# How to Install and Use Docker on Debian 10

### Introduction

Docker is an application that simplifies the process of managing application processes in containers. Containers let you run your applications in resource-isolated processes. They’re similar to virtual machines, but containers are more portable, more resource-friendly, and more dependent on the host operating system.

For a detailed introduction to the different components of a Docker container, check out The Docker Ecosystem: An Introduction to Common Components.

In this tutorial, you’ll install and use Docker Community Edition (CE) on Debian 10. You’ll install Docker itself, work with containers and images, and push an image to a Docker Repository.

## Prerequisites

To follow this tutorial, you will need the following:

- One Debian 10 server set up by following the Debian 10 initial server setup guide, including a sudo non-root user and a firewall.
- An account on Docker Hub if you wish to create your own images and push them to Docker Hub, as shown in Steps 7 and 8.

## Step 1 — Installing Docker

The Docker installation package available in the official Debian repository may not be the latest version. To ensure we get the latest version, we’ll install Docker from the official Docker repository. To do that, we’ll add a new package source, add the GPG key from Docker to ensure the downloads are valid, and then install the package.

First, update your existing list of packages:

    sudo apt update

Next, install a few prerequisite packages which let `apt` use packages over HTTPS:

    sudo apt install apt-transport-https ca-certificates curl gnupg2 software-properties-common

Then add the GPG key for the official Docker repository to your system:

    curl -fsSL https://download.docker.com/linux/debian/gpg | sudo apt-key add -

Add the Docker repository to APT sources:

    sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/debian $(lsb_release -cs) stable"

Next, update the package database with the Docker packages from the newly added repo:

    sudo apt update

Make sure you are about to install from the Docker repo instead of the default Debian repo:

    apt-cache policy docker-ce

You’ll see output like this, although the version number for Docker may be different:

Output of apt-cache policy docker-ce

    ocker-ce:
      Installed: (none)
      Candidate: 5:18.09.7~3-0~debian-buster
      Version table:
         5:18.09.7~3-0~debian-buster 500
            500 https://download.docker.com/linux/debian buster/stable amd64 Packages

Notice that `docker-ce` is not installed, but the candidate for installation is from the Docker repository for Debian 10 (`buster`).

Finally, install Docker:

    sudo apt install docker-ce

Docker is now installed, the daemon started, and the process enabled to start on boot. Check that it’s running:

    sudo systemctl status docker

The output will be similar to the following, showing that the service is active and running:

    Output● docker.service - Docker Application Container Engine
       Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
       Active: active (running) since Mon 2019-07-08 15:11:19 UTC; 58s ago
         Docs: https://docs.docker.com
     Main PID: 5709 (dockerd)
        Tasks: 8
       Memory: 31.6M
       CGroup: /system.slice/docker.service
               └─5709 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock

Installing Docker gives you not just the Docker service (daemon) but also the `docker` command line utility, or the Docker client. We’ll explore how to use the `docker` command later in this tutorial.

## Step 2 — Executing the Docker Command Without Sudo (Optional)

By default, the `docker` command can only be run the root user or by a user in the docker group, which is automatically created during Docker’s installation process. If you attempt to run the `docker` command without prefixing it with `sudo` or without being in the docker group, you’ll get an output like this:

    Outputdocker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?.
    See 'docker run --help'.

If you want to avoid typing `sudo` whenever you run the `docker` command, add your username to the `docker` group:

    sudo usermod -aG docker ${USER}

To apply the new group membership, log out of the server and back in, or type the following:

    su - ${USER}

You will be prompted to enter your user’s password to continue.

Confirm that your user is now added to the docker group by typing:

    id -nG

    Outputsammy sudo docker

If you need to add a user to the `docker` group that you’re not logged in as, declare that username explicitly using:

    sudo usermod -aG docker username

The rest of this article assumes you are running the `docker` command as a user in the docker group. If you choose not to, please prepend the commands with `sudo`.

Let’s explore the `docker` command next.

