Docker is a containerization technology that allows you to quickly  build, test and deploy applications as portable, self-sufficient  containers that can run virtually anywhere.

In this tutorial, we’ll go through how to install Docker CE on CentOS 7 and explore the basic Docker concepts and commands.

Prerequisites

Before proceeding with this tutorial, make sure that the following prerequisites are met:

  • CentOS 7 server
  • You are logged in as a non-root user with sudo privileges. You check this guide about how to create a new sudo user.

Install Docker on CentOS

Although  the Docker package is available in the official CentOS 7 repository, it  may not always be the latest version. The recommended approach is to  install Docker from the Docker’s repositories.

To install Docker on your CentOS 7 server follow the steps below:

Start by updating your system packages and install the required dependencies:

sudo yum update

Next, run the following command which will add the Docker stable repository to your system:

sudo yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo

Now that the Docker repository is enabled, install the latest version of Docker CE (Community Edition) using yum by typing:

sudo yum install docker-ce

Once the Docker package is installed, start the Docker daemon and enable it to automatically start at boot time:

sudo systemctl start docker

To verify that the Docker service is running type:

sudo systemctl status docker

The output should look something like this:

● docker.service - Docker Application Container Engine
   Loaded: loaded (/usr/lib/systemd/system/docker.service; enabled; vendor preset: disabled)
   Active: active (running) since Wed 2018-10-31 08:51:20 UTC; 7s ago
     Docs: https://docs.docker.com
 Main PID: 2492 (dockerd)
   CGroup: /system.slice/docker.service
           ├─2492 /usr/bin/dockerd
           └─2498 docker-containerd --config /var/run/docker/containerd/containerd.toml

At the time of writing, the current stable version of Docker is, 18.06.1, to print the Docker version type:

docker -v
Docker version 18.06.1-ce, build e68fc7a

Executing the Docker Command Without Sudo

By  default managing, Docker requires administrator privileges. If you want  to run Docker commands as a non-root user without prepending sudo you need to add your user to the docker group which is created during the installation of the Docker CE package. You can do that by typing:

sudo usermod -aG docker $USER

$USER is an environment variable that holds your username.

Log out and log back in so that the group membership is refreshed.

To  verify Docker is installed successfully and that you can run docker  commands without sudo, issue the following command which will download a  test image, run it in a container, print a “Hello from Docker” message and exit:

docker container run hello-world

The output should look like the following:

Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
9bb5a5d4561a: Pull complete
Digest: sha256:f5233545e43561214ca4891fd1157e1c3c563316ed8e237750d59bde73361e77
Status: Downloaded newer image for hello-world:latest

Hello from Docker!
This message shows that your installation appears to be working correctly.

Docker command line interface

Now that we have a working Docker installation, let’s go over the basic syntax of the docker CLI.

The docker command line take the following form:

docker [option] [subcommand] [arguments]

You can list all available commands by typing docker with no parameters:

docker

If you need more help on any [subcommand], just type:

docker [subcommand] --help

Docker Images

A Docker image is made up of a series of layers representing instructions in the image’s Dockerfile that make up an executable software application. An image is an  immutable binary file including the application and all other  dependencies such as binaries, libraries, and instructions necessary for  running the application. In short, a Docker image is essentially a  snapshot of a Docker container.

The Docker Hub is cloud-based  registry service which among other functionalities is used for keeping  the Docker images either in a public or private repository.

To search the Docker Hub repository for an image just use the search subcommand. For example, to search for the CentOS image, run:

docker search centos

The output should look like the following:

NAME                               DESCRIPTION                                     STARS               OFFICIAL            AUTOMATED
centos                             The official build of CentOS.                   4257                [OK]
ansible/centos7-ansible            Ansible on Centos7                              109                                     [OK]
jdeathe/centos-ssh                 CentOS-6 6.9 x86_64 / CentOS-7 7.4.1708 x86_…   94                                      [OK]
consol/centos-xfce-vnc             Centos container with "headless" VNC session…   52                                      [OK]
imagine10255/centos6-lnmp-php56    centos6-lnmp-php56                              40                                      [OK]
tutum/centos                       Simple CentOS docker image with SSH access      39

As you can see the search results prints a table with five columns, NAME, DESCRIPTION, STARS, OFFICIAL and AUTOMATED. The official image is an image that Docker develops in conjunction with upstream partners.

If we want to download the official build of CentOS 7, we can do that by using the image pull subcommand:

docker image pull centos
Using default tag: latest
latest: Pulling from library/centos
469cfcc7a4b3: Pull complete
Digest: sha256:989b936d56b1ace20ddf855a301741e52abca38286382cba7f44443210e96d16
Status: Downloaded newer image for centos:latest

Depending on your Internet speed, the download may take a  few seconds or a few minutes. Once the image is downloaded we can list  the images with:

docker image ls

The output should look something like the following:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
hello-world         latest              e38bc07ac18e        3 weeks ago         1.85kB
centos              latest              e934aafc2206        4 weeks ago         199MB

If for some reason you want to delete an image you can do that with the image rm [image_name] subcommand:

docker image rm centos
Untagged: centos:latest
Untagged: centos@sha256:989b936d56b1ace20ddf855a301741e52abca38286382cba7f44443210e96d16
Deleted: sha256:e934aafc22064b7322c0250f1e32e5ce93b2d19b356f4537f5864bd102e8531f
Deleted: sha256:43e653f84b79ba52711b0f726ff5a7fd1162ae9df4be76ca1de8370b8bbf9bb0

Docker Containers

An instance of an image is called a container. A container represents a runtime for a single application, process, or service.

It  may not be the most appropriate comparison but if you are a programmer  you can think of a Docker image as class and Docker container as an  instance of a class.

We can start, stop, remove and manage a container with the docker container subcommand.

The  following command will start a Docker container based on the CentoOS  image. If you don’t have the image locally, it will download it first:

docker container run centos

At  first sight, it may seem to you that nothing happened at all. Well,  that is not true. The CentOS container stops immediately after booting  up because it does not have a long-running process and we didn’t provide  any command, so the container booted up, ran an empty command and then  exited.

The switch -it allows us to interact with the container via the command line. To start an interactive container type:

docker container run -it centos /bin/bash

As you can see from the output once the container is started the command prompt is changed which means that you’re now working from inside the container:

[root@719ef9304412 /]#

To list running containers: , type:

docker container ls
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
79ab8e16d567        centos              "/bin/bash"         22 minutes ago      Up 22 minutes                           ecstatic_ardinghelli

If you don’t have any running containers the output will be empty.

To view both running and stopped containers, pass it the -a switch:

docker container ls -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                      PORTS               NAMES
79ab8e16d567        centos              "/bin/bash"              22 minutes ago      Up 22 minutes                                   ecstatic_ardinghelli
c55680af670c        centos              "/bin/bash"              30 minutes ago      Exited (0) 30 minutes ago                       modest_hawking
c6a147d1bc8a        hello-world         "/hello"                 20 hours ago        Exited (0) 20 hours ago                         sleepy_shannon

To delete one or more containers just copy the container ID (or IDs) from above and paste them after the container rm subcommand:

docker container rm c55680af670c

Conclusion

You  have learned how to install Docker on your CentOS 7 machine and how to  download Docker images and manage Docker containers. You may also want  to read about Docker Compose, which allows you to define and run multi-container Docker applications.

This  tutorial barely scratches the surface of the Docker ecosystem. In some  of our next articles, we will continue to dive into other aspects of  Docker. To learn more about Docker check out the official Docker documentation.

If you have any questions or remarks, please leave a comment below.