K:Kubernetes/Docker EntryPoint and Cmd

 

In Docker:

• ENTRYPOINT: Defines the main command that runs when the container starts. It cannot be easily overridden directly from the command line.
• CMD: Provides default arguments for the ENTRYPOINT or defines a command to run if no ENTRYPOINT is specified. It can be overridden by passing arguments to docker run.

In Kubernetes:

• command (in the Pod specification): This field overrides the ENTRYPOINT specified in the Dockerfile.
• args (in the Pod specification): This field overrides the CMD specified in the Dockerfile.

Example Dockerfile:

Dockerfile
FROM ubuntu
ENTRYPOINT ["echo"]
CMD ["Hello, World!"]

Running with Docker:

• Default behavior: docker run myimage will output Hello, World!.
• Overriding CMD: docker run myimage Kubernetes will output Kubernetes.
• Overriding ENTRYPOINT: Not directly possible via command line (requires a change in the Dockerfile or using the --entrypoint flag).

Running in Kubernetes:

yaml
apiVersion: v1
kind: Pod
metadata:
  name: example-pod
spec:
  containers:
  - name: example-container
    image: myimage
    command: ["echo"]
    args: ["Kubernetes is awesome!"]

• command field: Overrides the ENTRYPOINT (echo in this example).
• args field: Overrides the CMD ("Kubernetes is awesome!" instead of "Hello, World!").

Key Points:

• Overriding in Docker: You can override CMD by passing arguments to docker run. Overriding ENTRYPOINT requires the --entrypoint flag.
• Overriding in Kubernetes: You can override both ENTRYPOINT (using command) and CMD (using args) in the Pod specification.

Therefore, Kubernetes provides a more flexible way to control container startup commands directly through the Pod configuration, without needing to modify the Dockerfile or use specific command-line flags like in Docker.