Using skopeo for Container Image Management (with examples)

Inspecting a remote image from a registry

Code: skopeo inspect docker://registry_hostname/image:tag

Motivation: Inspecting a remote image can provide detailed information about the image, such as its architecture, OS, and layering scheme. This is useful for understanding the contents of an image before using it.

Explanation: The command skopeo inspect is used to inspect a remote container image. In this example, we specify the image’s location using the docker:// protocol followed by the registry hostname, image name, and tag.

Example output:

{
    "Name": "registry_hostname/image",
    "Digest": "sha256:abcdefg123456...",
    "Architecture": "amd64",
    "OS": "linux",
    ...
}

Listing available tags for a remote image

Code: skopeo list-tags docker://registry_hostname/image

Motivation: Listing available tags for a remote image allows users to see all the different versions or variations of an image that are available in the registry. This can be helpful when selecting a specific tag to use for downloading or pulling an image.

Explanation: The command skopeo list-tags is used to list the available tags for a remote container image. Similar to the previous example, we provide the image’s location using the docker:// protocol followed by the registry hostname and image name.

Example output:

{
    "Tags": [
        "tag1",
        "tag2",
        "tag3",
        ...
    ]
}

Downloading an image from a registry

Code: skopeo copy docker://registry_hostname/image:tag dir:path/to/directory

Motivation: Downloading an image from a registry allows users to have local access to the image. This is useful for scenarios where the image needs to be used on a different machine or shared with others.

Explanation: The skopeo copy command is used to download an image from a registry. In this example, we specify the source image using the docker:// protocol followed by the registry hostname, image name, and tag. We also provide the destination directory where the image will be downloaded using the dir:path/to/directory syntax.

Example output:

Copying image from docker://registry_hostname/image:tag to dir:path/to/directory

Copying an image from one registry to another

Code: skopeo copy docker://source_registry/image:tag docker://destination_registry/image:tag

Motivation: Copying an image from one registry to another allows users to migrate or replicate images between registries. This can be useful when transitioning to a new registry or when needing to distribute images across multiple registries.

Explanation: The skopeo copy command can also be used to copy an image from one registry to another. We specify the source image using the docker:// protocol followed by the source registry hostname, image name, and tag. Similarly, we provide the destination image location using the same docker:// protocol followed by the destination registry hostname, image name, and tag.

Example output:

Copying image from docker://source_registry/image:tag to docker://destination_registry/image:tag

Deleting an image from a registry

Code: skopeo delete docker://registry_hostname/image:tag

Motivation: Deleting an image from a registry allows users to remove unnecessary or outdated images, freeing up storage space and reducing clutter. This can be helpful in maintaining a clean and organized registry.

Explanation: The skopeo delete command is used to delete an image from a registry. We specify the image to delete using the docker:// protocol followed by the registry hostname, image name, and tag.

Example output:

Deleting image docker://registry_hostname/image:tag

Logging in to a registry

Code: skopeo login --username username registry_hostname

Motivation: Logging in to a registry is necessary to authenticate and access private or secured images. This allows users to pull or push images from/to the registry, depending on the user’s permissions.

Explanation: The skopeo login command is used to log in to a registry. We provide the --username option followed by the username and specify the registry hostname.

Example output:

Logged in to registry_hostname as username

Conclusion

In this article, we explored different use cases of the skopeo command for managing container images. We demonstrated how to inspect a remote image, list available tags, download an image, copy an image between registries, delete an image from a registry, and log in to a registry. These examples showcase the versatility and power of skopeo for container image management.