Linux – Exploring Docker container’s file system

dockerfilesystemslinux

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.

Best Answer

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

Related Solutions

Python – How to copy a file in Python

shutil has many methods you can use. One of which is:

from shutil import copyfile
copyfile(src, dst)

# 2nd option
copy(src, dst)  # dst can be a folder; use copy2() to preserve timestamp

Copy the contents of the file named src to a file named dst. Both src and dst need to be the entire filename of the files, including path.
The destination location must be writable; otherwise, an IOError exception will be raised.
If dst already exists, it will be replaced.
Special files such as character or block devices and pipes cannot be copied with this function.
With copy, src and dst are path names given as strs.

Another shutil method to look at is shutil.copy2(). It's similar but preserves more metadata (e.g. time stamps).

If you use os.path operations, use copy rather than copyfile. copyfile will only accept strings.

Docker – How is Docker different from a virtual machine

Docker originally used LinuX Containers (LXC), but later switched to runC (formerly known as libcontainer), which runs in the same operating system as its host. This allows it to share a lot of the host operating system resources. Also, it uses a layered filesystem (AuFS) and manages networking.

AuFS is a layered file system, so you can have a read only part and a write part which are merged together. One could have the common parts of the operating system as read only (and shared amongst all of your containers) and then give each container its own mount for writing.

So, let's say you have a 1 GB container image; if you wanted to use a full VM, you would need to have 1 GB x number of VMs you want. With Docker and AuFS you can share the bulk of the 1 GB between all the containers and if you have 1000 containers you still might only have a little over 1 GB of space for the containers OS (assuming they are all running the same OS image).

A full virtualized system gets its own set of resources allocated to it, and does minimal sharing. You get more isolation, but it is much heavier (requires more resources). With Docker you get less isolation, but the containers are lightweight (require fewer resources). So you could easily run thousands of containers on a host, and it won't even blink. Try doing that with Xen, and unless you have a really big host, I don't think it is possible.

A full virtualized system usually takes minutes to start, whereas Docker/LXC/runC containers take seconds, and often even less than a second.

There are pros and cons for each type of virtualized system. If you want full isolation with guaranteed resources, a full VM is the way to go. If you just want to isolate processes from each other and want to run a ton of them on a reasonably sized host, then Docker/LXC/runC seems to be the way to go.

For more information, check out this set of blog posts which do a good job of explaining how LXC works.

Why is deploying software to a docker image (if that's the right term) easier than simply deploying to a consistent production environment?

Deploying a consistent production environment is easier said than done. Even if you use tools like Chef and Puppet, there are always OS updates and other things that change between hosts and environments.

Docker gives you the ability to snapshot the OS into a shared image, and makes it easy to deploy on other Docker hosts. Locally, dev, qa, prod, etc.: all the same image. Sure you can do this with other tools, but not nearly as easily or fast.

This is great for testing; let's say you have thousands of tests that need to connect to a database, and each test needs a pristine copy of the database and will make changes to the data. The classic approach to this is to reset the database after every test either with custom code or with tools like Flyway - this can be very time-consuming and means that tests must be run serially. However, with Docker you could create an image of your database and run up one instance per test, and then run all the tests in parallel since you know they will all be running against the same snapshot of the database. Since the tests are running in parallel and in Docker containers they could run all on the same box at the same time and should finish much faster. Try doing that with a full VM.

From comments...

Interesting! I suppose I'm still confused by the notion of "snapshot[ting] the OS". How does one do that without, well, making an image of the OS?

Well, let's see if I can explain. You start with a base image, and then make your changes, and commit those changes using docker, and it creates an image. This image contains only the differences from the base. When you want to run your image, you also need the base, and it layers your image on top of the base using a layered file system: as mentioned above, Docker uses AuFS. AuFS merges the different layers together and you get what you want; you just need to run it. You can keep adding more and more images (layers) and it will continue to only save the diffs. Since Docker typically builds on top of ready-made images from a registry, you rarely have to "snapshot" the whole OS yourself.