Docker - Containers & Shells

Docker - Containers & Shells change how we build and use applications. It uses lightweight and portable containers. These containers hold everything we need to run software. This technology helps us make development easier. It also helps with scaling and keeps environments the same across different platforms.

In this chapter, we will look at the basics of Docker - Containers & Shells. We will talk about its structure, how to install it, and how to create and manage containers. We will also discuss more advanced topics like networking, debugging, and best practices for security. This will give us a good understanding of what Docker can do.

For more information about Docker’s structure and how to manage containers, we can check our detailed articles on Docker Architecture and Working with Containers.

Understanding Docker Architecture

Docker architecture is a strong system that helps us create, deploy, and manage containerized applications. It has three main parts: Docker Daemon, Docker Client, and Docker Registry.

• Docker Daemon (dockerd): This is the main part that runs in the background. It manages Docker containers, images, networks, and volumes. It listens for API requests and takes care of container events. If we want to learn more about setting up the Docker daemon, we can check Docker Daemon Configuration.

• Docker Client: The client is what we use to talk to Docker. It takes commands like docker run or docker build. Then it sends these commands to the Docker daemon using the Docker API.

• Docker Registry: This is where we store Docker images. The default public registry is Docker Hub. Here, we can pull or push images. We can also set up private registries for our own images.

Docker has a layered filesystem. This helps us store and share images in a smart way. Each image has layers. This makes caching better and deployments faster. To learn more about image layers and caching, we can visit Docker Image Layering and Caching.

With this architecture, Docker helps us manage resources well. It allows developers to build, ship, and run applications easily using containers. If we want to understand more, we can explore the complete Docker architecture.

Installing Docker

We start using Docker for containers and shells by installing Docker on our operating system. Docker works on different platforms like Windows, macOS, and Linux. Here are the steps for each platform.

For Windows:

1. Download Docker Desktop from the Docker Hub.
2. Run the Installer and follow the steps to finish the installation.
3. Enable WSL 2 if it asks you. This is needed for running Linux containers.

For macOS:

1. Download Docker Desktop from the Docker Hub.
2. Drag and drop the Docker icon into your Applications folder.
3. Launch Docker from Applications and follow the setup steps.

For Linux:

1. Update the Package Index:

   sudo apt-get update

2. Install Required Packages:

   sudo apt-get install apt-transport-https ca-certificates curl software-properties-common

3. Add Docker’s Official GPG Key:

   curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -

4. Set up the Stable Repository:

   sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

5. Install Docker Engine:

   sudo apt-get update
   sudo apt-get install docker-ce

After we install, we can check by running:

docker --version

For more details, we can look at the Docker Installation documentation. Once we install it, we can start to explore Docker containers and shells in a good way.

Creating Your First Container

Creating our first container in Docker is simple. It shows how powerful containerization is. First, we need to make sure Docker is installed on our system. We can check the installation guide here.

1. Open the terminal. We can pull a basic image like Ubuntu using this command:

   docker pull ubuntu

2. Run a container from the image we just pulled with this command:

   docker run -it ubuntu

   The -it flag helps us interact with the container’s shell.

3. Check if the container is running by looking at the active containers:

   docker ps

4. Leave the container by typing exit or pressing Ctrl + D. We can start the container again later using the container ID or name.

By following these steps, we have created our first Docker container. For more info on managing containers, we can look at the section on working with containers. Docker containers are light, portable, and easy to manage. They are very important in today’s development workflows.

Running Commands in Docker Containers

Running commands in Docker containers is very important for working with apps and services inside the container. Docker offers different ways to run commands. We can do this during the container’s runtime or by accessing the shell of a running container.

To run a command in a new container, we can use the docker run command followed by the command we want. For example:

docker run ubuntu echo "Hello, Docker!"

This command starts a new Ubuntu container and runs the echo command inside it.

For containers that are already running, we use the docker exec command. This allows us to run commands in a running container:

docker exec -it <container_id> /bin/bash

The -it flags let us interact with the shell of the container. We need to replace <container_id> with the real ID or name of our container.

We can also run scripts or commands directly using Docker’s CMD or ENTRYPOINT in a Dockerfile. These tell Docker which command to run when the container starts.

Knowing how to run commands in Docker containers is key for managing apps and fixing problems. For more information on working with containers, visit Working with Docker Containers.

Working with Docker Shells

We can work with Docker shells to talk with containers using the command line. Docker helps us access the shell of a running container easily. This lets us run commands, fix problems, and manage our apps without any hassle.

