Understanding Docker and Its Importance
Docker is a powerful platform that enables developers to create, deploy, and run applications in containers. Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and ship it all out as one package. This means that the application will run on any other Linux machine regardless of any customized settings that machine might have that could differ from the machine used for writing and testing the code.
In this context, Docker stands out by providing a standardized unit for software development, thereby simplifying the complexities of IT infrastructure. Docker’s containers are lightweight, especially compared to traditional virtual machines. This efficiency is due to containers sharing the host system’s kernel, rather than needing the entire operating system for themselves.
Prerequisites for Docker Installation on CentOS 7
Before diving into the installation process, it’s important to ensure that your CentOS 7 system meets the necessary prerequisites:
- A CentOS 7 server
- A user with sudo privileges
- Access to a terminal/command line
- An internet connection
- YUM or DNF package manager (YUM is the default on CentOS 7)
- SELinux set to permissive mode or disabled (Docker may not work correctly with SELinux enforcing mode)
Installing Docker on CentOS 7
The installation of Docker on CentOS 7 can be done through the yum package manager. The process involves several steps that need to be followed carefully.
Step 1: Update the System
Before installing any new software, it’s a good practice to update your system’s package database. You can do this by running the following command:
sudo yum update -y
Step 2: Add the Docker Repository
CentOS 7’s default repositories do not contain the Docker software, so you will need to add the official Docker repository to your system:
sudo yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo
Step 3: Install Docker CE (Community Edition)
With the Docker repository added, you can now install Docker Community Edition (CE) using yum:
sudo yum install docker-ce docker-ce-cli containerd.io
During the installation, you may be asked to accept the GPG key, and you should do so to continue with the installation.
Step 4: Start and Enable Docker
Once Docker is installed, you need to start the Docker service and enable it to launch every time your system boots:
sudo systemctl start docker
sudo systemctl enable docker
Step 5: Verify Docker Installation
To ensure that Docker has been installed correctly and is running, you can run the following command:
sudo docker run hello-world
This command will download a test image and run it in a container. If everything is set up correctly, you will see a message indicating that your Docker installation is working.
Post-Installation Steps and Configurations
After installing Docker, there are a few configurations that can enhance your experience and allow for smoother operations.
Managing Docker as a Non-root User
By default, the Docker daemon binds to a Unix socket instead of a TCP port. And by default, that Unix socket is owned by the user root. If you don’t want to preface the docker command with sudo, create a Unix group called docker and add users to it.
sudo groupadd docker
sudo usermod -aG docker $USER
After executing these commands, you will need to log out and log back in so that your group membership is re-evaluated.
Configuring Docker to Start on Boot
Earlier, we enabled Docker to start on boot using systemctl enable docker. However, if you ever wish to disable this behavior, you can use the following command:
sudo systemctl disable docker
Working with Docker Containers
With Docker installed, you can begin using containerization to run your applications. Below are some basic commands to get you started with Docker containers.
Pulling a Docker Image
To download a Docker image from Docker Hub, use the docker pull command:
sudo docker pull ubuntu
This command will pull the latest Ubuntu image to your system.
Running a Docker Container
To run a container using the pulled image, you can use the docker run command:
sudo docker run -it ubuntu
This will start an interactive shell inside the Ubuntu container.
Listing Docker Containers
To list all running containers, you can use:
sudo docker ps
To see all containers, including stopped ones, add the -a flag:
sudo docker ps -a
Advanced Docker Features and Tips
Docker is not just about running containers; it offers advanced features such as Docker Compose for defining and running multi-container Docker applications, Docker Swarm for clustering and scheduling containers, and Docker Volumes for persistent data storage.
Using Docker Compose
Docker Compose allows you to define and run multi-container Docker applications with a YAML file. To use Docker Compose, you need to install it separately as it does not come bundled with Docker CE.
Setting Up Docker Swarm
Docker Swarm is Docker’s native clustering and scheduling tool. With Swarm, you can turn a pool of Docker hosts into a single, virtual Docker host.
Creating and Managing Docker Volumes
Docker Volumes are the preferred way of handling persistent data created by and used by Docker containers. To create a volume, you can use:
sudo docker volume create my_volume
You can then use this volume in a container by specifying the -v flag in the docker run command.
Frequently Asked Questions
Can I install Docker on CentOS 7 without internet access?
Yes, you can install Docker on CentOS 7 without internet access by downloading the Docker package and its dependencies from another system with internet access and transferring them to your CentOS 7 system. However, this process is more complex and requires careful management of dependencies.
Is Docker free to use on CentOS 7?
Docker CE (Community Edition) is free to use and is suitable for developers and small teams looking to get started with Docker and experimenting with container-based apps.
How do I uninstall Docker from CentOS 7?
To uninstall Docker CE from CentOS 7, you can use the following commands:
sudo yum remove docker-ce docker-ce-cli containerd.io
sudo rm -rf /var/lib/docker
sudo rm -rf /var/lib/containerd
This will remove Docker and all its associated data.
What is the difference between Docker CE and Docker EE?
Docker CE (Community Edition) is the free, open-source version of Docker, which is ideal for developers and small teams. Docker EE (Enterprise Edition) is the premium version designed for enterprise development and IT teams who build, ship, and run business-critical applications in production at scale.
How do I update Docker on CentOS 7?
To update Docker on CentOS 7, you can use the yum package manager:
sudo yum update docker-ce docker-ce-cli containerd.io
This will update Docker to the latest version available in the repository.
References
- Docker Official Documentation: https://docs.docker.com/
- CentOS Official Documentation: https://www.centos.org/docs/
- Get Docker CE for CentOS: https://docs.docker.com/engine/install/centos/
- Docker Hub: https://hub.docker.com/
- Docker Compose Documentation: https://docs.docker.com/compose/
- Docker Swarm Documentation: https://docs.docker.com/engine/swarm/
