Step-by-Step Guide: How to Install Docker in Linux

What is Docker?

Docker is a platform used to develop, ship, and run applications inside containers. Containers are lightweight, portable, and self-sufficient environments that ensure your app runs smoothly on any system, regardless of differences in configuration.

If you’re a developer, sysadmin, or just curious, learning Docker is a huge step forward in modern DevOps and deployment workflows.

📌 Prerequisites

Before diving in, ensure the following:

• You are using a 64-bit Linux system.
• You have a terminal with sudo access.
• Your system is connected to the internet.
• Recommended: Ubuntu 20.04 LTS or later, or Debian 10+.

✅ Step 1: Update Your System

Why?
It’s important to update your local package list to ensure you’re working with the latest repositories and security patches.

sudo apt update && sudo apt upgrade -y

• apt update: Updates the list of available packages.
• apt upgrade: Installs the newest versions of packages on your system.

✅ Step 2: Install Required Dependencies

Docker requires some dependencies to handle repositories and HTTPS.

sudo apt install ca-certificates curl gnupg lsb-release -y

• ca-certificates: Ensures HTTPS connections are trusted.
• curl: Used to download files from the internet.
• gnupg: Required to add GPG keys for verifying packages.
• lsb-release: Provides info about your distribution, used in repo setup.

✅ Step 3: Add Docker’s Official GPG Key

Why?
This key is used to verify Docker packages and make sure they haven’t been tampered with.

sudo mkdir -p /etc/apt/keyrings
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | \
  sudo gpg --dearmor -o /etc/apt/keyrings/docker.gpg

• mkdir -p: Creates a directory if it doesn’t already exist.
• curl ... | gpg --dearmor: Downloads and converts the Docker GPG key.

✅ Step 4: Add the Docker Repository

We now add the Docker repository to our APT sources list so we can install Docker directly from Docker’s official servers.

echo \
  "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.gpg] \
  https://download.docker.com/linux/ubuntu \
  $(lsb_release -cs) stable" | \
  sudo tee /etc/apt/sources.list.d/docker.list > /dev/null

• dpkg --print-architecture: Returns your CPU architecture (e.g., amd64).
• lsb_release -cs: Returns your codename (e.g., focal, jammy).
• This command sets up the Docker repository specific to your system version.

✅ Step 5: Update APT and Install Docker Engine

Now that Docker’s repo is set up, you can install Docker using the following commands:

sudo apt update
sudo apt install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin -y

• docker-ce: The core Docker engine.
• docker-ce-cli: Command-line tools to interact with Docker.
• containerd.io: A core container runtime.
• docker-compose-plugin: Allows you to use docker compose commands (note the space, new syntax).
• docker-buildx-plugin: Used for building multi-platform images.

✅ Step 6: Verify That Docker Is Installed and Running

Check the Docker version:

sudo docker --version

Expected output:

Docker version 24.x.x, build xxxx

Run a test container:

sudo docker run hello-world

This command pulls a test image and runs a container from it. It verifies:

• Docker is installed correctly
• The Docker daemon is active
• The Docker engine can pull images from the internet

✅ Step 7: Use Docker Without sudo (Optional but Recommended)

By default, Docker needs sudo for security. You can allow your user to run Docker directly by adding them to the Docker group:

sudo usermod -aG docker $USER

• usermod -aG: Adds your user to a group (here, docker).
• $USER: Refers to your current username.

Important: Log out and log back in after this step. Otherwise, changes won’t take effect.

Then test without sudo:

docker run hello-world

✅ Step 8: Enable Docker to Start on System Boot

If you want Docker to always start after a reboot (which is useful for servers), run:

sudo systemctl enable docker

• This ensures Docker starts every time your machine does.

🎯 Additional: Useful Docker Commands

📌 Troubleshooting Tips

• If docker run hello-world fails, check:
  • Is the Docker daemon running? sudo systemctl status docker
  • Any firewall blocking Docker?
  • Try restarting: sudo systemctl restart docker.