Difference Between RUN and CMD in a Dockerfile

When writing Dockerfiles, two instructions often confuse beginners: RUN and CMD. While they may look similar (both execute commands), they serve different purposes in the container lifecycle.

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1️⃣ What is RUN?

• Purpose: Executes a command at build time.
• Effect: Creates a new image layer with the results of that command.
• Use Case: Installing dependencies, setting up environment, modifying the image.

📌 Example:

FROM ubuntu:20.04

RUN apt-get update && apt-get install -y python3

Here, RUN installs Python during image build. The resulting image now includes Python.

Think of RUN as:
👉 “Baking instructions into the image itself.”

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2️⃣ What is CMD?

• Purpose: Defines the default command that runs when a container starts.
• Effect: Does not create a new image layer, but sets a startup instruction.
• Use Case: Starting the main process inside the container.

📌 Example:

FROM ubuntu:20.04

CMD ["echo", "Hello from the container!"]

Here, CMD runs echo when the container is launched:

docker build -t cmd-example .
docker run cmd-example
# Output: Hello from the container!

Think of CMD as:
👉 “What should this container do by default when it runs?”

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3️⃣ Key Differences

Feature	RUN	CMD
When executed	At build time	At container runtime
Effect	Creates a new image layer	Sets default container startup command
Use case	Install software, configure environment	Define entrypoint or app execution
Runs once or always?	Runs during image build	Runs every time container starts
Overridable?	No	Yes (docker run <image> <command> overrides CMD)

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4️⃣ Example: RUN vs CMD Together

FROM python:3.9

# Install dependencies at build time
RUN pip install flask

# Default command at runtime
CMD ["python3", "-m", "flask", "run", "--host=0.0.0.0"]

• RUN pip install flask → Ensures Flask is already in the image.
• CMD [...] → Defines what happens when the container starts.

If you run the container with a custom command:

docker run my-flask-image python3 --version

The CMD is overridden, and Docker runs python3 --version instead.

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📌 Summary

• ✅ RUN → Build-time instruction, creates an image layer (install, configure).
• ✅ CMD → Runtime instruction, defines container startup command.
• ✅ CMD can be overridden at docker run, but RUN cannot.

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👉 A simple way to remember:

• RUN = Build step (permanent change to the image).
• CMD = Runtime step (default container behavior).