How to use the command 'ctr' (with examples)

The ctr command is used to manage containers and images in containerd, which is a high-performance container runtime. It provides a set of commands to list containers and images, pull images, and tag images.

Use case 1: List all containers (running and stopped)

Code:

ctr containers list

Motivation: This use case is useful when you want to get an overview of all the containers that are currently running or have been stopped.

Explanation:

• ctr - The command itself.
• containers - The subcommand that operates on containers.
• list - The specific action to list containers.

Example output:

CONTAINER ID     IMAGE NAME                                    PID     STATUS
8e7ad3f20739     registry.aliyuncs.com/library/busybox:latest   20417   Running
3a3cf70adb56     registry.aliyuncs.com/library/nginx:latest    23579   Exited

Use case 2: List all images

Code:

ctr images list

Motivation: This use case is helpful when you want to see a list of all the images available in containerd.

Explanation:

• ctr - The command itself.
• images - The subcommand that operates on images.
• list - The specific action to list images.

Example output:

REF                                                                              TYPE      DIGEST                                                                  SIZE      PLATFORMS
registry.aliyuncs.com/library/busybox:latest                                      application/vnd.docker.distribution.manifest.list.v2+json   sha256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx   2.6 MiB   linux/amd64
registry.aliyuncs.com/library/nginx:latest                                       application/vnd.docker.distribution.manifest.list.v2+json   sha256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx   191 MiB   linux/amd64

Use case 3: Pull an image

Code:

ctr images pull image

Motivation: Use this use case when you want to retrieve an image from a container registry or repository and make it available in containerd.

Explanation:

• ctr - The command itself.
• images - The subcommand that operates on images.
• pull - The specific action to pull an image.
• image - The name or reference of the image to pull.

Example output:

docker.io/library/alpine:latest:                   resolved       |+++++++++++++++++++++++++++++++++++++|
docker.io/library/alpine:latest:             download started       |+++++++++++++++++++++++++++++++++++++|
docker.io/library/alpine:latest:           download complete      |+++++++++++++++++++++++++++++++++++++| 2/2 MB
docker.io/library/alpine:latest:import complete

Use case 4: Tag an image

Code:

ctr images tag source_image:source_tag target_image:target_tag

Motivation: This use case is handy when you want to assign a different name and tag to an existing image.

Explanation:

• ctr - The command itself.
• images - The subcommand that operates on images.
• tag - The specific action to tag an image.
• source_image:source_tag - The original image and tag that you want to tag.
• target_image:target_tag - The new image and tag that you want to assign.

Example output:

Tagged [source_image:source_tag] as [target_image:target_tag]

Conclusion:

In this article, we explored the various use cases of the ctr command. We learned how to list containers, list images, pull images, and tag images using ctr. Each of these use cases is helpful in managing containers and images effectively in containerd.