Docker Comprehensive Guide: From Zero to Hero

This guide covers everything you need to know to get started with Docker. It explains commands in detail, breaks down arguments (flags), and provides real-world examples including web servers (Nginx) and databases (MySQL).

Installation & Setup

Before using Docker, you need to install and enable it on your system.

Install Docker

Command:

sudo yum install docker

• Explanation: This command uses yum, the package manager for RHEL/CentOS/Amazon Linux distributions, to download and install the Docker engine.

• Note: If you are on Ubuntu/Debian, you would typically use apt-get install docker.io.

Start Docker Service

Command:

sudo systemctl start docker

• Explanation: Installing Docker doesn't start it automatically. This command starts the Docker daemon (background process) so it can receive commands.

Enable Docker (auto-start)

Command:

sudo systemctl enable docker

• Explanation: This ensures that Docker starts automatically whenever you reboot your server or computer.

Check Docker Version

Command:

docker --version

• Explanation: Verifies that Docker is installed correctly and shows you which version you are running.

The Golden Command: docker Run

The docker run command is the most frequently used command. It effectively performs three steps in one:

1. Pull: Downloads the image from Docker Hub (if not found locally).

2. Create: Creates the container from that image.

3. Start: Starts the container.

Basic Run

Command:

docker run hello-world

• What it does: Downloads a tiny test image, runs it to print a "Hello" message, and then exits. Used to verify your installation works.

Running a Web Server (nginx)

Command:

docker run nginx

• Behavior: This runs Nginx in the foreground. Your terminal will be stuck showing Nginx logs, and you cannot type new commands until you stop it (Ctrl+C).

Running in Background (detached Mode)

Command:

docker run -d nginx

• Arguments:

  • -d (Detached): Crucial. It tells Docker to run the container in the background. The terminal remains free for you to use, and the container keeps running silently.

Running with a Specific Version (tags)

Command:

docker run -d nginx:1.19

• Explanation: By default, Docker looks for the latest tag. You can specify a version (e.g., 1.19, alpine) by adding a colon :.

Running with Port Mapping

Command:

docker run -d -p 8080:80 nginx

• Arguments:

  • -p (Publish/Port): Very Important. The format is HOST_PORT:CONTAINER_PORT.

  • Example: -p 8080:80 means "Forward traffic from port 8080 on my laptop (Host) to port 80 inside the container."

  • Result: You can access the website at http://localhost:8080.

Overriding the Default Command

Command:

docker run -d ubuntu sleep 5

• Explanation: Docker images have a default startup command. You can override it by typing a command after the image name.

• Example: This forces the Ubuntu container to simply sleep for 5 seconds and then exit (stop).

Managing Containers (lifecycle)

List Running Containers

Command:

docker ps

• Explanation: Shows only the containers that are currently UP and running.

• Columns: Shows Container ID, Image, Command, Created Time, Status, Ports, and Names.

List All Containers (running & Stopped)

Command:

docker ps -a

• Arguments:

  • -a (All): Shows active containers AND containers that have stopped, exited, or crashed.

List Only Container Ids

Command:

docker ps -aq

• Arguments:

  • -q (Quiet): Only displays the numeric IDs. Useful for scripts (see "Bulk Deletion" below).

Stop a Container

Command:

docker stop <container_id>
# OR
docker stop <container_name>

• Explanation: Gracefully stops a running container (sends a SIGTERM signal). It gives the application a moment to save data before shutting down.

Start a Stopped Container

Command:

docker start <container_id>

• Explanation: Starts a container that already exists but was stopped. (Note: docker run creates a new one; docker start resumes an existing one).

Restart a Container

Command:

docker restart <container_id>

• Explanation: Stops and immediately starts the container again. Useful if the app crashed or configs changed.

Removing & Cleanup

Remove a Stopped Container

Command:

docker rm <container_id>

• Explanation: Deletes the container (the instance), but not the image. The container must be stopped first.

Force Remove a Running Container

Command:

docker rm -f <container_id>

• Arguments:

  • -f (Force): Deletes the container immediately, even if it is currently running (sends SIGKILL). Use with caution.

