Docker is a software platform that allows you to build, test, and deploy applications quickly. Docker packages software into standardized units called containers that have everything the software needs to run including libraries, system tools, code, and runtime. Using Docker, you can quickly deploy and scale applications into any environment and know your code will run.

• Docker Engine − It is used for building Docker images and creating Docker containers.

• Docker Hub − This is the registry which is used to host various Docker images.

• Docker Compose − This is used to define applications using multiple Docker containers.

  Installation:

  Linux

• uname

  This method returns the system information about the Linux system.

  Syntax

    uname -a
  

  Options

  a − This is used to ensure that the system information is returned.

  Return Value

  This method returns the following information on the Linux system −

  • kernel name

  • node name

  • kernel release

  • kernel version

  • machine

  • processor

  • hardware platform

  • operating system

sudo apt-get update -y

• Step 3 − The next step is to install the necessary certificates that will be required to work with the Docker site later on to download the necessary Docker packages.

•   sudo apt-get install apt-transport-https ca-certificates
  

  apt-get update command

• sudo apt-get install docker.io

• Docker Hub is a registry service on the cloud that allows you to download Docker images that are built by other communities. You can also upload your own Docker built images to Docker hub. In this chapter, we will see how to download and the use the Jenkins Docker image from Docker hub.

  The official site for Docker hub is − https://www.docker.com/community-edition#/add_ons

  Step 1 − First you need to do a simple sign-up on Docker hub.

  

  Step 2 − Once you have signed up, you will be logged into Docker Hub.

  

  Step 3 − Next, let’s browse and find the Jenkins image.

  

  Step 4 − If you scroll down on the same page, you can see the Docker pull command. This will be used to download the Jenkins image onto the local Ubuntu server.

  

  Step 5 − Now, go to the Ubuntu server and run the following command −

    sudo docker pull jenkins
  

  

  To run Jenkins, you need to run the following command −

    sudo docker run -p 8080:8080 -p 50000:50000 jenkins
  

  Note the following points about the above sudo command −

• We are using the sudo command to ensure it runs with root access.

• Here, jenkins is the name of the image we want to download from Docker hub and install on our Ubuntu machine.

• -p is used to map the port number of the internal Docker image to our main Ubuntu server so that we can access the container accordingly.

    docker run hello-world
  

  • The Docker command is specific and tells the Docker program on the Operating System that something needs to be done.

  • The run command is used to mention that we want to create an instance of an image, which is then called a container.

  • Finally, "hello-world" represents the image from which the container is made.

Now let’s look at how we can use the CentOS image available in Docker Hub to run CentOS on our Ubuntu machine. We can do this by executing the following command on our Ubuntu machine −

    sudo docker run -it centos /bin/bash

Note the following points about the above sudo command −

• We are using the sudo command to ensure that it runs with root access.

• Here, centos is the name of the image we want to download from Docker Hub and install on our Ubuntu machine.

• ─it is used to mention that we want to run in interactive mode.

• /bin/bash is used to run the bash shell once CentOS is up and running.

Displaying Docker Images

To see the list of Docker images on the system, you can issue the following command.

    docker images

Downloading Docker Images

Images can be downloaded from Docker Hub using the Docker run command. Let’s see in detail how we can do this.

Syntax

The following syntax is used to run a command in a Docker container.

    docker run image

Removing Docker Images

The Docker images on the system can be removed via the docker rmi command. Let’s look at this command in more detail.

    docker rmi

This command is used to remove Docker images.

Syntax

    docker rmi ImageID

Return Value

The output will show only the Image ID’s of the images on the Docker host.

Example

    sudo docker images -q

docker inspect

This command is used see the details of an image or container.

Syntax

    docker inspect Repository

Options

• Repository − This is the name of the Image.

  Running a Container

  Running of containers is managed with the Docker run command. To run a container in an interactive mode, first launch the Docker container.

    sudo docker run –it centos /bin/bash
  

  Then hit Crtl+p and you will return to your OS shell.

