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 ensures 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.
Benefits of Using Docker
- Consistency: Docker ensures consistency across multiple development, release cycles, and standardizing your environment.
- Speed: Containers can reduce deployment to seconds, making it easy to scale up at a moment’s notice.
- Isolation: Docker ensures that applications are isolated not only from each other but also from the underlying system.
- Portability: Applications running in Docker containers can be deployed easily to any system running Docker, regardless of the environment.
- Microservices: Docker is ideal for microservices architecture as each service can be contained in its own container.
What is CentOS 7?
CentOS 7 is a Linux distribution that provides a free, enterprise-class, community-supported computing platform functionally compatible with its upstream source, Red Hat Enterprise Linux (RHEL). As a popular server distribution, CentOS 7 is a solid, predictable, manageable, and reproducible platform derived from the sources of Red Hat Enterprise Linux (RHEL).
Prerequisites for Installing Docker on CentOS 7
Before installing Docker on CentOS 7, it is important to ensure that the system meets the following requirements:
- 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)
Step-by-Step Guide to Installing Docker on CentOS 7
Step 1: Update the System
The first step is to update your system to ensure all existing packages are up to date. This can be done by running the following command:
sudo yum update -y
Step 2: Add the Docker Repository
Once the system is updated, the next step is to add the Docker repository to your CentOS system. This can be done by installing the required package yum-utils and setting up the Docker repository with the following commands:
sudo yum install -y yum-utils
sudo yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo
Step 3: Install Docker Engine
With the Docker repository in place, you can now install the Docker engine using the following command:
sudo yum install docker-ce docker-ce-cli containerd.io
During the installation process, 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
After the installation is complete, you need to start the Docker service. Additionally, you should enable it to start on boot using the following commands:
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 Tips
Managing Docker as a Non-Root User
By default, the Docker daemon binds to a Unix socket instead of a TCP port. By default, that Unix socket is owned by the user root, and so, by default, you need to have root access to communicate with the Docker daemon. To avoid this, you can add your user to the docker group with the following command:
sudo usermod -aG docker ${USER}
You will need to log out and log back in for this to take effect.
Configuring Docker to Start on Boot
Although we enabled Docker to start on boot in an earlier step, here’s how you can disable or re-enable this feature:
sudo systemctl disable docker
sudo systemctl enable docker
Updating Docker
To update Docker, you can simply update the package using your package manager:
sudo yum update docker-ce
Working with Docker Containers
Running a Container
To run a container, you can use the docker run command. For example, to run a container with an interactive shell, you can use:
sudo docker run -it ubuntu /bin/bash
Listing Containers
To list all running containers, you can use:
sudo docker ps
To list all containers, including stopped ones, add the -a flag:
sudo docker ps -a
Stopping and Removing Containers
To stop a running container, use:
sudo docker stop [CONTAINER_ID]
To remove a container, use:
sudo docker rm [CONTAINER_ID]
Advanced Docker Features and Commands
Docker Compose
Docker Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application’s services. Then, with a single command, you create and start all the services from your configuration.
Docker Images
Docker images are the basis of containers. An Image is an ordered collection of root filesystem changes and the corresponding execution parameters for use within a container runtime. To pull an image from Docker Hub, you can use the command:
sudo docker pull [IMAGE_NAME]
Building Your Own Docker Images
You can create your own Docker images using a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Using docker build users can create an automated build that executes several command-line instructions in succession.
Frequently Asked Questions
Can Docker run on any version of CentOS?
Docker is supported on CentOS 7 and later versions. It is not supported on CentOS 6 or earlier.
Is Docker free to use?
Yes, Docker CE (Community Edition) is free to use and is ideal 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, 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
Do I need to have root access to run Docker?
While you can run Docker as a non-root user by adding your user to the docker group, the Docker daemon itself will still require root privileges to run on the system.
How do I update Docker?
To update Docker, you can use your package manager to update the Docker packages. For CentOS 7, this would typically be:
sudo yum update docker-ce
References
- Docker Official Documentation: https://docs.docker.com/
- CentOS Official Website: https://www.centos.org/
- Docker Hub: https://hub.docker.com/
- YUM Package Manager: http://yum.baseurl.org/
- Docker Compose Documentation: https://docs.docker.com/compose/
