Docker is an application that makes it simple and easy to run application processes in a container, which are like virtual machines, only more portable, more resource-friendly, and more dependent on the host operating system.<\/p>\n\n\n\n
Note:<\/strong> Docker requires a 64-bit version of CentOS as well as a kernel version equal to or greater than 3.10.<\/p>\n\n\n\n All the commands in this tutorial should be run as a non-root user. If root access is required for the command, it will be preceded by The Docker installation package available 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 shows you how to do just that.<\/p>\n\n\n\n But first, let\u2019s update the package database:<\/p>\n\n\n\n Now run this command. It will add the official Docker repository, download the latest version of Docker, and install it:<\/p>\n\n\n\n After installation has completed, start the Docker daemon:<\/p>\n\n\n\n Verify that it\u2019s running:<\/p>\n\n\n\n The output should be similar to the following, showing that the service is active and running:<\/p>\n\n\n\n Lastly, make sure it starts at every server reboot:<\/p>\n\n\n\n Installing Docker now gives you not just the Docker service (daemon) but also the By default, running the If you want to avoid typing You will need to log out of the Droplet and back in as the same user to enable this change.<\/p>\n\n\n\n If you need to add a user to the The rest of this article assumes you are running the With Docker installed and working, now\u2019s the time to become familiar with the command line utility. Using To view all available subcommands, type:<\/p>\n\n\n\n As of Docker 1.11.1, the complete list of available subcommands includes:<\/p>\n\n\n\n To view the switches available to a specific command, type:<\/p>\n\n\n\n To view system-wide information, use:<\/p>\n\n\n\n Docker containers are run from Docker images. By default, it pulls these images from Docker Hub, a Docker registry managed by Docker, the company behind the Docker project. Anybody can build and host their Docker images on Docker Hub, so most applications and Linux distributions you\u2019ll need to run Docker containers have images that are hosted on Docker Hub.<\/p>\n\n\n\n To check whether you can access and download images from Docker Hub, type:<\/p>\n\n\n\n The output, which should include the following, should indicate that Docker in working correctly:<\/p>\n\n\n\n You can search for images available on Docker Hub by using the The script will crawl Docker Hub and return a listing of all images whose name match the search string. In this case, the output will be similar to this:<\/p>\n\n\n\n In the OFFICIAL<\/strong> column, OK<\/strong> indicates an image built and supported by the company behind the project. Once you\u2019ve identifed the image that you would like to use, you can download it to your computer using the After an image has been downloaded, you may then run a container using the downloaded image with the To see the images that have been downloaded to your computer, type:<\/p>\n\n\n\n The output should look similar to the following:<\/p>\n\n\n\n As you\u2019ll see later in this tutorial, images that you use to run containers can be modified and used to generate new images, which may then be uploaded (pushed<\/em> is the technical term) to Docker Hub or other Docker registries.<\/p>\n\n\n\n The As an example, let\u2019s run a container using the latest image of CentOS. The combination of the -i<\/strong> and -t<\/strong> switches gives you interactive shell access into the container:<\/p>\n\n\n\n Your command prompt should change to reflect the fact that you\u2019re now working inside the container and should take this form:<\/p>\n\n\n\n Important:<\/strong> Note the container id in the command prompt. In the above example, it is Now you may run any command inside the container. For example, let\u2019s install MariaDB server in the running container. No need to prefix any command with When you start up a Docker image, you can create, modify, and delete files just like you can with a virtual machine. The changes that you make will only apply to that container. You can start and stop it, but once you destroy it with the This section shows you how to save the state of a container as a new Docker image.<\/p>\n\n\n\n After installing MariaDB server inside the CentOS container, you now have a container running off an image, but the container is different from the image you used to create it.<\/p>\n\n\n\n To save the state of the container as a new image, first exit from it:<\/p>\n\n\n\n Then commit the changes to a new Docker image instance using the following command. The -m<\/strong> switch is for the commit message that helps you and others know what changes you made, while -a<\/strong> is used to specify the author. The container ID is the one you noted earlier in the tutorial when you started the interactive docker session. Unless you created additional repositories on Docker Hub, the repository is usually your Docker Hub username:<\/p>\n\n\n\n For example:<\/p>\n\n\n\n Note:<\/strong> When you commit<\/em> an image, the new image is saved locally, that is, on your computer. Later in this tutorial, you\u2019ll learn how to push an image to a Docker registry like Docker Hub so that it may be assessed and used by you and others.