• Listing of Containers

  One can list all of the containers on the machine via the docker ps command. This command is used to return the currently running containers.

    docker ps
  

    sudo docker ps -a
  

  docker history

  With this command, you can see all the commands that were run with an image via a container.

  Syntax

    docker history ImageID
  

  docker top

  With this command, you can see the top processes within a container.

  Syntax

    docker top ContainerID
  

  docker stop

  This command is used to stop a running container.

  Syntax

    docker stop ContainerID
  

  docker rm

  This command is used to delete a container.

  Syntax

    docker rm ContainerID
  

  docker stats

  This command is used to provide the statistics of a running container.

  Syntax

    docker stats ContainerID
  

  docker attach

  This command is used to attach to a running container.

  Syntax

    docker attach ContainerID
  

  docker pause

  This command is used to pause the processes in a running container.

  Syntax

    docker pause ContainerID
  

  docker unpause

  This command is used to unpause the processes in a running container.

  Syntax

    docker unpause ContainerID
  

  docker kill

  This command is used to kill the processes in a running container.

  Syntax

    docker kill ContainerID
  

  Docker – Container Lifecycle

  The following illustration explains the entire lifecycle of a Docker container.

  

  Architecture of virtualization.

  

  • service docker stop

    This command is used to stop the Docker daemon process.

    Syntax

      service docker stop
    

      sudo docker run –it centos /bin/bash
    

    We used this command to create a new container and then used the Ctrl+P+Q command to exit out of the container. It ensures that the container still exists even after we exit from the container.

  • A Docker File is a simple text file with instructions on how to build your images.

    The following steps explain how you should go about creating a Docker File.

    Step 1 − Create a file called Docker File and edit it using vim. Please note that the name of the file has to be "Dockerfile" with "D" as capital.

  • Step 2 − Build your Docker File using the following instructions.

      #This is a sample Image 
      FROM ubuntu 
      MAINTAINER demousr@gmail.com 
    
      RUN apt-get update 
      RUN apt-get install –y nginx 
      CMD [“echo”,”Image created”]
    

• The first line "#This is a sample Image" is a comment. You can add comments to the Docker File with the help of the # command

• The next line has to start with the FROM keyword. It tells docker, from which base image you want to base your image from. In our example, we are creating an image from the ubuntu image.

• The next command is the person who is going to maintain this image. Here you specify the MAINTAINER keyword and just mention the email ID.

• The RUN command is used to run instructions against the image. In our case, we first update our Ubuntu system and then install the nginx server on our ubuntu image.

• The last command is used to display a message to the user.

• docker build

  This method allows the users to build their own Docker images.

  Syntax

    docker build  -t ImageName:TagName dir
  

  Options

  • -t − is to mention a tag to the image

  • ImageName − This is the name you want to give to your image.

  • TagName − This is the tag you want to give to your image.

  • Dir − The directory where the Docker File is present.

Docker Hub:

Step 1 − Log into Docker Hub and create your repository. This is the repository where your image will be stored. Go to https://hub.docker.com/ and log in with your credentials.

• Step 1 − Log into Docker Hub and create your repository. This is the repository where your image will be stored. Go to https://hub.docker.com/ and log in with your credentials.

  

  Step 2 − Click the button "Create Repository" on the above screen and create a repository with the name demorep. Make sure that the visibility of the repository is public.

  

  Once the repository is created, make a note of the pull command which is attached to the repository.

  

  The pull command which will be used in our repository is as follows −

    docker pull demousr/demorep
  

  Step 3 − Now go back to the Docker Host. Here we need to tag our myimage to the new repository created in Docker Hub. We can do this via the Docker tag command.

  We will learn more about this tag command later in this chapter.

  Step 4 − Issue the Docker login command to login into the Docker Hub repository from the command prompt. The Docker login command will prompt you for the username and password to the Docker Hub repository.

  

  Step 5 − Once the image has been tagged, it’s now time to push the image to the Docker Hub repository. We can do this via the Docker push command. We will learn more about this command later in this chapter.

  docker tag

  This method allows one to tag an image to the relevant repository.

  Syntax

    docker tag imageID Repositoryname
  

  Options

  • imageID − This is the ImageID which needs to be tagged to the repository.

