🐳 Docker Fundamentals

⚙️ How Containers Work

• Containers do not consume pre-allocated resources like virtual machines.

• They use shared system resources (CPU, RAM, storage).

• If a container needs only 200MB of space, it will consume only 200MB, not more.

💡 Containerization is made possible through Docker.

🧠 Hypervisor vs Docker

• Hypervisor: Used in virtual machines (e.g., VirtualBox, VMware). It allocates separate OS and resources to each VM.

• Docker: Uses shared OS kernel (specifically Linux kernel) to run containers more efficiently with fewer resources.

🐧 Installing Docker (on Ubuntu)

sudo apt-get update

sudo apt-get install docker.io

sudo systemctl start docker

sudo systemctl enable docker

sudo systemctl status docker

🍏 Installing Docker (on macOS)

brew install --cask docker

docker --version

🔍 How Docker Works

Docker consists of the following components:

• Docker CLI → Command Line Interface

• Docker Daemon (dockerd) → Runs in the background and manages containers/images

• Containers → Lightweight isolated environments to run apps

• Docker Images → Blueprints used to create containers

Flow:

DOCKER CLI → Docker Daemon → Container

🧪 Common Docker Commands

• docker ps → Lists running containers

• cat /etc/group → Checks if you’re in the docker group

• If not in the group:

sudo usermod -aG docker $USER

newgrp docker # Refresh group membership

🧠 $USER is an environment variable representing the currently logged-in user.

🚀 Docker File → Image → Container

• A Dockerfile is used to create a Docker image.

• An image is like a blueprint.

• A container is a running instance of that image.

Without a Dockerfile, you can directly pull and run prebuilt images using DockerHub.

🛠️ Example Docker Run Commands

docker run -it ubuntu ,,,, Interactive terminal

docker run -itd ubuntu ,,,, Interactive + detached (runs in background)

docker run -d -p 80:80 nginx ,,,,, Run nginx in background and expose port 80

docker run -it -d -p 80:80 nginx …….Interactive + detached nginx

⛔ Stopping a Container

docker stop <container_id>