What is an environment variable?
An environment variable is a named value the operating system passes to programs, such as settings or secrets.
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An environment variable is a name and a value that live in the environment of a running process and are inherited by every program it starts. Think of sticky notes on the fridge that everyone in the house can read: HOME says where your files are, LANG which language to use, PATH where programs live.
Applications read configuration from them so the same code runs in development and production: DATABASE_URL, PORT, API_KEY. Docker, Kubernetes and CI systems all set them for your app. Files called .env hold them locally and should never be committed. In the shell you set one for the current session with export, or just for one command by prefixing it.
export APP_ENV=production
APP_ENV=test python app.py
printenv | sortSee also: What is PATH? and What is a token?
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