（ Excerpt from https://blog.csdn.net/llzk_/article/details/53813266 And then integrate ）
1.linux The conditions of the system
Linux It's a multi-user operating system , Every user will have a special running environment when they log in to the system , Usually, the default environment for each user is the same . The default environment is the definition of a set of environment variables . Each user can configure his own running environment by modifying environment variables .
2. Several instructions commonly used to set environment variables
Show environment variables . Format ：echo$PATH
Set new environment variables . Format ：export New environment variable name = Content
Delete an environment variable
unset MYNAME（ Environment variables are usually uppercase ）
Classification of environmental variables
1. Permanent variables that work for all users
operation ： use vim stay /etc/profile Add the environment variables we want in the file , Go back to the command screen and type source /etc/profile You can successfully set the permanent variables that are valid for all users
This kind of variable works for all users in the system , All users can use such variables . The scope of action is the whole system .
Setting mode ： use vim stay /etc/profile Add the environment variables we want in the file .
Of course , This file is only available in root（ The super user ） I can't change it until next week . We can do it in etc Use... In the directory ls -l View the users and permissions of this file .
utilize vim open /etc/ profile file , use export Instruction to add environment variables .
【 Be careful 】： The new environment variable will not take effect immediately after adding , Unless you call source /etc/profile The document will take effect . Otherwise, it will only take effect the next time you re-enter this user .
2. Permanent variable in effect for a single user
operation ： use vim Open file .bash_profile, Add environment variables to the file . Then go back to the command screen and type source .bash_profile You can successfully set persistent variables that work for a single user .
This kind of environment variable is only permanent to the current user . In other words, if the user A Set such environment variables , The only environment variable is A have access to . And for the rest B,C,D,E…. Users and so on , This variable doesn't exist .
Setup method ： In the user home directory ”~” Under the hidden file “.bash_profile” Add the environment variable you want in .
View hidden files ： ls -a or ls -al
utilize vim Open file , utilize export Add environment variables . Same as above .
Also note that , The new environment variable will not take effect immediately after adding , Unless you call source ./.bash_profile The document will take effect . Otherwise, it will only take effect the next time you re-enter this user .
You can see that I framed two files in red in the picture above ,.bashrc and .bash_profile. In principle, when setting such environment variables, you can add them to either of these two files .
~/.bash_profile It's interactive login Way in bash shell function .
~/ .bashrc It's interactive non-login Way in bash shell function .
The two settings are roughly the same . Popular point theory , Namely .bash_profile The file will only be read once when the user logs in , and .bashrc It is read every time the terminal is opened for a new session .
3. Temporary valid environment variables
operation ： Input export Variable name is enough
This kind of environment variable is only valid for the current shell It works . When we log out or close the terminal and reopen it , This environment variable will disappear . It's temporary .
Setup method ： Use it directly export Instruction add .
Sentiment ： Environment variables can be understood as a path . After all linux It's a multiuser system , We can set some environment variables to run our program .
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