Containers 101: Start a Container With contaiNERD CTL (nerdctl)

In this challenge, you will need to start a container without Docker (or its high-level alternatives like Podman). Instead, you will use a much more lightweight combination of containerd and a Docker-compatible CLI for containerd called nerdctl.

Various containerd CLI clients (ctr, nerdctl, crictl).

You can use any container image you like, but we recommend choosing a long-running container (e.g., nginx) because to complete this challenge you will also need to inspect the running container and answer a few questions about it.

Hint 1 💡

nerdctl tries to be as compatible with the Docker CLI as possible. If you're familiar with Docker, you already know how to start a container with contaiNERD CTL. And if you're not, try solving this Docker 101 challenge first.

To keep track of containers, nerdctl assigns a unique ID to each of them. Can you find the ID of the container that you've just started?

Hint 2 💡

It's an easy one - nerdctl --help will show you the way.

Now, when you have a running container, let's try to understand what it actually is.

Did you know that Linux containers are regular processes? Can you locate the main container's process? What is its PID?

Hint 3 💡

If containers are regular Linux processes, will they show up in ps or top output? 🤔

Hint 4 💡

Entered a PID of a process that definitely belongs to the container, but the solution checker doesn't accept it? One of the key Docker (hence, containerd) design principles is to run one service per container. However, it doesn't mean that every container will have only one process inside. Actually, more often than not, you'll find a whole process tree inside a container. Try identifying the root process of that tree. That's what the checker expects.

Hint 5 💡

Still having trouble? You can always fall back to nerdctl inspect to cross-check your findings.

Approximating containers to regular Linux processes is helpful, but it's not very accurate. Thinking of containers as of boxes for processes might be even more helpful at times.

Linux container visualized as a box for one application.

From inside the box, it may look like the containerized app is running on its own machine. In particular, such a virtualized environment will have its own network interface and an IP address. Can you find the IP address of the container that you've just started?

Hint 6 💡

There are many ways to find the container IP address. If you're a Linux guru, you can try your luck with ip netns and ip addr.

Hint 7 💡

Don't feel comfortable messing with Linux network namespaces? You can always use the nerdctl inspect command to find the container IP address.