linux_永久环境变量配置和删除-CFANZ编程社区

reference

How to Set Environment Variables in Linux {Step-by-Step Guide} (phoenixnap.com)

conclusion

设置永久有效的变量就创建并配置/etc/profile.d 目录下的某个 sh 文件;
- 当然,要想配置在/etc/profile 文件中也有效
- 关于/etc/profile.d 目录下的配置文件(开机运行的脚本文件),和/etc/profile 作用之间的关系和区别,有如下论证
- bash - What do the scripts in /etc/profile.d do? - Unix & Linux Stack Exchange
  - 将配置文件合适的分散到/etc/profile.d 目录中的独立脚本,可以提供维护时的方便(问题定位等等);
  - 同时让配置的修改更加灵活
  - 若通过/etc/profile 来调用/etc/profile.d 中的脚本,这使得位于不同文件中的脚本和位于同一个文件中的脚本效果一样
删除永久环境变量就从配置改变量的文件处移除即可

Most Common Environment Variables

PWD – Current working directory.
HOME – The user’s home directory location.
SHELL – Current shell (bash, zsh, etc.).
LOGNAME – Name of the user.
UID – User’s unique identifier.
HOSTNAME – Device’s hostname on the network.
MAIL – User’s mail directory.
EDITOR – The Linux system default text editor.
TEMP – Directory location for temporary files.

How to Check Environment Variables

View All Environment Variables

Use the printenv command to view all environment variables. Since there are many variables on the list, use the less command to control the view:
```
printenv | less
```
The output will show the first page of the list and then allow you to go further by pressing Space to see the next page or Enter to display the next line:
- Exit the view by pressing Q .

Search a Single Environment Variable

To check a single environment variable value, use the following command:
```
printenv VARIABLE_NAME
```

Alternatively, display the value of a variable by using the echo command. The syntax is:
- the $ signal precede variable
```
echo $[VARIABLE_NAME]
```

Search Specific Environment Variables

To find all the variables containing a certain character string, use the grep command:
```
printenv | grep [VARIABLE_NAME]
```
Another command you can use to check environment variables is set .
- However, this command will also include local variables, as well as shell variables and shell functions.
- ```
# cxxu_kali @ cxxuWin11 in /mnt/c/users/cxxu/temp [18:24:12]
$ set |head
'!'=0
'#'=0
'$'=713
'*'=(  )
-=569JNRXghikls
0=-zsh
'?'=0
@=(  )
ARGC=0
BG
```
Find an environment variable in the list that contains all the variables and shell functions by using set | grep :
```
set | grep [VARIABLE_NAME]
```

Note: Take a look at our in-depth guide on how to use the Linux set command to learn more about it.

Set an Environment Variable in Linux

set a shell variable

The simplest way to set a variable using the command line is to type its name followed by a value:
```
[VARIABLE_NAME]=[variable_value]
```

As an example, create a variable called EXAMPLE with a text value.
- If you type the command correctly, the shell does not provide any output.
The set | grep command confirms the creation of the variable. However, printenv does not return any output.

Setting a shell variable

This is because the variable created in this way is a shell variable.

Another way to confirm this is to type bash and start a child shell session. Using the echo command to search for the EXAMPLE variable now returns no output:

Note: In a child process, EXAMPLE is not an existing variable.

How to Export an Environment Variable

If you want to turn a shell variable into an environment variable, return to the parent shell and export it with the export command:

  export [VARIABLE_NAME]

Use printenv to confirm the successful export:

# cxxu_kali @ cxxuWin11 in /mnt/c/users/cxxu/temp [19:12:26]
$ printenv test_env
test env

If you open a child shell session now, echo will return the environment variable value:

# cxxu_kali @ cxxuWin11 in /mnt/c/users/cxxu/temp [18:42:04]
$ test_env="test env"
# cxxu_kali @ cxxuWin11 in /mnt/c/users/cxxu/temp [19:09:00] C:1
$ set|grep test_env
test_env='test env'
# 导出临时变量(使得子进程shell可以继承访问到该变量)
# cxxu_kali @ cxxuWin11 in /mnt/c/users/cxxu/temp [19:09:04]
$ export test_env

# cxxu_kali @ cxxuWin11 in /mnt/c/users/cxxu/temp [19:09:14]
$ bash
┌──(cxxu_kali㉿cxxuWin11)-[/mnt/c/users/cxxu/temp]
└─$ echo $test_env
test env

The environment variable created in this way disappears after you exit the current shell session.

Set an Environment Variable in Linux Permanently

set permanent environment variables for `a single user`

To set permanent environment variables for a single user, edit the .bashrc file:
```
sudo nano ~/.bashrc
```
Write a line for each variable you wish to add using the following syntax:
```
export [VARIABLE_NAME]=[variable_value]
```
Save and exit the file. The changes are applied after you restart the shell.
- If you want to apply the changes during the current session, use the source command:
```
source ~/.bashrc
```

set permanent environment variables for all users

To set permanent environment variables for all users, create an .sh file in the /etc/profile.d folder:
```
sudo vim /etc/profile.d/[filename].sh
```
The syntax to add variables to the file is the same as with .bashrc :

Editing the file in the /etc/profile.d folder in nano

Save and exit the file. The changes are applied at the next logging in.

How to Unset an Environment Variable

To unset an environment variable, use the unset command:
```
unset [VARIABLE_NAME]
```
This command permanently removes variables exported through a terminal command.
- Variables stored in configuration files are also removed from the current shell session.
- However, they are set again upon next logging in.
To permanently unset a variable you stored in a file, go to the file, and remove the line containing the variable definition.