  • Repositoryname − This is the repository name to which the ImageID needs to be tagged to.

Return Value

None

Example

    sudo docker tag ab0c1d3744dd demousr/demorep:1.0

Output

A sample output of the above example is given below.

docker push

This method allows one to push images to the Docker Hub.

Syntax

    docker push Repositoryname

Options

• Repositoryname − This is the repository name which needs to be pushed to the Docker Hub.

Return Value

The long ID of the repository pushed to Docker Hub.

Example

    sudo docker push demousr/demorep:1.0

Output

If you go back to the Docker Hub page and go to your repository, you will see the tag name in the repository.

Step 1 − First, you need to do a simple sign-up on Docker Hub.

Step 2 − Once you have signed up, you will be logged into Docker Hub.

Step 3 − Next, let’s browse and find the Jenkins image.

Step 4 − If you scroll down on the same page, you can see the Docker pull command. This will be used to download the Jenkins Image onto the local Ubuntu server.

Step 5 − Now go to the Ubuntu server and run the command −

sudo docker pull jenkins

Step 6 − To understand what ports are exposed by the container, you should use the Docker inspect command to inspect the image.

Let’s now learn more about this inspect command.

docker inspect

This method allows one to return low-level information on the container or image.

Syntax

docker inspect Container/Image

Options

• Container/Image − The container or image to inspect

Return Value

The low-level information of the image or container in JSON format.

Example

sudo docker inspect jenkins

Output

The output of the inspect command gives a JSON output. If we observe the output, we can see that there is a section of "ExposedPorts" and see that there are two ports mentioned. One is the data port of 8080 and the other is the control port of 50000.

To run Jenkins and map the ports, you need to change the Docker run command and add the ‘p’ option which specifies the port mapping. So, you need to run the following command −

sudo docker run -p 8080:8080 -p 50000:50000 jenkins

The left-hand side of the port number mapping is the Docker host port to map to and the right-hand side is the Docker container port number.

When you open the browser and navigate to the Docker host on port 8080, you will see Jenkins up and running.

Docker - Private Registries

Step 1 − Use the Docker run command to download the private registry. This can be done using the following command.

sudo docker run –d –p 5000:5000 –-name registry registry:2

The following points need to be noted about the above command −

• Registry is the container managed by Docker which can be used to host private repositories.

• The port number exposed by the container is 5000. Hence with the –p command, we are mapping the same port number to the 5000 port number on our localhost.

• We are just tagging the registry container as “2”, to differentiate it on the Docker host.

• The –d option is used to run the container in detached mode. This is so that the container can run in the background

Step 2 − Let’s do a docker ps to see that the registry container is indeed running.

We have now confirmed that the registry container is indeed running.

Step 3 − Now let’s tag one of our existing images so that we can push it to our local repository. In our example, since we have the centos image available locally, we are going to tag it to our private repository and add a tag name of centos.

sudo docker tag 67591570dd29 localhost:5000/centos

The following points need to be noted about the above command −

• 67591570dd29 refers to the Image ID for the centos image.

• localhost:5000 is the location of our private repository.

• We are tagging the repository name as centos in our private repository.

Step 4 − Now let’s use the Docker push command to push the repository to our private repository.

sudo docker push localhost:5000/centos

Here, we are pushing the centos image to the private repository hosted at localhost:5000.

Step 5 − Now let’s delete the local images we have for centos using the docker rmi commands. We can then download the required centos image from our private repository.

sudo docker rmi centos:latest 
sudo docker rmi 67591570dd29

Step 6 − Now that we don’t have any centos images on our local machine, we can now use the following Docker pull command to pull the centos image from our private repository.

sudo docker pull localhost:5000/centos

Here, we are pulling the centos image to the private repository hosted at localhost:5000.

Docker - Building a Web Server Docker File

Step 1 − The first step is to build our Docker File. Let’s use vim and create a Docker File with the following information.

