system and your home directory .

Realize the management of user account , The main tasks to be completed are as follows ：

• linux System user account 、 Management of user groups
• Resolve system files related to users and groups
• Set and modify user login password

One 、linux System user account 、 Management of user groups

linux User accounts are mainly divided into 3 class ：
The first category ：root( Super administrator ), This user has great privileges , A lot of restrictions can be ignored , Including read-write execution permissions . So this user should be careful , Because he has too much authority ;
The second category ： System users , Generally, it cannot be used to log in to the system ;
The third category ： It's ordinary users , Permission is limited , It can be used root Account to change your permissions ;

1、 Add a new user account ：

useradd   Options    user name 
 Option description ：
-u：  Specify the user's id Number 
-s：  Specify the user's login shell Interpreter 
-g：  Specify the user group to which the user belongs 
-G：  Specifies the additional group to which the user belongs 
-d：  Specify the home directory to which the user belongs 

2、 Delete user account ：

userdel  Options    user name 
 Option description ：
-r：  The most commonly used option , Its main function is to delete the user's home directory 

3、 Modify user account attributes ：

usermod   Options    user name 
 Option description ：
-u：  Specify the user's id Number 
-s：  Specify the user's login shell Interpreter 
-g：  Specify the user group to which the user belongs 
-G：  Specifies the additional group to which the user belongs , Reset if the user has additional groups , If not, add a new additional group 
-d：  Specify the home directory to which the user belongs 

4、 Add a new group account ：

groupadd   Options    User group 
 Option description ：
-g：   Specifies the group identity number for the new user group （GID）
-o：   Generally speaking, it is related to -g Option to use at the same time , Represents a new user group GID Can work with the system with existing user groups GID identical .

5、 Delete group account ：

groupdel    User group 

6、 Modify the related attributes of the group account ：

groupdel   Options    User group 
-g：    Specify a new group for the user group id Number 
-o：    Often with -g Option to use at the same time , New to the user group GID Can work with the system with existing user groups GID identical 
-n  The new user group ：   Change the name of the user group to the new name 

Two 、 Resolve system files related to users and groups

1、/etc/passwd： It saves the basic information of the user account ：

[[email protected] ~]# cat /etc/passwd
root:x:0:0:root:/root:/bin/bash  	
 user name : Password placeholder : user UID Number : Group ID Number : User description information : Home directory : Sign in Shell
...
 Sign in shell： After the user logs in , To start a process , Responsible for passing the user's operation to the kernel , This process is the command interpreter or a specific program that the user runs after logging in to the system , namely Shell.

2、/etc/group： Save all the information of the user group ：

[[email protected] ~]# cat /etc/group
root::0:root
 Group name : password : Group identification number : List of users in the group 
...

3、/etc/shadow： Save user password related information ：

[[email protected] ~]# cat /etc/shadow
abcdef:$6$PVAvS8qk$Gvz1s3335ChBkAbe.sV5q7ftdF.2R8QbrMyiKk/UY6PespLRYXteWei3fhIvC
q5P.oAmRAKQbigfNmsARG9zR.:18407:0:99999:7:::
 Login name : encrypted password : Last password change time : The shortest period of validity : The longest period of validity : Warning time : Disable time : Failure time : placeholder 
...

3、 ... and 、 Set and modify user login password

Set login password （linux System regulation ： The user must set the password , To log in ）：

（1） Set password interactively ： passwd [ user name ]

（2） Non interactive set password ：echo “ password ” | passwd --stdin user name