skopeo

---

skopeo is a command line utility that performs various operations on container images and image repositories.

skopeo can work with OCI images as well as the original Docker v2 images.

Skopeo works with API V2 registries such as Docker registries, the Atomic registry, private registries, local directories and local OCI-layout directories. Skopeo does not require a daemon to be running to perform these operations which consist of:

• Copying an image from and to various storage mechanisms. For example you can copy images from one registry to another, without requiring priviledge.
• Inspecting a remote image showing its properties including its layers, without requiring you to pull the image to the host.
• Deleting an image from an image repository.
• When required by the repository, skopeo can pass the appropriate credentials and certificates for authentication.

Skopeo operates on the following image and repository types:

• containers-storage:docker-reference An image located in a local containers/storage image store. Location and image store specified in /etc/containers/storage.conf

• dir:path An existing local directory path storing the manifest, layer tarballs and signatures as individual files. This is a non-standardized format, primarily useful for debugging or noninvasive container inspection.

• docker://docker-reference An image in a registry implementing the "Docker Registry HTTP API V2". By default, uses the authorization state in $HOME/.docker/config.json, which is set e.g. using (docker login).

• docker-archive:path[:docker-reference] An image is stored in the docker save formated file. docker-reference is only used when creating such a file, and it must not contain a digest.

• docker-daemon:docker-reference An image docker-reference stored in the docker daemon internal storage. docker-reference must contain either a tag or a digest. Alternatively, when reading images, the format can also be docker-daemon:algo:digest (an image ID).

• oci:path:tag An image tag in a directory compliant with "Open Container Image Layout Specification" at path.

• ostree:image[@/absolute/repo/path] An image in local OSTree repository. /absolute/repo/path defaults to /ostree/repo.

Inspecting a repository

skopeo is able to inspect a repository on a Docker registry and fetch images layers. The inspect command fetches the repository's manifest and it is able to show you a docker inspect-like json output about a whole repository or a tag. This tool, in contrast to docker inspect, helps you gather useful information about a repository or a tag before pulling it (using disk space). The inspect command can show you which tags are available for the given repository, the labels the image has, the creation date and operating system of the image and more.

Examples:

# show properties of fedora:latest
$ skopeo inspect docker://docker.io/fedora
{
    "Name": "docker.io/library/fedora",
    "Tag": "latest",
    "Digest": "sha256:cfd8f071bf8da7a466748f522406f7ae5908d002af1b1a1c0dcf893e183e5b32",
    "RepoTags": [
        "20",
        "21",
        "22",
        "23",
        "heisenbug",
        "latest",
        "rawhide"
    ],
    "Created": "2016-03-04T18:40:02.92155334Z",
    "DockerVersion": "1.9.1",
    "Labels": {},
    "Architecture": "amd64",
    "Os": "linux",
    "Layers": [
        "sha256:236608c7b546e2f4e7223526c74fc71470ba06d46ec82aeb402e704bfdee02a2",
        "sha256:a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4"
    ]
}

# show unverifed image's digest
$ skopeo inspect docker://docker.io/fedora:rawhide | jq '.Digest'
"sha256:905b4846938c8aef94f52f3e41a11398ae5b40f5855fb0e40ed9c157e721d7f8"

Copying images

skopeo can copy container images between various storage mechanisms, including:

• Docker distribution based registries

  • The Docker Hub, OpenShift, GCR, Artifactory, Quay ...

• Container Storage backends

  • Docker daemon storage

  • github.com/containers/storage (Backend for CRI-O, Buildah and friends)

• Local directories

• Local OCI-layout directories

$ skopeo copy docker://busybox:1-glibc atomic:myns/unsigned:streaming
$ skopeo copy docker://busybox:latest dir:existingemptydirectory
$ skopeo copy docker://busybox:latest oci:busybox_ocilayout:latest

Deleting images

For example,

$ skopeo delete docker://localhost:5000/imagename:latest

Private registries with authentication

When interacting with private registries, skopeo first looks for --creds (for skopeo inspect|delete) or --src-creds|--dest-creds (for skopeo copy) flags. If those aren't provided, it looks for the Docker's cli config file (usually located at $HOME/.docker/config.json) to get the credentials needed to authenticate. The ultimate fallback, as Docker does, is to provide an empty authentication when interacting with those registries.

Examples:

$ cat /home/runcom/.docker/config.json
{
	"auths": {
		"myregistrydomain.com:5000": {
			"auth": "dGVzdHVzZXI6dGVzdHBhc3N3b3Jk",
			"email": "stuf@ex.cm"
		}
	}
}

# we can see I'm already authenticated via docker login so everything will be fine
$ skopeo inspect docker://myregistrydomain.com:5000/busybox
{"Tag":"latest","Digest":"sha256:473bb2189d7b913ed7187a33d11e743fdc2f88931122a44d91a301b64419f092","RepoTags":["latest"],"Comment":"","Created":"2016-01-15T18:06:41.282540103Z","ContainerConfig":{"Hostname":"aded96b43f48","Domainname":"","User":"","AttachStdin":false,"AttachStdout":false,"AttachStderr":false,"Tty":false,"OpenStdin":false,"StdinOnce":false,"Env":null,"Cmd":["/bin/sh","-c","#(nop) CMD [\"sh\"]"],"Image":"9e77fef7a1c9f989988c06620dabc4020c607885b959a2cbd7c2283c91da3e33","Volumes":null,"WorkingDir":"","Entrypoint":null,"OnBuild":null,"Labels":null},"DockerVersion":"1.8.3","Author":"","Config":{"Hostname":"aded96b43f48","Domainname":"","User":"","AttachStdin":false,"AttachStdout":false,"AttachStderr":false,"Tty":false,"OpenStdin":false,"StdinOnce":false,"Env":null,"Cmd":["sh"],"Image":"9e77fef7a1c9f989988c06620dabc4020c607885b959a2cbd7c2283c91da3e33","Volumes":null,"WorkingDir":"","Entrypoint":null,"OnBuild":null,"Labels":null},"Architecture":"amd64","Os":"linux"}

