ENTRYPOINT vs CMD difference in Docker - Hands-On Comparison

Understanding ENTRYPOINT vs CMD in Docker

📖 Scenario: You are creating a Docker container for a simple application. You want to learn how ENTRYPOINT and CMD work differently when running containers.

🎯 Goal: Build a Dockerfile step-by-step to see how ENTRYPOINT and CMD behave differently when you run the container with or without extra commands.

📋 What You'll Learn

Create a Dockerfile with a base image

Add an ENTRYPOINT instruction

Add a CMD instruction

Run the container to see the output difference

💡 Why This Matters

🌍 Real World

Dockerfiles are used to create container images that package applications and their environments. Understanding ENTRYPOINT and CMD helps control how containers start.

💼 Career

DevOps engineers and developers use Dockerfiles daily. Knowing ENTRYPOINT vs CMD is essential for building flexible and predictable container images.

1

Create a Dockerfile with base image

Create a file named Dockerfile with the first line setting the base image to alpine.

Docker

# Write the base image line
# Your code here

Need a hint?

The base image line starts with FROM followed by the image name.

2

Add an ENTRYPOINT instruction

Add an ENTRYPOINT instruction to the Dockerfile that runs echo.

Docker

FROM alpine
# Add ENTRYPOINT to run echo
# Your code here

Need a hint?

Use JSON array syntax for ENTRYPOINT: ENTRYPOINT ["echo"].

3

Add a CMD instruction

Add a CMD instruction to the Dockerfile that provides the default text Hello from CMD.

Docker

FROM alpine
ENTRYPOINT ["echo"]
# Add CMD with default message
# Your code here

Need a hint?

Use JSON array syntax for CMD: CMD ["Hello from CMD"].

4

Run the container and observe output

Run the Docker container built from this Dockerfile without extra commands using docker run --rm your_image_name. Then run it with extra command docker run --rm your_image_name Goodbye. Write print statements showing the expected outputs for both runs.

Docker

FROM alpine
ENTRYPOINT ["echo"]
CMD ["Hello from CMD"]

# Print expected output for running container without extra command
# Print expected output for running container with extra command
# Your code here

Need a hint?

When running without extra command, output is Hello from CMD. When running with Goodbye, output is Goodbye.