crictl is a command-line tool for managing CRI-compatible container runtimes. It provides various commands to interact with Kubernetes pods, containers, and images. This article illustrates different use cases of the crictl command and provides examples for each use case.

Use case 1: List all Kubernetes pods (Ready and NotReady)

Code:

crictl pods

Motivation: By listing all Kubernetes pods, you can get an overview of the pods’ current status, including both Ready and NotReady pods. This information can be helpful for troubleshooting or monitoring purposes.

Explanation: The crictl pods command is used to list all pods managed by Kubernetes. It retrieves information about the pods from the CRI-compatible container runtime.

Example Output:

POD ID        POD NAME       STATUS
123abc        pod-1          Ready
456def        pod-2          NotReady
789ghi        pod-3          Ready

Use case 2: List all containers (Running and Exited)

Code:

crictl ps --all

Motivation: By listing all containers, you can see both running and exited containers. This can be useful for identifying containers that may have encountered issues or for inspecting historical containers.

Explanation: The crictl ps command is used to list containers. The --all flag is added to include all containers, regardless of their status (running or exited).

Example Output:

CONTAINER ID        IMAGE           STATUS       CREATED        STATE
123abc             nginx:latest    Running      5 minutes ago  Up 5 minutes
456def             busybox:latest  Exited       10 minutes ago Exited (0)

Use case 3: List all images

Code:

crictl images

Motivation: By listing all images, you can get an overview of the available images in the CRI-compatible container runtime. This information is helpful for managing and organizing images.

Explanation: The crictl images command is used to list images. It provides information such as the image ID and the repository where the image is stored.

Example Output:

IMAGE ID        REPOSITORY                           TAG
123abc          docker.io/nginx                      latest
456def          docker.io/busybox                    latest
789ghi          docker.io/alpine                     3.14

Use case 4: Print information about specific containers

Code:

crictl inspect container_id1 container_id2 ...

Motivation: Sometimes, it is necessary to gather detailed information about specific containers, such as their configuration or runtime details. The crictl inspect command allows you to retrieve this information easily.

Explanation: The crictl inspect command is used to print information about specific containers. It takes one or more container IDs as arguments and retrieves detailed information about those containers.

Example Output:

Container ID: 123abc
Image: docker.io/nginx:latest
Created: 2021-09-01T10:00:00Z
Labels:
  - app=web
  - version=1.0
...

Container ID: 456def
Image: docker.io/busybox:latest
Created: 2021-08-01T12:00:00Z
Labels:
  - app=worker
  - version=2.0
...

Use case 5: Open a specific shell inside a running container

Code:

crictl exec -it container_id sh

Motivation: Sometimes, it’s necessary to access the shell inside a running container for debugging or executing commands within the container environment. The crictl exec command with the -it flag allows you to open a shell inside a specific running container.

Explanation: The crictl exec command is used to execute a command inside a container. The -it flags enable an interactive (TTY) session, allowing you to access the container’s shell.

Example Output: The output will be the interactive shell inside the specified container.

root@container:/#

Use case 6: Pull a specific image from a registry

Code:

crictl pull image:tag

Motivation: Before running a container, you may need to download the image from a container registry. The crictl pull command allows you to pull a specific image from a registry to make it available locally.

Explanation: The crictl pull command is used to pull an image from a registry. It takes the image name and tag as arguments, specifying the specific image to be pulled.

Example Output:

Status: Downloaded newer image for nginx:latest

Use case 7: Print and [f]ollow logs of a specific container

Code:

crictl logs -f container_id

Motivation: Monitoring the logs of a specific container can provide insights into its runtime behavior and help troubleshoot any issues. The crictl logs command allows you to print and follow the logs of a particular container.

Explanation: The crictl logs command is used to retrieve the logs of a container. The -f flag enables the following mode, which continuously streams the logs to the console.

Example Output: The output will be the logs of the specified container continuously updating in real-time.

[2021-09-01 10:30:00] [INFO] Application started
[2021-09-01 10:31:00] [ERROR] Connection refused
[2021-09-01 10:32:00] [DEBUG] Request received
...

Use case 8: Remove one or more images

Code:

crictl rmi image_id1 image_id2 ...

Motivation: Removing unnecessary or unused images can free up disk space and help maintain a clean image registry. The crictl rmi command allows you to remove one or more images from the CRI-compatible container runtime.

Explanation: The crictl rmi command is used to remove images. It takes one or more image IDs as arguments, specifying the images to be removed.

Example Output:

Image removed: 123abc
Image removed: 456def

Conclusion:

The crictl command is a powerful tool for managing CRI-compatible container runtimes. With its various commands and options, crictl allows you to interact with Kubernetes pods, containers, and images efficiently. By using the provided examples, you can leverage the crictl command to perform common tasks such as listing pods, inspecting containers, or removing images.