CMD instruction for default command in Docker - Mini Project: Build & Apply

Using CMD Instruction for Default Command in Dockerfile

📖 Scenario: You are creating a Docker image for a simple web server application. You want to set a default command that runs the server when the container starts.

🎯 Goal: Build a Dockerfile that sets up a base image and uses the CMD instruction to specify the default command to run the web server.

📋 What You'll Learn

Use the official python:3.12-slim base image

Create a CMD instruction that runs python3 -m http.server 8000 as the default command

💡 Why This Matters

🌍 Real World

Setting a default command in a Dockerfile is common when packaging applications so that containers start the app automatically.

💼 Career

Understanding how to use the <code>CMD</code> instruction and run containers is essential for DevOps roles managing containerized applications.

1

Create the base image

Write a Dockerfile line to use the official python:3.12-slim image as the base image with the FROM instruction.

Docker

# Your code here

Need a hint?

The FROM instruction sets the base image for your Dockerfile.

2

Add the CMD instruction

Add a CMD instruction to the Dockerfile that runs python3 -m http.server 8000 as the default command when the container starts.

Docker

FROM python:3.12-slim
# Your code here

Need a hint?

Use the JSON array syntax for CMD to specify the command and its arguments.

3

Build the Docker image

Write the Docker CLI command to build an image named my-web-server from the current directory.

Docker

FROM python:3.12-slim
CMD ["python3", "-m", "http.server", "8000"]
# Your code here

Need a hint?

Use docker build -t image_name . to build the image from the current folder.

4

Run the Docker container

Write the Docker CLI command to run a container from the my-web-server image, mapping port 8000 on the host to port 8000 in the container.

Docker

FROM python:3.12-slim
CMD ["python3", "-m", "http.server", "8000"]
docker build -t my-web-server .
# Your code here

Need a hint?

Use docker run -p host_port:container_port image_name to map ports and start the container.