## Step 3 — Using the Docker Command

Using `docker` consists of passing it a chain of options and commands followed by arguments. The syntax takes this form:

    docker [option] [command] [arguments]

To view all available subcommands, type:

    docker

As of Docker 18, the complete list of available subcommands includes:

    Outputattach      Attach local standard input, output, and error streams to a running container
    build       Build an image from a Dockerfile
    commit      Create a new image from a container's changes
    cp          Copy files/folders between a container and the local filesystem
    create      Create a new container
    diff        Inspect changes to files or directories on a container's filesystem
    events      Get real time events from the server
    exec        Run a command in a running container
    export      Export a container's filesystem as a tar archive
    history     Show the history of an image
    images      List images
    import      Import the contents from a tarball to create a filesystem image
    info        Display system-wide information
    inspect     Return low-level information on Docker objects
    kill        Kill one or more running containers
    load        Load an image from a tar archive or STDIN
    login       Log in to a Docker registry
    logout      Log out from a Docker registry
    logs        Fetch the logs of a container
    pause       Pause all processes within one or more containers
    port        List port mappings or a specific mapping for the container
    ps          List containers
    pull        Pull an image or a repository from a registry
    push        Push an image or a repository to a registry
    rename      Rename a container
    restart     Restart one or more containers
    rm          Remove one or more containers
    rmi         Remove one or more images
    run         Run a command in a new container
    save        Save one or more images to a tar archive (streamed to STDOUT by default)
    search      Search the Docker Hub for images
    start       Start one or more stopped containers
    stats       Display a live stream of container(s) resource usage statistics
    stop        Stop one or more running containers
    tag         Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE
    top         Display the running processes of a container
    unpause     Unpause all processes within one or more containers
    update      Update configuration of one or more containers
    version     Show the Docker version information
    wait        Block until one or more containers stop, then print their exit codes

To view the options available to a specific command, type:

    docker docker-subcommand --help

To view system-wide information about Docker, use:

    docker info

Let’s explore some of these commands. We’ll start by working with images.

## Step 4 — Working with Docker Images

Docker containers are built from Docker images. By default, Docker pulls these images from Docker Hub, a Docker registry managed by Docker, the company behind the Docker project. Anyone can host their Docker images on Docker Hub, so most applications and Linux distributions you’ll need will have images hosted there.

To check whether you can access and download images from Docker Hub, type:

    docker run hello-world

The output will indicate that Docker in working correctly:

    OutputUnable to find image 'hello-world:latest' locally
    latest: Pulling from library/hello-world
    1b930d010525: Pull complete
    Digest: sha256:41a65640635299bab090f783209c1e3a3f11934cf7756b09cb2f1e02147c6ed8
    Status: Downloaded newer image for hello-world:latest

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

Docker was initially unable to find the `hello-world` image locally, so it downloaded the image from Docker Hub, which is the default repository. Once the image downloaded, Docker created a container from the image and the application within the container executed, displaying the message.

You can search for images available on Docker Hub by using the `docker` command with the `search` subcommand. For example, to search for the Ubuntu image, type:

    docker search ubuntu

The script will crawl Docker Hub and return a listing of all images whose name match the search string. In this case, the output will be similar to this:

    OutputNAME                                                      DESCRIPTION                                     STARS               OFFICIAL            AUTOMATED
    ubuntu                                                    Ubuntu is a Debian-based Linux operating sys…   9704                [OK]
    dorowu/ubuntu-desktop-lxde-vnc                            Docker image to provide HTML5 VNC interface …   319                                     [OK]
    rastasheep/ubuntu-sshd                                    Dockerized SSH service, built on top of offi…   224                                     [OK]
    consol/ubuntu-xfce-vnc                                    Ubuntu container with "headless" VNC session…   183                                     [OK]
    ubuntu-upstart                                            Upstart is an event-based replacement for th…   99                  [OK]
    ansible/ubuntu14.04-ansible                               Ubuntu 14.04 LTS with ansible                   97                                      [OK]
    neurodebian                                               NeuroDebian provides neuroscience research s…   57                  [OK]
    1and1internet/ubuntu-16-nginx-php-phpmyadmin-mysql-5      ubuntu-16-nginx-php-phpmyadmin-mysql-5          50                                      [OK]
    ubuntu
    ...

In the OFFICIAL column, OK indicates an image built and supported by the company behind the project. Once you’ve identified the image that you would like to use, you can download it to your computer using the `pull` subcommand.

Execute the following command to download the official `ubuntu` image to your computer:

    docker pull ubuntu

You’ll see the following output:

    OutputUsing default tag: latest
    latest: Pulling from library/ubuntu
    5b7339215d1d: Pull complete
    14ca88e9f672: Pull complete
    a31c3b1caad4: Pull complete
    b054a26005b7: Pull complete
    Digest: sha256:9b1702dcfe32c873a770a32cfd306dd7fc1c4fd134adfb783db68defc8894b3c
    Status: Downloaded newer image for ubuntu:latest

After an image has been downloaded, you can then run a container using the downloaded image with the `run` subcommand. As you saw with the `hello-world` example, if an image has not been downloaded when `docker` is executed with the `run` subcommand, the Docker client will first download the image, then run a container using it.

