In this blog, I will be explaining what docker pull does, and we will see a couple of practical demonstrations of the docker pull command. We will also understand what image digest is and where the docker images get saved. So before going deep dive into the docker pull command, let’s first understand a few basic terminologies.
What is dockerhub?
Dockerhub is the world’s largest repository for container images. In Dockerhub, you will find images from the developer, open-source contributors as well as official images. You can even create your images and push them to dockerhub.
In dockerhub, you can create public and private repo’s to push your docker images. Dockerhub provides lots of advantages, some of them are:
- You can easily push and pull images anytime.
- You have the flexibility to create a private repo to store your customized images.
- You can pull high-quality images from the official vendors.
- You can pull images from external vendors as well.
- You can create an organization and add your teammates.
How to find the images in the docker hub
To find the available images in the docker hub, goto Dockerhub
On the search bar, search for any images. let’s search for the Nginx image
As you can see, we have more than 8k images available for Nginx. It is always recommended to use official images.
Now we have a pretty good idea about how to find images in Dockerhub. Now let’s proceed further to understand what docker pull does.
What docker pull does?
Docker pulls basically pulls an image or a repository from a registry. Docker will pull an image from the docker hub if you do not specify any registry. While pulling images, you have the flexibility to select the version as well. If the version is not specified docker pull command will always pull the latest image.
Now let’s proceed further and see the syntax of the docker pull
docker pull [OPTIONS] NAME[:TAG|@DIGEST]
There are many options available for image pull. To get a list of options, type.
➜ ~ docker pull --help Usage: docker pull [OPTIONS] NAME[:TAG|@DIGEST] Pull an image or a repository from a registry Options: -a, --all-tags Download all tagged images in the repository --disable-content-trust Skip image verification (default true) --platform string Set platform if server is multi-platform capable -q, --quiet Suppress verbose output ➜ ~
docker pull hello-world
In this session, we will be pulling a hello-world docker image from the dockerhub. Before proceeding further, make sure the docker is installed in your system and is running.
Now let’s pull the hello-world image from the dockerhub
docker pull hello-world Using default tag: latest latest: Pulling from library/hello-world Digest: sha256:9f6ad537c5132bcce57f7a0a20e317228d382c3cd61edae14650eec68b2b345c Status: Image is up to date for hello-world:latest docker.io/library/hello-world:latest
The above command will download the hello-world image from dockerhub to your local system. Run the below command to verify if the image gets successfully pulled.
docker image ls REPOSITORY TAG IMAGE ID CREATED SIZE hello-world latest d1165f221234 3 months ago 13.3kB
What to do after docker pull?
Once you have pulled an image from the dockerhub, the next step is to run the image using the docker run command. The run command creates an isolated container that runs on your host machine.
Let’s run the hello-world image which we pulled earlier
➜ ~ docker run hello-world 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 are getting output as Hello from Docker! It means your hello-world image is working as expected.
The hello-world image was a basic image to verify the docker setup. Now let’s pull another image from the dockerhub and run it using the docker run command.
Pull an nginx image
In this session, we will be pulling an Nginx image from the dockerhub. More information about the Nginx image can be found here.
Run the below command to pull an Nginx image
docker pull nginx Using default tag: latest latest: Pulling from library/nginx 69692152171a: Already exists 30afc0b18f67: Pull complete 596b1d696923: Pull complete febe5bd23e98: Pull complete 8283eee92e2f: Pull complete 351ad75a6cfa: Pull complete Digest: sha256:6d75c99af15565a301e48297fa2d121e15d80ad526f8369c526324f0f7ccb750 Status: Downloaded newer image for nginx:latest docker.io/library/nginx:latest
Verify if the image is by running the below command:
docker images REPOSITORY TAG IMAGE ID CREATED SIZE nginx latest d1a364dc548d 3 weeks ago 133MB
Let’s run the Nginx image and forward the request to port 8080
docker run --name mynginx -d -p 8080:80 nginx:latest ecdefe6d6e2661e8fc44a13d4f036fefefd2959658fa79c24d74eafe075bcd3a
goto http://localhost:8080 to access the Nginx web UI
If you are getting the above page, it means your Nginx setup is working as expected.
Let’s run the below command and check if there is any container that gets created for the Nginx image
docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES ecdefe6d6e26 nginx:latest "/docker-entrypoint.…" 2 minutes ago Up 2 minutes 0.0.0.0:8080->80/tcp mynginx
There is one container named mynginx that is running for Nginx installation.
Now we understand how to pull an image from the public dockerhub repo, let’s proceed further and understand how to pull an image from a private repo.
docker pull from a private repo?
To pull an image from the private repo, first, you need to login into your git repo with your credentials.
docker login Login with your Docker ID to push and pull images from Docker Hub. If you don't have a Docker ID, head over to https://hub.docker.com to create one. Username: naiveskill Password:
Once you are logged in, you can run the below command to pull the image from the private repo.
docker pull myrepo/myimage
You can also specify the tag while pulling the image
docker pull myrepo.com/myimage:tag
I hope now you are comfortable with pulling and running a docker image. In the next session, we will understand another important concept related to docker pull which is image digest
docker image digest
Docker containers are basically present in the form of [namespace/] followed by a tag. Docker digest is provided as a hash of a Docker image supported by the Docker v2 registry format. Thesha256 generates the hash and is deterministic based on the image build.
So instead of passing the image name, docker images can be directly run using digest
To list down the image digest, type the below command:
docker images --digests REPOSITORY TAG DIGEST IMAGE ID CREATED SIZE nginx latest sha256:6d75c99af15565a301e48297fa2d121e15d80ad526f8369c526324f0f7ccb750 d1a364dc548d 3 weeks ago 133MB
Now let’s try to run this image using digest
docker run --name mydigestnginx -d -p 8081:80 nginx:latest@sha256:6d75c99af15565a301e48297fa2d121e15d80ad526f8369c526324f0f7ccb750 a4f6e3db8597712b9e6c9a3f17cda55c87a1fe259af56183080859fe182d5fed
go to http://localhost:8081 to access the Nginx web UI.
To get more information about the image digest, go to this link.
where does docker pull stores images?
When you pull an image from the dockerhub, the images will be downloaded into your system and reside in memory. The next question that might come to your mind is where the images get stored on a hard disk.
So the images stored in your system are based on the OS on which the docker is running. Please find below the image path for different operating systems:
I hope you have found this article useful. We have started with the basics of dockerhub, and we deep dive into the docker pull command. We also pulled and ran hello-world and Nginx images from the dockerhub. And finally, we understood what image digest is and where the docker images save on disk. Please do let me know in the comment box if you need more clarification on any topic.