This is a giant subject. I’ll continue updating this article as I find and understand more about how Linux does all of this.
Most articles show how to set them in the terminal, then when you close the terminal and try to use a variable it doesn’t exist leaving you confused.
Well those posts are how to set a temporary environmental variable accessible in your terminal until it closes only. To set a lasting Environmental variable you need to use a file.
This is often needed to store sensitive api key secrets, passwords etc. You should never put any of that information in a file for your project directly, meaning don’t put it in a file that is stored in your code repository.
Developer App Variables
The idea is to create environmental variables to hold this info on your local machine, then when you put your code into production you add the necessary environmental variables to whatever controls them for your app. You can use kubernetes secrets or Hashicorp vault or if you are using something like Gitlab or Openshift continuous integration/delivery pipeline workflow will have a way to enter these values securely.
One problem you will run into with apps is how Nginx or Apache either pass or don’t pass the variables to your scripts. It can be confusing. Apache gladly passes values from the environment to our scripts, but Nginx not so much. With nginx you need to set your vars in 2 locations or run a hacky feeling 3rd party Perl script which takes your variables and passes them to Nginx. Nginx has better performance than Apache though. The story is an entire article for later though, back to the subject.
Overall Environment variables
There are many ways to set environment variables in linux and each depends on what needs to be done. Each user has their own files that set variables for them. There are also other files that set variables for programs that run on your computer like cronjob. There are also files to set variables system wide, but it depends on what is operating whether it can access it.
You can create a file with any name you want with extension .sh like example-app.sh and place it in /etc/profile.d directory.
As described in this link Linux will read all of the files in that directory and create the environmental variables.
This is basically what I have in a simple text file
You simply add a new line for each variable you want to have created for your app.
Each user on your Linux system has a .bashrc file the root, regular user etc. all have one. On Ubuntu root is disabled so settings in that .bashrc file are probably ignored. But the user you usually login as, the default name that shows in your terminal when you open it, that home directory has a .bashrc file. Located at /home/username/.bashrc If you set a variable in this file you must log out and log back into your system for them to take effect or restart your computer.
As mentioned in this article, the shell looks in several places when the terminal is invoked. Where to place it depends on what you need to do.
There are login shells, interactive shells, non interactive shells, each looks in different locations. This article has a good discussion of interactive vs non interactive. Basically interactive is when you type into the terminal, non interactive is when you run a shell script or other program that does the interaction with bash ( or other shell ) for you.
This article describes the many locations and files used to set environment variables. Not all versions of Linux will have all of these or use them, each version is different and you should read the documentation for your specific distribution. The bottom of that article gives you a better idea of where to set the variables.
For scripts like shell scripts set the environmental variables in .bashrc files. For interactive use in the terminal where the user or their commands need the variables set them in the .profile file or profile.d directory as mentioned above.
Here is a much more in depth article about Linux Environment variables.
As you can see in this link, this whole process depends on many things.
Here is another good, long explanation of Environment and Variables by digital ocean.