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Query user information command
View currently logged in users
Instructions for adding groups
Shutdown & Restart Commands
basic introduction
shutdown -h now it is time to shutdown
shutdown -h will shutdown in 1 minute"
shutdown -r now restarts the computer now
halt shutdown, the effect is the same as above
reboot now restarts the computer
sync synchronizes memory data to disk
Attention to detail
1. Start the system and shut down the system. First, run the sync command to write the data in the memory to the disk.
2. The current shutdown/reboot/halt and other commands have been synced before shutdown.
User login and logout
basic introduction
1. When logging in, try to log in with the root account as little as possible, because it is the system administrator and has the greatest authority to avoid operation errors. You can log in as a normal user, and then use the "su-username" command to switch to the system administrator identity after logging in.
2. Enter logout at the prompt to log out the user. Ordinary users can switch to root and fall back to ordinary users through logout.
usage details
1. The logout logout command is invalid at the graphical run level, but valid at run level 3.
2. The concept of run level will be introduced later.
logout test
The Linux virtual machine cannot logout continuously, using exit just exits the terminal
On Xshell, you can logout continuously, and you can exit the system by returning to the last one.
User Management
basic introduction
Linux system is a multi-user and multi-task operating system. Any user who wants to use system resources must first apply for an account to the system administrator, and then enter the system as this account.
Add user
Basic syntax:
useradd username
Applications
Case 1: Add a user milan, the default home directory of the user is /home/milan
Details
1. When the user is successfully created, the home directory with the same name as the user will be automatically created.
2. You can also specify a new user name through useradd -d to specify a home directory for the newly created user.
Specify/modify password
basic grammar
passwd username (without writing the username, the default is root to change the password)
show which directory pwd is currently in
Example: Specify password 123456 for milan
pwd shows which directory it is currently in
delete users
Basic syntax:
userdel username
Example
1. Delete the user milan, but keep the home directory (userdel milan)
2. Delete the user and the user's home directory (userdel -r milan) This operation will delete all the directories under the user, use it with caution
Normally keep home directory
Query user information command
Basic syntax:
id username
Application example
Query root information
When the user does not exist, the system will prompt that there is no such user
switch user
Introduction: In operating linux, if the current user has insufficient permissions, you can switch to an advanced user, such as root, through su - user
basic grammar
su - switch user (- with a space before and after)
Example:
Go to a user first, then switch to root
Details:
1. Switching from a user with high authority to a user with low authority does not require entering a password, and vice versa
2. When you need to return to the original user, use the logout/exit command
View currently logged in users
basic grammar
whoami/who am I
illustrate:
whoami is to view the currently logged in user
who am I is to view the last login, and logging in with su is also to view the previous user
user group
introduce
Similar to roles, the system can manage multiple users with commonality in a unified manner
Instructions for adding groups
groupadd group name
delete group command
groupdel group name
Add groups directly when adding users
Command: useradd -g user group username
Modify user group
grammar:
usermod -g user group username
User and group related files
/etc/passwd file
User (user) configuration file, recording various information
of the user Meaning of each line: Username: Password: User ID: Group ID: Commentary Description: Home Directory: Login Shell
vim /etc/passwd view, all bash, this file retains user information
/etc/shadow file password configuration file
Meaning of each line: Login Name: Encrypted Password: Last Modified Time: Minimum Time Interval: Maximum Time Interval: Warning Time: Inactivity Time: Expiration Time: Flag
vim /etc/shadow encrypted password
/etc/group file
The configuration file of the group (group) records the information of the groups contained in Linux The
meaning of each line: group name: password: group identification number: list of users in the group
specify runlevel
Basic introduction runlevel description:
0: Shutdown
1 Single user [Retrieve lost password]
2: Multi-user status without network service
3: Multi-user status with network service
4: The system is not used and reserved for users
5: Graphical interface
6: System restart
The common run levels are 3 and 5, and the default run level can also be specified, which will be demonstrated later
Application example
Command: init[0123456] Application example: Switch between different run levels through init, such as running 5-3, and then shutting down.
Description of run levels
after CentOS7 Before centos7, the /etc/inittab file was simplified as follows:
multi-user.target : analogous to runlevel 3
graphical.target : analogous to runlevel 5
# To view current default target, run:
systemctl get-default
# To set a default target, run:
systemctl set-default TARGET.target
retrieve root password
interview questions
how to retrieve root password
1. First, start the system to enter the boot interface, press e in the interface to enter the editing interface
2. Enter the editing interface, use the up and down keys on the keyboard to move the cursor down, find the number of lines where the content at the beginning of "Linux16" is located, and enter at the end of the line: init=/bin/sh, press the shortcut key ctrl+X after completion enter single user mode
3. Then enter: mount -o remount,rw / (note: there is a space between each word) in the position where the cursor is flashing, press the Enter key of the keyboard after completion, enter passwd after the new one, and press the Enter key after completion , enter the password, and then confirm the password next time, (the password length is preferably more than 8 digits, but less than 8 digits is also acceptable), after the password is successfully modified, the style of passwd will be displayed, indicating that the root password has been modified successfully.
4. Enter: touch /.autorelabel (note: there is a space after touch and /) in the position where the mouse is blinking, and press the Enter key after completion
5. Continue to the next input: exec /sbin/init (note: there is a space after exec and /) and press Enter after completion
In this way, the password of lroot is retrieved, and you can log in to root directly with the password you just used.
help command
manGet help information
Basic syntax: man command or configuration file ( function description: get help information)
Example: Query the help information of the ls command man ls (press space to display the next page)
Under linux, hidden files start with ., options can be used in combination, such as ls -a to display all files, ls -l to display in a single column, then ls -la or -al can display all files in a single column, such as ls -al /root
help help command
Basic syntax: help command (function description: get shell to get help information of shell built-in commands)
Or Baidu direct search related usage