How to install and use Docker on CentOS 7
Introduction
Docker is an application that makes it easy to run application processes in a container, just like a virtual machine, but more portable, more resource-friendly, and more dependent on the host operating system.
There are two ways to install Docker on CentOS 7. One way is to install it on an existing installation of the operating system. The other involves starting the server with a tool called Docker Machine, which automatically installs Docker on it.
In this tutorial, you will learn how to install and use it in an existing CentOS 7 installation.
Preparation
- 64 CentOS 7 Tencent Cloud CVM
- A CentOS server with a non-root account that can use the
sudocommand has been set up, and the firewall has been turned on. Students who don’t have a server can buy it from here, but I personally recommend you to use the free Tencent Cloud Developer Lab for experiments, and then buy server.
**Note: ** Docker requires a 64-bit version of CentOS 7 and a kernel version equal to or greater than 3.10. The default 64-bit CentOS 7 Tencent Cloud CVM meets these requirements.
All commands in this tutorial should be run as a non-root user.
Step 1-Install Docker
The Docker installation package provided in the official CentOS 7 repository may not be the latest version. To get the latest and greatest version, install Docker from the official Docker repository. This section will show you how to do this.
But first, let's update the package database:
sudo yum check-update
Now run this command. It will add the official Docker repository, download the latest version of Docker, and install it:
curl -fsSL https://get.docker.com/| sh
After the installation is complete, start the Docker daemon:
sudo systemctl start docker
Verify that it is running:
sudo systemctl status docker
The output should be similar to the following, indicating that the service is active and running:
● docker.service - Docker Application Container Engine
Loaded:loaded(/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
Active:active(running) since Sun 2016-05-0106:53:52 CDT;1 weeks 3 days ago
Docs: https://docs.docker.com
Main PID:749(docker)
Finally, make sure it starts every time the server restarts:
sudo systemctl enable docker
Installing Docker now can not only provide you with Docker service (daemon), but also provide you with a docker command line utility or Docker client. We will discuss how to use this command with docker later in this tutorial.
Step 2-Execute Docker commands without Sudo (optional)
By default, running the docker command requires root privileges-that is, you must add sudo before the command. It can also be run by a user in the docker group, which is automatically created during Docker installation. If you try to run the docker command without using sudo or without prefixing it in the docker group, you will get the following output:
docker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?.
See 'docker run --help'.
If you want sudo to avoid typing when running the docker command, please add your username to the docker group:
sudo usermod -aG docker $(whoami)
You need to log out of the Droplet and return as the same user to enable this change.
If you need to add a user to a group where you are not logged in to docker, please use the following method to clearly declare the user name:
sudo usermod -aG docker username
The rest of this article assumes that you docker runs the command as a user in the docker user group. If you choose not to do this, please add the command sudo in front.
Step 3-Use Docker commands
With Docker installed and working, it's time to get familiar with the command line utilities. Using docker involves passing a series of options and subcommands, followed by parameters. The syntax takes the following form:
docker [option][command][arguments]
To view all available subcommands, type:
docker
Starting with Docker 1.11.1, the complete list of available subcommands includes:
attach Attach to a running container
build Build an image from a Dockerfile
commit Create a newimagefrom a container's changes
cp Copy files/folders between a container and the local filesystem
create Create a newcontainer
diff Inspect changes on a container's filesystem
events Get real time events from the server
exec Run a command in a running container
export Export a container's filesystem as a tar archive
history Show the history of an image
images List images
import Import the contents from a tarball to create a filesystem image
info Display system-wide information
inspect Return low-level information on a container or image
kill Kill a running container
load Load an image from a tar archive or STDIN
login Log in to a Docker registry
logout Log out from a Docker registry
logs Fetch the logs of a container
network Manage Docker networks
pause Pause all processes within a container
port List port mappings or a specific mapping for the CONTAINER
ps List containers
pull Pull an image or a repository from a registry
push Push an image or a repository to a registry
rename Rename a container
restart Restart a container
rm Remove one or more containers
rmi Remove one or more images
run Run a command in a newcontainer
save Save one or more images to a tar archive
search Search the Docker Hub for images
start Start one or more stopped containers
stats Display a live stream ofcontainer(s) resource usage statistics
stop Stop a running container
tag Tag an image into a repository
top Display the running processes of a container
unpause Unpause all processes within a container
update Update configuration of one or more containers
version Show the Docker version information
volume Manage Docker volumes
wait Block until a container stops, then print its exit code
To view the switches available for a specific command, type:
docker docker-subcommand --help
To view system-wide information, use:
docker info
Step 4-Use Docker Mirror
The Docker container runs from the Docker image. By default, it gets these images from Docker Hub, which is a Docker registry managed by Docker, the company behind the Docker project. Anyone can build and host their Docker images on Docker Hub, so most of the applications and Linux distributions needed to run Docker containers have images hosted on Docker Hub.
