CODEWITHSUNDEEP
docker
Gravy
Gravy

Reputation: 12445

override default docker run from host with options/arguments

FROM alpine:3.5
CMD ["echo", "hello world"]

So after building docker build -t hello . I can run hello by calling docker run hello and I get the output hello world.

Now let's assume I wish to run ls or sh - this is fine. But what I really want is to be able to pass arguments. e.g. ls -al, or even tail -f /dev/null to keep the container running without having to change the Dockerfile

How do I go about doing this? my attempt at exec mode fails miserably... docker run hello --cmd=["ls", "-al"]

Upvotes: 1

Views: 390

Answers (2)

Robert
Robert

Reputation: 36823

Q. But what I really want is to be able to pass arguments. e.g. ls -al, or even tail -f /dev/null to keep the container running without having to change the Dockerfile

This is just achieved with:

 docker run -d hello tail -f /dev/null

So the container is running in background, and it let you to execute arbitrary commands inside it:

docker exec <container-id> ls -la

And, for example a shell:

docker exec -it <container-id> bash

Also, I recommend you what @BMitch says.

Upvotes: 0

BMitch
BMitch

Reputation: 264591

Anything after the image name in the docker run command becomes the new value of CMD. So you can run:

docker run hello ls -al

Note that if an ENTRYPOINT is defined, the ENTRYPOINT will receive the value of CMD as args rather than running CMD directly. So you can define an entrypoint as a shell script with something like:

#!/bin/sh

echo "running the entrypoint code"

# if no args are passed, default to a /bin/sh shell
if [ $# -eq 0 ]; then
  set -- /bin/sh
fi

# run the "CMD" with exec to replace the pid 1 of this shell script
exec "$@"

Upvotes: 2

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