Docker: HelloWorld Application

Step 1: Lets pull hello-world image to our local system by executing ‘docker run hello-world’ command.

‘docker run’ command starts a new container with the specified image.

$docker run hello-world
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
1b930d010525: Pull complete 
Digest: sha256:92695bc579f31df7a63da6922075d0666e565ceccad16b59c3374d2cf4e8e50e
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

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

As you see the output, docker tries to find the ‘hello-world’ image in your computer, since it is not available, it pulls the image from internet. Once the image is downloaded, it ran the image and prints the output of the container ‘Hello from Docker!’ in console.

You can check the same by running the command ‘docker ps -a’.

$docker ps -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                     PORTS                    NAMES
ecc56427245c        hello-world         "/hello"                 3 minutes ago       Exited (0) 3 minutes ago                            dazzling_dewdney

As you see the output, the image ‘hello-world’ ran 3 minutes ago.

You can get the same using ‘docker info’ command.

$docker info
Containers: 1
 Running: 0
 Paused: 0
 Stopped: 1
Images: 1
…..
…..

As you see the output, there is 1 container that is in stopped state.

What happened internally?

When you execute the command ‘docker run hello-world’, docker client receives this command and calls the respective docker daemon API.

Docker daemon checks whether the image ‘hello-world’ is available in localhost or not. If the image is not available, then docker daemon search the image in ‘https://hub.docker.com/’ and pull the image from docker hub to local system.

Once the image is available in local system, docker daemon starts new container with the image.

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