Where can I find the sha256 code of a docker image?

I'd like to pull the images of CentOS, Tomcat, ... using their sha256 code, like in

docker pull myimage@sha256:0ecb2ad60

But I can't find the sha256-code to use anywhere.

I checked the DockerHub repository for any hint of the sha256-code, but couldn't find any. I downloaded the images by their tag

docker pull tomcat:7-jre8

and checked the image with docker inspect to see if there's a sha256 code in the metadata, but there is none (adding the sha256 code of the image would probably change the sha256 code).

Do I have to compute the sha256 code of an image myself and use that?

Latest answer

Edit suggested by OhJeez in the comments.

docker inspect --format='{{index .RepoDigests 0}}' $IMAGE

Original answer

I believe you can also get this using

docker inspect --format='{{.RepoDigests}}' $IMAGE

Works only in Docker 1.9 and if the image was originally pulled by the digest. Details are on the docker issue tracker.

You can get it by docker images --digests

REPOSITORY          TAG    DIGEST                                                                    IMAGE ID     CREATED        SIZE
docker/ucp-agent    2.1.0  sha256:a428de44a9059f31a59237a5881c2d2cffa93757d99026156e4ea544577ab7f3   583407a61900 3 weeks ago    22.3 MB

Simplest and most concise way is:

docker images --no-trunc --quiet $IMAGE

This returns only the sha256:... string and nothing else.

e.g.:

$ docker images --no-trunc --quiet debian:stretch-slim
sha256:220611111e8c9bbe242e9dc1367c0fa89eef83f26203ee3f7c3764046e02b248

Edit:

NOTE: this only works for images that are local. You can docker pull $IMAGE first, if required.

Just saw it:

When I pull an image, the sha256 code is diplayed at the bottom of the output (Digest: sha....):

docker pull tomcat:7-jre8
7-jre8: Pulling from library/tomcat
902b87aaaec9: Already exists 
9a61b6b1315e: Already exists 
...   
4dcef5c50d60: Already exists 
Digest: sha256:c34ce3c1fcc0c7431e1392cc3abd0dfe2192ffea1898d5250f199d3ac8d8720f
Status: Image is up to date for tomcat:7-jre8

This sha code

> sha256:c34ce3c1fcc0c7431e1392cc3abd0dfe2192ffea1898d5250f199d3ac8d8720f

can be used to pull the image afterwards with

> docker pull tomcat@sha256:c34ce3c1fcc0c7431e1392cc3abd0dfe2192ffea1898d5250f199d3ac8d8720f

This way you can be sure that the image is not changed and can be safely used for production.

In addition to the existing answers, you can use the --digests option while doing docker images to get a list of digests for all the images you have.

docker images --digests

You can add a grep to drill down further

docker images --digests | grep tomcat

I found the above methods to not work in some cases. They either:

• don't deal well with multiple images with the same hash (in the case of .RepoDigests suggestion - when you want to use a specific registry path)
• don't work well when pushing the image to registries (in the case of .Id where it's a local hash, not the hash in the registry).

The below method is delicate, but works for extracting the specific full 'name' and hash for a specific pushed container.

Here's the scenario - An image is uploaded separately to 2 different projects in the same repo, so querying RepoDigests returns 2 results.

$ docker inspect --format='{{.RepoDigests}}' gcr.io/alpha/homeapp:latest

[gcr.io/alpha/homeapp@sha256:ce7395d681afeb6afd68e73a8044e4a965ede52cd0799de7f97198cca6ece7ed gcr.io/beta/homeapp@sha256:ce7395d681afeb6afd68e73a8044e4a965ede52cd0799de7f97198cca6ece7ed]

I want to use the alpha result, but I can't predict which index it will be. So I need to manipulate the text output to remove the brackets and get each entry on a separate line. From there I can easily grep the result.

$ docker inspect --format='{{.RepoDigests}}' gcr.io/alpha/homeapp:latest | sed 's:^.\(.*\).$:\1:' | tr " " "\n" | grep alpha

gcr.io/alpha/homeapp@sha256:ce7395d681afeb6afd68e73a8044e4a965ede52cd0799de7f97198cca6ece7ed 

You can find it at the time of pulling the image from the respective repository. Below command mentions Digest: sha256 at the time of pulling the docker image.

