Common DockerFile directives:
Estimated reading: 4 minutes 297 views
« PreviousNext »

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

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.

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

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.

CMD vs ENTRYPOINT – Key Differences

FeatureCMDENTRYPOINT
PurposeDefault arguments or commandMain command
Overridable Yes Not by default
Common UsageApp execution, default scriptsLong-running services or core logic
TogetherYes, CMD provides args to ENTRYPOINTENTRYPOINT runs regardless

CMD Override at Runtime

docker run python-app echo "Overridden CMD"

This completely overrides the Dockerfile CMD.

Real-World Use Case: Running Nginx

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

Keeps the Nginx server running in the foreground by default.

DockerFile CMD Command – Functional Table

Function Description Example
1. Set default commandDefines what to run at container startCMD ["python", "app.py"]
2. Shell form usageUses shell for executionCMD echo "Hello World"
3. Exec form usageDirect command without shellCMD ["echo", "Hello"]
4. Default args for ENTRYPOINTWorks with ENTRYPOINTENTRYPOINT ["nginx"] CMD ["-g", "daemon off;"]
5. Overridable at runtimeCustom command replaces CMDdocker run myimage ls -l
6. One CMD per DockerfileOnly last CMD is usedCMD ["last one wins"]
7. ENV variable supportSupports ENV in shell formCMD echo "User: $USER"
8. Runs at container startExecutes on docker rundocker run myimage
9. Fallback behaviorActs as default if no command is passedCMD ["npm", "start"]
10. Useful for testingRun ping, echo, sleep, etc.CMD ["ping", "google.com"]
11. PositioningPlace CMD at the end of DockerfileCMD ["start.sh"]
12. Affects exit codeA failing CMD affects container statusCMD ["false"] results in code 1
13. Works with ENTRYPOINTCan be paired for flexibilityUse --entrypoint to override ENTRYPOINT
14. Runs scriptsCan launch a shell scriptCMD ["./start.sh"]
15. Ignored if overriddenCMD is skipped if custom command givendocker run myimage bash

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

FAQs – DockerFile CMD Command

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

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.

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.

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

docker run image-name custom-command

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.

« PreviousNext »
Share Now :
Share

DockerFile CMD

Or Copy Link

CONTENTS
Scroll to Top