To check if you can access and download the image from Docker Hub, type:
docker run hello-world
The output should include the following and should indicate that Docker is working properly:
Hello from Docker.
This message shows that your installation appears to be working correctly....
You can use the search command with the docker command to search for images available on Docker Hub. For example, to search for CentOS mirrors, type:
docker search centos
This script will crawl Docker Hub and return a list of all mirrors whose names match the search string. In this case, the output will be similar to:
NAME DESCRIPTION STARS OFFICIAL AUTOMATED
centos The official build of CentOS.2224[OK]
jdeathe/centos-ssh CentOS-66.7 x86_64 / CentOS-77.2.1511 x8...22[OK]
jdeathe/centos-ssh-apache-php CentOS-66.7 x86_64 / Apache / PHP / PHP M...17[OK]
million12/centos-supervisor Base CentOS-7with supervisord launcher, h...11[OK]
nimmis/java-centos This is docker images of CentOS 7with dif...10[OK]
torusware/speedus-centos Always updated official CentOS docker imag...8[OK]
nickistre/centos-lamp LAMP on centos setup 3[OK]
...
In the OFFICIAL column, OK represents the image built and supported by the company behind the project. Once you have determined the image you want to use, you can use the pull subcommand to download it to your computer as shown below:
docker pull centos
After downloading the image, you can run the container using the downloaded image with the run subcommand. If the image has not been downloaded when docker is executed with the run subcommand, the Docker client will download the image first, and then use it to run the container:
docker run centos
To view the image that has been downloaded to the computer, type:
docker images
The output should be similar to the following:
[ secondary_lable Output]
REPOSITORY TAG IMAGE ID CREATED SIZE
centos latest 778a53015523 5 weeks ago 196.7 MB
hello-world latest 94df4f0ce8a4 2 weeks ago 967 B
As you will see later in this tutorial, the image used to run the container can be modified and used to generate a new image, which can then be uploaded (push is a technical term) to Docker Hub or other Docker registry.
Step 5-Run the Docker container
After sending the test message, running the hello-world container in your previous step is an example of the container running and exiting. However, containers are more useful than this, they can be interactive. After all, they are similar to virtual machines, but more resource-friendly.
For example, let's run the container using the latest image of CentOS. The combination of -i and -t switches gives you interactive shell access to the container:
docker run -it centos
Your command prompt should change to reflect the fact that you are now working in the container, and should take the following form:
[ root@59839a1b7de2 /]#
**Important: **Please note the container ID in the command prompt. In the example above, it is 59839a1b7de2.
Now you can run any command inside the container. For example, let us install the MariaDB server in a running container. There is no need to prefix any commands with sudo, because you are operating in a container with root privileges:
yum install mariadb-server
Step 6-Commit the changes in the container to the Docker image
When you start the Docker image, you can create, modify and delete files just like using a virtual machine. The changes you made only apply to this container. You can start and stop it, but once you destroy it with the docker rm command, the changes will be lost forever.
This section describes how to save the state of the container as a new Docker image.