09:33 AM##~::>docker --version
Docker version 19.03.4, build 9013bf5

Digest: sha256:6e9f67fa63b0323e9a1e587fd71c561ba48a034504fb804fd26fd8800039835d

09:28 AM##~::>docker pull ubuntu
Using default tag: latest
latest: Pulling from library/ubuntu
7ddbc47eeb70: Pull complete
c1bbdc448b72: Pull complete
8c3b70e39044: Pull complete
45d437916d57: Pull complete
**Digest: sha256:6e9f67fa63b0323e9a1e587fd71c561ba48a034504fb804fd26fd8800039835d**
Status: Downloaded newer image for ubuntu:latest
docker.io/library/ubuntu:latest

Once, the image is downloaded, we can do the following

"ubuntu@sha256:6e9f67fa63b0323e9a1e587fd71c561ba48a034504fb804fd26fd8800039835d"

09:36 AM##~::>docker inspect ubuntu | grep -i sha256
        "Id": "sha256:775349758637aff77bf85e2ff0597e86e3e859183ef0baba8b3e8fc8d3cba51c",
            **"ubuntu@sha256:6e9f67fa63b0323e9a1e587fd71c561ba48a034504fb804fd26fd8800039835d"**
            "Image": "sha256:f0caea6f785de71fe8c8b1b276a7094151df6058aa3f22d2902fe6b51f1a7a8f",
            "Image": "sha256:f0caea6f785de71fe8c8b1b276a7094151df6058aa3f22d2902fe6b51f1a7a8f",
                "sha256:cc967c529ced563b7746b663d98248bc571afdb3c012019d7f54d6c092793b8b",
                "sha256:2c6ac8e5063e35e91ab79dfb7330c6154b82f3a7e4724fb1b4475c0a95dfdd33",
                "sha256:6c01b5a53aac53c66f02ea711295c7586061cbe083b110d54dafbeb6cf7636bf",
                "sha256:e0b3afb09dc386786d49d6443bdfb20bc74d77dcf68e152db7e5bb36b1cca638"

This should have been the Id field, that you could see in the old deprecated Docker Hub API

GET /v1/repositories/foo/bar/images HTTP/1.1
  Host: index.docker.io
  Accept: application/json

Parameters:

namespace – the namespace for the repo
repo_name – the name for the repo

> Example Response:

HTTP/1.1 200
Vary: Accept
Content-Type: application/json

[{"id": "9e89cc6f0bc3c38722009fe6857087b486531f9a779a0c17e3ed29dae8f12c4f",
"checksum": "b486531f9a779a0c17e3ed29dae8f12c4f9e89cc6f0bc3c38722009fe6857087"},
{"id": "ertwetewtwe38722009fe6857087b486531f9a779a0c1dfddgfgsdgdsgds",
"checksum": "34t23f23fc17e3ed29dae8f12c4f9e89cc6f0bsdfgfsdgdsgdsgerwgew"}]

BUT: this is not how it is working now with the new docker distribution.
See issue 628: "Get image ID with tag name"

> > The /v1/ registry response /repositories/<repo>/tags used to list the image ID along with the tag handle.
/v2/ only seems to give the handle.

> > It would be useful to get the ID to compare to the ID found locally. The only place I can find the ID is in the v1Compat section of the manifest (which is overkill for the info I want)

The current (mid 2015) answer is:

> This property of the V1 API was very computationally expensive for the way images are stored on the backend. Only the tag names are enumerated to avoid a secondary lookup.
In addition, the V2 API does not deal in Image IDs. Rather, it uses digests to identify layers, which can be calculated as property of the layer and are independently verifiable.

As mentioned by @zelphir, using digests is not a good way since it doesn't exist for a local-only image. I assume the image ID sha is the most accurate and consistent across tags/pull/push etc.

docker inspect --format='{{index .Id}}' $IMAGE

Does the trick.

Just issue docker pull tomcat:7-jre8 again and you will get what you want.