Linux usage operation[3]

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Environment variables are some key information recorded by the operating system (Windows, Linux, Mac) when it is running to assist the system in running.
Execute the env command in the Linux system to view the environment variables recorded in the current system
Environment variables are a KeyValue type structure, that is, names and values, as shown below:
No matter what the current working directory is, the program /usr/bin/cd can be executed. This is done with the help of the value of the PATH item in the environment variable.
PATH records the search path for any command executed by the system. When any command is executed, the ontology of the program to be executed will be searched from the above path in order.

In Linux systems, $symbols are used to get the value of "variables". The information recorded by environment variables is not only used by the operating system itself, but can also be used if we want to access it.
To obtain the value of an environment variable, you can obtain it through the syntax: ·$environment variable name·
For example: echo $PATHyou can get the value of the PATH environment variable and output it through the echo statement.

Linux environment variables can be set by users themselves, which are divided into:

Temporary setting, syntax: export variable name = variable value
Effective permanently
- Effective for the current user, configured in the current user's: ~/.bashrcfile
- Effective for all users, configured in the system:/etc/profile文件中
- And use the syntax: source 配置文件, it will take effect immediately, or log in to FinalShell again to take effect.

Use the FinalShell tool to conveniently exchange data with virtual machines.
In the lower form of the FinalShell software, the Linux file system view is provided, which can be conveniently:
- Browse the file system, find the appropriate file, right-click to download, and transfer it to your local computer
- Browse the file system, find the appropriate directory, and expand the files on the local computer to easily upload data to Linux.

Installation command
```
yum -y install lrzsz
```
rz and sz are command line tools for file transfer in Linux systems, usually used with a terminal emulator such as xterm or gnome-terminal.

rz command:
- The rz command is used to upload files from the local computer to a remote computer, usually used in a terminal emulator.
- After you launch the rz command using a terminal emulator, it waits for files to be sent from the remote computer.
- Select Send File from the terminal emulator's menu and select the file to upload. rz will then receive and save the file to the remote computer's current directory.
sz command:
- The sz command is used to download files from a remote computer to the local computer, usually used in a terminal emulator.
- After you launch the sz command using a terminal emulator, it will wait for you to specify the file to download. You use menus or commands in the terminal emulator to select files to download. sz will then send the selected files to the local computer's current directory.

There are two compression formats commonly used in Linux and Mac systems, and the suffix names are:
- .tar, called tarball, is an archive file that simply assembles files into a .tar file. It does not reduce the file size much, it is just a simple package.
- .gz, also commonly known as .tar.gz, gzip format compressed files, that is, using the gzip compression algorithm to compress the files into one file, which can greatly reduce the compressed volume.
For both formats, you can use the tar command to perform compression and decompression operations.

Basic syntax:

tar [选项] [目标文件] [文件或目录...]

Unzip test.tar and extract the file to the current directory
```
tar -xvf test.tar
```
Unzip test.tar and extract the file to the specified directory (/home/it)
```
tar -xvf test.tar -C /home/it
```
Decompress test.tar.gz in Gzip mode and extract the file to the specified directory (/home/it)
```
tar -zxvf test.tar.gz -C /home/it
```

You can use the zip command, and the compressed file is a zip compressed package.
Basic syntax:
```
zip [-r] 参数1 参数2 ... 参数N
```

Example:

Compress a.txt b.txt c.txt into the test.zip file
```
zip test.zip a.txt b.txt c.txt
```
Compress the test and itheima folders and the a.txt file into the test.zip file
```
zip -r test.zip test itheima a.txt
```