FROM ubuntu 
RUN apt-get update 
RUN apt-get install –y apache2 
RUN apt-get install –y apache2-utils 
RUN apt-get clean 
EXPOSE 80 CMD [“apache2ctl”, “-D”, “FOREGROUND”]

Step 2 − Run the Docker build command to build the Docker file. It can be done using the following command −

sudo docker build –t=”mywebserver” .

Step 3 − Now that the web server file has been built, it’s now time to create a container from the image. We can do this with the Docker run command.

sudo docker run –d –p 80:80 mywebserver

The following points need to be noted about the above command −

• The port number exposed by the container is 80. Hence with the –p command, we are mapping the same port number to the 80 port number on our localhost.

• The –d option is used to run the container in detached mode. This is so that the container can run in the background.

  Docker - Instruction Commands

• CMD Instruction

  This command is used to execute a command at runtime when the container is executed.

  Syntax

    CMD command param1
  

  Options

  • command − This is the command to run when the container is launched.

  • param1 − This is the parameter entered to the command.

Return Value

The command will execute accordingly.

Example

In our example, we will enter a simple Hello World echo in our Docker File and create an image and launch a container from it.

Step 1 − Build the Docker File with the following commands −

    FROM ubuntu 
    MAINTAINER demousr@gmail.com 
    CMD [“echo” , “hello world”]

ENTRYPOINT

This command can also be used to execute commands at runtime for the container. But we can be more flexible with the ENTRYPOINT command.

Syntax

    ENTRYPOINT command param1

Options

• command − This is the command to run when the container is launched.

• param1 − This is the parameter entered into the command.

Step 1 − Build the Docker File with the following commands −

    FROM ubuntu 
    MAINTAINER demousr@gmail.com 
    ENTRYPOINT [“echo”]

Step 2 − Build the image using the Docker build command.

Step 3 − Run a container from the image.

ENV

This command is used to set environment variables in the container.

Syntax

    ENV key value

Options

• Key − This is the key for the environment variable.

• value − This is the value for the environment variable.

Return Value

The command will execute accordingly.

Example

In our example, we will enter a simple echo command in our Docker File and create an image and launch a container from it.

Step 1 − Build the Docker File with the following commands −

    FROM ubuntu 
    MAINTAINER demousr@gmail.com 
    ENV var1=Tutorial var2=point

Step 2 − Build the image using the Docker build command.

Step 3 − Run a container from the image.

Step 4 − Finally, execute the env command to see the environment variables.

WORKDIR

This command is used to set the working directory of the container.

Syntax

    WORKDIR dirname

Options

• dirname − The new working directory. If the directory does not exist, it will be added.

Return Value

The command will execute accordingly.

Example

In our example, we will enter a simple echo command in our Docker File and create an image and launch a container from it.

Step 1 − Build the Docker File with the following commands −

    FROM ubuntu 
    MAINTAINER demousr@gmail.com 
    WORKDIR /newtemp 
    CMD pwd

Step 2 − Build the image using the Docker build command.

Step 3 − Run a container from the image.

Docker - Container Linking

Container Linking allows multiple containers to link with each other. It is a better option than exposing ports. Let’s go step by step and learn how it works.

Step 1 − Download the Jenkins image, if it is not already present, using the Jenkins pull command.

• Step 1 − Download the Jenkins image, if it is not already present, using the Jenkins pull command.

  

  Step 2 − Once the image is available, run the container, but this time, you can specify a name to the container by using the –-name option. This will be our source container.

  

  Step 3 − Next, it is time to launch the destination container, but this time, we will link it with our source container. For our destination container, we will use the standard Ubuntu image.

  

  When you do a docker ps, you will see both the containers running.

  Step 4 − Now, attach to the receiving container.

  

  Then run the env command. You will notice new variables for linking with the source container.

  

  

  Docker - Storage

• Storage Drivers

  Docker has multiple storage drivers that allow one to work with the underlying storage devices. The following table shows the different storage drivers along with the technology used for the storage drivers.

  | Technology | Storage Driver | | --- | --- | | OverlayFS | overlay or overlay2 | | AUFS | aufs | | Btrfs | brtfs | | Device Manager | devicemanager | | VFS | vfs | | ZFS | zfs |

  Data Volumes

  In Docker, you have a separate volume that can shared across containers. These are known as data volumes. Some of the features of data volume are −

  • They are initialized when the container is created.

