Docker First Container

1.0 Running your first container

Now that you have everything setup, it's time to get our hands dirty. In this section, you are going to run an Alpine Linux container (a lightweight linux distribution) on your system and get a taste of the docker run command.

To get started, let's run the following in our terminal:

docker pull alpine

Note: Depending on how you've installed docker on your system, you might see a permission denied error after running the above command. Try the commands from the Getting Started tutorial to verify your installation. If you're on Linux, you may need to prefix your docker commands with sudo. Alternatively you can create a docker group to get rid of this issue.

The pull command fetches the alpine image from the Docker registry and saves it in our system. You can use the docker images command to see a list of all images on your system.

docker images

1.1 Docker Run

Great! Let's now run a Docker container based on this image. To do that you are going to use the docker run command.

docker run alpine ls -l

What happened? Behind the scenes, a lot of stuff happened. When you call run,

1. The Docker client contacts the Docker daemon

2. The Docker daemon checks local store if the image (alpine in this case) is available locally, and if not, downloads it from Docker Store. (Since we have issued docker pull alpine before, the download step is not necessary)

3. The Docker daemon creates the container and then runs a command in that container.

4. The Docker daemon streams the output of the command to the Docker client

When you run docker run alpine, you provided a command (ls -l), so Docker started the command specified and you saw the listing.

Let's try something more exciting.

docker run alpine echo "hello from alpine"

OK, that's some actual output. In this case, the Docker client dutifully ran the echo command in our alpine container and then exited it. If you've noticed, all of that happened pretty quickly. Imagine booting up a virtual machine, running a command and then killing it. Now you know why they say containers are fast!

Try another command.

docker run alpine /bin/sh

Wait, nothing happened! Is that a bug? Well, no. These interactive shells will exit after running any scripted commands, unless they are run in an interactive terminal - so for this example to not exit, you need to docker run -it alpine /bin/sh.

You are now inside the container shell and you can try out a few commands like ls -l, uname -a and others. Exit out of the container by giving the exit command.

Ok, now it's time to see the docker ps command. The docker ps command shows you all containers that are currently running.

docker ps

Since no containers are running, you see a blank line. Let's try a more useful variant: docker ps -a

docker ps -a

What you see above is a list of all containers that you ran. Notice that the STATUS column shows that these containers exited a few minutes ago. You're probably wondering if there is a way to run more than just one command in a container. Let's try that now:

docker run -it alpine /bin/sh

To view the dead container logs

docker logs <container-id>

The below command will help you to become a root user.

sudo -i

cat /var/lib/docker/containers/<container id>/<container id>-json.log

Running the run command with the -it flags attaches us to an interactive tty in the container. Now you can run as many commands in the container as you want. Take some time to run your favorite commands.

That concludes a whirlwind tour of the docker run command which would most likely be the command you'll use most often. It makes sense to spend some time getting comfortable with it. To find out more about run, use docker run --help to see a list of all flags it supports. As you proceed further, we'll see a few more variants of docker run.

1.2 Terminology

In the last section, you saw a lot of Docker-specific jargon which might be confusing to some. So before you go further, let's clarify some terminology that is used frequently in the Docker ecosystem.

• Images - The file system and configuration of our application which are used to create containers. To find out more about a Docker image, run docker inspect alpine. In the demo above, you used the docker pull command to download the alpine image. When you executed the command docker run hello-world, it also did a docker pull behind the scenes to download the hello-world image.

• Containers - Running instances of Docker images — containers run the actual applications. A container includes an application and all of its dependencies. It shares the kernel with other containers, and runs as an isolated process in user space on the host OS. You created a container using docker run which you did using the alpine image that you downloaded. A list of running containers can be seen using the docker ps command.

• Docker daemon - The background service running on the host that manages building, running and distributing Docker containers.

• Docker client - The command line tool that allows the user to interact with the Docker daemon.

• Docker Store - A registry of Docker images, where you can find trusted and enterprise ready containers, plugins, and Docker editions. You'll be using this later in this tutorial.