CMD instruction for default command in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the CMD instruction in a Dockerfile?

CMD sets the default command that runs when a container starts from the image.

beginner

How many CMD instructions can a Dockerfile have?

Only one CMD instruction is allowed; if multiple are present, the last one takes effect.

intermediate

What happens if you run a Docker container and provide a command after the image name?

The provided command overrides the CMD instruction in the Dockerfile for that container run.

beginner

Show the syntax of CMD using exec form in a Dockerfile.

CMD ["executable", "param1", "param2"] - This form runs the command directly without a shell.

intermediate

What is the difference between CMD and ENTRYPOINT in Docker?

CMD provides default arguments for ENTRYPOINT or a default command, while ENTRYPOINT sets a fixed command that always runs.

What does the CMD instruction do in a Dockerfile?

A Sets the default command to run in the container

B Builds the Docker image

C Copies files into the image

D Defines environment variables

CMD sets the default command that runs when the container starts.

If a Dockerfile has multiple CMD instructions, which one is used?

A The last CMD instruction

B The first CMD instruction

C All CMD instructions run in order

D None, it causes an error

Only the last CMD instruction in the Dockerfile is used.

How can you override the CMD command when running a container?

A By changing the ENTRYPOINT

B By specifying a command after the image name in 'docker run'

C By editing the Dockerfile after building

D By using the ENV instruction

Providing a command after the image name in 'docker run' overrides CMD.

Which CMD syntax runs the command without a shell?

A CMD executable param1 param2

B CMD (executable param1 param2)

C CMD "executable param1 param2"

D CMD ["executable", "param1", "param2"]

The exec form uses JSON array syntax and runs the command directly.

What is the main difference between CMD and ENTRYPOINT?

A CMD copies files; ENTRYPOINT runs commands

B CMD builds image; ENTRYPOINT runs container

C CMD sets default command; ENTRYPOINT sets fixed command

D CMD sets environment; ENTRYPOINT sets volumes

CMD provides default command or arguments; ENTRYPOINT sets the command that always runs.

Explain how the CMD instruction works in a Dockerfile and how it interacts with commands given at container run time.

Think about what happens if you run a container with or without extra commands.

Describe the difference between the exec form and shell form of the CMD instruction.

Consider how the command is executed inside the container.