Use variable interpolation with defaults and required checks in a Docker Compose file (${VAR:-default}, ${VAR:?error})

domain: docs.docker.com/compose/how-tos/environment-variables/variable-interpolation/ · 5 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗

Documented steps

  1. Reference values with `${VAR}` (direct), `${VAR:-default}` (default when unset OR empty), `${VAR-default}` (default only when unset), `${VAR:?err}` (fail with err if unset/empty), or `${VAR:+rep}` (replacement when set/non-empty).
  2. Compose resolves vars from the shell environment (highest precedence), then a file given by `--env-file`, then `.env` in the project directory.
  3. Inject variables into the container environment via the service `environment:` attribute using the same interpolation.
  4. Verify resolution with `docker compose config` (resolved YAML) or `docker compose config --environment`.
  5. Docs: https://docs.docker.com/compose/how-tos/environment-variables/variable-interpolation/

Known gotchas

Related routes

Use variable interpolation with defaults, required-value, and alternate-value forms in a Compose file
docs.docker.com/compose/how-tos/environment-variables/variable-interpolation · 7 steps · unrated
Manage Docker Compose variables with the .env file and --env-file per-environment overrides
docs.docker.com/compose/how-tos/environment-variables/variable-interpolation/ · 5 steps · unrated
Load environment variables from multiple env_file files in Docker Compose (order and precedence)
docs.docker.com/compose/compose-file/05-services/ · 6 steps · unrated

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