After installing MariaDB server in the CentOS container, you now have a container that runs the image, but the container is different from the image you used to create it.
To save the state of the container as a new image, please exit from it first:
exit
Then use the following command to commit the changes to the new Docker image instance. The -m switch is the submission information, which can help you and others know the changes you made, while the -a is used to specify the author. When you start an interactive docker session, the container ID is the container ID you mentioned earlier in this tutorial. Unless you have created other repositories on Docker Hub, the repository is usually your Docker Hub username:
docker commit -m "What did you do to the image"-a "Author Name" container-id repository/new_image_name
E.g:
docker commit -m "added mariadb-server"-a "Sunday Ogwu-Chinuwa" 59839a1b7de2 finid/centos-mariadb
**Note: **When you submit the image, the new image is saved locally, that is, on your computer. Later in this tutorial, you will learn how to push an image to a Docker registry such as Docker Hub so that you and others can evaluate and use it.
After completing the operation, immediately listing the Docker image on the computer should show the new image and the old image derived from it:
docker images
The output should look like this:
REPOSITORY TAG IMAGE ID CREATED SIZE
finid/centos-mariadb latest 23390430ec73 6 seconds ago 424.6 MB
centos latest 778a53015523 5 weeks ago 196.7 MB
hello-world latest 94df4f0ce8a4 2 weeks ago 967 B
In the above example, centos-mariadb is the new image, which comes from the existing CentOS image of Docker Hub. The size difference reflects the changes made. In this example, the change is that MariaDB server is installed. Therefore, next time you need to run a container with CentOS pre-installed with MariaDB server, you can use the new image. The image can also be built from a so-called Dockerfile. But this is a very complicated process that is beyond the scope of this article. We will explore this in a future article.
Step 7-List Docker containers
After using Docker for a while, you will have many active (running) and inactive containers on your computer. To view the active ones, use:
docker ps
You will see output similar to the following:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
f7c79cc556dd centos "/bin/bash"3 hours ago Up 3 hours silly_spence
To see all containers-active and inactive, pass the -a switch to it:
docker ps -a
To see the latest container you created, pass it to the -l switch:
docker ps -l
Stopping a running or active container is as simple as typing:
docker stop container-id
You can use the docker ps command to find the container-id in the output.
Step 8-Push the Docker image to the Docker repository
The next logical step after creating a new image from an existing image is to share it with a few friends of your choice, the entire world on Docker Hub, or other Docker registries that you can access. To push an image to Docker Hub or any other Docker registry, you must have an account there.
This section describes how to push Docker images to Docker Hub.
To create an account with Docker Hub, register in Docker Hub. Then, in order to push your image, first log in to Docker Hub. You will be prompted for identity verification:
docker login -u docker-registry-username
If you specified the correct password, authentication should be successful. Then you can use the following methods to push your own image
docker push docker-registry-username/docker-image-name
It will take some time to complete, and when complete, the output will look like this:
The push refers to a repository [docker.io/finid/centos-mariadb]
670194 edfaf5: Pushed
5 f70bf18a086: Mounted from library/centos
6 a6c96337be1: Mounted from library/centos
...
After pushing the image to the registry, it should be listed on the dashboard of your account as shown in the image below.
If the push attempt results in this type of error, you may not be logged in:
The push refers to a repository [docker.io/finid/centos-mariadb]
e3fbbfb44187: Preparing
5 f70bf18a086: Preparing
a3b5c80a4eba: Preparing
7 f18b442972b: Preparing
3 ce512daaf78: Preparing
7 aae4540b42d: Waiting
unauthorized: authentication required
Log in and repeat the push attempt.
in conclusion
Docker is much more than what is given in this article, but it is enough to get you started using it on CentOS 7. Like most open source projects, Docker is built from a rapidly developing code base, so make a habit of visiting the project blog page to get the latest information.
For more CentOS tutorials, please go to [Tencent Cloud + Community] (https://cloud.tencent.com/developer?from=10680) to learn more.
Reference: "How To Install and Use Docker on CentOS 7"
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