Introduction
Docker is an application that makes it simple and easy to run application processes in a container, which are like virtual machines, only 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.
There are two methods for installing Docker on Ubuntu 16.04. One method involves installing it on an existing installation of the operating system. The other involves spinning up a server with a tool called Docker Machine that auto-installs Docker on it.
In this tutorial, you’ll learn how to install and use it on an existing installation of Ubuntu 16.04.
Prerequisites
To follow this tutorial, you will need the following:
One Ubuntu 16.04 server set up with a non-root user with sudo privileges and a basic firewall, as explained in the Initial Setup Guide for Ubuntu 16.04
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 Ubuntu 16.04 repository may not be the latest version. To get this latest version, install Docker from the official Docker repository. This section shows you how to do just that.
First, in order to ensure the downloads are valid, 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 $(lsb_release -cs) stable"
Next, update the package database with the Docker packages from the newly added repo:
sudo apt-get update
Make sure you are about to install from the Docker repo instead of the default Ubuntu 16.04 repo:
apt-cache policy docker-ce
You should see output similar to the follow:
of apt-cache policy docker-ce
docker-ce:
Installed: (none)
Candidate: 18.06.1~ce~3-0~ubuntu
Version table:
18.06.1~ce~3-0~ubuntu 500
500 https://download.docker.com/linux/ubuntu xenial/stable amd64 Packages
Notice that docker-ce is not installed, but the candidate for installation is from the Docker repository for Ubuntu 16.04 (xenial).
Finally, install Docker:
sudo apt-get install -y 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 Thu 2018-10-18 20:28:23 UTC; 35s ago
Docs: https://docs.docker.com
Main PID: 13412 (dockerd)
CGroup: /system.slice/docker.service
├─13412 /usr/bin/dockerd -H fd://
└─13421 docker-containerd --config /var/run/docker/containerd/containerd.toml
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, running the docker command requires root privileges — that is, you have to prefix the command with sudo. It can also be run by a user in the docker group, which is automatically created during the installation of Docker. 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, you can log out of the server and back in, or you can type the following:
su - ${USER}
You will be prompted to enter your user’s password to continue. Afterwards, you can confirm that your user is now added to the docker group by typing:
id -nG
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:
sudo usermod -aG docker username
The rest of this article assumes you are running the docker command as a user in the docker user group. If you choose not to, please prepend the commands with sudo.
Step 3 — Using the Docker Command
With Docker installed and working, now’s the time to become familiar with the command line utility. 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 18.06.1, the complete list of available subcommands includes:
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 switches available to a specific command, type:
docker docker-subcommand --help
To view system-wide information about Docker, use:
docker info
Step 4 — Working with Docker Images
Docker containers are run from Docker images. By default, it pulls these images from Docker Hub, a Docker registry managed by Docker, the company behind the Docker project. Anybody can build and host their Docker images on Docker Hub, so most applications and Linux distributions you’ll need to run Docker containers have images that are hosted on Docker Hub.
To check whether you can access and download images from Docker Hub, type:
docker run hello-world
In the output, you should see the following message, which indicates that Docker is working correctly:
...
Hello from Docker!
This message shows that your installation appears to be working correctly.
...
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 matches the search string. In this case, the output will be similar to this:
NAME DESCRIPTION STARS OFFICIAL AUTOMATED
ubuntu Ubuntu is a Debian-based Linux operating sys… 8564 [OK]
dorowu/ubuntu-desktop-lxde-vnc Ubuntu with openssh-server and NoVNC 230 [OK]
rastasheep/ubuntu-sshd Dockerized SSH service, built on top of offi… 176 [OK]
consol/ubuntu-xfce-vnc Ubuntu container with "headless" VNC session… 129 [OK]
ansible/ubuntu14.04-ansible Ubuntu 14.04 LTS with ansible 95 [OK]
ubuntu-upstart Upstart is an event-based replacement for th… 91 [OK]
neurodebian NeuroDebian provides neuroscience research s… 54 [OK]
1and1internet/ubuntu-16-nginx-php-phpmyadmin-mysql-5 ubuntu-16-nginx-php-phpmyadmin-mysql-5 48 [OK]
ubuntu-debootstrap debootstrap --variant=minbase --components=m… 39 [OK]
nuagebec/ubuntu Simple always updated Ubuntu docker images w… 23 [OK]
tutum/ubuntu Simple Ubuntu docker images with SSH access 18
i386/ubuntu Ubuntu is a Debian-based Linux operating sys… 14
1and1internet/ubuntu-16-apache-php-7.0 ubuntu-16-apache-php-7.0 13 [OK]
ppc64le/ubuntu Ubuntu is a Debian-based Linux operating sys… 12
eclipse/ubuntu_jdk8 Ubuntu, JDK8, Maven 3, git, curl, nmap, mc, … 6 [OK]
1and1internet/ubuntu-16-nginx-php-5.6-wordpress-4 ubuntu-16-nginx-php-5.6-wordpress-4 6 [OK]
codenvy/ubuntu_jdk8 Ubuntu, JDK8, Maven 3, git, curl, nmap, mc, … 4 [OK]
darksheer/ubuntu Base Ubuntu Image -- Updated hourly 4 [OK]
pivotaldata/ubuntu A quick freshening-up of the base Ubuntu doc… 2
1and1internet/ubuntu-16-sshd ubuntu-16-sshd 1 [OK]
smartentry/ubuntu ubuntu with smartentry 1 [OK]
ossobv/ubuntu Custom ubuntu image from scratch (based on o… 0
paasmule/bosh-tools-ubuntu Ubuntu based bosh-cli 0 [OK]
1and1internet/ubuntu-16-healthcheck ubuntu-16-healthcheck 0 [OK]
pivotaldata/ubuntu-gpdb-dev Ubuntu images for GPDB development 0
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. Try this with the ubuntu image, like so:
docker pull ubuntu
After an image has been downloaded, you may then run a container using the downloaded image with the run subcommand. 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:
docker run ubuntu
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 ea4c82dcd15a 16 hours ago 85.8MB
hello-world latest 4ab4c602aa5e 5 weeks 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.