Bulk Delete (delete All Containers)

Command:

docker rm -f $(docker ps -aq)

• How it works:

  1. $(docker ps -aq) runs first and lists ALL container IDs.

  2. docker rm -f then receives that list and deletes them all at once.

  3. Warning: This deletes everything. Be careful!

Working with Images

Pull an Image (download Only)

Command:

docker pull nginx

• Explanation: Downloads the image from Docker Hub to your local machine but does not run it.

List Local Images

Command:

docker images

• Explanation: Shows all images currently downloaded on your machine (Repository, Tag, Image ID, Size).

Create a Container (without Starting)

Command:

docker create nginx

• Explanation: Creates the container layer ready to use but status remains "Created", not "Up". You must use docker start afterwards.

• Logic: docker run = docker pull + docker create + docker start.

Delete an Image

Command:

docker rmi <image_name>
# OR
docker image rm <image_name>

• Explanation: Deletes the actual image file from your disk to save space. You cannot delete an image if a container is using it (you must delete the container first).

Advanced Interactions & Inspection

Inspecting Details

Command:

docker inspect <container_id>

• Explanation: Returns a huge JSON file with every low-level detail about the container: IP address, environment variables, mounted volumes, paths, etc.

Viewing Logs

Command:

docker logs <container_id>

• Explanation: Shows the output (stdout/stderr) of the container. Essential for debugging why an app failed.

• Useful Flag: docker logs -f <id> (Follow mode - streams logs live like tail -f).

Executing Commands Inside a Container

Command:

docker exec -it <container_id> bash

• Arguments:

  • -i (Interactive): Keeps STDIN open even if not attached.

  • -t (TTY): Allocates a pseudo-terminal (makes it look like a real shell).

  • bash: The command to run (opens the Bash shell).

• Use Case: "Logging into" the container to fix files or check configurations manually.

Persistent Storage (volumes)

Command:

docker run -d -p 8090:80 -v /data:/tmp nginx

• Arguments:

  • -v (Volume): Crucial for Databases. Format is HOST_PATH:CONTAINER_PATH.

  • Explanation: Maps the folder /data on your computer to /tmp inside the container.

  • Why? If the container is deleted, files in /tmp are gone. But with this mapping, the files are safely stored in /data on your host machine.

Real World Scenario: Mysql (from Uploaded Images)

This section explains the complex commands found in the screenshots you provided.

Step 1: Running the Mysql Container

Command:

docker run -d \
  --name mysql-server \
  -e MYSQL_ROOT_PASSWORD=your_password \
  -e MYSQL_DATABASE=mydb \
  -e MYSQL_USER=myuser \
  -e MYSQL_PASSWORD=mypassword \
  -p 3306:3306 \
  -v mysql_data:/var/lib/mysql \
  mysql:latest

Detailed Breakdown of Arguments:

1. docker run -d: Run container in detached mode (background).

2. --name mysql-server: Naming. Instead of a random name (like silly_sammet), we name it mysql-server. This makes it easier to reference later.

3. -e (Environment Variable): Configuration. This is how we pass settings to the container.

   • MYSQL_ROOT_PASSWORD: Sets the root password (Mandatory for MySQL image).

   • MYSQL_DATABASE: Creates a default database named mydb.

   • MYSQL_USER: Creates a generic user myuser.

   • MYSQL_PASSWORD: Sets the password for myuser.

4. -p 3306:3306: Port Mapping. Allows external tools (like Workbench) on your laptop to connect to the DB.

5. -v mysql_data:/var/lib/mysql: Volume. Maps a named volume mysql_data to the container's data folder. Even if you delete this container, your database data remains safe in mysql_data.

6. mysql:latest: The image name and tag.

Step 2: Connecting to Mysql (interactive Mode)

Command:

docker exec -it mysql-server mysql -u root -p

Detailed Breakdown:

1. docker exec: Execute a command inside an existing running container.

2. -it: Interactive terminal mode (allows you to type the password).

3. mysql-server: The name of the container we created in Step 1.

4. mysql -u root -p: The actual Linux command running inside the container. It launches the MySQL CLI client logging in as root.

5. Result: You will see the mysql> prompt where you can run SQL queries directly.

Summary Cheatsheet