Exploring Docker container s file system

Question

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

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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  gt  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

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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

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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  lt container id gt  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

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My preferred way to understand what is going on inside container is    expose -p 8000  docker run -it -p 8000 8000 image  Start server inside it  python -m SimpleHTTPServer

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another trick is to use the atomic tool to do something like   mkdir -p  path to mnt  amp  amp  atomic mount IMAGE  path to mnt   The Docker image will be mounted to  path to mnt for you to inspect it

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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  lt container-name gt   lt path inside container gt   lt path on host  gt    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

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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

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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

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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  lt name gt  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

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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  lt image gt     returns container ID the container is never started  docker cp  lt container ID gt   lt source path gt   lt destination path gt  docker rm  lt container ID gt  cd  lt destination path gt   amp  amp  ls -lsah  see docker exec documentation see docker-compose exec documentation see docker inspect documentation see docker create documentation

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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

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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

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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

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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 voil   you re inside the container   Basically you can as root cd into  proc  lt PID gt  root  folder as long as that process is run by the container  Beware symlinks will not make sense wile using that mode

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you can use dive to view the image content interactively with TUI  https   github com wagoodman dive

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

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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  gt  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

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Try using  docker exec -it  lt container-name gt   bin bash   There might be possibility that bash is not implemented  for that you can use  docker exec -it  lt container-name gt  sh

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Here are a couple different methods    A  Use docker exec  easiest  Docker version 1 3 or newer supports the command exec that behave similar to nsenter  This command can run new process in already running container  container must have PID 1 process running already   You can run  bin bash to explore container state  docker exec -t -i mycontainer  bin bash  see Docker command line documentation B  Use Snapshotting You can evaluate container filesystem this way    find ID of your running container  docker ps    create image  snapshot  from container filesystem docker commit 12345678904b5 mysnapshot    explore this filesystem using bash  for example  docker run -t -i mysnapshot  bin bash  This way  you can evaluate filesystem of the running container in the precise time moment  Container is still running  no future changes are included  You can later delete snapshot using  filesystem of the running container is not affected    docker rmi mysnapshot  C  Use ssh If you need continuous access  you can install sshd to your container and run the sshd daemon   docker run -d -p 22 mysnapshot  usr sbin sshd -D      you need to find out which port to connect   docker ps  This way  you can run your app using ssh  connect and execute what you want   D  Use nsenter Use nsenter  see Why you don t need to run SSHd in your Docker containers  The short version is  with nsenter  you can get a shell into an existing container  even if that container doesn   t run SSH or any kind of special-purpose daemon

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In my case no shell was supported in container except sh  So  this worked like a charm  docker exec -it  lt container-name gt  sh

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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

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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

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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   Find out the process id  PID  of the container and store into some variable  PID   docker inspect -f     State Pid    your-container-name-here    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

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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

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This will launch a bash session for the image   docker run --rm -it --entrypoint  bin bash

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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

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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

[linux] Exploring Docker container's file system

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