Exploring Docker container’s file system

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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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