In these series of Docker tutorials, i will walk you through a hands on experimentation with Docker. The operating system I am working on is Ubuntu 16.04.

Docker is a containerization technology which allows deployment of applications in containers. Its advantage is speed, a docker container hosting an application would be up and running in a few milliseconds.

As opposed to Virtual machines, containers run on top of the host OS. They share the host kernel. Thus you can only run a Linux container on a Linux host or machine.

Docker Installation – use this link for instructions on how to install Docker.

Installation Limitation – Docker runs on 64-bit OS only and it supports Linux kernel version 3.10 and above. You can verify this using the commands below –

root@lindell:~# arch
x86_64
root@lindell:~# uname -r
4.4.0-47-generic

Docker – terminology

    Images – are the building blocks of Docker. Once created or built, they can be shared, updated and used to launch containers. No image, no containers.

    Containers – are images in action. Containers give images life, containers are image plus all the ecosystem the Operating system need to run the application.

    Registry – where images are stored. They can be public or private. DockerHub is a typical example of public registry.

    Data volumes – persistent storage used by containers.
    Dockerfile – file containing instructions to be read by Docker for building a Docker image.

    Node – physical or virtual machine running Docker engine.

Our first Docker container
After installing docker and making sure that the Docker engine is running, run the commands below to check the Docker images(‘docker images’) available or if any Docker containers are running(‘docker ps’). Both commands should not return any results if this is a first time installation.

root@lindell:~# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
root@lindell:~# docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED
             STATUS              PORTS               NAMES

The next step is to get a Docker image from Docker Hub. For security reasons, we are going to use only official images –

root@lindell:~# docker search --filter=is-official=true ubuntu
NAME                 DESCRIPTION                                     STARS     OFFICIAL   AUTOMATED
ubuntu               Ubuntu is a Debian-based Linux operating s...   5238      [OK]
ubuntu-upstart       Upstart is an event-based replacement for ...   69        [OK]
ubuntu-debootstrap   debootstrap --variant=minbase --components...   27        [OK]


root@lindell:~# docker run -ti ubuntu /bin/bash
Unable to find image 'ubuntu:latest' locally
latest: Pulling from library/ubuntu

b3e1c725a85f: Pull complete
4daad8bdde31: Pull complete
63fe8c0068a8: Pull complete
4a70713c436f: Pull complete
bd842a2105a8: Pull complete
Digest: sha256:7a64bc9c8843b0a8c8b8a7e4715b7615e4e1b0d8ca3c7e7a76ec8250899c397a
Status: Downloaded newer image for ubuntu:latest

root@d1b13e2c3d3f:/# docker images
bash: docker: command not found

root@d1b13e2c3d3f:/# hostname -f
d1b13e2c3d3f

root@d1b13e2c3d3f:/# uname -r
4.4.0-47-generic

We just downloaded an official Ubuntu image and started an Ubuntu container by running /bin/bash inside the newly started container. The ‘-ti’ option runs bash interactively(-i) by allocating a pseudo-TTY(-t).

Note that – the kernel version on the container is the same as the host’s kernel version. During first run, Docker will try to find Ubuntu image in our local storage, if it can’t find it, it downloads it from Docker Hub. On next runs, starting the containers will be much faster.

If we check the images and processes running now –

root@lindell:~# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
ubuntu              latest              104bec311bcd        5 days ago          129 MB
root@lindell:~# docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED              STATUS              PORTS               NAMES
d1b13e2c3d3f        ubuntu              "/bin/bash"         About a minute ago   Up About a minute

At this point, if we exit from the container, docker ps will no longer show the container as it has been terminated. We use ‘docker ps -a’ instead to view it and then use ‘docker start’ command to start the container –

root@d1b13e2c3d3f:/# exit
exit


root@lindell:~# docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

root@lindell:~# docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                     PORTS               NAMES
d1b13e2c3d3f        ubuntu              "/bin/bash"         5 minutes ago       Exited (0) 5 seconds ago

root@lindell:~# docker start d1b13e2c3d3f
root@lindell:~# docker exec -ti d1b13 /bin/bash
root@d1b13e2c3d3f:/# uptime
 01:39:46 up  1:18,  0 users,  load average: 0.39, 0.39, 0.37
root@d1b13e2c3d3f:/#

On Part 2 of quick introduction to Docker, we will walk through using Dockerfile to automate image creation. We will see how quickly we can go from development to deployment.

You might also find some of the questions I answered in Stackoverflow about Docker.