[linux] Exploring Docker container's file system

I've noticed with docker that I need to understand what's happening inside a container or what files exist in there. One example is downloading images from the docker index - you don't have a clue what the image contains so it's impossible to start the application.

What would be ideal is to be able to ssh into them or equivalent. Is there a tool to do this, or is my conceptualisation of docker wrong in thinking I should be able to do this.

UPDATE: EXPLORING!

This command should let you explore a running docker container:

docker exec -it name-of-container bash

The equivalent for this in docker-compose would be:

docker-compose exec web bash

(web is the name-of-service in this case and it has tty by default.)

Once you are inside do:

ls -lsa

or any other bash command like:

cd ..

This command should let you explore a docker image:

docker run --rm -it --entrypoint=/bin/bash name-of-image

once inside do:

ls -lsa

or any other bash command like:

cd ..

The -it stands for interactive... and tty.

This command should let you inspect a running docker container or image:

docker inspect name-of-container-or-image

You might want to do this and find out if there is any bash or sh in there. Look for entrypoint or cmd in the json return.

NOTE: This answer relies on commen tool being present, but if there is no bash shell or common tools like ls present you could first add one in a layer if you have access to the Dockerfile: example for alpine:

RUN apk add --no-cache bash

Otherwise if you don't have access to the Dockerfile then just copy the files out of a newly created container and look trough them by doing:

docker create <image>  # returns container ID the container is never started.
docker cp <container ID>:<source_path> <destination_path>
docker rm <container ID>
cd <destination_path> && ls -lsah

see docker exec documentation

see docker-compose exec documentation

see docker inspect documentation

see docker create documentation

Only for LINUX

The most simple way that I use was using proc dir, which is the container is must be running in order to inspect the docker container files.

1. Find out the process id (PID) of the container and store into some variable

   PID=$(docker inspect -f '{{.State.Pid}}' your-container-name-here)

2. Make sure the container process is running, and use the variable name to get into the container folder

   cd /proc/$PID/root

If you want to get through the dir without finding out the PID number, just us this long command

cd /proc/$(docker inspect -f '{{.State.Pid}}' your-container-name-here)/root

Tips:

After you get inside of the container, everything you do will affect the actual process of the container, such as stopping the service or changing the port number.

Hope it helps

Note:

This method only works if the container is still running, otherwise the directory wouldn't exist anymore if the container has stopped or removed

In my case no shell was supported in container except sh. So, this worked like a charm

docker exec -it <container-name> sh

I use another dirty trick that is aufs/devicemapper agnostic.

I look at the command that the container is running e.g. docker ps and if it's an apache or java i just do the following:

sudo -s
cd /proc/$(pgrep java)/root/

and voilá you're inside the container.

Basically you can as root cd into /proc/<PID>/root/ folder as long as that process is run by the container. Beware symlinks will not make sense wile using that mode.

Try using

docker exec -it <container-name> /bin/bash

There might be possibility that bash is not implemented. for that you can use

docker exec -it <container-name> sh

another trick is to use the atomic tool to do something like:

mkdir -p /path/to/mnt && atomic mount IMAGE /path/to/mnt

The Docker image will be mounted to /path/to/mnt for you to inspect it.

This answer will help those (like myself) who want to explore the docker volume filesystem even if the container isn't running.

List running docker containers:

docker ps

=> CONTAINER ID "4c721f1985bd"

Look at the docker volume mount points on your local physical machine (https://docs.docker.com/engine/tutorials/dockervolumes/):

docker inspect -f {{.Mounts}} 4c721f1985bd

=> [{ /tmp/container-garren /tmp true rprivate}]

This tells me that the local physical machine directory /tmp/container-garren is mapped to the /tmp docker volume destination.

Knowing the local physical machine directory (/tmp/container-garren) means I can explore the filesystem whether or not the docker container is running. This was critical to helping me figure out that there was some residual data that shouldn't have persisted even after the container was not running.

On newer versions of Docker you can run docker exec [container_name] which runs a shell inside your container

So to get a list of all the files in a container just run docker exec [container_name] ls

If you are using Docker v19.03, you follow the below steps.

# find ID of your running container:

  docker ps

# create image (snapshot) from container filesystem

  docker commit 12345678904b5 mysnapshot

# explore this filesystem 

  docker run -t -i mysnapshot /bin/sh

This will launch a bash session for the image:

docker run --rm -it --entrypoint=/bin/bash

The file system of the container is in the data folder of docker, normally in /var/lib/docker. In order to start and inspect a running containers file system do the following:

hash=$(docker run busybox)
cd /var/lib/docker/aufs/mnt/$hash

And now the current working directory is the root of the container.

The docker exec command to run a command in a running container can help in multiple cases.

Usage:  docker exec [OPTIONS] CONTAINER COMMAND [ARG...]

Run a command in a running container

Options:
  -d, --detach               Detached mode: run command in the background
      --detach-keys string   Override the key sequence for detaching a
                             container
  -e, --env list             Set environment variables
  -i, --interactive          Keep STDIN open even if not attached
      --privileged           Give extended privileges to the command
  -t, --tty                  Allocate a pseudo-TTY
  -u, --user string          Username or UID (format:
                             [:])
  -w, --workdir string       Working directory inside the container

For example :

1) Accessing in bash to the running container filesystem :

docker exec -it containerId bash 

2) Accessing in bash to the running container filesystem as root to be able to have required rights :

docker exec -it -u root containerId bash  

This is particularly useful to be able to do some processing as root in a container.

