🐳 Docker Basics Every Beginner Should Know

What is Docker?

Docker is a containerization platform that allows you to package an application together with all of its dependencies. This ensures the application runs consistently across different environments-whether it's your local machine, a testing server, or production. One of Docker's biggest advantages is the famous phrase: "It works on my machine." → With Docker, it works on every machine.

Docker Architecture

Before learning the commands, it's helpful to understand how Docker works behind the scenes.

Docker Client

The Docker Client is the command-line interface (CLI) that we use every day. Whenever we run commands like:

• docker run
• docker ps
• docker pull

the Docker Client sends those requests to the Docker Daemon.

Docker Daemon

The Docker Daemon (dockerd) runs in the background. It is responsible for:

• Building images
• Running containers
• Managing images
• Managing networks
• Managing volumes

Think of it as the engine that powers Docker.

Docker Image

A Docker Image is a read-only template that contains everything required to run an application:

• Application code
• Runtime
• Libraries
• Dependencies
• Configuration

Images are used to create containers.

Docker Container

A Container is a running instance of a Docker image. Containers are:

• Lightweight
• Isolated
• Portable
• Fast to start

You can create multiple containers from the same image.

Docker Registry

A Docker Registry is where Docker images are stored and shared. The most commonly used public registry is Docker Hub, where thousands of official and community images are available.

What is Containerization?

Containerization means packaging an application together with all of its dependencies so that it runs the same way everywhere. Instead of worrying about different operating systems or missing libraries, Docker provides a consistent environment.

Essential Docker Commands

1. Check Docker Installation

• Check Docker Version
  docker -v
  Displays the installed Docker version.

• Show Docker Client & Daemon Version
  docker version
  Shows both the Docker Client and Docker Daemon versions.

• Display Docker Information
  docker info
  Displays detailed information about your Docker installation.

2. Working with Images

• Download an Image
  docker pull hello-world:latest
  Downloads the image from Docker Hub.
  Note: If the image already exists locally, Docker won't download it again.

• View Local Images
  docker images or docker image ls
  Lists all images stored locally.

• Remove an Image
  docker image rm <image_name_or_id> or docker rmi <image_name_or_id>
  Deletes an image from your local machine.

3. Creating and Running Containers

• Create a Container
  docker create ubuntu
  Creates a container but does not start it.

• Start an Existing Container
  docker start <container_id>
  Starts a previously created container.

• Create and Run a Container
  docker run hello-world
  If the image doesn't exist locally, Docker downloads it first and then creates and starts the container.

• Run an Interactive Ubuntu Container
  docker run -it ubuntu
  Downloads the Ubuntu image (if needed), starts the container, and opens an interactive terminal where you can execute Linux commands.

• Run a Container in Detached Mode
  docker run -d nginx
  Runs the container in the background.

4. Managing Containers

• View Running Containers
  docker ps
  Lists only running containers.

• View All Containers
  docker ps -a
  Lists both running and stopped containers.

• Stop a Container
  docker stop <container_id>
  Stops a running container.

• Restart a Container
  docker restart <container_id>
  Restarts a container.

• Remove a Container
  docker rm <container_id>
  Deletes a stopped container.

5. Execute Commands Inside a Running Container

Open a Bash shell inside a running container:

docker exec -it <container_id> bash

For example, if you're running Redis:

docker exec -it <container_id> redis-cli

6. Viewing Logs

• Display Container Logs
  docker logs <container_id>
  Shows logs generated by the container.

• Follow Logs in Real Time
  docker logs -f <container_id>
  Streams logs as they are generated.

7. Cleaning Up Docker Resources

• Remove Unused Resources
  docker system prune
  This removes:

  • Stopped containers
  • Unused networks
  • Dangling images
  • Build cache

• Remove All Unused Images
  docker system prune -a
  Removes all unused images, stopped containers, unused networks, and build cache.

Quick Summary

Final Thoughts

Docker is one of the most important tools in modern software development and DevOps. While there are many advanced topics to explore-such as Dockerfiles, Docker Compose, volumes, networking, Kubernetes, and container orchestration-mastering these fundamentals will make your learning journey much smoother. I'm currently learning Docker as part of my DevOps roadmap, and I'll continue sharing practical notes and examples as I progress. If you have suggestions, best practices, or beginner tips, feel free to share them in the comments. Happy Learning!