1. Users & User Groups

Linux is a multi-user and multi-task operating system. The Linux system supports multiple users to log on at the same time. Different users can perform different tasks within their permissions without affecting each other.

Corresponding relationships between users and user groups:

• One-to-one: A user can exist in a user group and be the only member in the group
• One-to-many: A user can exist in more than one user group, which has common permissions
• Many-to-one: Multiple users may exist in a group that has the same permissions as the group
• Many-to-many: Multiple users can exist in multiple groups

2. User Management

Common user management commands for Linux: useradd, usermod, userdel, passwd

command	describe
useradd	Create User
usermod	Modify User
userdel	delete user
passwd	Modify user password

2.1 useradd

Syntax format: useradd [options] username

Function description: New users, self-built user UID s since 1000

Option parameters:

option	describe
-u UID	UID of the specified user
-d Home Directory	Specify the user's home directory (absolute path)
-c User Description	Specifies the descriptive content of the fifth field of each user's information in the /etc/passwd file, which can be configured at will
-g group name	Initial group of the specified user
-G Group Name	Additional groups for specified users
-s shell	Login Shell for the specified user, default is/bin/bash
-e Date	Specify the expiration date for the user in the format "YYYY-MM-DD" (equivalent to the eighth field of the / etc/shadow file)
-o	Allow users to be created with the same UID
-m	Force the user's home directory to be created when the user is created (default option)
-r	Create a system user, that is, a user whose UID is between 1 and 499 for use by system programs

🌟 Default Configuration: Use useradd-D to view default values for new users

[root@joel ~]# useradd -D

# Default group for new users
GROUP=100
# Default location of user home directory
HOME=/home
# Set grace days after password expires, -1 is permanent
INACTIVE=-1
# Set the password expiration time, blank means permanent
EXPIRE=
# Set the default Shell s for new users to be/bin/bash
SHELL=/bin/bash
# After setting up a new user, the profile that needs to be copied into the user's home directory
SKEL=/etc/skel
# Set up a mailbox file for new users at / var/spool/mail/
CREATE_MAIL_SPOOL=yes

🌟 Extension command: useradd-D [options] parameter (modify configuration file)

1. -b HOME: Set the location of the home directory where the user was created (absolute path), such as useradd-D-b/home
2. -e EXPIRE: Set the password expiration time in yyyy-MM-dd format, such as useradd-D-e 2021-11-12
3. -f INACTIVE: Set grace days for password expiration, such as useradd-D-f 7
4. -g GROUP: Set the initial group of new users
5. -s SHELL: Set the default shell for new users

Case demo:

When creating a new user JOEL:

1. A line of data related to the JOEL user is created in the /etc/passwd file
2. A password information related to the JOEL user is created in the / etc/shadow file
3. A user group with the same name as JOEL is created in the / etc/group file
4. A password information related to the new user group is created in the /etc/gshadow file
5. Mailbox with the same name as JOEL under / var/spod/mail/path
6. Copies the configuration file from the /etc/skel directory to the home directory of the JOEL user/home/JOEL

[root@joel ~]# useradd JOEL
[root@joel ~]# grep "JOEL" /etc/passwd /etc/shadow /etc/group /etc/gshadow 
# 1001 for UID, 1002 for GID, /home/JOEL for user home directory, /bin/bash for user's login Shell
/etc/passwd:JOEL:x:1001:1002::/home/JOEL:/bin/bash
# No password, so yes!!, 99999 represents the number of days the password will be valid, and 7 represents the grace days after the password expires
/etc/shadow:JOEL:!!:18943:0:99999:7:::
/etc/group:JOEL:x:1002:
/etc/gshadow:JOEL:!::

2.2 usermod

Syntax format: usermod [options] username

Function description: New users, self-built user UID s since 1000

Option parameters:

option	describe
-c	Modify user's description information
-d	Modify the user's home directory
-e	Modify user's expiration date
-g	Modify the user's initial group
-u	Modify user's UID
-G	Modify additional groups of users
-I	Modify user's name
-L	Temporary Lock User
-U	Unlock User
-s	Modify user's login Shell

Case demo:

# Add JOEL to admin user group
[root@joel ~]# usermode -G amdin JOEL
[root@joel ~]# grep "JOEL" /etc/passwd
JOEL:x:1001:1002::/home/JOEL:/bin/bash
[root@joel ~]# grep "admin" /etc/group
admin:x:1000:JOEL

