CODEWITHSUNDEEP
docker
basickarl
basickarl

Reputation: 40464

Accessing file stored in a Docker volume

I was reading the following https://developer.atlassian.com/blog/2016/06/common-dockerfile-mistakes/ and got caught up on a section:

Volumes in your image are added when you run your container, not when you build it. You should never interact with your declared volume in your build process as it should only be used when you run your container.

Example: Creating a file in my build process and then running cat on the file once the image is run works fine:

FROM ubuntu:12.04
RUN echo "Hello Charlie!" > /test.txt

CMD ["cat", "/test.txt"]

$ docker run test-build-volume
Hello Charlie!

If I attempt to do the same thing for a file stored in a volume then it won't work:

FROM ubuntu:12.04
RUN echo "Hello Charlie!" > /var/data/test.txt

CMD ["cat", "/var/data/test.txt"]

$ docker run test-build-volume
cat: can't open '/var/data/test.txt': No such file or directory

I don't see what is wrong with the second example? Is it incorrect (typo?). The only difference I see is /test.txt replaced with /var/data/test.txt, I fail to see the problem they should work the same? Is there something fundamental in Docker I'm missing?

Upvotes: 1

Views: 11654

Answers (2)

tgogos
tgogos

Reputation: 25152

GSUgambit is right about echo silently failing, but (deleted the relative post).

IMHO the example is not verifiable, it lacks the details of how to Start a container with a volume, doesn't make clear that /var/data doesn't exist at the time of build and that it is later created when you run a container (see case 1 below).

A few details which might help:

Volumes, Source, Destination fields & 2 different cases:

When you docker inspect a container with a volume notice the two fields:

  • Source (host location of files)
  • Destination (container location of files)

for example:

"Mounts": [
    {
        "Type": "volume",
        "Name": "myvol2",
        "Source": "/var/lib/docker/volumes/myvol2/_data",
        "Destination": "/app",
         ...
    }
],

Case 1: destination location doesn't exist

In this case, docker run ... with a volume option will create a new destination directory in the container, for example:

$ docker run --rm -it -v test_volume:/this_folder_will_be_created ubuntu
root@6b80b9895dcc:/# cd /this_folder_will_be_created/
root@6b80b9895dcc:/this_folder_will_be_created#

Case 2: destination location exists

Populate a volume using a container

If you start a container which creates a new volume, as above, and the container has files or directories in the directory to be mounted (such as /app/ above), the directory’s contents are copied into the volume.

Upvotes: 1

Gorav Singal
Gorav Singal

Reputation: 538

/var/data does not exists. Your docker build must be giving this warning.

Try replace /var/data with /var/log This will work.

Upvotes: 0

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