Research and Development
This tutorial was written having Ubuntu 18.04 LTS in mind.
To ensure we get the latest version, we’ll install Docker from the official Docker repository.
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 software-properties-common
Then add the GPG key for the official Docker repository to your system:
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
Add the Docker repository to APT sources:
sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu bionic 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 Ubuntu repo:
apt-cache policy docker-ce
You’ll see output like this, although the version number for Docker may be different:
docker-ce:
Installed: (none)
Candidate: 5:19.03.8~3-0~ubuntu-bionic
Version table:
5:19.03.8~3-0~ubuntu-bionic 500
500 https://download.docker.com/linux/ubuntu bionic/stable amd64 Packages
5:19.03.7~3-0~ubuntu-bionic 500
500 https://download.docker.com/linux/ubuntu bionic/stable amd64 Packages
5:19.03.6~3-0~ubuntu-bionic 500
500 https://download.docker.com/linux/ubuntu bionic/stable amd64 Packages
...
Notice that docker-ce is not installed, but the candidate for installation is from the Docker repository for Ubuntu 18.04 (bionic).
Finally, install Docker:
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:
sudo systemctl status docker
The output should be similar to the following, showing that the service is active and running:
● docker.service - Docker Application Container Engine
Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
Active: active (running) since Sat 2020-04-18 20:11:57 UTC; 6s ago
Docs: https://docs.docker.com
Main PID: 14930 (dockerd)
Tasks: 13
CGroup: /system.slice/docker.service
└─14930 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock
Installing Docker now gives not just the Docker service (daemon) but also the docker command line utility, or the Docker client.
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:
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:
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
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.
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
To view the options available to a specific command, type:
docker docker-subcommand --help
To view system-wide information about Docker, use:
docker info
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:
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
0e03bdcc26d7: Pull complete
Digest: sha256:8e3114318a995a1ee497790535e7b88365222a21771ae7e53687ad76563e8e76
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 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
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:
REPOSITORY TAG IMAGE ID CREATED SIZE
ubuntu latest 4e5021d210f6 4 weeks ago 64.2MB
hello-world latest bf756fb1ae65 3 months ago 13.3kB
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.
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:
root@919ca7eff79a:/#
Note the container id in the command prompt. In this example, it is 919ca7eff79a. 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.
Any changes you make inside the container only apply to that container.
To exit the container, type exit at the prompt.
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
To view all containers, active and inactive, run docker ps with the -a switch:
docker ps -a
To view the latest container you created, pass it the -l switch:
docker ps -l
To start a stopped container, use docker start, followed by the container ID or the container’s name.
The container will start, and you can use docker ps to see its status.
To stop a running container, use docker stop, followed by the container ID or name.
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 and name.
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.
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 zerocool, with the container ID of 919ca7eff79a, the command would be:
docker commit -m "Added Node.js" -a "zerocool" 919ca7eff79a zerocool/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
Example output:
REPOSITORY TAG IMAGE ID CREATED SIZE
zerocool/ubuntu-nodejs latest ac78ec1721ec 6 seconds ago 153MB
ubuntu latest 4e5021d210f6 4 weeks ago 64.2MB
hello-world latest bf756fb1ae65 3 months ago 13.3kB
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.
To push the ubuntu-nodejs image to the zerocool repository, the command would be:
docker push zerocool/ubuntu-nodejs
The process may take some time to complete as it uploads the images, but when completed, the output will look like this:
The push refers to repository [docker.io/zercool/ubuntu-nodejs]
db0a08b76287: Pushed
16542a8fc3be: Mounted from library/ubuntu
6597da2e2e52: Mounted from library/ubuntu
...
After pushing an image to a registry, it should be listed on your account’s dashboard.
You can now pull the image to a new machine and use it to run a new container:
docker pull zerocool/ubuntu-nodejs