### Introduction [Docker](https://www.docker.com/) 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](https://www.digitalocean.com/community/tutorials/the-docker-ecosystem-an-introduction-to-common-components). In this tutorial, you'll install and use Docker Community Edition (CE) on Ubuntu 20.04. 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 Ubuntu 20.04 server set up by following [the Ubuntu 20.04 initial server setup guide](https://www.digitalocean.com/community/tutorials/initial-server-setup-with-ubuntu-20-04), including a sudo non-root user and a firewall. - An account on [Docker Hub](https://hub.docker.com/) 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 Ubuntu 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: ```command sudo apt update ``` Next, install a few prerequisite packages which let `apt` use packages over HTTPS: ```command sudo apt install apt-transport-https ca-certificates curl software-properties-common ``` Then add the GPG key for the official Docker repository to your system: ```command curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add - ``` Add the Docker repository to APT sources: ```command sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu <^>focal<^> stable" ``` This will also update our package database with the Docker packages from the newly added repo. Make sure you are about to install from the Docker repo instead of the default Ubuntu repo: ```command apt-cache policy docker-ce ``` You'll see output like this, although the version number for Docker may be different: ``` [label Output of apt-cache policy docker-ce] docker-ce: Installed: (none) Candidate: 5:<^>19.03.9~3-0~ubuntu-focal<^> Version table: 5:<^>19.03.9~3-0~ubuntu-focal<^> 500 500 https://download.docker.com/linux/ubuntu focal/stable amd64 Packages ``` Notice that `docker-ce` is not installed, but the candidate for installation is from the Docker repository for Ubuntu 20.04 (`focal`). Finally, install Docker: ```command sudo apt install docker-ce ``` Docker should now be installed, the daemon started, and the process enabled to start on boot. Check that it's running: ```command sudo systemctl status docker ``` The output should be similar to the following, showing that the service is active and running: ``` [secondary_label Output] ● docker.service - Docker Application Container Engine Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled) Active: <^>active<^> (running) since Tue 2020-05-19 17:00:41 UTC; 17s ago TriggeredBy: ● docker.socket Docs: https://docs.docker.com Main PID: 24321 (dockerd) Tasks: 8 Memory: 46.4M CGroup: /system.slice/docker.service └─24321 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock ``` Installing Docker now 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: ``` [secondary_label Output] docker: 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: ```command sudo usermod -aG docker ${USER} ``` To apply the new group membership, log out of the server and back in, or type the following: ```command 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: ```command groups ``` ``` [secondary_label Output] <^>sammy<^> 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: ```command 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: ```command docker [option] [command] [arguments] ``` To view all available subcommands, type: ```command docker ``` As of Docker 19, the complete list of available subcommands includes: ``` [secondary_label Output] attach 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: ```command docker <^>docker-subcommand<^> --help ``` To view system-wide information about Docker, use: ```command 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](https://hub.docker.com), 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: ```command docker run hello-world ``` The output will indicate that Docker in working correctly: ``` [secondary_label Output] Unable to find image 'hello-world:latest' locally latest: Pulling from library/hello-world 0e03bdcc26d7: Pull complete Digest: sha256:6a65f928fb91fcfbc963f7aa6d57c8eeb426ad9a20c7ee045538ef34847f44f1 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: ```command 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: ``` [secondary_label Output] NAME DESCRIPTION STARS OFFICIAL AUTOMATED ubuntu Ubuntu is a Debian-based Linux operating sys… 10908 [OK] dorowu/ubuntu-desktop-lxde-vnc Docker image to provide HTML5 VNC interface … 428 [OK] rastasheep/ubuntu-sshd Dockerized SSH service, built on top of offi… 244 [OK] consol/ubuntu-xfce-vnc Ubuntu container with "headless" VNC session… 218 [OK] ubuntu-upstart Upstart is an event-based replacement for th… 108 [OK] ansible/ubuntu14.04-ansible Ubuntu 14.04 LTS with ... ``` 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: ```command docker pull ubuntu ``` You'll see the following output: ``` [secondary_label Output] Using default tag: latest latest: Pulling from library/ubuntu d51af753c3d3: Pull complete fc878cd0a91c: Pull complete 6154df8ff988: Pull complete fee5db0ff82f: Pull complete Digest: sha256:747d2dbbaaee995098c9792d99bd333c6783ce56150d1b11e333bbceed5c54d7 Status: Downloaded newer image for ubuntu:latest docker.io/library/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: ```command docker images ``` The output will look similar to the following: ``` [secondary_label Output] REPOSITORY TAG IMAGE ID CREATED SIZE <^>ubuntu<^> latest 1d622ef86b13 3 weeks ago 73.9MB hello-world latest bf756fb1ae65 4 months ago 13.3kB ``` 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: ```command 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: ``` [secondary_label Output] [environment second] root@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: ```custom_prefix(root@d9b100f2f636:/#) [environment second] apt update ``` Then install any application in it. Let's install Node.js: ```custom_prefix(root@d9b100f2f636:/#) [environment second] 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: ```custom_prefix(root@d9b100f2f636:/#) [environment second] node -v ``` You'll see the version number displayed in your terminal: ``` [secondary_label Output] [environment second] v10.19.0 ``` 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: ```command docker ps ``` You will see output similar to the following: ``` [secondary_label Output] CONTAINER 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: ```command docker ps -a ``` You'll see output similar to this: ``` 1c08a7a0d0e4 ubuntu "/bin/bash" 2 minutes ago Exited (0) 8 seconds ago quizzical_mcnulty a707221a5f6c hello-world "/hello" 6 minutes ago Exited (0) 6 minutes ago youthful_curie ``` To view the latest container you created, pass it the `-l` switch: ```command docker ps -l ``` ```command CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 1c08a7a0d0e4 ubuntu "/bin/bash" 2 minutes ago Exited (0) 40 seconds ago quizzical_mcnulty ``` 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 `1c08a7a0d0e4`: ```command docker start <^>1c08a7a0d0e4<^> ``` The container will start, and you can use `docker ps` to see its status: ``` [secondary_label Output] CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 1c08a7a0d0e4 ubuntu "/bin/bash" 3 minutes ago Up 5 seconds quizzical_mcnulty ``` 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 `quizzical_mcnulty`: ```command docker stop <^>quizzical_mcnulty<^> ``` 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. ```command docker rm <^>youthful_curie<^> ``` 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. ```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: ```command docker commit -m "added Node.js" -a "<^>sammy<^>" <^>d9b100f2f636<^> <^>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: ```command docker images ``` You'll see output like this: ``` [secondary_label Output] REPOSITORY TAG IMAGE ID CREATED SIZE <^>sammy<^>/ubuntu-nodejs latest 7c1f35226ca6 7 seconds ago 179MB ... ``` 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 18.04](https://www.digitalocean.com/community/tutorials/how-to-set-up-a-private-docker-registry-on-ubuntu-18-04). To push your image, first log into Docker Hub. ```command 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] **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: ```command docker tag <^>sammy<^>/ubuntu-nodejs <^>docker-registry-username<^>/ubuntu-nodejs ``` <$> Then you may push your own image using: ```command docker push <^>docker-registry-username<^>/<^>docker-image-name<^> ``` To push the `ubuntu-nodejs` image to the **sammy** repository, the command would be: ```command 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: ``` [secondary_label Output] The 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. ![New Docker image listing on Docker Hub](https://assets.digitalocean.com/articles/docker_1804/ec2vX3Z.png) If a push attempt results in an error of this sort, then you likely did not log in: ``` [secondary_label Output] The 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](https://www.digitalocean.com/community/tags/docker?type=tutorials) in the DigitalOcean Community.

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