3) Accessing in bash to the running container filesystem with a specific working directory :

docker exec -it -w /var/lib containerId bash 

You can run a bash inside the container with this: $ docker run -it ubuntu /bin/bash

For docker aufs driver:

The script will find the container root dir(Test on docker 1.7.1 and 1.10.3 )

if [ -z "$1" ] ; then
 echo 'docker-find-root $container_id_or_name '
 exit 1
fi
CID=$(docker inspect   --format {{.Id}} $1)
if [ -n "$CID" ] ; then
    if [ -f  /var/lib/docker/image/aufs/layerdb/mounts/$CID/mount-id ] ; then
        F1=$(cat /var/lib/docker/image/aufs/layerdb/mounts/$CID/mount-id)
       d1=/var/lib/docker/aufs/mnt/$F1
    fi
    if [ ! -d "$d1" ] ; then
        d1=/var/lib/docker/aufs/diff/$CID
    fi
    echo $d1
fi

The most upvoted answer is working for me when the container is actually started, but when it isn't possible to run and you for example want to copy files from the container this has saved me before:

docker cp <container-name>:<path/inside/container> <path/on/host/>

Thanks to docker cp (link) you can copy directly from the container as it was any other part of your filesystem. For example, recovering all files inside a container:

mkdir /tmp/container_temp
docker cp example_container:/ /tmp/container_temp/

Note that you don't need to specify that you want to copy recursively.

The most voted answer is good except if your container isn't an actual Linux system.

Many containers (especially the go based ones) don't have any standard binary (no /bin/bash or /bin/sh). In that case, you will need to access the actual containers file directly:

Works like a charm:

name=<name>
dockerId=$(docker inspect -f {{.Id}} $name)
mountId=$(cat /var/lib/docker/image/aufs/layerdb/mounts/$dockerId/mount-id)
cd /var/lib/docker/aufs/mnt/$mountId

Note: You need to run it as root.

On Ubuntu 14.04 running Docker 1.3.1, I found the container root filesystem on the host machine in the following directory:

/var/lib/docker/devicemapper/mnt/<container id>/rootfs/

Full Docker version information:

Client version: 1.3.1
Client API version: 1.15
Go version (client): go1.3.3
Git commit (client): 4e9bbfa
OS/Arch (client): linux/amd64
Server version: 1.3.1
Server API version: 1.15
Go version (server): go1.3.3
Git commit (server): 4e9bbfa

If you are using the AUFS storage driver, you can use my docker-layer script to find any container's filesystem root (mnt) and readwrite layer :

# docker-layer musing_wiles
rw layer : /var/lib/docker/aufs/diff/c83338693ff190945b2374dea210974b7213bc0916163cc30e16f6ccf1e4b03f
mnt      : /var/lib/docker/aufs/mnt/c83338693ff190945b2374dea210974b7213bc0916163cc30e16f6ccf1e4b03f

Edit 2018-03-28 :
docker-layer has been replaced by docker-backup

In case your container is stopped or doesn't have a shell (e.g. hello-world mentioned in the installation guide, or non-alpine traefik), this is probably the only possible method of exploring the filesystem.

You may archive your container's filesystem into tar file:

docker export adoring_kowalevski > contents.tar

Or list the files:

docker export adoring_kowalevski | tar t

Do note, that depending on the image, it might take some time and disk space.

None of the existing answers address the case of a container that exited (and can't be restarted) and/or doesn't have any shell installed (e.g. distroless ones). This one works as long has you have root access to the Docker host.

For a real manual inspection, find out the layer IDs first:

docker inspect my-container | jq '.[0].GraphDriver.Data'

In the output, you should see something like

"MergedDir": "/var/lib/docker/overlay2/03e8df748fab9526594cfdd0b6cf9f4b5160197e98fe580df0d36f19830308d9/merged"

Navigate into this folder (as root) to find the current visible state of the container filesystem.

For me, this one works well (thanks to the last comments for pointing out the directory /var/lib/docker/):

chroot /var/lib/docker/containers/2465790aa2c4*/root/

Here, 2465790aa2c4 is the short ID of the running container (as displayed by docker ps), followed by a star.

My preferred way to understand what is going on inside container is:

1. expose -p 8000

   docker run -it -p 8000:8000 image
   

2. Start server inside it

   python -m SimpleHTTPServer
   

you can use dive to view the image content interactively with TUI

https://github.com/wagoodman/dive

I wanted to do this, but I was unable to exec into my container as it had stopped and wasn't starting up again due to some error in my code.

What worked for me was to simply copy the contents of the entire container into a new folder like this:

docker cp container_name:/app/ new_dummy_folder

I was then able to explore the contents of this folder as one would do with a normal folder.

Before Container Creation :

If you to explore the structure of the image that is mounted inside the container you can do

sudo docker image save image_name > image.tar
tar -xvf image.tar

This would give you the visibility of all the layers of an image and its configuration which is present in json files.

After container creation :

For this there are already lot of answers above. my preferred way to do this would be -

docker exec -t -i container /bin/bash

For an already running container, you can do:

dockerId=$(docker inspect -f {{.Id}} [docker_id_or_name])

cd /var/lib/docker/btrfs/subvolumes/$dockerId

You need to be root in order to cd into that dir. If you are not root, try 'sudo su' before running the command.

Edit: Following v1.3, see Jiri's answer - it is better.