🐳 DockerFile CMD Instruction – The Complete Guide with Examples & Best Practices

🧲 Introduction – Why Learn the CMD Instruction in Docker?

When building Docker images, defining the default behavior of your container is crucial. The CMD instruction in a Dockerfile allows you to specify what command should run when a container starts—unless overridden at runtime.

It’s a key component for scripting lightweight, reusable containers. Whether you’re launching a Python app, running a web server, or testing a CLI tool, CMD makes it easy to automate default actions.

🎯 In this article, you’ll learn:

• What CMD does and how it works
• Differences between Shell form, Exec form, and combined use with ENTRYPOINT
• How to override CMD at runtime
• Real-world use cases and best practices

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📌 What is the CMD Instruction in a DockerFile?

The CMD instruction provides default arguments or commands that Docker executes when a container is started without specifying a command.

🧠 Important Note:
If you provide a command during docker run, it overrides the CMD.

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🔍 Syntax of the CMD Instruction

Docker supports three forms of CMD:

✅ 1. Shell Form

CMD echo "Hello, World"

• Executes via shell: /bin/sh -c
• Supports shell features like &&, variables, redirection

✅ 2. Exec Form (Recommended)

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

• Executes directly without a shell
• More secure and predictable
• Preferred for most use cases

✅ 3. Parameter Form (Used with ENTRYPOINT)

ENTRYPOINT ["nginx"]
CMD ["-g", "daemon off;"]

• CMD supplies default arguments to the ENTRYPOINT
• Easily overridden with runtime parameters

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💡 Example: CMD in Action

🧾 Dockerfile:

FROM python:3.11
COPY app.py /app.py
CMD ["python", "/app.py"]

▶️ Build & Run:

docker build -t python-app .
docker run python-app

✅ The container runs /app.py by default using the CMD line.

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🆚 CMD vs ENTRYPOINT – Key Differences

Feature	CMD	ENTRYPOINT
Purpose	Default arguments or command	Main command
Overridable	✅ Yes	🚫 Not by default
Common Usage	App execution, default scripts	Long-running services or core logic
Together	Yes, CMD provides args to ENTRYPOINT	ENTRYPOINT runs regardless

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🔄 CMD Override at Runtime

docker run python-app echo "Overridden CMD"

🔁 This completely overrides the Dockerfile CMD.

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📦 Real-World Use Case: Running Nginx

FROM nginx:alpine
CMD ["nginx", "-g", "daemon off;"]

✅ Keeps the Nginx server running in the foreground by default.

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🧰 DockerFile CMD Command – Functional Table

🛠️ Function	📝 Description	📌 Example
1. Set default command	Defines what to run at container start	CMD ["python", "app.py"]
2. Shell form usage	Uses shell for execution	CMD echo "Hello World"
3. Exec form usage	Direct command without shell	CMD ["echo", "Hello"]
4. Default args for ENTRYPOINT	Works with ENTRYPOINT	ENTRYPOINT ["nginx"] CMD ["-g", "daemon off;"]
5. Overridable at runtime	Custom command replaces CMD	docker run myimage ls -l
6. One CMD per Dockerfile	Only last CMD is used	CMD ["last one wins"]
7. ENV variable support	Supports ENV in shell form	CMD echo "User: $USER"
8. Runs at container start	Executes on docker run	docker run myimage
9. Fallback behavior	Acts as default if no command is passed	CMD ["npm", "start"]
10. Useful for testing	Run ping, echo, sleep, etc.	CMD ["ping", "google.com"]
11. Positioning	Place CMD at the end of Dockerfile	CMD ["start.sh"]
12. Affects exit code	A failing CMD affects container status	CMD ["false"] results in code 1
13. Works with ENTRYPOINT	Can be paired for flexibility	Use --entrypoint to override ENTRYPOINT
14. Runs scripts	Can launch a shell script	CMD ["./start.sh"]
15. Ignored if overridden	CMD is skipped if custom command given	docker run myimage bash

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🧵 Final Thoughts

The CMD instruction is your way of telling Docker, “If the user doesn’t specify anything, this is what I want the container to do.”

✅ Use CMD for flexible defaults
✅ Combine it with ENTRYPOINT for more control
✅ Always test behavior using docker run

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❓ FAQs – DockerFile CMD Command

❓ 1. Can I use multiple CMD instructions in one DockerFile?
🚫 No. Only the last CMD will be used.

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❓ 2. When should I use CMD over ENTRYPOINT?
✅ Use CMD when you want to set a default that can be overridden. Use ENTRYPOINT when the command should always run.

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❓ 3. Can I use both CMD and ENTRYPOINT together?
✅ Yes, and it’s a common best practice. ENTRYPOINT defines the command; CMD defines the default arguments.

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❓ 4. How do I override CMD at runtime?
▶️ Provide a new command at the end:

docker run image-name custom-command

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❓ 5. What happens if I don’t use CMD or ENTRYPOINT?
📭 The container starts and exits immediately unless a command is passed during docker run.

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