1. Basic introduction

Linux system is a multi-user and multi task operating system. Any user who wants to use system resources must first apply to the system administrator for an account, and then enter the system as this account.

2. Add user

• Basic grammar

  useradd username

• case

  Add a user milan.

  [root@hostName01 ~]# cd /home
  [root@hostName01 home]# ls
  tom
  [root@hostName01 home]# useradd milan
  [root@hostName01 home]# ls
  milan  tom
  

• be careful

  1. When the user is created successfully, a home directory with the same name as the user will be automatically created. By default, the user's home directory is [/ home/milan]. After logging in with a new user, it will be switched to this directory by default.

  2. You can specify the home directory for the newly created user through [useradd -d].

     [root@hostName01 home]# useradd -d /home/test king
     [root@hostName01 home]# ls
     milan  test  tom
     

3. Specify / modify password

• Basic grammar

  passwd user name

• case

  Assign a password to [milan].

  [root@hostName01 home]# passwd milan
   Change user milan Your password.
  New password:
  Invalid password: password is less than 8 characters
   Re enter the new password:
  passwd: All authentication tokens have been successfully updated.
  

  When [invalid password: password less than 8 characters] is reminded, the system only reminds that the password is too simple, but it can still be set successfully. Of course, in practical work, the password should be set more complex.

4. Display the directory of the current user

• Basic grammar

  pwd

• case

  Log in to the [milan] user and display the directory of the user.

  [milan@hostName01 ~]$ pwd
  /home/milan
  

5. Delete user

• Basic grammar

  userdel user name

• case

  1. Delete the user [milan], but keep the home directory.

     Syntax: userdel milan

     [root@hostName01 ~]# cd /home
     [root@hostName01 home]# ls
     milan  test  tom
     [root@hostName01 home]# userdel milan
     [root@hostName01 home]# ls
     milan  test  tom
     

     [milan] the user cannot log in, but the user's home directory is still there.

  2. Delete the user tom and its home directory.

     Syntax: userdel -r tom

     [root@hostName01 home]# ls
     milan  test  tom
     [root@hostName01 home]# userdel -r tom
     [root@hostName01 home]# ls
     milan  test
     

     This method will delete all work data under the [tom] user, which should be operated with caution.

• be careful

  1. Deleting a user requires the permission of the [root] user.

  2. Think: do you want to keep the home directory?

     Data are valuable and generally recommended to be retained.

6. Query user information

• Basic grammar

  id user name

• case

  Query the information of the [root] user.

  [root@hostName01 home]# id root
  uid=0(root) gid=0(root) group=0(root)
  [root@hostName01 home]# id king
  uid=1002(king) gid=1002(king) group=1002(king)
  [root@hostName01 home]# id milan
  id: milan: no such user
  

• be careful

  When the user does not exist, return no such user.

7. Switch users

• introduce

  In Linux operation, if the current user's permission is insufficient, you can switch to a high authority user through the [su -] command, such as [root].

• Basic grammar

  su - switch user name

• case

  Create a [jack] user, specify the password, and then switch to [jack].

  [root@hostName01 home]# useradd jack
  [root@hostName01 home]# passwd jack
   Change user jack Your password.
  New password:
  Invalid password: password is less than 8 characters
   Re enter the new password:
  passwd: All authentication tokens have been successfully updated.
  [root@hostName01 home]# su - jack
  [jack@hostName01 ~]$ su - root
   password:
  Last login: October 23, 2006:02:55 CST 2021 From 192.168.233.1pts/0 upper
  [root@hostName01 ~]# logout
  [jack@hostName01 ~]$ exit
   Logout
  

• be careful

  1. When a user with high permission switches to a user with low permission, it is not necessary to enter a password, otherwise it is required.
  2. When you need to return to the original user, use the [exit/logout] command.

8. View the current login user

• Basic grammar

  1. who am i

     Query the users who log in to the system.

  2. whoami

     Query valid users of the current system

• case

  1. who am i

     [root@hostName01 ~]# who am i
     root     pts/0        2021-10-23 10:24 (192.168.233.1)
     [root@hostName01 ~]# su - jack
      Last login: October 23, 2006:20:12 CST 2021pts/0 upper
     [jack@hostName01 ~]$ who am i
     root     pts/0        2021-10-23 10:24 (192.168.233.1)
     

  2. whoami

     [root@hostName01 ~]# whoami
     root
     [root@hostName01 ~]# su - jack
      Last login: October 23, 2006:28:16 CST 2021 From 192.168.233.1pts/0 upper
     [jack@hostName01 ~]$ whoami
     jack
     

• be careful

  This command queries the logged in user, not the current user, that is, the switched user will not be queried, but the user used when logging in the system for the first time.

9. User group

<1> Introduction

Similar to roles, the system can uniformly manage multiple users with common (common permissions). For example, tom, jack and milan have the same permissions. We can divide them into the same group for unified management.

<2> Add and delete groups

• New group

  Instruction:

  groupadd group name

  Case:

  [root@hostName01 ~]# groupadd wudang
  

• delete group

  Instruction:

  groupdel group name

  Case:

  [root@hostName01 ~]# groupdel wudang
  

<3> Add groups directly when adding users

Instruction:

useradd -g user group user name

Case:

Add a user [zwj] and directly assign the user to [wudang]

[root@hostName01 ~]# groupadd wudang
[root@hostName01 ~]# useradd -g wudang zwj
[root@hostName01 ~]# id zwj
uid=1004(zwj) gid=1004(wudang) group=1004(wudang)

be careful:

If a new user does not specify a group, the system will automatically create a group with the same name by default and divide the new user into groups with the same name.

[root@hostName01 ~]# id king
uid=1002(king) gid=1002(king) group=1002(king)

<4> Modify user's group

Instruction:

usermod -g user group user name

Case:

Create a group [mojiao] and put the user [zwj] into the [mojiao] group.

[root@hostName01 ~]# id zwj
uid=1004(zwj) gid=1004(wudang) group=1004(wudang)
[root@hostName01 ~]# groupadd mojiao
[root@hostName01 ~]# usermod -g mojiao zwj
[root@hostName01 ~]# id zwj
uid=1004(zwj) gid=1005(mojiao) group=1005(mojiao)

10. User and group related documents

• [/ etc/passwd] file

  The user's configuration file records various user information.

  Meaning of each line: user name: Password: user ID: group ID: annotative Description: Home Directory: login shell

  king:x:1002:1002::/home/test:/bin/bash
  jack:x:1003:1003::/home/jack:/bin/bash
  zwj:x:1004:1005::/home/zwj:/bin/bash
  

• [/ etc/shadow] file

  Password profile.

  Meaning of each line: Login Name: encryption password: last modification time: minimum time interval: maximum time interval: warning time: inactive time: expiration time: Flag

  king:!!:18923:0:99999:7:::
  jack:$6$VjRt0agK$lXdDlSt6bQAh4GT/6xEE2TGK28wWMnHjrBs4lIk64fU6mVwr0lYYLNBksA6eb4t9szs78uPk8AN8KhID73wmP0:18923:0:99999:7:::
  zwj:!!:18923:0:99999:7:::
  

  [!] indicates that no password is set.

• [/ etc/group] file

  group configuration file, which records the information of groups contained in Linux.

  Meaning of each line: Group Name: Password: group ID number: list of users in the group

  king:x:1002:
  jack:x:1003:
  wudang:x:1004:
  mojiao:x:1005:
  

Reference video: https://www.bilibili.com/video/BV1Sv411r7vd?p=21