<\/p>\n\n\n\n After that operation has completed, listing the Docker images now on your computer should show the new image, as well as the old one that it was derived from:<\/p>\n\n\n\n The output should be of this sort:<\/p>\n\n\n\n In the above example, centos-mariadb<\/strong> is the new image, which was derived from the existing CentOS image from Docker Hub. The size difference reflects the changes that were made. And in this example, the change was that MariaDB server was installed. So next time you need to run a container using CentOS with MariaDB server pre-installed, you can just use the new image. Images may also be built from what\u2019s called a Dockerfile. But that\u2019s a very involved process that\u2019s well outside the scope of this article. We\u2019ll explore that in a future article.<\/p>\n\n\n\n After using Docker for a while, you\u2019ll have many active (running) and inactive containers on your computer. To view the active ones, use:<\/p>\n\n\n\n You will see output similar to the following:<\/p>\n\n\n\n To view all containers \u2014 active and inactive, pass it the To view the latest container you created, pass it the Stopping a running or active container is as simple as typing:<\/p>\n\n\n\n The The next logical step after creating a new image from an existing image is to share it with a select few of your friends, the whole world on Docker Hub, or other Docker registry that you have access to. To push an image to Docker Hub or any other Docker registry, you must have an account there.<\/p>\n\n\n\n This section shows you how to push a Docker image to Docker Hub.<\/p>\n\n\n\nsudo<\/code>.<\/p>\n\n\n\nStep 1 \u2014 Installing Docker<\/h2>\n\n\n\n
sudo yum check-update\n\n<\/code><\/pre>\n\n\n\ncurl -fsSL https:\/\/get.docker.com\/ | sh\n\n<\/code><\/pre>\n\n\n\nsudo systemctl start docker\n\n<\/code><\/pre>\n\n\n\nsudo systemctl status docker\n\n<\/code><\/pre>\n\n\n\nOutput\u25cf docker.service - Docker Application Container Engine\n Loaded: loaded (\/lib\/systemd\/system\/docker.service; enabled; vendor preset: enabled)\n Active: active (running)<\/mark> since Sun 2016-05-01 06:53:52 CDT; 1 weeks 3 days ago\n Docs: https:\/\/docs.docker.com\n Main PID: 749 (docker)\n<\/code><\/pre>\n\n\n\nsudo systemctl enable docker\n\n<\/code><\/pre>\n\n\n\ndocker<\/code> command line utility, or the Docker client. We\u2019ll explore how to use the docker<\/code> command later in this tutorial.<\/p>\n\n\n\nStep 2 \u2014 Executing Docker Command Without Sudo (Optional)<\/h2>\n\n\n\n
docker<\/code> command requires root privileges \u2014 that is, you have to prefix the command with sudo<\/code>. It can also be run by a user in the docker<\/strong> group, which is automatically created during the installation of Docker. If you attempt to run the docker<\/code> command without prefixing it with sudo<\/code> or without being in the docker group, you\u2019ll get an output like this:<\/p>\n\n\n\nOutputdocker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?.\nSee 'docker run --help'.\n<\/code><\/pre>\n\n\n\nsudo<\/code> whenever you run the docker<\/code> command, add your username to the docker group:<\/p>\n\n\n\nsudo usermod -aG docker $(whoami)\n\n<\/code><\/pre>\n\n\n\ndocker<\/code> group that you\u2019re not logged in as, declare that username explicitly using:<\/p>\n\n\n\nsudo usermod -aG docker username<\/mark>\n\n<\/code><\/pre>\n\n\n\ndocker<\/code> command as a user in the docker user group. If you choose not to, please prepend the commands with sudo<\/code>.<\/p>\n\n\n\nStep 3 \u2014 Using the Docker Command<\/h2>\n\n\n\n
docker<\/code> consists of passing it a chain of options and subcommands followed by arguments. The syntax takes this form:<\/p>\n\n\n\ndocker [option] [command] [arguments]\n\n<\/code><\/pre>\n\n\n\ndocker\n\n<\/code><\/pre>\n\n\n\nOutput\n attach Attach to a running container\n build Build an image from a Dockerfile\n commit Create a new image from a container's changes\n cp Copy files\/folders between a container and the local filesystem\n create Create a new container\n diff Inspect changes on a container's filesystem\n events Get real time events from the server\n exec Run a command in a running container\n export Export a container's filesystem as a tar archive\n history Show the history of an image\n images List images\n import Import the contents from a tarball to create a filesystem image\n info Display system-wide information\n inspect Return low-level information on a container or image\n