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Docker Cheat Sheet

Why

Why Should I Care (For Developers)

"Docker interests me because it allows simple environment isolation and repeatability. I can create a run-time environment once, package it up, then run it again on any other machine. Furthermore, everything that runs in that environment is isolated from the underlying host (much like a virtual machine). And best of all, everything is fast and simple."

TL;DR, I just want a dev environment

Prequisites

Use Homebrew.

ruby -e "$(curl -fsSL https://raw.github.com/mxcl/homebrew/go)"

Installation

Install VirtualBox and Vagrant using Brew Cask.

brew tap phinze/homebrew-cask
brew install brew-cask
brew cask install virtualbox
brew cask install vagrant

We use the pre-built vagrant box: http://blog.phusion.nl/2013/11/08/docker-friendly-vagrant-boxes/

mkdir mydockerbox
cd mydockerbox
vagrant init docker https://oss-binaries.phusionpassenger.com/vagrant/boxes/ubuntu-12.04.3-amd64-vbox.box
vagrant up
vagrant ssh

In the VM:

sudo su -
sh -c "curl https://get.docker.io/gpg | apt-key add -"
sh -c "echo deb http://get.docker.io/ubuntu docker main > /etc/apt/sources.list.d/docker.list"
apt-get update
apt-get install -y lxc-docker

Verify:

docker run -i -t ubuntu /bin/bash

That's it, you have a running Docker container.

Containers

Your basic isolated Docker process. Containers are to Virtual Machines as threads are to processes. Or you can think of them as chroots on steroids.

Some common misconceptions it's worth correcting:

  • Containers are not transient. docker run doesn't do what you think.
  • Containers are not limited to running a single command or process. It's just encouraged.

Lifecycle

If you want to run and then interact with a container, docker start then docker attach to get in.

If you truly want a transient container, docker run -rm will remove the container after it stops.

If you just want to poke around in an image, docker run -rm -t -i <myshell> to open a tty.

If you just want to map a directory on the host to a docker container, docker run -v $HOSTDIR:$DOCKERDIR

Info

docker ps -a shows running and stopped containers.

Import / Export

Images

Images are just templates for docker containers.

Lifecycle

  • docker images shows all images.
  • docker import creates an image from a tarball.
  • docker build creates image from Dockerfile.
  • docker commit creates image from a container.
  • docker rmi removes an image.
  • docker insert inserts a file from URL into image. (kind of odd, you'd think images would be immutable after create)
  • docker load loads an image from a tar archive as STDIN, including images and tags (as of 0.7).
  • docker save saves an image to a tar archive stream to STDOUT with all parent layers, tags & versions (as of 0.7).

docker import and docker commit only set up the filesystem, not Dockerfile info like CMD or ENTRYPOINT or EXPOSE. See bug.

Info

Registry & Repository

A repository is a hosted collection of tagged images that together create the file system for a container.

A registry is a host -- a server that stores repositories and provides an HTTP API for managing the uploading and downloading of repositories.

Docker.io hosts its own index to a central registry which contains a large number of repositories.

Dockerfile

The configuration file. Sets up a Docker container when you run docker build on it. Vastly preferable to docker commit.

Best to look at http://github.com/wsargent/docker-devenv and the best practices for more details.

Layers

The versioned filesystem in Docker is based on layers. They're like git commits or changesets for filesystems.

Links

Links are how Docker containers talk to each other. Linking into Redis is the only real example.

If you have a docker container with the name CONTAINER (specified by docker run -name CONTAINER) and in the Dockerfile, it has an exposed port:

EXPOSE 1337

Then if we create another container called LINKED like so:

docker run -d -link CONTAINER:ALIAS -name LINKED user/wordpress

Then the exposed ports and aliases of CONTAINER will show up in LINKED with the following environment variables:

$ALIAS_PORT_1337_TCP_PORT
$ALIAS_PORT_1337_TCP_ADDR

And you can connect to it that way.

To delete links, use docker rm -link .

Volumes

Docker volumes are free-floating filesystems. They don't have to be connected to a particular container.

Volumes are useful in situations where you can't use links (which are TCP/IP only). For instance, if you need to have two docker instances communicate by leaving stuff on the filesystem.

You can mount them in several docker containers at once, using docker run -volume-from

See advanced volumes for more details.

Tips

Sources:

Last Ids

alias dl='docker ps -l -q'
docker run ubuntu echo hello world
docker commit `dl` helloworld

Commit with command (needs Dockerfile)

docker commit -run='{"Cmd":["postgres", "-too -many -opts"]}' `dl` postgres

Get IP address

docker inspect `dl` | grep IPAddress | cut -d '"' -f 4

or

wget http://stedolan.github.io/jq/download/source/jq-1.3.tar.gz
tar xzvf jq-1.3.tar.gz
cd jq-1.3
./configure && make && sudo make install
docker inspect `dl` | jq -r '.[0].NetworkSettings.IPAddress'

Get Environment Settings

docker run -rm ubuntu env 

Delete old containers

docker ps -a | grep 'weeks ago' | awk '{print $1}' | xargs docker rm

Delete stopped containers

docker rm `docker ps -a -q`

Show image dependencies

docker images -viz | dot -Tpng -o docker.png
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