To get to the shell of a container, we can use this command:

docker exec -it <container_id_or_name> /bin/bash

This command opens a terminal inside the container we choose. If the container is light, we might need to use /bin/sh instead of /bin/bash.

Here are some key points when we work with Docker shells:

• Isolation: Each container works in its own space. This means the shell session will not mess with other containers.
• Temporary Changes: Changes we make in the container’s shell are temporary. They go away unless we save them to an image.
• Troubleshooting: We can use the shell to check for problems, look at logs, or change settings.

To manage our apps in containers well, we need to understand how to use Docker shells. For more information on working with Docker containers, we can look at the resources given.

Using Dockerfile for Custom Images

A Dockerfile is a simple script that tells how to build a Docker image. With Dockerfiles, we can make custom images that fit our application’s needs. This helps us keep things the same in different environments. The syntax of Dockerfile is easy. We can choose a base image, add files, set environment variables, and say what commands to run when the image starts.

Basic Dockerfile Structure:

# Use an official base image
FROM ubuntu:20.04

# Set environment variables
ENV APP_HOME /usr/src/app

# Set the working directory
WORKDIR $APP_HOME

# Copy files from the host to the container
COPY . .

# Install dependencies
RUN apt-get update && \
    apt-get install -y python3

# Define the command to run the application
CMD ["python3", "app.py"]

Key Instructions:

• FROM: This tells which base image to use.
• WORKDIR: This sets where we work for the next steps.
• COPY: This is for copying files into the image.
• RUN: This runs commands while we build the image.
• CMD: This says what command to run when the container starts.

For more info about Docker images, check Docker Image Layering and Caching. When we learn to use Dockerfiles well, we can manage our Docker container life better and make our development process smoother.

Managing Container Lifecycle

Managing the container lifecycle in Docker is very important for good application deployment and resource management. The lifecycle has different states that a container goes through. These states are created, running, paused, stopped, and deleted. When we understand these states, we can manage Docker containers better.

1. Creating Containers: We use the docker create command to set up a container without starting it. For example:

   docker create --name my_container nginx

2. Starting and Stopping: To run a container, we can use docker start. To stop a container that is running, we use docker stop:

   docker start my_container
   docker stop my_container

3. Pausing and Resuming: The docker pause command stops the processes in a container. The docker unpause command starts them again:

   docker pause my_container
   docker unpause my_container

4. Removing Containers: When we do not need a container anymore, we can use docker rm to delete it:

   docker rm my_container

5. Inspecting Container State: To see all containers and their states, we use docker ps -a. This helps us manage the lifecycle.

For more about container management, we can check working with containers. Managing the container lifecycle well is key for using Docker fully.

Sharing Data Between Containers

Sharing data between Docker containers is important for communication and keeping data for applications. Docker gives us a few ways to share data. The main ones are volumes and bind mounts.

Volumes: These are saved in a part of the host filesystem that Docker manages. This part is /var/lib/docker/volumes/. We can share volumes among many containers. They are good for keeping data safe over time. To create and use a volume, we can run:

docker volume create my_volume
docker run -d -v my_volume:/data my_image

Bind Mounts: With bind mounts, we can choose an exact path on the host. This path will be mounted into the container. This is helpful for development when we need changes in files to show up right away:

docker run -d -v /host/path:/container/path my_image

Data Containers: Another way is to make a special data container that holds shared data. Other containers can then use this data by mounting it from this container.

To sum up, we can share data between containers using:

• Volumes: These keep data safe and are managed by Docker.
• Bind mounts: These directly link host folders.
• Data containers: These hold shared data in one container.

For more information about working with containers, we can check out the full guide.

Networking in Docker

Networking in Docker is very important. It helps containers talk to each other, to the host, and to outside networks. Docker gives us different networking choices. We can pick what works best for our applications.

Network Types:

1. Bridge Network:

   • This is the default network for containers.
   • It keeps containers isolated but lets them talk to each other on the same host.
   • It is good for simple applications.

2. Host Network:

   • Containers share the host’s network.
   • This gives high performance and low delay.
   • Be careful with this one. It does not keep the network separate.

3. Overlay Network:

   • This helps containers communicate across different Docker hosts.
   • It is very important for applications that run on multiple hosts, especially in swarm mode.

4. Macvlan Network:

   • It gives a MAC address to a container. This makes it look like a real device on the network.
   • It is helpful for older applications that need special network settings.

