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:
Output Unable 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:
Output NAME 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:
Output Using 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:
The output should look similar to the following:
Output REPOSITORY 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.
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:
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:
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:
You’ll see the version number displayed in your terminal:
Any changes you make inside the container only apply to that container. To exit the container, type exit at the prompt.
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:
You will see output similar to the following:
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:
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.
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:
You’ll see output like this:
Output REPOSITORY 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.