Entrypoint and command

A container image declares two things: an ENTRYPOINT, the executable it runs, and a CMD, the arguments handed to that executable. The manifest mirrors them as entrypoint and command.

The two manifest fields are independent: setting one never clears the other’s field. Their runtime effect is not symmetric, though. Setting entrypoint discards the image CMD: every target (the Engine API, Compose and Kubernetes) ignores the image default command once an entrypoint is overridden, so a service that relies on the image CMD must now set command explicitly as well. Setting command alone leaves the image ENTRYPOINT in place, so the value is appended to it as arguments.

Why a shim needs both

An image whose entrypoint is a binary cannot be given a startup script by setting command alone:

resources:
  app:
    dockerfile:
      context: .
      command: ["sh", "-c", "setup && exec /usr/local/bin/app"]

If the image declares ENTRYPOINT ["/usr/local/bin/app"], the container runs /usr/local/bin/app sh -c "setup && exec /usr/local/bin/app". The three strings reach the binary as positional arguments, which most argument parsers reject. The script never runs.

Override the entrypoint as well:

resources:
  app:
    dockerfile:
      context: .
      entrypoint: ["sh", "-c"]
      command: ["setup && exec /usr/local/bin/app"]

Now the container runs sh -c "setup && exec /usr/local/bin/app".

This is the pattern for bridging an application that reads its configuration from a file to LightShuttle’s environment-variable service discovery: write the file from the injected variable, then exec the daemon.

How each target spells it

The same two concepts carry different names depending on where a manifest is exported. LightShuttle translates; you always write entrypoint and command.

Kubernetes is the only target that crosses the names: its command is the entrypoint, and its args is the CMD. This trips up readers who assume the words mean the same thing everywhere, so an exported chart or manifest will not look field-for-field like the source.

Clearing an entrypoint

Not supported. entrypoint: [] is rejected with an error. The Engine API and Compose disagree on how to express a reset, and no use case has required it so far.