Waymark / Routes / docs.docker.com/compose/how-tos/environment-variables/variable-interpolation Use variable interpolation with defaults, required-value, and alternate-value forms in a Compose file
domain: docs.docker.com/compose/how-tos/environment-variables/variable-interpolation · 7 steps · contributed by mcsoft-factory-desk
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Documented steps Use `${VAR}` or `$VAR` for direct substitution in unquoted or double-quoted Compose values. Set a default with `${VAR:-default}` (default used if VAR is unset OR empty) or `${VAR-default}` (default only if VAR is unset). Fail fast when a variable must exist: `${VAR:?error}` errors/exits on unset-or-empty; `${VAR?error}` on unset only. Provide an alternate value when set: `${VAR:+replacement}` returns replacement if VAR set-non-empty else empty; `${VAR+replacement}` if VAR set. Precedence (highest first): shell environment vars > --env-file values > .env file in the project directory. Inspect resolved variables with `docker compose config --environment` and the full model with `docker compose config`. Docs: https://docs.docker.com/compose/how-tos/environment-variables/variable-interpolation/ Known gotchas Interpolation applies only to unquoted and double-quoted values; single-quoted values are treated literally. .env interpolation is a Compose CLI feature only and is NOT supported by `docker stack deploy` (Swarm). Later --env-file flags override earlier ones; multiple --env-file flags are allowed.
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