CODEWITHSUNDEEP
docker
jab
jab

Reputation: 5823

Dockerfile run entrypoint before shell entrypoint

I'm wanting to do some last minute setup on run before passing arguments to the shell entrypoint, to accomplish this I have the following in mind.

ENTRYPOINT ./run_binary ${args}; /bin/sh -c
CMD ./run_binary

However, by doing this, it doesn't seem that any CMD arguments make it to the shell entrypoint. Is there a way around this? I'm just wanting to run a setup step on a binary before handing back control to the shell entrypoint (and then to the USER via CMD).

Upvotes: 1

Views: 1577

Answers (2)

ldg
ldg

Reputation: 9402

You could use an intermediate build image that triggers an ONBUILD statement from your original Dockerfile, see: https://docs.docker.com/engine/reference/builder/#/onbuild

The ONBUILD instruction adds to the image a trigger instruction to be executed at a later time, when the image is used as the base for another build. The trigger will be executed in the context of the downstream build, as if it had been inserted immediately after the FROM instruction in the downstream Dockerfile.

This is useful if you are building an image which will be used as a base to build other images, for example an application build environment or a daemon which may be customized with user-specific configuration.

Regarding CMD and ENTRYPOINT, see: https://docs.docker.com/engine/reference/builder/#understand-how-cmd-and-entrypoint-interact

  1. Dockerfile should specify at least one of CMD or ENTRYPOINT commands.
  2. ENTRYPOINT should be defined when using the container as an executable.
  3. CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.
  4. CMD will be overridden when running the container with alternative arguments.

Upvotes: 1

alebianco
alebianco

Reputation: 2555

CMD becomes a list of arguments to send to ENTRYPOINT when both are specified, see the manual, so that's not the way to go

but you could use a .sh script as ENTRYPOINT, that first executes your binary command and then forwards the received arguments to a shell

I haven't tried it but something along the lines of 

#!/bin/sh

./run_binary
/bin/sh -c "$@"

Upvotes: 2

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