Exploring and Running Containers

Overview

Teaching: 20 min
Exercises: 10 min

Questions

• How do I interact with a Docker container on my computer?

Objectives

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

• Be able to download new Docker images.

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

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

Reminder of terminology: 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 list and modify Docker 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 images listed.

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

$ docker pull hello-world

You should see output like this:

Using default tag: latest
latest: Pulling from library/hello-world
b8dfde127a29: Pull complete
Digest: sha256:9f6ad537c5132bcce57f7a0a20e317228d382c3cd61edae14650eec68b2b345c
Status: Downloaded newer image for hello-world:latest
docker.io/library/hello-world:latest

Docker Hub

Where did the hello-world image come from? It came from the Docker Hub website, which is a place to share Docker 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 image had downloaded successfully and was on your computer? Give it a try before checking the solution.

Solution

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

$ docker image ls

Note that the downloaded hello-world image is not in the folder where you are in the terminal! (Run ls by itself to check.) The 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 images you have on your computer.

Running the hello-world container

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

$ docker 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 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 run to get the image

We could have skipped the docker pull step; if you use the docker run command and you don’t already have a copy of the Docker image, Docker will automatically pull the 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 image to explore what else we can do with the docker 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 and running the alpine Docker container. You can do it in two steps, or one. What are they?

What happened when you ran the Alpine Docker container?

$ docker run alpine

If you never used the alpine docker image on your computer, docker probably printed a message that it couldn’t find the image and had to download it. If you used the alpine 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 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 the 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 run alpine echo 'Hello World'

So here, we see another option – we can provide commands at the end of the docker 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 shut down 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 flag -it to the docker run command and provide a shell (bash,sh, etc.) as our command. The alpine docker image doesn’t include bash so we need to use sh.

$ docker 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 bash, 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, just type exit.

/ # exit

Practice Makes Perfect

Can you find out the version of Linux installed on the busybox container? (Hint: If you search online, you’ll find that there are a few different ways to find out what version of Linux a computer or container is running. Because the busybox container is very simplified, you’ll want to use a command that prints out the contents of the file /proc/version.)

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

Solution 1 – Interactive

Run the busybox container interactively – you can use docker pull first, or just run it with this command:

$ docker run -it busybox sh

Then try, running these commands

/# cat /proc/version
/# busybox --help

Exit when you’re done.

/# exit

Solution 2 – Run commands

Run the busybox container, first with a command to read out the Linux version:

$ docker run busybox cat /proc/version

Then run the container again with a command to print out the busybox help:

$ docker run busybox busybox --help

Conclusion

So far, we’ve seen how to download Docker 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 images that are out there.

Key Points

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

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

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

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