To see the images that have been downloaded to your computer, type:

    docker images

The output should look similar to the following:

    OutputREPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
    ubuntu              latest              4c108a37151f        2 weeks ago         64.2MB
    hello-world         latest              fce289e99eb9        6 months ago        1.84kB

As you’ll see later in this tutorial, images that you use to run containers can be modified and used to generate new images, which may then be uploaded (pushed is the technical term) to Docker Hub or other Docker registries.

Let’s look at how to run containers in more detail.

## Step 5 — Running a Docker Container

The `hello-world` container you ran in the previous step is an example of a container that runs and exits after emitting a test message. Containers can be much more useful than that, and they can be interactive. After all, they are similar to virtual machines, only more resource-friendly.

As an example, let’s run a container using the latest image of Ubuntu. The combination of the -i and -t switches gives you interactive shell access into the container:

    docker run -it ubuntu

Your command prompt should change to reflect the fact that you’re now working inside the container and should take this form:

    Outputroot@d9b100f2f636:/#

Note the container id in the command prompt. In this example, it is `d9b100f2f636`. You’ll need that container ID later to identify the container when you want to remove it.

Now you can run any command inside the container. For example, let’s update the package database inside the container. You don’t need to prefix any command with `sudo`, because you’re operating inside the container as the root user:

    apt update

Then install any application in it. Let’s install Node.js:

    apt install nodejs

This installs Node.js in the container from the official Ubuntu repository. When the installation finishes, verify that Node.js is installed:

    node -v

You’ll see the version number displayed in your terminal:

    Outputv8.10.0

Any changes you make inside the container only apply to that container.

To exit the container, type `exit` at the prompt.

Let’s look at managing the containers on our system next.

## Step 6 — Managing Docker Containers

After using Docker for a while, you’ll have many active (running) and inactive containers on your computer. To view the active ones, use:

    docker ps

You will see output similar to the following:

    OutputCONTAINER ID        IMAGE               COMMAND             CREATED

In this tutorial, you started two containers; one from the `hello-world` image and another from the `ubuntu` image. Both containers are no longer running, but they still exist on your system.

To view all containers — active and inactive, run `docker ps` with the `-a` switch:

    docker ps -a

You’ll see output similar to this:

    CONTAINER ID        IMAGE               COMMAND             CREATED              STATUS                      PORTS               NAMES
    d42d0bbfbd35        ubuntu              "/bin/bash"         About a minute ago   Exited (0) 20 seconds ago                       friendly_volhard
    0740844d024c        hello-world         "/hello"            3 minutes ago        Exited (0) 3 minutes ago                        elegant_neumann

To view the latest container you created, pass it the `-l` switch:

    docker ps -l

    CONTAINER ID        IMAGE               COMMAND             CREATED              STATUS                      PORTS               NAMES
    d42d0bbfbd35        ubuntu              "/bin/bash"         About a minute ago   Exited (0) 34 seconds ago                       friendly_volhard

To start a stopped container, use `docker start`, followed by the container ID or the container’s name. Let’s start the Ubuntu-based container with the ID of `d9b100f2f636`:

    docker start d42d0bbfbd35

The container will start, and you can use `docker ps` to see its status:

    CONTAINER ID        IMAGE               COMMAND             CREATED              STATUS              PORTS               NAMES
    d42d0bbfbd35        ubuntu              "/bin/bash"         About a minute ago   Up 8 seconds                            friendly_volhard

To stop a running container, use `docker stop`, followed by the container ID or name. This time, we’ll use the name that Docker assigned the container, which is `friendly_volhard`:

    docker stop friendly_volhard

Once you’ve decided you no longer need a container anymore, remove it with the `docker rm` command, again using either the container ID or the name. Use the `docker ps -a` command to find the container ID or name for the container associated with the `hello-world` image and remove it.

    docker rm elegant_neumann

You can start a new container and give it a name using the `--name` switch. You can also use the `--rm` switch to create a container that removes itself when it’s stopped. See the `docker run help` command for more information on these options and others.

Containers can be turned into images which you can use to build new containers. Let’s look at how that works.

