Here’s a comprehensive guide to Docker concepts and commands, complete with explanations to help you understand each one.
1. Basic Docker Commands
1.1. Check Docker Version
Explanation: This command displays the installed Docker version. It's useful for verifying that Docker is installed and checking its version.
1.2. List Running Containers
Explanation: Lists all currently running containers. By default, it shows the container ID, image, command, creation time, status, ports, and names.
1.3. List All Containers (including stopped ones)
Explanation: Lists all containers, both running and stopped. This helps in managing and inspecting containers that are not currently active.
1.4. Pull an Image from Docker Hub
Explanation: Downloads the nginx image from Docker Hub (the default image repository). If the image is already on your local machine, Docker will pull the latest version.
1.5. Run a Container
docker run -d -p 80:80 --name webserver nginx
Explanation:
-d: Runs the container in detached mode (in the background).-p 80:80: Maps port 80 on the host to port 80 in the container.--name webserver: Assigns the name "webserver" to the container.nginx: Specifies the image to use.
1.6. Stop a Container
Explanation: Stops the running container named "webserver." It sends a SIGTERM signal, followed by SIGKILL after a grace period if the container doesn’t stop.
1.7. Remove a Container
Explanation: Removes the stopped container named "webserver." The container must be stopped before it can be removed.
1.8. Remove an Image
Explanation: Deletes the nginx image from your local Docker repository. If any containers are using this image, Docker will prevent its removal unless forced.
2. Dockerfile Basics
A Dockerfile is a script used to automate the building of Docker images.
2.1. Simple Dockerfile
Create a file named Dockerfile with the following content:
# Use an official Python runtime as a parent image
FROM python:3.9-slim
# Set the working directory in the container
WORKDIR /app
# Copy the current directory contents into the container at /app
COPY . /app
# Install any needed packages specified in requirements.txt
RUN pip install --no-cache-dir -r requirements.txt
# Make port 80 available to the world outside this container
EXPOSE 80
# Define environment variable
ENV NAME World
# Run app.py when the container launches
CMD ["python", "app.py"]
Explanation:
FROM python:3.9-slim: Sets the base image to Python 3.9 slim version.WORKDIR /app: Sets the working directory inside the container.COPY . /app: Copies files from the current directory to the container’s /app directory.RUN pip install --no-cache-dir -r requirements.txt: Installs Python dependencies listed in requirements.txt.EXPOSE 80: Documents that the container listens on port 80.ENV NAME World: Sets an environment variable NAME with value World.CMD ["python", "app.py"]: Specifies the default command to run when the container starts.
2.2. Build the Docker Image
docker build -t my-python-app .
Explanation: Builds an image from the Dockerfile in the current directory (.) and tags it as my-python-app.
2.3. Run a Container from the Image
docker run -p 4000:80 my-python-app
Explanation: Runs a container from the my-python-app image, mapping port 4000 on the host to port 80 in the container.
3. Docker Compose
Docker Compose is a tool for defining and running multi-container Docker applications.
3.1. Basic docker-compose.yml File
Create a file named docker-compose.yml with the following content:
version: '3'
services:
web:
image: nginx
ports:
- "8080:80"
redis:
image: redis
Explanation:
version: '3': Specifies the version of Docker Compose syntax.services: Defines the services (containers) to run.web: A service using the nginx image, exposing port 8080.redis: A service using the redis image.
3.2. Start Services with Docker Compose
Explanation: Starts the services defined in docker-compose.yml. If the images are not available locally, Docker Compose will pull them.
3.3. Stop and Remove Containers with Docker Compose
Explanation: Stops and removes the containers defined in docker-compose.yml. It also removes the associated networks and volumes.
4. Docker Networking
Docker networking allows containers to communicate with each other and with the outside world.
4.1. Create a Network
docker network create my-network
Explanation: Creates a new network named my-network. Containers connected to this network can communicate with each other.
4.2. Run Containers on a Custom Network
docker run -d --name db --network my-network mongo
docker run -d --name app --network my-network my-python-app
Explanation: Runs mongo and my-python-app containers on the my-network network, allowing them to communicate directly.
4.3. Inspect a Network
docker network inspect my-network
Explanation: Shows detailed information about the my-network network, including connected containers and configuration.
5. Docker Volumes
Volumes are used to persist data across container restarts and to share data between containers.
5.1. Create a Volume
docker volume create my-volume
Explanation: Creates a new Docker volume named my-volume. Volumes are stored in a part of the host filesystem managed by Docker.
5.2. Run a Container with a Volume
docker run -d -v my-volume:/data --name my-container nginx
Explanation: Runs a container with the volume my-volume mounted to /data in the container. This allows data to persist and be shared.
5.3. List Volumes
Explanation: Lists all Docker volumes on the system.
5.4. Inspect a Volume
docker volume inspect my-volume
Explanation: Provides detailed information about the my-volume volume, including its mount point and usage.
5.5. Remove a Volume
docker volume rm my-volume
Explanation: Deletes the my-volume volume. It can only be removed if no containers are using it.
6. Advanced Docker Commands
6.1. Build an Image with Build Arguments
docker build --build-arg MY_ARG=value -t my-image .
Explanation: Passes build arguments to the Dockerfile. You can use MY_ARG in the Dockerfile with ARG directive.
6.2. Tag an Image
docker tag my-image my-repo/my-image:latest
Explanation: Tags an image (my-image) with a new name and tag (my-repo/my-image:latest). This helps in organizing and managing images.
6.3. Push an Image to Docker Hub
docker push my-repo/my-image:latest
Explanation: Uploads the tagged image to Docker Hub or another Docker registry.
6.4. Pull an Image from a Private Repository
docker login
docker pull my-repo/my-image:latest
Explanation: Logs into a Docker registry and pulls an image from it. Authentication is required for private repositories.
6.5. Create and Manage Docker Swarm
docker swarm init
docker service create --name my-service -p 80:80 nginx
docker service ls
docker service ps my-service
docker service rm my-service
docker swarm leave --force
Explanation:
docker swarm init: Initializes a Docker Swarm cluster.docker service create --name my-service -p 80:80 nginx: Creates a new service named my-service using the nginx image.docker service ls: Lists all services in the swarm.docker service ps my-service: Lists tasks (containers) of the specified service.docker service rm my-service: Removes the specified service.docker swarm leave --force: Forces the current node to leave the swarm.
7. Debugging and Logs
7.1. View Container Logs
Explanation: Displays the logs of the specified container, useful for debugging issues.
7.2. Attach to a Running Container
docker attach my-container
Explanation: Attaches your terminal to the running container’s process, allowing you to interact with it directly.
7.3. Exec into a Running Container
docker exec -it my-container /bin/bash
Explanation: Opens an interactive terminal session (-it) inside the container, allowing you to run commands directly.
This guide covers a broad range of Docker commands and concepts, giving you a solid foundation to work with Docker in various scenarios.
