Environment variable

docker run -e <KEY>=<value> <image>
docker run --env-file <file> <image>

docker run -e NODE_ENV=production -e DB_HOST=db myapp:1.0
docker run --env-file .env myapp:1.0

Note Using --env-file keeps secrets out of your shell history and process list. Each line in the file should be KEY=value with no quotes needed around the value.

ENV <key>=<value> [<key>=<value>...]

ENV NODE_ENV=production
ENV APP_PORT=3000 LOG_LEVEL=info

Note ENV values persist into the running container and into child images. If you only need a variable during build time (e.g., a version number), use ARG instead to avoid leaking it into the final image.

env
export VAR=value
printenv VAR

env | grep PATH
export DATABASE_URL='postgres://user:pass@db:5432/mydb'
printenv HOME

/home/deploy

Note env lists all environment variables. export makes a variable available to child processes. Variables set without export are local to the current shell. printenv retrieves a single variable. Avoid putting secrets in environment variables on shared systems; prefer a secrets manager.

import os
os.path.join()
os.environ

import os

# Environment variables
db_host = os.environ.get("DB_HOST", "localhost")
print(f"DB: {db_host}")

# Path operations (prefer pathlib for new code)
print(os.path.expanduser("~"))
print(os.path.splitext("report.pdf"))
print(os.getcwd())

DB: localhost
/Users/username
('report', '.pdf')
/current/directory

Note Prefer pathlib.Path for path manipulation in new code. os.environ provides direct access to environment variables. Use os.getenv() for safe access with defaults.