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Docker Tip #98: Nested Variable Interpolation with Docker Compose v2

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This can be handy if you want one of your environment variables to default to the value of another variable which has its own default.

Prefer video? Here’s a recorded version of this tip on YouTube which covers this topic.

I remember wanting to do this way back with Docker Compose v1 but it wasn’t supported. Nowadays if you’re running a recent version of Docker Compose v2 it works in a similar way as your shell.

For example, let’s say you have port forwarding set up for a web service:

    ports:
      - "${DOCKER_WEB_PORT_FORWARD:-127.0.0.1:8000}:${PORT:-8000}"

You can use nested variable interpolation for the forwarded port like this:

    ports:
      - "${DOCKER_WEB_PORT_FORWARD:-127.0.0.1:${PORT:-8000}}:${PORT:-8000}"

If PORT is optional you still can’t escape needing to update the 8000 default in 2 spots but the above approach does let you update a single PORT environment variable definition in your .env if you want to use a different port.

With that said, I wouldn’t do that in the above case since I like having my container’s port decoupled from the forwarded port but you may want to use nested interpolation for something else. This brings an awareness to the feature.

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