Exploring and Running Containers

Overview

Teaching: 20 min
Exercises: 10 min

Questions

• How do I interact with Docker containers and container images on my computer?

Objectives

• Use the correct command to see which Docker container images are on your computer.

• Be able to download new Docker container images.

• Demonstrate how to start an instance of a container from a container image.

• Describe at least two ways to execute commands inside a running Docker container.

Reminder of terminology: container images and containers

Recall that a container image is the template from which particular instances of containers will be created.

Let’s explore our first Docker container. The Docker team provides a simple container image online called hello-world. We’ll start with that one.

Downloading Docker images

The docker image command is used to interact with Docker container images. You can find out what container images you have on your computer by using the following command (“ls” is short for “list”):

$ docker image ls

If you’ve just installed Docker, you won’t see any container images listed.

To get a copy of the hello-world Docker container image from the internet, run this command:

$ docker image pull hello-world

You should see output like this:

Using default tag: latest
latest: Pulling from library/hello-world
1b930d010525: Pull complete
Digest: sha256:f9dfddf63636d84ef479d645ab5885156ae030f611a56f3a7ac7f2fdd86d7e4e
Status: Downloaded newer image for hello-world:latest
docker.io/library/hello-world:latest

Docker Hub

Where did the hello-world container image come from? It came from the Docker Hub website, which is a place to share Docker container images with other people. More on that in a later episode.

Exercise: Check on Your Images

What command would you use to see if the hello-world Docker container image had downloaded successfully and was on your computer? Give it a try before checking the solution.

Solution

To see if the hello-world container image is now on your computer, run:

$ docker image ls

Note that the downloaded hello-world container image is not in the folder where you are in the terminal! (Run ls by itself to check.) The container image is not a file like our normal programs and documents; Docker stores it in a specific location that isn’t commonly accessed, so it’s necessary to use the special docker image command to see what Docker container images you have on your computer.

Running the hello-world container

To create and run containers from named Docker container images you use the docker container run command. Try the following docker container run invocation. Note that it does not matter what your current working directory is.

$ docker container run hello-world

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
    (amd64)
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
    to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

What just happened? When we use the docker container run command, Docker does three things:

The hello-world container is set up to run an action by default – namely to print this message.

Using docker container run to get the image

We could have skipped the docker image pull step; if you use the docker container run command and you don’t already have a copy of the Docker container image, Docker will automatically pull the container image first and then run it.

Running a container with a chosen command

But what if we wanted to do something different with the container? The output just gave us a suggestion of what to do – let’s use a different Docker container image to explore what else we can do with the docker container run command. The suggestion above is to use ubuntu, but we’re going to run a different type of Linux, alpine instead because it’s quicker to download.

Run the Alpine Docker container

Try downloading the alpine container image and using it to run a container. You can do it in two steps, or one. What are they?

What happened when you ran the Alpine Docker container?

$ docker container run alpine

If you have never used the alpine Docker container image on your computer, Docker probably printed a message that it couldn’t find the container image and had to download it. If you used the alpine container image before, the command will probably show no output. That’s because this particular container is designed for you to provide commands yourself. Try running this instead:

$ docker container run alpine cat /etc/os-release

You should see the output of the cat /etc/os-release command, which prints out the version of Alpine Linux that this container is using and a few additional bits of information.

Hello World, Part 2

Can you run a copy of the alpine container and make it print a “hello world” message?

Give it a try before checking the solution.

Solution

Use the same command as above, but with the echo command to print a message.

$ docker container run alpine echo 'Hello World'

So here, we see another option – we can provide commands at the end of the docker container run command and they will execute inside the running container.

Running containers interactively

In all the examples above, Docker has started the container, run a command, and then immediately stopped the container. But what if we wanted to keep the container running so we could log into it and test drive more commands? The way to do this is by adding the interactive flags -i and -t (usually combined as -it) to the docker container run command and provide a shell (bash,sh, etc.) as our command. The alpine Docker container image doesn’t include bash so we need to use sh.

$ docker container run -it alpine sh

Technically…

Technically, the interactive flag is just -i – the extra -t (combined as -it above) is the “pseudo-TTY” option, a fancy term that means a text interface. This allows you to connect to a shell, like sh, using a command line. Since you usually want to have a command line when running interactively, it makes sense to use the two together.

Your prompt should change significantly to look like this:

/ #

That’s because you’re now inside the running container! Try these commands:

• pwd
• ls
• whoami
• echo $PATH
• cat /etc/os-release

All of these are being run from inside the running container, so you’ll get information about the container itself, instead of your computer. To finish using the container, type exit.

/ # exit

Practice Makes Perfect

Can you find the busybox program in a container created from the busybox container image? What does it do? (Hint: try passing --help to almost any command will give you more information.)

Solution 1 – Interactive

Run an interactive busybox container – you can use docker image pull first, or just run it with this command:

$ docker container run -it busybox sh

Then try, running this command

/# busybox --help

Exit when you’re done.

/# exit

Solution 2 – Run commands

Run a busybox container with a command to print out the busybox help:

$ docker container run busybox busybox --help

Even More Options

There are many more options, besides -it that can be used with the docker container run command! A few of them will be covered in later episodes and we’ll share two more common ones here:

• --rm: this option guarantees that any running container is completely removed from your computer after the container is stopped. Without this option, Docker actually keeps the “stopped” container around, which you’ll see in a later episode. Note that this option doesn’t impact the container images that you’ve pulled, just running instances of containers.

• --name=: By default, Docker assigns a random name and ID number to each container instance that you run on your computer. If you want to be able to more easily refer to a specific running container, you can assign it a name using this option.

Conclusion

So far, we’ve seen how to download Docker container images, use them to run commands inside running containers, and even how to explore a running container from the inside. Next, we’ll take a closer look at all the different kinds of Docker container images that are out there.

Key Points

• The docker image pull command downloads Docker container images from the internet.

• The docker image ls command lists Docker container images that are (now) on your computer.

• The docker container run command creates running containers from container images and can run commands inside them.

• When using the docker container run command, a container can run a default action (if it has one), a user specified action, or a shell to be used interactively.