Extract file from docker image?

Question

I'd like to extract a file from a Docker image without having to run the image.

The docker save option is not currrently a viable option for me as it's saving too huge of a file just to un-tar a specific file.

Answer 1

You can extract files from an image with the following commands:

container_id=$(docker create "$image")
docker cp "$container_id:$source_path" "$destination_path"
docker rm "$container_id"

According to the docker create documentation, this doesn't run the container:

The docker create command creates a writeable container layer over the specified image and prepares it for running the specified command. The container ID is then printed to STDOUT. This is similar to docker run -d except the container is never started. You can then use the docker start <container_id> command to start the container at any point.

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For reference (my previous answer), a less efficient way of extracting a file from an image is the following:

docker run some_image cat "$file_path" > "$output_path"

Answer 2

None of the above worked for me. The complete working command is:

docker run --rm --entrypoint /bin/sh image_name -c "cat /path/filename" > output_filename

Without quotes cat is passed without filename, so it does not know what to show. Also it is a good idea to delete the container after command is finished.

Answer 3

If storing the full output of docker save isn't an option, you could use pipelines to extract just the needed file from it.

Unfortunately, because the output is a "tar of tars", it can be a slightly iterative process.

What I did when I needed to extract a file just now was:

1. Determine which version of the image the file you are interested in changed most recently (how you do this probably depends on your image), and the date it was created / saved

2. Get the full table of contents from the output of the docker save command with:

   docker save IMAGE_NAME | tar -tvf -

3. Find the layer.tar file(s) in the output of that command that match the date of the image that you determined in step 1. (you can add | grep layer.tar to just show those files)

4. Extract that layer.tar file to standard out, and get the table of contents of it:

   docker save IMAGE_NAME | tar -xf - -O CHECKSUM_FROM_LIST/layer.tar | tar -tvf -

5. Verify the file you want is listed, and extract it once you find the name:

   docker save IMAGE_NAME | tar -xf - -O CHECKSUM_FROM_LIST/layer.tar | tar -xf - PATH/TO/YOUR/FILE

If there are more than one layer.tar files matching the date you are looking for in step 2/3, you may need to repeat step 4 for each one of them until you find the right one

Replace the text in capitals in the commands above with the correct image names, checksums and filenames for your case.

Answer 4

I believe that Docker containers store cached files created in the following directory for Ubuntu:

/var/lib/docker/aufs/diff/<container_id>

From there you should be able to access the file system and retrieve your file(s).