For example, we open our own graphical interface. Because it is a graphical interface, the run level is 5. We right-click to open the terminal, and enter init 3

We enter the carriage return and it will become the command line format:

2. Operation level description after CentOS7

#查看当前运行级别
systemctl get-default

#运行级别3
multi-user.target

#运行级别5
graphical.target


#设置运行级别，其中TARGET就是对应上面的级别3和5
systemctl set-default TARGET.target

Let's look at an example:

We first check that the current run level is 5, and then we set it to 3

2. Retrieve the root password (classic interview questions)

What should I do if you forget the root password and cannot log in to the root account?

1. Start the system and quickly press "e" to enter the editing page

On this page, we press e, a little faster, or we will skip to the next step by ourselves.

Once we press e, we will enter this edit page

2. Find the linux16 line, enter init=/bin/sh, and then press ctrl+x to enter the single-user mode

After entering the edit page, find the line headed by linux16. If you look at the above picture carefully, you may find that it does not have the line linux16, you need to pull down, and there are still things below.

We add after this line

init=/bin/sh

After the input is complete, press ctrl+x to enter the single-user mode, and the page looks like this:

3. Single user mode for input

Enter the following in single-user mode: (recommended to copy here, please pay attention to spaces)

mount -o remount,rw /

Enter after pressing enter:

passwd

There will be a passwd ... line at this time.

Continue to enter:

touch /.autorelabel

Then enter:

exec /sbin/init

At this time you will get this page:

Note at this time: don't move, wait. This may take a while, don’t move, everyone! ! !

Then, the page will automatically restart:

We can log in with root and the new password

4. Modify the root password and change it to your favorite

passwd root

3. Commonly used commands (emphasis)

1.man: get help information

Basic syntax:

man [命令或配置文件]

For example, if we enter man ls, the following will appear:

To see the next page, press the space bar.

To return to the command line state, press "q"

Note: Hidden files under linux start with "."

2.help: Get help information for shell built-in commands

3.psw: Display the absolute path of the current working directory

4.ls: View content

ls [选项] [目录或者文件]

-a: Display the directory of all files in the current directory, including hidden

-l: display information in a list

5.cd: switch to the specified directory

Back to the previous directory:

cd ..

Back to the home directory: cd

6.mkdir: create a directory

mkdir [选项] 要创建的目录

-p: Create a multi-set directory

7.rmdir: delete empty directories

If there is content in the directory, use: (recursively force deletion)

rm -rf 要删除的目录

8.touch: Create an empty file

touch 文件名称

9.cp: copy files to the specified directory

cp [选项] source dest

-r: copy the entire folder recursively

If you have the original file, and you copy it again, there will be a problem of overwriting. Of course, you can enter y to overwrite. When there are many files, this is troublesome. We can force overwrite without prompting:

\cp [选项] source dest

10.rm: Remove files or directories

rm [选项] 要删除的文件和目录

-r: delete the entire folder recursively

-f: no prompt for forced deletion

11.mv: Move files and directories, or rename

mv oldNameFile newNameFile

If the two file directories are in the same directory, it means renaming, otherwise it means moving.

12.cat: View the contents of the file

cat [选项] 要查看的文件

-n: display line number

Note: cat can only browse files, not modify files. For the convenience of browsing, it will usually bring "|more": (press enter to view the next line, press space to view the next page)

cat [选项] 要查看的文件 | more

13.more: paging display text content

more 要查看的文件

Because it is paging, there are some operation instructions after more:

Space bar: view the next page

Enter: view the next line

q: leave more

ctrl+f: scroll down one screen

ctrl+b: return to the previous screen

=: output the current line number

:f: output file name and line number of the current line

14.less: View file content in split screen

less 要查看的文件

The function is similar to more, but more powerful than more, and supports various real terminals. It is recommended to use less to view larger files

Space bar: view the next page

pagedown: page down

pageup: page up

/String: search downwards, n: search downwards, N: search upwards

?String: search upwards, n: search upwards, N: search downwards

q: leave

15.echo: output content to the console

echo [选项] [输出内容]

16.head: Display the beginning of the file, the first 10 lines of the file are displayed by default

head 文件名

#查看指定行数文件
head -n 数字 文件名

17.tail: display the end of the file, the last 10 lines are displayed by default

tail 文件名

#查看指定行数文件
tail -n 数字 文件名

#实时追踪文档的更新状况
tail -f 文件名

(1) Exit the monitoring state: ctrl+c

(2) In the virtual machine, we write something to the file through the echo command, and then the monitored tail will be updated:

One arrow indicates coverage, and two arrows indicate append .

(3) Display calendar information: cal

18.ln: soft link, also called symbolic link. Similar to the shortcuts in windows, it mainly stores the path to link other files

#给原文件创建一个软链接名
ln -s [原文件] [软链接名]

Example:

Make a shortcut for root under home and delete it

19.History command: view the history commands that have been executed, and you can also execute history commands

#查看所有历史命令
history

#查看最后10条历史命令
history 10

#执行历史编号为5的指令
!5

We first find out the historical commands in history, and then execute the historical commands through !5

20.date command: date

(1) Display the current date

#显示当前时间
date 

#显示年
date +%Y

#显示月
date +%m

#显示日
date +%d

#显示年月日时分秒
date "+%Y-%m-%d %H:%M:%S"

(2) Set the date

date -s 字符串时间

(3) Display calendar

#显示当前日历
cal

#显示整年的日历
cal 2021

21.find instruction: recursively traverse all subdirectories from the specified directory downwards, and display the files or directories that meet the conditions on the terminal

find [搜索范围] [选项]

-name: search by file name

-user: search by user

-size: search according to file size (+n: greater than -n: less than n: equal to) (file unit k, M, G)

When we view the file through ls, in order to see the file size conveniently, we can add the h parameter .

22.locate instruction: quickly locate the file path

#如果是第一次使用，需要使用updatedb创建locate数据库

locate 搜索文件

Note: Because locate is based on the database for query, before the first run, you must use the updatedb command to create the locate database .

23.which instruction: view which directory a certain instruction is in

Check which directory ls is in:

24. grep instructions and pipe symbols

Pipe symbol: "|", which means that the processing result output of the previous command is passed to the subsequent command processing .

grep [选项] 查找内容 源文件

-n: display matching line and line number

-i: Ignore letter case

25.gzip/gunzip command: compression/decompression

#将文件压缩成*.gz文件
gzip 文件

#解压缩
gunzip 文件.gz

26.zip/unzip instructions: compression/decompression, often used for project packaging and release

#压缩
zip [选项] xx.zip 将要压缩的内容

#解压
unzip [选项] xx.zip

Common options for zip:

-r: Recursive compression, that is, compressed directory

Common options for unzip:

-d <directory>: the directory where the decompressed files are stored

27.tar command: decompression is tar

The tar instruction is a packaging instruction, and the final packaged file is the xx.tar.gz file

tar [选项] xx.tar.gz 打包的内容

-c: Generate .tar package file

-v: display detailed information

-f: Specify the compressed file name

-z: Pack and compress at the same time

-x: Unpack the .tar file

-C: Unzip to the specified directory