Docker is an open-source platform that enables software developers to build, deploy and manage containerized applications. It means the applications are packaged into container which is combination of source codes, libraries and other dependencies required to run in any operating system.
In addition, docker makes it simpler, smoother and more secure when working with container.
In this tutorial, I will go through all steps of installing Docker on Ubuntu 20.04 and learn the fundamental concepts when working with docker containers/images.
In the official software repository of Ubuntu 20.04, the docker installation package is available by default. However, it isn't the up-to-date version. It's a recommendation to install the latest version of docker from the official docker repository.
Firstly, you need to update the current packages list and install the prerequisite packages in order to add new repository to your system by running the following commands:
$ sudo apt update $ sudo apt install apt-transport-https ca-certificates curl software-properties-common gnupg-agent
Next, you have to import the GPG key of Docker repository:
$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
Then, add the docker repository to APT sources on your system:
$ sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
Run the following command to update the newly added repository:
$ sudo apt update
Ensure that you are going to install docker from its repository instead of the Ubuntu repository:
$ apt-cache policy docker-ce
You will see the output is something like this:
docker-ce: Installed: (none) Candidate: 5:19.03.11~3-0~ubuntu-focal Version table: 5:19.03.11~3-0~ubuntu-focal 500 500 https://download.docker.com/linux/ubuntu focal/stable amd64 Packages 5:19.03.10~3-0~ubuntu-focal 500 500 https://download.docker.com/linux/ubuntu focal/stable amd64 Packages 5:19.03.9~3-0~ubuntu-focal 500 500 https://download.docker.com/linux/ubuntu focal/stable amd64 Packages
The above output indicates that the 'docker-ce' from the Docker repo for Ubuntu 20.04 is going to be installed on your system.
Finally, install docker-ce (community edition):
$ sudo apt install docker-ce
In order to verify whether docker was successfully installed and running properly or not, run the following command:
$ sudo systemctl status docker
The output will show you that the docker service is active (running):
● docker.service - Docker Application Container Engine Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled) Active: active (running) since Wed 2020-06-17 15:29:48 +07; 8min ago TriggeredBy: ● docker.socket Docs: https://docs.docker.com Main PID: 136640 (dockerd) Tasks: 17 Memory: 50.8M CGroup: /system.slice/docker.service └─136640 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock ...
You've just installed docker successfully on your Ubuntu machine. We will deep dive into the way of using the docker command in the following sections of this tutorial.
Execute docker without sudo privilege
By default, you have to run docker commands with sudo privilege or by a user in the docker group. If you try to run the docker commands without sudo or with a privilege of a user doesn't in docker group, you'll get the error like this:
docker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?. See 'docker run --help'.
The workaround is adding the user to the 'docker' group:
$ sudo usermod -aG docker <user-name>
Logout the current session then login again to apply the membership. Verifying that the user is in 'docker' group by running the command:
$ groups ubuntu sudo docker
From now on, you can run the docker command without 'sudo'.
This section will explain how to use docker command. Before going into detail, let's take look at the syntax of 'docker' command:
$ docker [options] [sub-commands] [arguments]
To list all available sub-commands of docker, run:
There is a lot of options, sub-commands and arguments need to be remembered. Let's delve into some of them in the following sections of this tutorial.
Management Commands: builder Manage builds config Manage Docker configs container Manage containers context Manage contexts engine Manage the docker engine image Manage images network Manage networks node Manage Swarm nodes plugin Manage plugins secret Manage Docker secrets service Manage services stack Manage Docker stacks swarm Manage Swarm system Manage Docker trust Manage trust on Docker images volume Manage volumes Commands: attach Attach local standard input, output, and error streams to a running container build Build an image from a Dockerfile commit Create a new image from a container's changes cp Copy files/folders between a container and the local filesystem create Create a new container diff Inspect changes to files or directories 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 Docker objects kill Kill one or more running containers 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 pause Pause all processes within one or more containers 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 one or more containers rm Remove one or more containers rmi Remove one or more images run Run a command in a new container save Save one or more images to a tar archive (streamed to STDOUT by default) search Search the Docker Hub for images start Start one or more stopped containers stats Display a live stream of container(s) resource usage statistics stop Stop one or more running containers tag Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE top Display the running processes of a container unpause Unpause all processes within one or more containers update Update configuration of one or more containers version Show the Docker version information wait Block until one or more containers stop, then print their exit codes Run 'docker COMMAND --help' for more information on a command.
Docker images consist of source code, libraries, and all necessary dependencies that the application requires to run as a container. Docker images can be built from scratch, but you can pull these images down from a container registry. The registry is where you can store images and download them using a unique name. By default, the container registry of docker is Docker Hub.
