What are environment variables?

// 01What they actually are

An environment variable is a key-value pair — a name and a value — stored in your operating system's running environment. Every terminal session starts with a set of them already loaded. You can see them all right now.

These variables are inherited by every program your terminal launches. So when a Node.js script calls process.env.OPENAI_API_KEY, or a Python script calls os.environ["OPENAI_API_KEY"], they're reading from this same pool. The program reads the setting; you never hard-code it.

// 02Why they exist (the reason makes everything else clear)

Environment variables exist to solve two problems at once:

Problem 1: Secrets in code

If you put your API key directly in a Python file, that file gets committed to git, pushed to GitHub, and read by every bot that crawls public repositories. Keys in code get stolen. Environment variables live outside the code — they never touch git history.

Problem 2: Config that changes between environments

Your code runs in multiple places: your laptop (development), maybe a friend's laptop, maybe a cloud server (production). The database URL is different in each place. The API key might be different. Rather than editing the code every time you switch contexts, you change one environment variable. The code stays the same.

// 03The .env file — how you set them for a project

Setting environment variables in your shell manually works, but it's annoying — you'd have to re-set them every time you open a terminal. The standard solution is a .env file: a plain-text file in your project root, with one variable per line.

# AI service keys
OPENAI_API_KEY=sk-proj-abc123...
ANTHROPIC_API_KEY=sk-ant-api03-xyz789...

# App settings
PORT=3000
NODE_ENV=development

Your code loads this file at startup using a library (dotenv for Node.js, python-dotenv for Python). After that, every variable in the file is available as an environment variable — exactly as if you'd set them in the shell.

// 04How to read them in code

Each language has its own way to read environment variables. You'll see one of these patterns in every AI guide on this site:

# Python
import os
key = os.environ["OPENAI_API_KEY"]          # raises error if missing
key = os.environ.get("OPENAI_API_KEY", "")  # returns empty string if missing

// JavaScript / Node.js
const key = process.env.OPENAI_API_KEY;      // undefined if missing

// Shell (bash/zsh)
echo $OPENAI_API_KEY

# PowerShell (Windows)
$key = $env:OPENAI_API_KEY

When a guide says "the script reads OPENAI_API_KEY from the environment," this is what's happening. The variable name is a contract between the code and the person running it: you set that name in your .env file, the code looks it up by that exact name.

// 05Now you. Set a variable, read it back.

Five minutes. We'll set an environment variable in the shell and in a .env file, then read it back both ways. No API key needed — we'll use a fake one for the exercise.