[SOLVED] Piping tar archive to host fails on docker run but works on docker exec

Piping tar archive to host fails on docker run but works on docker exec

I'm trying to get some libraries out of a Docker image, since I temporarily need it in another environment.

Since I need symlinks to persist through this, I'm trying to tar the directory and pipe it to my hot system to immediately untar it there. In the end I want to pipe them directly into a running Kubernetes Pod.

Since I don't have any environment running with the image, that I need the files from, I tried to do the following:

docker run --rm bitnami/postgresql:11.22.0-debian-11-r4 \
    tar cf - -C /usr/lib/x86_64-linux-gnu . | tar xf -

This results in

tar: This does not look like a tar archive
tar: Skipping to next header
tar: Exiting with failure status due to previous errors
write /dev/stdout: broken pipe

But when I do this in an already running container it works.

Start a docker container with the image and open a bash (so it does not start the postgres service but stays alive):

docker run --rm -it --name my-postgresql bitnami/postgresql:11.22.0-debian-11-r4 -- bash

No open up a new terminal on your host and execute the following:

docker exec my-postgresql \
    tar cf - -C /usr/lib/x86_64-linux-gnu/ . |\
    tar xf -

This works like a charm.

Can anyone explain to me, what I do wrong in the former example? Thanks in advance!

Solution

The bitnami postgres image includes an ENTRYPOINT script:

$ docker image inspect bitnami/postgresql:11.22.0-debian-11-r4   -f '{{ .Config.Entrypoint }}'
[/opt/bitnami/scripts/postgresql/entrypoint.sh]

That script produces some output on stdout before tar produces any output; this results in data that can't be processed by the reading tar command. You can avoid that problem by replacing the entrypoint:

docker run --rm --entrypoint tar bitnami/postgresql:11.22.0-debian-11-r4 \
  cf - -C /usr/lib/x86_64-linux-gnu . |
  tar xf -