Thursday, February 26, 2015

Doing something simple with Docker

A couple posts back I walked you through setting up an Ubuntu VM, and installing the latest version of Docker.  Then, I left you hanging.

Docker is interesting.  These container things are a cross between a VM and a user process.  There is still a base OS there (of some type) to bootstrap the application.  What I thought was interesting when I first poked Docker is that each container is a network isolation zone.

Docker has a great tutorial:

And that was fine.  I did something.  But I really didn't understand it until I tried to really use it.

What does it take to get an application into an image and run it?  And this Docker Hub that is chock full of images, and Dockerfile - what is that?

Lets begin with an easy example as we get the language down.

I want to run a Docker container.  I want the OS in this Docker container to be Ubuntu (yes, Ubuntu within Ubuntu).

Returning to my Ubuntu VM from before, I logon as my user and try a couple Docker commands:
sudo docker images  - this lists the Docker images that have been built / downloaded to this machine and these images are used to run containers.

Notice that language - a container is a running instance of an image.  The image is analogous to a virtual disk with something in it.  The image consumes space on the disk of my Ubuntu VM.

sudo docker ps - If you have been around Linux before you have run across ps - processes.  The ps command lists the containers, and containers being processes only exist when they run.

Enough of that, lets get confusing and run an instance of Ubuntu on Ubuntu in the same command window where I ran my Docker command (the key here, watch the bouncing command prompt).

sudo docker run -i -t ubuntu:latest /bin/bash

Like most commands, lets read this on from right to left (not left to right).
Run a BASH shell, in the image 'ubuntu:latest', run a tty session, keep STDIN open (send input).
What this accomplishes is: the image is checked if it is local, if not it is pulled from the hub.  Then open the tty in the console session (where I ran the command) and run bash.

Notice when you do this that your prompt changed at your console.  That console window is now the container process.  What you do now is inside the container process and image.

I you really want to realize that you are somewhere else, type ifconfig at the prompt.  By default you should get a class B address in the 172 range.  There will be more later on this, but right now that VM can get out, but there are no incoming ports open to it.

When you are ready to get out of the image use exit
This actually stops the container in this case.  Since it closes the tty.

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