Networking Commands:

• To see networks:

  docker network ls

• To create a network:

  docker network create my_network

• To connect a container to a network:

  docker network connect my_network my_container

For more details on Docker networking, we can look at the official Docker documentation. Knowing these networking ideas is very important for managing our Docker containers and helping them communicate well.

Debugging Containers

We think that debugging Docker containers is very important for developers and system admins. When problems happen in containers, good debugging can help us save time and resources. Here are some simple ways and tools to help us debug Docker containers:

1. Container Logs: We can use the command docker logs <container_id> to see the standard output and error messages of a container. This helps us understand what the application inside the container is doing.

2. Interactive Shell Access: We can open an interactive shell inside a container by running:

   docker exec -it <container_id> /bin/bash

   This lets us fix problems while processes are running.

3. Docker Events: We can check real-time events from the Docker daemon by using:

   docker events

   This is helpful to track changes and problems as they happen.

4. Inspecting Containers: We should use the command:

   docker inspect <container_id>

   This shows us detailed information about the container’s setup, network settings, and storage.

5. Network Troubleshooting: If we think there are networking problems, we can use tools like ping, curl, or telnet from inside the container to check if we can connect.

6. Resource Monitoring: We can use docker stats to check how much resources the container is using. This can help us find performance issues.

By learning these debugging methods, we can fix problems in our Docker containers quickly. This helps us deliver applications smoothly. For more detailed information about working with Docker containers, we can look at our article on working with containers.

Best Practices for Container Security

We know that keeping container security is very important. It helps us keep our applications safe and private when they run in Docker containers. Here are some simple best practices to make our Docker environment more secure:

• Use Official Images: We should always start with official or trusted images from Docker Hub. This helps us lower risks. We must also update these images often to add security fixes.

• Minimize Image Size: We can use small base images like Alpine. This reduces the chance for attacks. Smaller images have fewer packages, which means less risk.

• Run as Non-Root User: It is better to run the container as a non-root user. This limits what the container can do and reduces the damage from any security issues.

• Implement Resource Limits: We should set resource limits for Docker, like CPU and memory. This stops any single container from using all the resources and helps prevent denial-of-service attacks.

• Use Docker Secrets: For sensitive information like passwords and API keys, we should use Docker Secrets. This is better than putting them directly in our images or environment variables.

• Regularly Scan Images: We can use tools like Docker Bench for Security or other solutions to check our images for known vulnerabilities regularly.

• Keep Docker Updated: We need to update the Docker engine to the latest stable version regularly. This way, we get the newest security fixes.

By following these simple best practices for container security, we can lower the risk of security problems in our Docker containers. For more details on managing containers, please check working with containers.

Docker - Containers & Shells - Full Example

Let us show the power of Docker - Containers & Shells. We will walk through a simple example. We will create a web application using Docker. This example will show how to build a Docker image, run a container, and use its shell.

1. Create a Simple Node.js Application:
   First, we make a folder for our application. In this folder, we create a simple app.js file:

   // app.js
   const http = require("http");
   const port = 3000;
   
   const requestHandler = (req, res) => {
     res.end("Hello from Docker Container!");
   };
   
   const server = http.createServer(requestHandler);
   server.listen(port, () => {
     console.log(`Server running at http://localhost:${port}`);
   });

2. Create a Dockerfile:
   In the same folder, we create a Dockerfile to define the image:

   # Use the official Node.js image
   FROM node:14
   
   # Set the working directory
   WORKDIR /usr/src/app
   
   # Copy package.json and install dependencies
   COPY package*.json ./
   RUN npm install
   
   # Copy application code
   COPY . .
   
   # Expose the port
   EXPOSE 3000
   
   # Command to run the app
   CMD ["node", "app.js"]

3. Build the Docker Image:
   In your terminal, go to the directory and build the image:

   docker build -t my-node-app .

4. Run the Container:
   We start a container from the image:

   docker run -d -p 3000:3000 my-node-app

5. Access the Application:
   Open your browser and go to http://localhost:3000. You will see the message “Hello from Docker Container!”.

This example shows how to use Docker - Containers & Shells. We create, manage, and use a Docker container. For more details, check Docker - Containers and Docker - Working with Containers.

Conclusion

In this article about Docker - Containers & Shells, we looked at how Docker works, how to install it, and how to create and manage containers. Knowing how to run commands in Docker containers and use Docker shells helps us develop and deploy applications better. Also, using tools like Docker Compose and Docker Hub can make our work easier. For more information, we can check our guides on Docker Architecture and Working with Containers.