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
Note: The default behavior for the run command is to start a new container. Once you run the preceding the command, you will open up the shell interface of a second ubuntu container.
Your command prompt should change to reflect the fact that you’re now working inside the container and should take this form:
root@9b0db8a30ad1:/#
Note: Remember the container id in the command prompt. In the preceding example, it is 9b0db8a30ad1. 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-get update
Then install any application in it. Let’s install Node.js:
apt-get install -y 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:
v8.10.0
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:
CONTAINER ID IMAGE COMMAND CREATED
In this tutorial, you started three containers; one from the hello-world image and two from the ubuntu image. These 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
9b0db8a30ad1 ubuntu "/bin/bash" 21 minutes ago Exited (0) About a minute ago xenodochial_neumann
d7851eb12e23 ubuntu "/bin/bash" 24 minutes ago Exited (0) 24 minutes ago boring_chebyshev
d54945b6510b hello-world "/hello" 32 minutes ago Exited (0) 32 minutes ago youthful_roentgen
To view the latest container you created, pass it the -l switch:
docker ps -l
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
9b0db8a30ad1 ubuntu "/bin/bash" 22 minutes ago Exited (127) About a minute ago xenodochial_neumann
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 9b0db8a30ad1 :
docker start 9b0db8a30ad1
The container will start, and you can use docker ps to see its status:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
9b0db8a30ad1 ubuntu "/bin/bash" 23 minutes ago Up 11 seconds xenodochial_neumann
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 xenodochial_neumann:
docker stop xenodochial_neumann
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 youthful_roentgen
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.
To do this, commit the changes to a new Docker image instance using the following command structure:
docker commit -m "What did you do 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" d9b100f2f636 sammy/ubuntu-nodejs
Note: When you commit an image, the new image is saved locally, that is, on your computer. Later in this tutorial, you’ll learn how to push an image to a Docker registry like Docker Hub so that it can be assessed and used by you and others.
After that operation is completed, listing the Docker images now on your computer should show the new image, as well as the old one that it was derived from:
docker images
The output should be similar to this:
REPOSITORY TAG IMAGE ID CREATED SIZE
sammy/ubuntu-nodejs latest 6a1784a63edf 2 minutes ago 170MB
ubuntu latest ea4c82dcd15a 17 hours ago 85.8MB
hello-world latest 4ab4c602aa5e 5 weeks ago 1.84kB
In the above 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. In this example, the change was that Node.js was installed. Next time you need to run a container using Ubuntu with Node.js 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 another 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 can push your own image using:
docker push docker-registry-username/ubuntu-nodejs
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:
The push refers to repository [docker.io/sammy/ubuntu-nodejs]
1aa927602b6a: Pushed
76c033092e10: Pushed
2146d867acf3: Pushed
ae1f631f14b7: Pushed
102645f1cf72: Pushed
latest: digest: sha256:2be90a210910f60f74f433350185feadbbdaca0d050d97181bf593dd85195f06 size: 1362
After pushing an image to a registry, it should be listed on your account’s dashboard, like that shown in the image below.
If a push attempt results in the following error, it is likely that you are not logged in:
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, then repeat the push attempt.
Conclusion
In this tutorial, you’ve learned the basics to get you started working with Docker on Ubuntu 16.04. Like most open source projects, Docker is built from a fast-developing codebase, so make a habit of visiting the project’s blog page for the latest information.
For further exploration, check out the other Docker tutorials in the DigitalOcean Community.
