• Linux is a multi-user and multi-task operating system. Any user who wants to use system resources must apply for an account from the system administrator and use the account to access the system.
• On the one hand, the user account can help the system administrator to track the users using the system and control their access to system resources. On the other hand, it can also help users organize files and provide security protection for users.
• Each user account has a unique user name and its own password.
• After the user enters the correct user name and password during login, the user can enter the system and its home directory.
• In order to achieve the management of user accounts, the work to be completed mainly includes the following aspects:
• Add, delete and modify user accounts and manage user passwords.
• Manage user groups.

Note three files:

• /etc/passwdStore key user information
• /etc/groupStore key information about user groups
• /etc/shadowStore user password information

• Common syntax:# useradd option user name
• Common options:
  
  -g: indicates the primary user group of the specified user. The value can be of the user groupidCan also be a group name
  
  -g: indicates the additional user group of the specified user. The value can be of the user groupidCan also be a group name
  
  – u:uid, the user’sid(User’s identifier),By default, they are allocated from 500 to 500uid, if you do not want to use the system allocation, you can use this option to customize [similar to Tencent QQ’s self-selected numbers]
  
  – c:commenT Add a comment
• Example: Creating a userzhangsan, without any options
  
  
• Verification is successful:
  
  A. validation/etc/passwdThe last line of thezhangsanThe information;
  
  B. Check whether there is a home directory (after the user is created in Centos, a home directory with the same name is generated);

Expansion: knowledgepasswdfile

• User name: Password: User ID: User group ID: Note: Home directory: interpreter shell
• User name: Create a user name that you need to enter during login.
• Password: The password position is usually “X”, indicating the password placeholder.
• User ID: the user’s identifier;
• User group ID: indicates the ID of the primary group to which the user belongs.
• Comment: explain what the user does;
• Home directory: the default location after the user logs in to the system;
• Interpreter shell: After the user enters the system, the interpreter will collect the user’s input instructions and pass them to the kernel for processing.

Note: When no options are added, useradd is followed by a series of actions

• A. Create a home directory with the same name.
• B. Create a user group with the same name.

• Example: add option, create user lisi, add group 501 to lisi, add group 500 to LISi, select id 666.
  
  
  
  
  
  
• Pay attention to: View the primary group of the userpasswdFiles can be viewed by viewing attached groupsgroupFile.

• Linux does not allow users to log in to the system without a password, so the previously created users are currently locked and require a password to log in to the computer.
• Common syntax:# passwd Specifies the user name
• Case: SetupwangermaUser password
  
  
• When setting the password, there is no input prompt. Please enter the password and press Enter to ensure that the password entered twice is the same.
• You can also use a weak password, but it is not recommended, otherwise you will see the following prompt:
  
  
• After setting the passwordshadowIn the document: can be seenlisiThe user doesn’t have a password.
  
  
• You can log in to an account after setting a user password, as required herewangerma

• Switch user command:# su [username]Switch (user)
• If the user name is not specified, the system switches torootThe user.

• Note the following when switching users:
  
  From a.rootSwitching to a normal user does not require a password, but vice versarootPassword;
  
  B. After the user is switched, the working path remains unchanged.
  
  C. Common users cannot access itrootUser home directory, but vice versa;

• Each user has a user group. The system can manage all users in a user group in a centralized manner. For example, users in Linux belong to a user group with the same name. This user group is created at the same time when users are created.
• User group management involves adding, deleting, and modifying user groups. The addition, deletion, and modification of groups are actually pairs/etc/groupUpdate files.
  
  
  
  File structure:
  
  User group name: Password: User group ID: indicates the user name in the group
  
  Password:XRepresents a placeholder. Although passwords can be set for user groups, most of the time no password is set.
  
  Intra-group user name: Indicates the user name of the additional group.

• Common syntax:# groupmod Option User group name
• Common options are as follows: -g: similar to -u in user modification. -g indicates that you need to set a user-defined user group ID. -n: Similar to -l in user modification, you need to set a new user group name
• Case: ModificationAdministratorsUser group, change the group ID from 502 to 520, change the group name toadmins
  
  

• First know the location of the nic configuration file:/etc/sysconfig/network-scripts
  
  
• Name the configuration file of the nic in the directory:ifcfg– Nic name
  
  
• ONBOOT: Indicates whether to start
• BOOTPROTO: indicates the IP address assignment mode. DHCP indicates the dynamic host assignment protocol
• HWADDR: Hardware address, MAC address

• What do I do if I need to restart the NIC?

# service network restart
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• There may be no service command to quickly manipulate services in some branch versions, but there is a common directory:/etc/init.d
  
  This directory contains shortcuts to the right services.
• The reboot command can also be used here:

# /etc/init.d/network restart
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• Extension 1: If you modify the configuration file of a NETWORK adapter but the directory level of the configuration file is very deep, you can create a shortcut (soft connection) in the shallow directory to facilitate future search

# ln -s Path to the original file shortcut pathCopy the code

• throughls -lThe following effects can be listed:
  
  
  
  L in the file type position indicates that the file type is link, and -> indicates the original file path.
• Extension 2: How do I restart a single NIC?
  
  Stopping a network adapter:# ifDown Indicates the name of the nic
  
  Enable a NETWORK adapter:# ifup Indicates the name of the nic
• For example, to stop and start (restart) eth0, enter the value

# ifdown eth0
# ifup eth0
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• Tip: do not ban network cards at will in the actual work.