Engineering Craft › Developer Tooling · also in Backend Basics

Environment Variable

A key-value setting passed to processes from their environment.

Also known as: env var, env vars, environment variables, ENV, .env

An environment variable is a named string setting that the operating system passes to every program it starts. It lets you change how a program behaves, such as which database to talk to, without changing its code.

export DATABASE_URL="postgres://localhost/app_dev"
python app.py                            # the program inherits DATABASE_URL

API_KEY=abc123 python app.py             # set for this one command only

Reading them in code:

import os
db_url = os.environ["DATABASE_URL"]            # KeyError if missing: fails fast
debug = os.environ.get("DEBUG", "false") == "true"

const dbUrl = process.env.DATABASE_URL;        // undefined if missing

Why they’re used

• The same code runs everywhere (laptop, test, production) with different settings.
• Secrets stay out of source code and Git. Passwords and API keys belong in the environment or a secrets manager, never in the repository.

Points to remember

• Values are always strings. "false" and "0" are still non-empty strings, so convert them yourself.
• Missing variables should fail loudly at startup, not as a confusing error later.
• .env files are a convenience for local development. Tools such as python-dotenv or your framework load them into the environment. Add .env to .gitignore and commit a .env.example with fake values.
• They’re inherited, not shared. Setting a variable in one terminal doesn’t affect another; changing it after a program starts has no effect on that program.
• They can show up in logs, crash reports and process listings, so don’t print them.

PATH is the best-known one: see PATH.