Environment variables

Reading them in code

Every language can read them. The value is always text, or missing.

// Node.js
const port = Number(process.env.PORT ?? 3000);
const apiKey = process.env.API_KEY;

# Python
import os

port = int(os.environ.get("PORT", "3000"))
api_key = os.environ["API_KEY"]  # raises KeyError if it's missing

Two habits help. Give optional settings a sensible default, like the port above. For required ones, such as a key, fail at start-up with a clear message instead of carrying on without it.

Because values are text, DEBUG=false is the string "false", which counts as true in both languages. Compare it to a string: process.env.DEBUG === "true".

Naming them

Names are case-sensitive on macOS and Linux, so api_key and API_KEY are different variables. The convention is upper case with underscores, and letters, digits and underscores only, not starting with a digit. Hosting platforms usually set a few for you, such as PORT, the port your app should listen on (lesson 1.4.3).

Pick one name per setting and use it everywhere: in your code, your .env file and the platform's settings.

Setting them in your shell

A variable set in a terminal applies to programs started from that terminal, until you close it.

export API_KEY="demo-not-a-real-key-0000"
node server.js

$env:API_KEY = "demo-not-a-real-key-0000"
node server.js

.env files, for your own machine

Typing every variable each time gets old. A .env file keeps them in one place for local development:

# .env: local development only, never committed
DATABASE_URL=postgres://app:localpass@localhost:5432/shop
API_KEY=demo-not-a-real-key-0000
LOG_LEVEL=debug

Your app doesn't read this file by itself. Something has to load it: node --env-file=.env server.js (Node 20.6 or later), the python-dotenv package, or your framework, which often does it for you.

Two rules keep it safe:

1. Add .env to .gitignore before your first commit, so it never reaches the repository.
2. Commit a .env.example instead, with every name and a placeholder value, so a teammate knows what to set.

On a hosting platform, there's no .env file. You set the same names in its settings, and it passes them to your app when it starts.