## Step 7 — Committing Changes in a Container to a Docker Image

When you start up a Docker image, you can create, modify, and delete files just like you can with a virtual machine. The changes that you make will only apply to that container. You can start and stop it, but once you destroy it with the `docker rm` command, the changes will be lost for good.

This section shows you how to save the state of a container as a new Docker image.

After installing Node.js inside the Ubuntu container, you now have a container running off an image, but the container is different from the image you used to create it. But you might want to reuse this Node.js container as the basis for new images later.

Then commit the changes to a new Docker image instance using the following command.

    docker commit -m "What you did to the image" -a "Author Name" container_id repository/new_image_name

The -m switch is for the commit message that helps you and others know what changes you made, while -a is used to specify the author. The `container_id` is the one you noted earlier in the tutorial when you started the interactive Docker session. Unless you created additional repositories on Docker Hub, the `repository` is usually your Docker Hub username.

For example, for the user sammy, with the container ID of `d9b100f2f636`, the command would be:

    docker commit -m "added Node.js" -a "sammy" d42d0bbfbd35 sammy/ubuntu-nodejs

When you commit an image, the new image is saved locally on your computer. Later in this tutorial, you’ll learn how to push an image to a Docker registry like Docker Hub so others can access it.

Listing the Docker images again will show the new image, as well as the old one that it was derived from:

    docker images

You’ll see output like this:

    OutputREPOSITORY            TAG                 IMAGE ID            CREATED             SIZE
    sammy/ubuntu-nodejs   latest              d441c62350b4        10 seconds ago      152MB
    ubuntu                latest              4c108a37151f        2 weeks ago         64.2MB
    hello-world           latest              fce289e99eb9        6 months ago        1.84kB

In this example, `ubuntu-nodejs` is the new image, which was derived from the existing `ubuntu` image from Docker Hub. The size difference reflects the changes that were made. And in this example, the change was that NodeJS was installed. So next time you need to run a container using Ubuntu with NodeJS pre-installed, you can just use the new image.

You can also build Images from a `Dockerfile`, which lets you automate the installation of software in a new image. However, that’s outside the scope of this tutorial.

Now let’s share the new image with others so they can create containers from it.

## Step 8 — Pushing Docker Images to a Docker Repository

The next logical step after creating a new image from an existing image is to share it with a select few of your friends, the whole world on Docker Hub, or other Docker registry that you have access to. To push an image to Docker Hub or any other Docker registry, you must have an account there.

This section shows you how to push a Docker image to Docker Hub. To learn how to create your own private Docker registry, check out How To Set Up a Private Docker Registry on Ubuntu 14.04.

To push your image, first log into Docker Hub.

    docker login -u docker-registry-username

You’ll be prompted to authenticate using your Docker Hub password. If you specified the correct password, authentication should succeed.

Note: If your Docker registry username is different from the local username you used to create the image, you will have to tag your image with your registry username. For the example given in the last step, you would type:

    docker tag sammy/ubuntu-nodejs docker-registry-username/ubuntu-nodejs

Then you may push your own image using:

    docker push docker-registry-username/docker-image-name

To push the `ubuntu-nodejs` image to the sammy repository, the command would be:

    docker push sammy/ubuntu-nodejs

The process may take some time to complete as it uploads the images, but when completed, the output will look like this:

    OutputThe push refers to a repository [docker.io/sammy/ubuntu-nodejs]
    e3fbbfb44187: Pushed
    5f70bf18a086: Pushed
    a3b5c80a4eba: Pushed
    7f18b442972b: Pushed
    3ce512daaf78: Pushed
    7aae4540b42d: Pushed

    ...

After pushing an image to a registry, it should be listed on your account’s dashboard, like that show in the image below.

If a push attempt results in an error of this sort, then you likely did not log in:

    OutputThe push refers to a repository [docker.io/sammy/ubuntu-nodejs]
    e3fbbfb44187: Preparing
    5f70bf18a086: Preparing
    a3b5c80a4eba: Preparing
    7f18b442972b: Preparing
    3ce512daaf78: Preparing
    7aae4540b42d: Waiting
    unauthorized: authentication required

Log in with `docker login` and repeat the push attempt. Then verify that it exists on your Docker Hub repository page.

You can now use `docker pull sammy/ubuntu-nodejs` to pull the image to a new machine and use it to run a new container.

## Conclusion

In this tutorial you installed Docker, worked with images and containers, and pushed a modified image to Docker Hub. Now that you know the basics, explore the other Docker tutorials in the DigitalOcean Community.