Docker is an open-source tool that makes creating & managing Linux containers(LXC) easy. Containers are like lightweight VMs which can be started & stopped in milliseconds. Dockers help the system admin & coders to develop their application in a container and can further scale up to 1000 of nodes.

The main difference between container and VM(Virtual machine) is that dockers provide process based isolation , whereas VM provides full isolation of resources. Virtual machine takes a minute to start where as container can be started in a second or less than a second. Container uses the Kernel of host OS , whereas VM uses the separate Kernel.

One of the limitation of Docker is that it can be used only on 64bit hosts OS.

In this post we will discuss how to install docker in CentOS 7.x

Installation of Docker on CentOS 7

Docker package is included in the default CentOS-Extras repository. So to install docker , simply run below yum command :

[root@localhost ~]# yum install docker

Installation Steps of Docker CE (Community Edition) on CentOS 7

If you looking for stable and latest version Docker then you should go for Docker CE. Below are the steps how to install Docker CE.

Set the repository for Docker Community Edition

First install the required packages for setting up Docker Community edition repo using the following command

[root@localhost ~]# yum install yum-utils device-mapper-persistent-data lvm2 -y

Run the beneath command to set the stable repository of Docker CE (Community edition)

[root@localhost ~]# yum-config-manager --add-repo  https://download.docker.com/linux/centos/docker-ce.repo

Now use the following yum command to install Docker CE.

[root@localhost ~]# yum install docker-ce -y

Verify the Docker Version using the beneath command

[root@localhost ~]# docker --version

Docker version 19.03.12, build 48a66213fe

OR

[root@testserver ~]# docker version

Client: Docker Engine - Community
 Version:           19.03.12
 API version:       1.40
 Go version:        go1.13.10
 Git commit:        48a66213fe
 Built:             Mon Jun 22 15:46:54 2020
 OS/Arch:           linux/amd64
 Experimental:      false
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
[root@testserver ~]# 

[root@localhost ~]#

Start the Docker Service

Once the Installation is finished , start docker service and enable it at boot using below commands

[root@localhost ~]# systemctl start docker && systemctl enable docker

Get the Docker Complete Information by docker info command

[root@testserver ~]# docker info

Client:
 Debug Mode: false

Server:
 Containers: 0
  Running: 0
  Paused: 0
  Stopped: 0
 Images: 0
 Server Version: 19.03.12
 Storage Driver: overlay2
  Backing Filesystem: xfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 3.10.0-1127.18.2.el7.x86_64
 Operating System: CentOS Linux 7 (Core)
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 1.777GiB
 Name: testserver.com
 ID: NHPO:AZ7D:UOYK:Z45F:DNLF:75FJ:MLTH:YIAE:7PD6:BJOL:R5DV:EUV6
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

List all images by docker images Command

[root@testserver ~]# docker images

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE

Download the official Centos images Locally

[root@testserver ~]# docker pull centos

Using default tag: latest
latest: Pulling from library/centos
6910e5a164f7: Pull complete 
Digest: sha256:4062bbdd1bb0801b0aa38e0f83dece70fb7a5e9bce223423a68de2d8b784b43b
Status: Downloaded newer image for centos:latest
docker.io/library/centos:latest
[root@testserver ~]# 

 Verify CentOS images that have been fetched :

[root@testserver ~]# docker images

REPOSITORY					TAG                 IMAGE ID            CREATED             SIZE
ubuntu						latest              1e4467b07108        2 weeks ago         73.9MB
centos						latest              831691599b88        7 weeks ago         215MB
thanishinfotech/javagitmaven			latest              73ae15627927        7 months ago        690MB
[root@testserver ~]# 

Run a Docker Container :

[root@localhost ~]# docker run -i -t centos /bin/bash

[root@dbf66395436d /]#

As we can see centos container has been started and we got the bash shell. In docker command we have used options like '-i attaches stdin and stdout' and '-t allocates a terminal or console' . To disconnect from container type exit .

[root@cd05639b3f5c /]# cat /etc/redhat-release 

CentOS Linux release 7.0.1406 (Core) 

[root@cd05639b3f5c /]# exit

exit

[root@localhost ~]#

We can also search Containers based on fedora & ubuntu OS.

[root@localhost ~]# docker search ubuntu

Display the list of running containers