In order to check whether you can pull images from Docker Hub, you can run the following command:
$ docker run hello-world
As you can see in the following output: docker could not find the 'hello-world' image locally and it pulled the image from Docker Hub. After the image has been downloaded, docker creates a container from that image. The application in the container will be executed and display the message:
Unable to find image 'hello-world:latest' locally latest: Pulling from library/hello-world 0e03bdcc26d7: Pull complete Digest: sha256:d58e752213a51785838f9eed2b7a498ffa1cb3aa7f946dda11af39286c3db9a9 Status: Downloaded newer image for hello-world:latest Hello from Docker! This message shows that your installation appears to be working correctly. To generate this message, Docker took the following steps: 1. The Docker client contacted the Docker daemon. 2. The Docker daemon pulled the "hello-world" image from the Docker Hub. (amd64) 3. The Docker daemon created a new container from that image which runs the executable that produces the output you are currently reading. 4. The Docker daemon streamed that output to the Docker client, which sent it to your terminal. To try something more ambitious, you can run an Ubuntu container with: $ docker run -it ubuntu bash ...
In order to search for an image on the Docker Hub, you can use the 'docker' command with 'search' sub-command. Assuming that you want to search for the 'alpine' image, run:
$ docker search alpine
As you can see, the output returns a list of all images whose name has 'alpine'.
Moreover, you can download a docker image by running command 'docker image pull' follow with the name of the image. For example:
$ docker image pull alpine
Depending on the speed of the Internet, the download may take a minute.
Using default tag: latest latest: Pulling from library/alpine df20fa9351a1: Pull complete Digest: sha256:185518070891758909c9f839cf4ca393ee977ac378609f700f60a771a2dfe321 Status: Downloaded newer image for alpine:latest docker.io/library/alpine:latest
In addition, if you want to list all the download images, run the following command:
$ docker image ls
The output will show you the detail of the images includes: repository, tag, image id, created time and file size.
REPOSITORY TAG IMAGE ID CREATED SIZE alpine latest a24bb4013296 2 weeks ago 5.57MB hello-world latest bf756fb1ae65 5 months ago 13.3kB
Sometimes, you need to delete docker image, you can run the command:
$ docker image rm <image-name>
$ docker image rm <image-id>
A container is a running instance of a docker image. A container performs a runtime of single application or process. You can interact with a container and start/stop or remove it by running the 'docker container' sub-command.
For example, in order to start a docker container based on the 'Ubuntu' image, run:
ubuntu@linoxide:~$ docker container run ubuntu
After downloading a newer image for Ubuntu from Docker Hub and starting the Ubuntu container, it returns system command prompt because the Ubuntu container has been stopped immediately after booting up.
Unable to find image 'ubuntu:latest' locally latest: Pulling from library/ubuntu a4a2a29f9ba4: Pull complete 127c9761dcba: Pull complete d13bf203e905: Pull complete 4039240d2e0b: Pull complete Digest: sha256:35c4a2c15539c6c1e4e5fa4e554dac323ad0107d8eb5c582d6ff386b383b7dce Status: Downloaded newer image for ubuntu:latest ubuntu@linoxide:~$
In order to interact with the Ubuntu container through the command line, you can start the container with
$ docker container run -it ubuntu
Now, you've gone into the container and accessed to its interactive shell.
So far, you've already run many container on your Ubuntu system. In order to list all of the active (running) containers, run the following command:
$ docker container ls CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
As the result return an empty list, so there is not any container running. If you want to list all of the containers: active and inactive, run:
$ docker container ls -a
The output is similar as:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 45cc68d4395e ubuntu "/bin/bash" 14 minutes ago Exited (0) 11 minutes ago stoic_dhawan fc91e50d223e ubuntu "/bin/bash" 26 minutes ago Exited (0) 26 minutes ago sleepy_ritchie ce0d81682913 alpine "/bin/bash" 26 minutes ago Created trusting_kowalevski 4291477f5ac2 alpine "/bin/sh" 27 minutes ago Exited (130) 27 minutes ago priceless_wozniak 86c0fd44a4f4 alpine "/bin/sh" 28 minutes ago Exited (0) 28 minutes ago objective_agnesi 3fdcaa196fca hello-world "/hello" 31 minutes ago Exited (0) 31 minutes ago eloquent_rhodes
Once you want to delete/remove a container, run:
$ docker container rm <container-id>
In this tutorial, we learned how to install Docker on Ubuntu 20.04 and how to work with images, containers through examples. They are the basics of Docker concepts and be used usually by developers.
Thanks for reading and please leave your suggestion in the below comment section.