How to install Docker on Ubuntu 20.04

Roustabout It is an open source platform that enables software developers to build, deploy, and manage containerized applications. This means packaging the application into a container, which is a combination of source code, libraries, and other dependencies required to run on any operating system.

In addition, when using containers, docker makes it simpler, smoother, and safer.

In this tutorial, I will introduce all the steps to install Docker on Ubuntu 20.04 and learn the basic concepts of using Docker containers/images.

Install docker

In the official software repository of Ubuntu 20.04, docker can be used to install software packages by default. However, it is not the latest version. It is recommended to install the latest version of docker from the official docker repository.

First, you need to update the current package list and install the prerequisite packages to add the new repository to the system by running the following command:

$ sudo apt update
$ sudo apt install apt-transport-https ca-certificates curl software-properties-common gnupg-agent

Next, you must import the GPG key of the Docker repository:

$ curl -fsSL | sudo apt-key add -

Then, Add docker repository To the APT source on the system:

$ sudo add-apt-repository "deb [arch=amd64] $(lsb_release -cs) stable"

Run the following command to update the newly added repository:

$ sudo apt update

Make sure to install docker from its repository and not the Ubuntu repository:

$ apt-cache policy docker-ce

You will see the output looks like this:

  Installed: (none)
  Candidate: 5:19.03.11~3-0~ubuntu-focal
  Version table:
     5:19.03.11~3-0~ubuntu-focal 500
        500 focal/stable amd64 Packages
     5:19.03.10~3-0~ubuntu-focal 500
        500 focal/stable amd64 Packages
     5:19.03.9~3-0~ubuntu-focal 500
        500 focal/stable amd64 Packages

The above output indicates that “docker-ce” from the Docker repository from Ubuntu 20.04 will be installed on your system.

Last install Roustabout (Community Edition):

$ sudo apt install docker-ce

In order to verify that docker has been successfully installed and running correctly, please run the following command:

$ sudo systemctl status docker

The output will show 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
   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

Check docker service status

Docker service status

You have just successfully installed docker on your Ubuntu computer. In the following part of this tutorial, we will discuss in depth how to use the docker command.

Execute docker without sudo privileges

By default, you must run docker commands with sudo privileges or by users in the docker group. If you try to run the docker command without sudo or without the user rights of the docker group, you will get the following error:

docker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?.
See 'docker run --help'.

The solution is to add the user to the “docker” group:

$ sudo usermod -aG docker 

Log out of the current session and log in again as an application member. Verify that the user is in the “docker” group by running the following command:

$ groups
ubuntu sudo docker

From now on, you can run the docker command without “sudo”.

Docker commands

This section will explain how to use the docker command. Before going into details, let’s take a look at the syntax of the “docker” command:

$ docker [options] [sub-commands] [arguments]

To list all available subcommands of docker, run:

$ docker

There are many options, subcommands and parameters to remember. 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

  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 image

The Docker image is composed of source code, libraries, and applications as all necessary dependencies for the container to run. It is possible to build Docker images from scratch, but you can extract these images from the container registry. You can store images in the registry and download them with unique names. By default, Docker’s container registry is Docker Hub.

In order to check whether the image can be pulled from Docker Center, You can run the following command:

$ docker run hello-world

As you can see in the following output: docker cannot find the “hello-world” image locally, it extracted the image from Docker Hub. After downloading the image, docker creates a container from the image. The application in the container will be executed with the following 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.
 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

To search for images on Docker Hub, you can use the’docker’ command with the’search’ subcommand. Suppose you want to search for “mountain” images, run:

$ docker search alpine

Docker searches for high mountains

docker search command

As you can see, the output returns a list of all images named “alpine”.

In addition, you can run the command “Docker picture pull” Then enter the picture name. E.g:

$ 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

In addition, if you want to list all downloaded images, run the following command:

$ docker image ls

The output will display the detailed information of the image, including: repository, label, image ID, creation 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 Delete docker Picture, you can run the following command:

$ docker image rm 


$ docker image rm 

Docker container

Containers are running instances of docker images. The container executes the runtime of a single application or process. You can interact with the container and start/stop or delete it by running the “docker container” subcommand.

For example, to start the docker container based on the “Ubuntu” image, run:

[email protected]:~$ docker container run ubuntu

After downloading the newer image of Ubuntu from Docker Hub and starting the Ubuntu container, it will return to the system command prompt because the Ubuntu container has stopped immediately after starting.

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
[email protected]:~$

In order to interact with the Ubuntu container through the command line, you can use the following command to start the container -it Options:

$ docker container run -it ubuntu

You have now entered the container and accessed its interactive shell.

[email protected]:/# 

So far, you have run many containers on the Ubuntu system. To list all active (running) containers, run the following command:

$ docker container ls

CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

The result is an empty list, so there are no containers running. If you want to list all containers: active and inactive, run:

$ docker container ls -a

The output is similar to:

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/delete the container, run:

$ docker container rm 

in conclusion

In this tutorial, we learned how to install Docker on Ubuntu 20.04 and how to use images and containers through examples. They are the foundation of the Docker concept and are usually used by developers.

Thanks for reading, please leave your suggestions in the comment section below.