# let's try now to fake a non existent Docker's config file
$ cat /home/runcom/.docker/config.json
{}

$ skopeo inspect docker://myregistrydomain.com:5000/busybox
FATA[0000] unauthorized: authentication required

# passing --creds - we can see that everything goes fine
$ skopeo inspect --creds=testuser:testpassword docker://myregistrydomain.com:5000/busybox
{"Tag":"latest","Digest":"sha256:473bb2189d7b913ed7187a33d11e743fdc2f88931122a44d91a301b64419f092","RepoTags":["latest"],"Comment":"","Created":"2016-01-15T18:06:41.282540103Z","ContainerConfig":{"Hostname":"aded96b43f48","Domainname":"","User":"","AttachStdin":false,"AttachStdout":false,"AttachStderr":false,"Tty":false,"OpenStdin":false,"StdinOnce":false,"Env":null,"Cmd":["/bin/sh","-c","#(nop) CMD [\"sh\"]"],"Image":"9e77fef7a1c9f989988c06620dabc4020c607885b959a2cbd7c2283c91da3e33","Volumes":null,"WorkingDir":"","Entrypoint":null,"OnBuild":null,"Labels":null},"DockerVersion":"1.8.3","Author":"","Config":{"Hostname":"aded96b43f48","Domainname":"","User":"","AttachStdin":false,"AttachStdout":false,"AttachStderr":false,"Tty":false,"OpenStdin":false,"StdinOnce":false,"Env":null,"Cmd":["sh"],"Image":"9e77fef7a1c9f989988c06620dabc4020c607885b959a2cbd7c2283c91da3e33","Volumes":null,"WorkingDir":"","Entrypoint":null,"OnBuild":null,"Labels":null},"Architecture":"amd64","Os":"linux"}

# skopeo copy example:
$ skopeo copy --src-creds=testuser:testpassword docker://myregistrydomain.com:5000/private oci:local_oci_image

If your cli config is found but it doesn't contain the necessary credentials for the queried registry you'll get an error. You can fix this by either logging in (via docker login) or providing --creds or --src-creds|--dest-creds.

Obtaining skopeo

skopeo may already be packaged in your distribution, for example on Fedora 23 and later you can install it using

$ sudo dnf install skopeo

Otherwise, read on for building and installing it from source:

To build the skopeo binary you need at least Go 1.5 because it uses the latest GO15VENDOREXPERIMENT flag.

There are two ways to build skopeo: in a container, or locally without a container. Choose the one which better matches your needs and environment.

Building without a container

Building without a container requires a bit more manual work and setup in your environment, but it is more flexible:

• It should work in more environments (e.g. for native macOS builds)
• It does not require root privileges (after dependencies are installed)
• It is faster, therefore more convenient for developing skopeo.

Install the necessary dependencies:

Fedora$ sudo dnf install gpgme-devel libassuan-devel btrfs-progs-devel device-mapper-devel ostree-devel
macOS$ brew install gpgme

Make sure to clone this repository in your GOPATH - otherwise compilation fails.

$ git clone https://github.com/projectatomic/skopeo $GOPATH/src/github.com/projectatomic/skopeo
$ cd $GOPATH/src/github.com/projectatomic/skopeo && make binary-local

Building in a container

Building in a container is simpler, but more restrictive:

• It requires the docker command and the ability to run Linux containers
• The created executable is a Linux executable, and depends on dynamic libraries which may only be available only in a container of a similar Linux distribution.

$ make binary # Or (make all) to also build documentation, see below.

Building documentation

To build the manual you will need go-md2man.

Debian$ sudo apt-get install go-md2man
Fedora$ sudo dnf install go-md2man

Then

$ make docs

Installation

Finally, after the binary and documentation is built:

$ sudo make install

TODO

• list all images on registry?
• registry v2 search?
• show repo tags via flag or when reference isn't tagged or digested
• support rkt/appc image spec

NOT TODO

• provide a format flag - just use the awesome jq

CONTRIBUTING

Dependencies management

Make sure vndr is installed.

In order to add a new dependency to this project:

• add a new line to vendor.conf according to vndr rules (e.g. github.com/pkg/errors master)
• run make vendor

In order to update an existing dependency:

• update the relevant dependency line in vendor.conf
• run make vendor

When new PRs for containers/image break skopeo (i.e. containers/image tests fail in make test-skopeo):

• create out a new branch in your skopeo checkout and switch to it
• update vendor.conf. Find out the containers/image dependency; update it to vendor from your own branch and your own repository fork (e.g. github.com/containers/image my-branch https://github.com/runcom/image)
• run make vendor
• make any other necessary changes in the skopeo repo (e.g. add other dependencies now requied by containers/image, or update skopeo for changed containers/image API)
• optionally add new integration tests to the skopeo repo
• submit the resulting branch as a skopeo PR, marked “DO NOT MERGE”
• iterate until tests pass and the PR is reviewed
• then the original containers/image PR can be merged, disregarding its make test-skopeo failure
• as soon as possible after that, in the skopeo PR, restore the containers/image line in vendor.conf to use containers/image:master
• run make vendor
• update the skopeo PR with the result, drop the “DO NOT MERGE” marking
• after tests complete succcesfully again, merge the skopeo PR

License

skopeo is licensed under the Apache License, Version 2.0. See LICENSE for the full license text.