Common commands for Linux user and group management

In Linux system management, we often add and delete information about users and groups, so let's learn how to create, delete, modify, and Group

Next, let's briefly describe what is a group and what is a user concept?

User:

In fact, a simple understanding is an identifier created by a file or directory.

GROUP:

A simple understanding of a group is a container that is used to give users a container for directories or files. Therefore, a group cannot log on to the system and cannot use resources.

Account and group management:

/Etc/login. defs
Shadow password suite configuration.

/Etc/passwd
User account information.
Root: x: 0: 0: root:/bin/bash

Field 1: account: Login Name
Field 2: password -->/etc/shadow
Section 3: UID:
Section 4: GID: basic group ID
Section 5: comment: account comment
Section 6: home dir: User Account HOME directory
Section 7: SHELL: the user's default shell

/Etc/shadow
Secure user account information.
Root: $1 $ AKMibarg $ G/nw/HgmH3geg66k8/keF1: 15761: 0: 99999: 7 :::

Field 1: account: Login Name
Field 2: encrypted password (if */! appears */!, The account cannot log on to the system)
Field 3: Date on which the password was recently changed
Field 4: Number of days when the password cannot be changed
Field 5: Number of days for Password Change
Field 6: warning information before the Password Change Period
Field 7: password expiration time
Field 8: Account Expiration Date
Field 9: Reserved

/Etc/group
Group account information.
Daemon: x: 2: root, bin, daemon

Field 1: account
Field 2: password
Field 3: GID-> User Group ID
Field 4: All accounts added to this group

Create a user:
Useradd [options] LOGINNAME
-U: UID (the default value is greater than or equal to 500 and cannot be repeated)
-G: GID (basic group): this group exists beforehand.
-G: GID... additional group
-C: "comment": Specifies the comment information.
-D:/path/to/directory, specifying the user's home directory
-S: Specifies shell->/etc/shells (current)
-M: When the specified home directory does not exist,-m indicates the user's home directory.
-M: do not create a home directory
Comprehensive Utilization:
Create a system user tom with the annotation information "This is a text account", and specify the ID as 111. The home directory is/home/tom3.
Change shell to/sbin/nologin and add it to mygroup.
# Adduser-r-c "This is a text account:-d/home/tom3-m-s/sbin/nologin-G mygroup

ID:
Function: view User details
Option:
-G: ID of all groups
-U: displays the user ID.
-G: Display gid
-N: Display name
For example:
View the account ID of the tom User:
# Id tom can be displayed

Userdel:
Function: delete a user.
Option:
-R: the user is deleted along with the home directory.
For example
Delete the tom user and delete the user's home directory together:
# Userdel-r tom
Usermod:
Function: Modify user account settings.
Option:
-U: UID
-G: the GID must be in the/etc/group.
-G: overwrites the existing additional group by default.
-A: append an additional group to change user groups that users can support.
-C: Specifies the comment information.
-D: Specifies the home directory.
-M: Specifies the location of the home directory. 5. Move the contents of the home directory to the current directory.
-S: Modify shell
-L: Change the username --> corresponding to the first column in the/etc/passwd file.
-E: change the password expiration time, followed by the date parameter is MM/DD/YY or YYYY-MM-DD
-L: (lock) lock the user account
-U: (unlock) unlock the user account
For example, lock the jack user account information and change the comment to "download file" # usermod-c "beause download file"-L jack

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