kill Kill a running container\n load Load an image from a tar archive or STDIN\n login Log in to a Docker registry\n logout Log out from a Docker registry\n logs Fetch the logs of a container\n network Manage Docker networks\n pause Pause all processes within a container\n port List port mappings or a specific mapping for the CONTAINER\n ps List containers\n pull Pull an image or a repository from a registry\n push Push an image or a repository to a registry\n rename Rename a container\n restart Restart a container\n rm Remove one or more containers\n rmi Remove one or more images\n run Run a command in a new container\n save Save one or more images to a tar archive\n search Search the Docker Hub for images\n start Start one or more stopped containers\n stats Display a live stream of container(s) resource usage statistics\n stop Stop a running container\n tag Tag an image into a repository\n top Display the running processes of a container\n unpause Unpause all processes within a container\n update Update configuration of one or more containers\n version Show the Docker version information\n volume Manage Docker volumes\n wait Block until a container stops, then print its exit code\n<\/code><\/pre>\n\n\n\ndocker docker-subcommand<\/mark> --help\n\n<\/code><\/pre>\n\n\n\ndocker info\n\n<\/code><\/pre>\n\n\n\nStep 4 \u2014 Working with Docker Images<\/h2>\n\n\n\n
docker run hello-world\n\n<\/code><\/pre>\n\n\n\nOutputHello from Docker.\nThis message shows that your installation appears to be working correctly.\n...\n<\/code><\/pre>\n\n\n\ndocker<\/code> command with the search<\/code> subcommand. For example, to search for the CentOS image, type:<\/p>\n\n\n\ndocker search centos\n\n<\/code><\/pre>\n\n\n\nOutputNAME DESCRIPTION STARS OFFICIAL AUTOMATED\ncentos The official build of CentOS. 2224 [OK] \njdeathe\/centos-ssh CentOS-6 6.7 x86_64 \/ CentOS-7 7.2.1511 x8... 22 [OK]\njdeathe\/centos-ssh-apache-php CentOS-6 6.7 x86_64 \/ Apache \/ PHP \/ PHP M... 17 [OK]\nmillion12\/centos-supervisor Base CentOS-7 with supervisord launcher, h... 11 [OK]\nnimmis\/java-centos This is docker images of CentOS 7 with dif... 10 [OK]\ntorusware\/speedus-centos Always updated official CentOS docker imag... 8 [OK]\nnickistre\/centos-lamp LAMP on centos setup 3 [OK]\n\n...\n<\/code><\/pre>\n\n\n\npull<\/code> subcommand, like so:<\/p>\n\n\n\ndocker pull centos\n\n<\/code><\/pre>\n\n\n\nrun<\/code> subcommand. If an image has not been downloaded when docker<\/code> is executed with the run<\/code> subcommand, the Docker client will first download the image, then run a container using it:<\/p>\n\n\n\ndocker run centos\n\n<\/code><\/pre>\n\n\n\ndocker images\n\n<\/code><\/pre>\n\n\n\n[secondary_lable Output]\nREPOSITORY TAG IMAGE ID CREATED SIZE\ncentos latest 778a53015523 5 weeks ago 196.7 MB\nhello-world latest 94df4f0ce8a4 2 weeks ago 967 B\n<\/code><\/pre>\n\n\n\nStep 5 \u2014 Running a Docker Container<\/h2>\n\n\n\n
hello-world<\/code> container you ran in the previous step is an example of a container that runs and exits, after emitting a test message. Containers, however, can be much more useful than that, and they can be interactive. After all, they are similar to virtual machines, only more resource-friendly.<\/p>\n\n\n\ndocker run -it centos\n\n<\/code><\/pre>\n\n\n\nOutput[root@59839a1b7de2 \/]#\n<\/code><\/pre>\n\n\n\n59839a1b7de2<\/code>.<\/p>\n\n\n\nsudo<\/code>, because you\u2019re operating inside the container with root privileges:<\/p>\n\n\n\nyum install mariadb-server\n\n<\/code><\/pre>\n\n\n\nStep 6 \u2014 Committing Changes in a Container to a Docker Image<\/h2>\n\n\n\n
docker rm<\/code> command, the changes will be lost for good.<\/p>\n\n\n\nexit\n\n<\/code><\/pre>\n\n\n\ndocker commit -m \"What did you do to the image\" -a \"Author Name\" container-id<\/mark> repository<\/mark>\/new_image_name<\/mark>\n\n<\/code><\/pre>\n\n\n\ndocker commit -m \"added mariadb-server\" -a \"Sunday Ogwu-Chinuwa\" 59839a1b7de2 finid\/centos-mariadb\n\n<\/code><\/pre>\n\n\n\ndocker images\n\n<\/code><\/pre>\n\n\n\nOutputREPOSITORY TAG IMAGE ID CREATED SIZE\nfinid\/centos-mariadb latest 23390430ec73 6 seconds ago 424.6 MB\ncentos latest 778a53015523 5 weeks ago 196.7 MB\nhello-world latest 94df4f0ce8a4 2 weeks ago 967 B\n<\/code><\/pre>\n\n\n\nStep 7 \u2014 Listing Docker Containers<\/h2>\n\n\n\n
docker ps\n\n<\/code><\/pre>\n\n\n\nOutputCONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES\nf7c79cc556dd centos \"\/bin\/bash\" 3 hours ago Up 3 hours silly_spence\n<\/code><\/pre>\n\n\n\n-a<\/code> switch:<\/p>\n\n\n\ndocker ps -a\n\n<\/code><\/pre>\n\n\n\n-l<\/code> switch:<\/p>\n\n\n\ndocker ps -l\n\n<\/code><\/pre>\n\n\n\ndocker stop container-id<\/mark>\n\n<\/code><\/pre>\n\n\n\ncontainer-id<\/code><\/mark> can be found in the output from the docker ps<\/code> command.<\/p>\n\n\n\nStep 8 \u2014 Pushing Docker Images to a Docker Repository<\/h2>\n\n\n\n