Docker is a set of platform as a service (PaaS) products that use OS-level virtualization to deliver software in packages called containers.
Docker containers are built from Docker images. These images are retrieved from Docker Hub, a registry managed by the Docker project. You can host Docker images on the Hub. It’s the most popular place to grab images with more than 100,000 container images.
However, a huge number of these images are unmaintained, so you need to be selective about which images to explore.
Run a Docker Image
Let’s test that our Docker installation can retrieve images from Docker Hub. At the command line type:
$ docker run hello-world
By default, a docker installation doesn’t have any images stored locally. The
docker command is therefore unable to find the hello-world image locally when we try to run the image. The program contacts Docker Hub and downloads the hello-world image. It then proceeds to create a container from the image, and run the application stored in the container, with the output shown below.
sde@ganges:~$ docker run hello-world Unable to find image 'hello-world:latest' locally latest: Pulling from library/hello-world 2db29710123e: Pull complete Digest: sha256:37a0b92b08d4919615c3ee023f7ddb068d12b8387475d64c622ac30f45c29c51 Status: Downloaded newer image for hello-world:latest Hello from Docker! This message shows that your installation appears to be working correctly. To generate this message, Docker took the following steps: 1. The Docker client contacted the Docker daemon. 2. The Docker daemon pulled the "hello-world" image from the Docker Hub. (amd64) 3. The Docker daemon created a new container from that image which runs the executable that produces the output you are currently reading. 4. The Docker daemon streamed that output to the Docker client, which sent it to your terminal. To try something more ambitious, you can run an Ubuntu container with: $ docker run -it ubuntu bash Share images, automate workflows, and more with a free Docker ID: https://hub.docker.com/ For more examples and ideas, visit: https://docs.docker.com/get-started/
If you want only to download a Docker image replace the run command with pull. As you can see, docker reports that our hello-world image is up to date.
sde@ganges:~$ docker pull hello-world Using default tag: latest latest: Pulling from library/hello-world Digest: sha256:37a0b92b08d4919615c3ee023f7ddb068d12b8387475d64c622ac30f45c29c51 Status: Image is up to date for hello-world:latest docker.io/library/hello-world:latest
List installed images
Listing the installed images is simple. At the command line type:
$ docker images
sde@ganges:~$ docker images REPOSITORY TAG IMAGE ID CREATED SIZE hello-world latest feb5d9fea6a5 6 weeks ago 13.3kB
At the command line, type:
$ docker ps -a
sde@ganges:~$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 808ccbb253c8 hello-world "/hello" 14 hours ago Exited (0) 14 hours ago blissful_goodall
If we omit the -a option the command only shows containers that are running.
Remove an installed image
Let’s suppose we want to remove the hello-world container. At the command line, type:
$ docker rmi -f hello-world
The -f option forces the removal of the image.
Search for Images
We can use the command search to look for matches on Docker Hub. For example, if we want to search for Nextcloud (a suite of client-server software for creating and using file hosting service), we type:
$ docker search nextcloud
When experimenting with docker images, we recommend you start with matches that have [OK] in the OFFICIAL column. The vast majority of docker images stored on Docker Hub are unmaintained.
All articles in this series:
|Getting Started with Docker|
|Installing Docker Engine||Let's start with the basics. We install Docker Engine on Ubuntu|
|Run Docker without sudo||Run Docker without the security privileges of root|
|Commands||A brief overview of the 40 Docker commands|
|Images||A Docker image is a file used to execute code in a Docker container|
|Portainer CE||Install this interface to manage different Docker environments|
|Dry||Interactive CLI for Docker containers|