  • They can be shared and also reused amongst many containers.

  • Any changes to the volume itself can be made directly.

  • They exist even after the container is deleted.

Let’s look at our Jenkins container. Let’s do a docker inspect to see the details of this image. We can issue the following command to write the output of the docker inspect command to a text file and then view the file accordingly.

    sudo docker inspect Jenkins > tmp.txt

When you view the text file using the more command, you will see an entry as JENKINS_HOME=/var/Jenkins_home.

This is the mapping that is done within the container via the Jenkins image.

Now suppose you wanted to map the volume in the container to a local volume, then you need to specify the –v option when launching the container. An example is shown below −

    sudo docker run –d –v /home/demo:/var/jenkins_home –p 8080:8080 –p 50000:50000 jenkins

The –v option is used to map the volume in the container which is /var/jenkins_home to a location on our Docker Host which is /home/demo.

Now if you go to the /home/demo location on your Docker Host after launching your container, you will see all the container files present there.

Changing the Storage Driver for a Container

If you wanted to change to the storage driver used for a container, you can do so when launching the container. This can be done by using the –volume-driver parameter when using the docker run command. An example is given below −

    sudo docker run –d --volume-driver=flocker 
       –v /home/demo:/var/jenkins_home –p 8080:8080 –p 50000:50000 jenkins

The –volume-driver option is used to specify another storage driver for the container.

To confirm that the driver has been changed, first let’s use the docker ps command to see the running containers and get the container ID. So, issue the following command first −

    sudo docker ps

Then issue a docker inspect against the container and put the output in a text file using the command.

    sudo docker inspect 9bffb1bfebee > temp.txt

If you browse through the text file and go to the line which says VolumeDriver, you will see that the driver name has been changed.

Creating a Volume

A volume can be created beforehand using the docker command. Let’s learn more about this command.

Syntax

    docker volume create –-name=volumename –-opt options

Options

• name − This is the name of the volume which needs to be created.

• opt − These are options you can provide while creating the volume.

Return Value

The command will output the name of the volume created.

Example

    sudo docker volume create –-name = demo –opt o = size = 100m

In the above command, we are creating a volume of size 100MB and with a name of demo.

Output

The output of the above command is shown below −

Listing all the Volumes

You can also list all the docker volumes on a docker host. More details on this command is given below −

Syntax

    docker volume ls

Options

None

Return Value

The command will output all the volumes on the docker host.

Example

    sudo docker volume ls

Output

The output of the above command is shown below −

Docker - Networking

Docker takes care of the networking aspects so that the containers can communicate with other containers and also with the Docker Host. If you do an ifconfig on the Docker Host, you will see the Docker Ethernet adapter. This adapter is created when Docker is installed on the Docker Host.

• This is a bridge between the Docker Host and the Linux Host. Now let’s look at some commands associated with networking in Docker.

• Listing All Docker Networks

  This command can be used to list all the networks associated with Docker on the host.

  Syntax

    docker network ls
  

  Inspecting a Docker network

  If you want to see more details on the network associated with Docker, you can use the Docker network inspect command.

  Syntax

    docker network inspect networkname
  

  Return Value

  The command will output all the details about the network.

  Example

    sudo docker network inspect bridge
  

    sudo docker run –it ubuntu:latest /bin/bash
  

  Now if we inspect our network name via the following command, you will now see that the container is attached to the bridge.

    sudo docker network inspect bridge
  

  Creating Your Own New Network

  One can create a network in Docker before launching containers. This can be done with the following command −

  Syntax

    docker network create –-driver drivername name
  

  • drivername − This is the name used for the network driver.

  • name − This is the name given to the network.

  • You can now attach the new network when launching the container. So let’s spin up an Ubuntu container with the following command −

      sudo docker run –it –network=new_nw ubuntu:latest /bin/bash
    

    And now when you inspect the network via the following command, you will see the container attached to the network.

      sudo docker network inspect new_nw