2.3 userdel

Syntax format: userdel [options] username

Function description: Delete user

Option parameters:

option	describe
-r	Delete the user's home directory when deleting the user

Case demo:

[root@joel ~]# userdel -r JOEL

2.4 passwd

Syntax format: passwd [options] username

Function description: Set user password

Option parameters:

option	describe
-S	Query the user password status, /etc/shadow file for the contents of this user password
-l	Lock the user temporarily and add'!'before specifying the user's encrypted password string in the / etc/shadow file
-u	Unlock User
–stdin	User's password is the data output through the pipe character
-n	Set how long it will take the user to change his or her password before he or she can change it again, that is, the fourth field of each row of passwords in the /etc/shadow file
-x	Set the password validity period for the user, corresponding to the fifth field of each row of passwords in the / etc/shadow file
-w	Set the number of warning days before user password expires, for the sixth field of each row password in the / etc/shadow file
-i	Set the user password expiration date, corresponding to the 7th field of each line password in the / etc/shadow file

Case demo:

[root@joel ~]# passwd admin
Changing password for user admin.
New password: 
BAD PASSWORD: The password is shorter than 8 characters
Retype new password: 
Sorry, passwords do not match.

[root@joel ~]# passwd -S admin
# User Name, Password, Set Time, Warning Time, Password Validity Period, Password Grace Time, Password Not Invalid
admin LK 2021-11-06 0 99999 7 -1 (Password locked.)

3. User Group Management

Common user group management commands for Linux: groupadd, groupmod, groupdel, gpasswd

3.1 groupadd

Syntax format: groupadd [options] user group name

Functional description: Add user groups

Option parameters:

option	describe
-g	Specify GID
-r	Create System Groups

Case demo:

[root@joel ~]# groupadd -g 1003  groupdemo
[root@joel ~]# grep "groupdemo" /etc/group
groupdemo:x:1003:

3.2 groupmod

Syntax format: groupmod [options] user group name

Function description: Modify user groups

Option parameters:

option	describe
-g	Modify GID
-n	Modify User Group Name

Case demo:

[root@joel ~]# groupmod -n grouptest groupdemo
[root@joel ~]# grep "grouptest" /etc/group
grouptest:x:1003:

3.3 groupdel

Syntax format: groupdel user group name

Function description: Delete user groups

Case demo:

[root@joel ~]# groupdel grouptest
[root@joel ~]# grep "grouptest" /etc/group

3.4 gpasswd

Syntax format: gpasswd [options] user group name

Function description: User group configuration, configure a group administrator to complete user management within a group instead of root

Option parameters:

option	describe
	When the option is empty, the group is given a password, which is available only to root users
-A user1,...	Give control of the group to user1,... Etc. user management, that is, set user1,... Etc. user as group administrator, only root user is available
-M user1,...	Add user1,... To this group, only root users are available
-r	Remove passwords for groups, available only to root users
-R	Invalidate group password, available only to root users
-a user	Join user to group
-d user	Remove user from group

Case demo:

[root@joel ~]# groupadd groupdemo

#  Set Password
[root@joel ~]# gpasswd groupdemo
Changing the password for group groupdemo
New Password: 
Re-enter new password: 
[root@joel ~]# useradd groupdemouser1

# Make groupdemouser1 the administrator of groupdemo
[root@joel ~]# gpasswd -A  groupdemouser1 groupdemo
[root@joel ~]# grep "groupdemouser1" /etc/group /etc/gshadow
/etc/group:groupdemouser1:x:1003:
/etc/gshadow:groupdemo:$6$aiKcsEFQv$WA28tABqOdTnjz67I91CyZiME0okjes2V/mzyZj/e7FtbbLLvJrD4gPinJu8C.sa8.mmX9xUnPbdJC7XVQe7t/:groupdemouser1:
/etc/gshadow:groupdemouser1:!::

[root@joel ~]# useradd groupdemouser2
[root@joel ~]# su groupdemouser1
[groupdemouser1@joel root]$ gpasswd -a groupdemouser2 groupdemo
Adding user groupdemouser2 to group groupdemo
[groupdemouser1@joel root]$ grep "groupdemouser2" /etc/group
groupdemo:x:1002:groupdemouser2
groupdemouser2:x:1004: