Pass ARG to ENTRYPOINT

Question

Say I have this in a Dockerfile:

ARG FOO=1
ENTRYPOINT ["docker.r2g", "run"]

where I build the above with:

docker build -t "$tag" --build-arg FOO="$(date +%s)" .

is there a way to do something like:

ENTRYPOINT ["docker.r2g", "run", ARG FOO]  // something like this

I guess the argument could also be passed with docker run instead of during the docker build phase?

score 7 · Accepted Answer · answered Jun 19 '18 at 05:48

7

You could combine ARG and ENV in your Dockerfile, as I mention in "ARG or ENV, which one to use in this case?"

ARG FOO
ENV FOO=${FOO}

That way, you docker.r2g can access the ${FOO} environment variable.

I guess the argument could also be passed with docker run instead of during the docker build phase?

That is also possible, if it makes more sense to give FOO a value at runtime:

docker run -e FOO=$(...) ...

answered Jun 19 '18 at 05:48

VonC

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another solution might be to use `--entrypoint` with the docker run command and pass in the flag there? if that works, feel free to add that to your answer, idk – Alexander Mills Jun 19 '18 at 06:47
@AlexanderMills Yes, but if your entry point is still "docker.r2g", you can override CMD instead (since CMD are parameters passed to entrypoint) – VonC Jun 19 '18 at 06:52
I am not sure if you already saw my answer, but I had to try a lot of things to get this to work properly. Turns out, trying to include a list of arguments in a single env variable is not that robust, so passing arguments using `$@` with bash, is probably the best way. – Alexander Mills Jun 19 '18 at 21:05
thanks yeah I figured, I updated with a final addition, using `docker run --entrypoint` we can override ENTRYPOINT which is probably the absolute best way to pass in a dynamic list of arguments. – Alexander Mills Jun 19 '18 at 21:10

Alexander Mills · Answer 2 · 2018-06-19T21:12:24.040

This simple technique works for me:

FROM node:9
# ...
ENTRYPOINT dkr2g run "$dkr2g_run_args"

then we launch the container with:

docker run \
    -e dkr2g_run_args="$run_args" \
    --name "$container_name" "$tag_name"

there might be some edge case issues with spreading an env variable into command line arguments, but should work for the most part.

ENTRYPOINT can work either like so:

ENTRYPOINT ["foo", "--bar", "$baz"]  # $baz will not be interpreted

or like so:

ENTRYPOINT foo --bar $baz

not sure why the latter is not preferred - but env variable interpolation/interpretation is only possible using the latter. See: How do I use Docker environment variable in ENTRYPOINT array?

However, a more robust way of passing arguments is to use $@ instead of an env variable. So what you should do then is override --entrypoint using the docker run command, like so:

docker run --entrypoint="foo" <tag> --bar $@

To learn the correct syntax of how to properly override entrypoint, you have to look that up, to be sure, but in general it's weird - you have to put --entrypoint="foo" before the tag name, and the arguments to --entrypoint, after the tag name. weird.

Pass ARG to ENTRYPOINT

2 Answers2