Docker Compose is a tool that helps you define and run multi-container applications. Instead of you building, running and connecting containers from separate Dockerfiles, it allows you to use a YAML file (called the Compose file) to define all of your application’s services, after which you can create and start them by using a single command.

Some of the most popular uses of Docker Compose include single application deployments, automated testing and local development.

In this tutorial we’ll learn how to install Docker Compose by the end of it, you should have a basic understanding of some of its’ commands and how it works.

NOTE: Even though this tutorial is written for Ubuntu 18.04, the commands should apply to other operating systems as well.

Prerequisites

  1. A non-root sudo user. You can check our tutorial [How to Create a Sudo User on Ubuntu] to see how to set it up.
  2. Docker installed and optionally allowing non-sudo access to Docker. To do this, you can follow Step 1 and Step 2 from our previous tutorial – [How to Install & Use Docker on Ubuntu 18.04]

NOTE: Since it’s optional for readers to allow non-sudo access to Docker, the command examples will have sudo appended to the docker command, for those who have not enabled non-sudo access.

Step 1 — Install Docker Compose

Check the latest Docker Compose releases here https://github.com/docker/compose/releases, and replace the version in the following command ( 1.23.2 ) with the version you see tagged as the Latest release, to download the latest version:

Set executable permissions to the Docker Compose binary:

Check if the installation was successful by checking the Compose version:

The output should look something like this:

Step 2 — Docker Compose Basics

With Docker Compose installed we can get to running our first container using it. We’ll use the well known Hello World example. To do this we’ll create our first Compose file.

The Compose files are YAML files, and one of the default names that Docker Compose looks for is called docker-compose.yml.

If you want to name the Compose file something else, then you have to use the -f argument followed by the alternate file name.

Now let’s create a directory for the YAML file and move into it:

Use your preferred text editor and create the docker-compose.yml file:

Add the following contents in the file:

Let’s see what each line means:

my-test – will be used as part of the container name
image: hello-world – we’re telling Docker Compose to use the image by the name hello-world. Just like when we use docker run hello-world, it will search locally and if it doesn’t find it, it’ll try to pull it from Docker Hub.

Save and exit the file when you’re finished.

While we’re still in the ~/hello-world directory, we’ll execute the following command:

Now, just like when running docker run hello-world, it’ll search locally for the hello-world image, and download it from Docker Hub if it doesn’t find it.

The output will look something like this when it’s finished:

After pulling the image, Docker Compose creates a container from it, attaches, and runs the [hello] program, which then outputs the message confirming that the installation is working and explains the steps that Docker ran through:

Since Docker containers only run as long as the command is active, the container stopped after the hello program finished running.

We can see that the container is stopped by checking active containers:

Output:

And if we check all of the containers, by using the -a flag, we can see the container created:

Output:

Using this information, we can remove the container when we’re done with it, to keep things clean. We can remove the container by using the docker rm command, followed by the name or ID of the container:

OR

When all the containers referencing an image are removed, we can also remove the image by using the docker rmi command:

You can check out the details and examples on how the rmi subcommand works in the official Docker documentation

Step 3 — Docker Compose Commands

So we ran docker-compose up to run a container which ran the hello program that displayed the output message and then exited. This is not ideal, however. What we want is for docker-compose to act more like a service.

The Hello World container exits after it’s run, so we’ll need a container that keeps running.

Starting a Container as a Background Process

For this example we’ll use nginx.

Let’s set up a new directory for nginx and move into it:

Now create the docker-compose.ymlfile and paste the following content:

Save and exit the file when you’re finished.

Now we’ll run the nginx container as a background process by running docker-compose up and by adding the -d switch, which tells it do detach.

NOTE: If we don’t add the -d switch, then we’d have to type Ctrl+C to exit the process.

To start the container and detach, run:

The nginx image will be downloaded and the container will be started in the background.

We can see our active containers with the command:

The output looks something like this:

Interacting with a Running Container

If you need to interact with a running container, we can do so by using the docker exec command, followed by its’ ID or name, to start a shell inside the running container:

The -t switch opens up a terminal, the -i switch makes it interactive, and by using the /bin/bash option we’re opening a bash shell in the running container.

You’ll notice your prompt change to root@container_id:/# :

You can now work from the command prompt.

IMPORTANT: Keep in mind, however, that unless you are in a directory that is part of a data volume ( a linked path on the host machine that can be used by the container ) , your changes will disappear when the container restarts.

Conclusion

That covers installing Docker Compose and a basic usage of it, however there’s more to learn in order to really have fun with it.

To see the full list of configuration options for the docker-compose.yml file, please check the official Docker Documentation and refer to the Compose file reference.

If you’ve found any issues with this tutorial, or have any questions, then please do not hesitate to contact us.

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Vlad

Tech Support