CMD instruction for default command in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Dockerfile CMD instruction?

Given the Dockerfile snippet below, what will be the output when running the container without any additional command?

Docker

FROM alpine
CMD ["echo", "Hello World"]

A echo Hello World

B No output, container exits immediately

C Hello World

D Error: CMD syntax invalid

💡 Hint

CMD defines the default command and arguments for the container.

The CMD instruction specifies the default command to run when the container starts. Here, it runs 'echo Hello World', so the output is 'Hello World'.

🧠 Conceptual

intermediate

What happens if you specify CMD multiple times in a Dockerfile?

Consider a Dockerfile with multiple CMD instructions. Which statement is true about how Docker handles them?

A Only the last CMD instruction is used as the default command.

B All CMD instructions run sequentially when the container starts.

C Docker throws an error during build for multiple CMD instructions.

D The first CMD instruction is used and others are ignored.

💡 Hint

Think about how Docker layers and instructions override each other.

Docker uses only the last CMD instruction in the Dockerfile as the default command for the container.

🔀 Workflow

advanced

How to override the CMD default command when running a container?

You have a Docker image with CMD ["python", "app.py"]. How can you run the container to execute 'python test.py' instead?

A docker run image_name -c python test.py

B docker run image_name python test.py

C docker run image_name CMD python test.py

D docker run image_name --cmd python test.py

💡 Hint

The CMD can be overridden by specifying a command after the image name.

When running a container, any command after the image name overrides the CMD default command.

❓ Troubleshoot

advanced

Why does this Docker container exit immediately after starting?

Dockerfile: FROM ubuntu CMD ["python"] You run the container but it exits immediately. What is the most likely cause?

A The python command is not installed in the image.

B The container has no foreground process and exits immediately.

C The CMD syntax is incorrect and causes an error.

D Docker daemon is not running.

💡 Hint

Check if the command in CMD exists inside the container.

Ubuntu base image does not have 'python' installed by default, so the command fails and container exits immediately.

✅ Best Practice

expert

What is the recommended form of CMD instruction for best compatibility?

Which CMD instruction form is best practice to avoid shell processing issues and signal handling problems?

A CMD echo "Hello World"

B CMD echo Hello World

C CMD /bin/sh -c "echo Hello World"

D CMD ["echo", "Hello World"]

💡 Hint

Consider how Docker executes commands and handles signals.

The exec form (JSON array) of CMD avoids shell wrapping, improving signal handling and avoiding shell parsing issues.