The third lesson of MySQL database------simple commands and then advanced-------become a must for a big cow

Author's Foreword

 Welcome little cuties to come and learn from my gtiee Qin Boss (qin-laoda) - Gitee.com

————————————————————————————————

Table of contents

A brief introduction to the command

1. Redirect command

2.cat

3. ln

4.grep

5. Wildcards

6.find

7.tar

——————————————————————————

Interstitial knowledge

There are some small mistakes in the last blog

 1. rm -r is to delete a directory

2. The parameters of the command can be written consecutively or at the end, such as mv -i a.py ./b/b.py or mv a.py ./b/b.py -i

3. The default permission is generally designed to be 755

A brief introduction to the command

Immediately after the last blog, I briefly introduced ls (file information) pwd (view the absolute path of the current file) mkdir (create folder) touch (create file) cp mv rm chmod (modify permissions) and other related commands and and related parameters

Next, I will introduce other commands

Redirect command: >

    Redirect command execution results to a file.

    Why are there redirects? In the operation of Linux , formulating the redirection language through code is like a log record, which will uniformly record your operations. When you can't find the file you want or forget your previous operation, you can see your previous operation by viewing the redirected file. Maybe this file has been deleted by you.

Simply put , it is to write the result of the command to a file and save it.

ll > 文件路径

 

 It can be seen that when redirection is used to write to the file, the running result is not displayed on the terminal, but written directly to the file, so only the contents of the file can be viewed

But when we write another result into it, we will find that the latter content covers the previous content

 

 See if it's covered

If you want to add content, you can try >> this

ll  >> 文件路径

 

 Check if content is added

cat

View or merge file contents

view content

cat 文件名称

 See if you can view the content. Cat is generally used for small files, and it is generally not recommended for large files.

Merge and view (that is, view the contents of multiple files)

cat 文件1  文件2

 

Create a linked file: ln

Linux link files are similar to shortcuts under Windows .

Link files are divided into soft links and hard links.

Soft link: Soft link does not occupy disk space, and the soft link becomes invalid when the source file is deleted. Similar to shortcuts under Windows

Hard links: Hard links can only link ordinary files, not directories.

A shortcut:

These are shortcuts. If you delete these, the file can still run. When you delete the corresponding file, it will not work, and the shortcut is useless.

硬连接：ln 源文件 链接文件
软连接：ln -s 源文件 链接文件

Soft link:

 Equivalent to creating a shortcut

Hard link:

 How it looks on the desktop

  The relationship between these two links and the source file, the modification of the source file will affect the two links, and the modification of the other two links will also affect the source file 

What is the difference between the two?

 We can see that there is one type of entry into the aa.py file, and there are two types of content into the b.py file and into the content of cc.py

aa.py is b.py, which can be understood as aa.py with b.py

What happens if the source file is deleted

 It can be seen that the source file is deleted, the soft link is useless, but the hard link can still be used

 

Why did it cause such a result?

 

 It is a bit like the address of Python. Deleting b.py is equivalent to disconnecting the pointing, that is, deleting the address pointing to this content

If there is a pointer that knows the C language, it can be understood very clearly

So the difference between hard links and soft links is here.

grep

Text search, grep allows pattern finding in text files. If matching data is found, grep prints all lines containing this data. Simply put, it matches the content of the text

 

The general format of grep is: grep [-option ] ' search content string ' file name

When entering string parameters in the grep command, it is best to enclose them in quotes or double quotes

grep  "被查找的内容"   被查找的文件

 It can be seen from the figure that the result returned by grep is to return only the lines with the searched content, and the lines that cannot be matched will be discarded.

Related parameters

 -n Displays the matching line in the first line of the file content

grep "查找内容"   文件  -n

 See if it shows up

-i ignore case

grep -i  "匹配的内容"   文件

 -v does not match related content

grep -i  "匹配的内容"   文件

Can be understood as matching negation

 There is another interesting command

ps aux| grep py

View the running py file and the running file containing py

 wildcard

A special character used for fuzzy queries.

Note that there are universal characters

*：代表0个或多个任意字符
?：代表任意一个字符
[列举的字符]：代表的任意一个字符

Generally, it is used in combination with search commands, such as ls and find commands

find (find file name)

It is used to search for qualified files in a specific directory, and can also search for files owned by a specific user.

 Writing:

find -name  文件名称或者某些标记

 See if it matches a lot, that is, it matches the current directory, as long as it is in the current directory. No matter how many layers there are, it will match

find -name '*.txt'

Find all files ending with ".txt" in the current directory

In the displayed results, not only the files meeting the conditions in the current directory are found, but also the files meeting the conditions in the subdirectories are found.

In fact, *.txt here is a way of writing regular expressions, and regular expressions will be explained in detail later.

tar

Pack and compress files

Packing: It is equivalent to folding clothes and putting them in boxes without changing the storage size

Compression: pack first and then compress, which is equivalent to putting a piece of clothing in a bag and venting the air, the clothes will become smaller and the storage size will change

Compression must be packaged, packaging is not necessarily compressed

 We can roughly divide it into packing, compressing, unpacking, and decompressing

Pack

If we want to package, we only need to use the three parameters -c -v -f, remember that the f parameter must be written later

tar -cvf  打包文件名称   被打包的文件

 If you dislike this method, you can use * to represent all files

unpack

tar -xf 打包文件名称  

 

It can still be seen that this command is unpacked to the current directory

compression

Compression must be packed

tar -czf  文件名称.tar.gz   要压缩的文件

 

decompress 

tar -xf 要解压的文件

 This command is to unzip to the current directory

Unzip to the specified directory

-C Note that this C is uppercase

tar -xf 要解压的文件名称  -C  解压到指定目录

other compression methods

bz2 

bz2
压缩： tar -jcvf 压缩包包名 文件
解压： tar -jxvf 压缩包包名

decompress

 

decompress

 

zip

zip
zip压缩的目标文件不需要指定扩展名，默认扩展名为zip。
压缩：zip 目标文件(没有扩展名) 源文件
解压：unzip -d 解压到的目标目录文件 压缩文件

compression

 decompress

 Decompression is a bit different here  

When there is no -d parameter, it will be decompressed to the current directory by default.

Summarize:

This is the end of the above introduction. What we need to pay attention to is that the grep and find -name commands need to use strings, and the way of decompressing zip is somewhat different.