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README.md
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README.md

title Learn Dockerfile CMD Instructions Practically
description Understand how to use the CMD instruction in Dockerfiles, and how to override CMD during the 'docker run' command.

Learn Dockerfile CMD Instructions Practically

Introduction

In this guide, you will:

  • Create an Nginx Dockerfile with the CMD instruction.
  • Understand how to override the CMD instruction during the docker run command.
  • Build the Docker image and verify its functionality.

Step 1: Create Dockerfile and Custom index.html

  • Directory: DockerFiles

Create a Dockerfile with the following content:

# Use nginx:alpine-slim as base Docker Image
FROM nginx:alpine-slim

# OCI Labels
LABEL org.opencontainers.image.authors="Kalyan Reddy Daida"
LABEL org.opencontainers.image.title="Demo: CMD Instruction in Docker"
LABEL org.opencontainers.image.description="A Dockerfile demo illustrating the use of the CMD instruction"
LABEL org.opencontainers.image.version="1.0"

# Copy a custom index.html to the Nginx HTML directory
COPY index.html /usr/share/nginx/html

# Default CMD to start Nginx in the foreground
CMD ["nginx", "-g", "daemon off;"]

Create a simple index.html:

<!DOCTYPE html> 
<html> 
  <body style='background-color:rgb(227, 213, 180);'> 
    <h1>Welcome to StackSimplify - CMD  Dockerfile Instruction</h1> 
    <p>Learn technology through practical, real-world demos.</p> 
    <p>Application Version: V1</p>     
    <p>CMD: Specify default commands.</p>     
  </body>
</html>

Step 2: Build Docker Image and Run It

# Change to the directory containing your Dockerfile
cd DockerFiles

# Build the Docker Image
docker build -t [IMAGE_NAME]:[TAG] .

# Example:
docker build -t demo10-dockerfile-cmd:v1 .

# Run the Docker Container
docker run --name my-cmd-demo1 -p 8080:80 -d demo10-dockerfile-cmd:v1

# Verify Nginx is running inside the container
docker exec -it my-cmd-demo1 ps aux

# Expected Output:
# You should see the Nginx process running with 'nginx: master process nginx -g daemon off;'

# Access the application in your browser
http://localhost:8080

Observations:

  1. The Nginx process should be running inside the container.
  2. The CMD ["nginx", "-g", "daemon off;"] defined in the Dockerfile is executed as-is.

Step 3: Run Docker Container by Overriding CMD

# Run Docker Container by overriding the CMD instruction
docker run --name my-cmd-demo2 -it demo10-dockerfile-cmd:v1 /bin/sh

# Run inside container ps aux
ps aux

# Expected Output:
# Nginx is not running because the CMD has been overridden with '/bin/sh'

# You can start Nginx manually if desired:
nginx -g 'daemon off;'

# To exit the container shell:
exit

Observations:

  1. After connecting to the Docker container, Nginx is not running.
  2. The CMD instruction has been overridden with /bin/sh during docker run.

Step 4: Stop and Remove Containers and Images

# Stop and remove the containers
docker rm -f my-cmd-demo1
docker rm -f my-cmd-demo2

# Remove the Docker images from local machine
docker rmi [DOCKER_USERNAME]/[IMAGE_NAME]:[TAG]
docker rmi [IMAGE_NAME]:[TAG]

# Example:
docker rmi demo10-dockerfile-cmd:v1

# List Docker Images to confirm removal
docker images

Conclusion

You have successfully:

  • Created an Nginx Dockerfile using the CMD instruction.
  • Built the Docker image and ran it, observing the default CMD execution.
  • Overridden the CMD instruction during docker run and verified its effect.
  • Tagged and pushed the Docker image to Docker Hub.

Additional Notes

  • CMD Instruction:

    • The CMD instruction specifies the default command to run when starting a container from the image.
    • It can be overridden by specifying a different command during docker run.
    • Only the last CMD instruction in the Dockerfile takes effect.

  • Overriding CMD:

    • When you specify a command at the end of the docker run command, it overrides the CMD specified in the Dockerfile.
    • This is useful when you want to run different commands using the same image.

  • Best Practices:

    • Use CMD to specify the default command for the container.
    • Use ENTRYPOINT when you want to define a fixed command and allow additional parameters.
    • Avoid using both ENTRYPOINT and CMD unless necessary.

Additional Resources

Happy Dockerizing!