When you run a container in a pod, you might want to run a command at the start-up. The process consists of 2 stages – at the container (docker) level and at the Kubernetes level.
Docker – Commands and Entry points
CMD directive
The “CMD” instruction can be one of three forms:
- Executable form – Preferred
- CMD [“executable”, “param1”, “param2”]
- Default parameters form to ENTRYPOINT
- CMD [“param1”, “param2”]
- Shell form
- CMD executable parm1 parm2
Please make sure the differences of each form.
- In the shell form, the CMD will execute in “/bin/sh -c“.
- In the array form, the CMD will not be invoked in a command shell.
FROM ubuntu
CMD echo "Hello World"
FROM ubuntu
CMD ["sh", "-c", "echo 'Hello World'"]
ENTRYPOINT directive
The ENTRYPOINT instruction allows you to configure a container to run an executable.
- Executable form – Preferred
- ENTRYPOINT [“executable”, “param1”, “param2”]
- Shell form
- ENTRYPOINT executable parm1 parm2
FROM ubuntu
ENTRYPOINT ["sh", "-c", "echo 'Hello World'"]
ENTRYPOINT + CMD
ENTRYPOINT and CMD look quite similar and redundant. The power of the configuration comes when both directives are used at the same time.
- ENTRYPOINT: specifies the executable and stable default parameters
- CMD: specifies additional default parameters but might be replaced at runtime
[Note] You need to use only the array form here.
A container will run the combined command ENTRYPOINT + CMD.
Here are the rules of how ENTRYPOINT and CMD interact together.
- Dockerfile should specify at least one of CMD or ENTRYPOINT.
- ENTRYPOINT should be defined when using the container as an executable.
- CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.
- CMD will be overridden when running the container with alternative arguments.
| No ENTRYPOINT | ENTRYPOINT [“exec1”, “p1”] | |
| No CMD | error | exec1 p1 |
| CMD [“exec2”, “p2”] | exec2 p2 | – |
| CMD [“p2”] | – | exec1 p1 p2 |
FROM ubuntu
ENTRYPOINT ["sh", "-c"]
CMD ["echo 'Hello World'"]
When you run the image above with arguments, CMD parameters will be replaced.
docker run my-ubuntu sleep 10
Kubernetes – Commands
You can specify the docker CMD argument or ENTRYPOINT argument in the Kubernetes definition file.
- ENTRYPOINT = “spec.containers.command“
- CMD =”spec.containers.args“
apiVersion: v1
kind: Pod
metadata:
name: ubuntu-pod
spec:
containers:
- name: ubuntu-container
image: my-ubuntu
command: ["sh", "-c", "echo"]
args: ["Hello World"]
Scriptorium 