Brian Morearty recently gave a lightning talk at Twitter HQ about Docker. You should really check out his slides, they are fun and have a lot of character. He also runs a hands-on training course for Docker.

But as a reference, here are the 15 tips he suggests:

1. Getting the id of the last-run container.

  • You could do this:
$ ID=$(docker run ubuntu echo hello world)
hello world
$ docker commit $ID hello world
  • But you have to keep assigning IDs. Try this instead:
$ alias dl='docker ps -l -q'
$ docker run ubuntu echo hello world
hello world
$ dl
$ docker commit 'dl' hello world

2. Why you always want a Dockerfile, even if you install everything in the shell.

  • Docker commit commands can get unwieldy:
$ docker run -i -t ubuntu bash
root@db0c3978abf8:/# apt-get install postgresql -y
root@db0c3978abf8:/# exit
$ docker commit -run='{"Cmd":["postgres", "-too -many -opts"]}' `dl` postgres # ugly long command, easier to do within a Dockerfile
  • Instead, make a wee little Dockerfile that is FROM the image you made interactively
  • There you can set CMD, ENTRYPOINT, VOLUME, etc.

3. Su-su-sudo

  • Here is how to run Docker without root:
$ sudo groupadd docker
$ sudo gpasswd -a myusername docker
$ sudo service docker restart
$ exit

4. Take out the garbage

  • Here is how to delete all stopped containers (useful if you need to re-run named containers):
$ docker rm $(docker ps -a -q)
  • Tries, but conveniently fails to delete still-running containers.

5. jq – the gangsta way to parse docker inspect's output

  • Here is how to grab the IP address in one line of unix:
$ docker inspect `dl` | grep IPAddress | cut -d '"' -f 4
$ docker inspect `dl` | jq -r '.[0].NetworkSettings.IPAddress'

6. What environment variables does an image have?

  • This is especially nice when using docker run --link to connect containers:
$ docker run ubuntu env
RUN vs. CMD instruction

7. Here is the difference between RUN and CMD in the Dockerfile:

FROM an_existing_image
MAINTAINER my_email_address
\# docker build will execute these commands:
RUN apt-get update
RUN apt-get install softwares
\# docker run will run this command by default:
CMD ["softwares"]

8. CMD vs ENTRYPOINT instruction

  • CMD arguments can be over-ridden:
cat Dockerfile
FROM ubuntu
CMD ["echo"]
$ docker run imagename echo hello
  • ENTRYPOINT arguments can NOT be over-ridden:
cat Dockerfile
FROM ubuntu
$ docker run imagename echo hello
echo hello

9. Does a Docker container have its own IP address?

  • Sim. It is like a process with an internal IP address
$ ip -4 -o addr show eth0
2: eth0 inet
$ docker run ubuntu ip -4 -o addr show eth0
149: eth0 inet

10. Docker's architecture

  • Docker has a thin CLI client that has binaries for Linux and OS X, and a REST server daemon with binaries for Linux over UNIX socket
$ nc -U /var/run/docker.sock
GET /images/json HTTP/1.1
HTTP/1.1 200 OK
Content-Type: application/json
Date: Tue, 05 Nov 2013 23:18:09 GMT
Transfer-Encoding: chunked

11. Graph the dependencies among your images

  • Here is how to generate an image dependency diagram
$ docker images -viz | dot -Tpng -o docker.png
  • Then to see it, run this command and go to
$ python -m SimpleHTTPServer

12. Where does Docker store everything?

  • In /var/lib/docker… images are stored in the graph folder
$ sudo -i
\# cd /var/lib/docker
\# ls -F
containers/ graph/ repositories volumes/

13. Docker source code: Go, Go, Go, Grabote

  • docker/docker.go – the CLI
  • – the REST API router
  • – the Docker server

14. RUN some daemons, exit the container. What happens?

  • When you docker run -i -t it will override the RUN you have in your Dockerfile
$ cat Dockerfile
FROM ubuntu
RUN pg_ctl start
$ docker run -i -t postgresimage bash
root@08d363f57161:/# ps aux
\# No postgres daemon running

15. Letting containers talk to each other

$ docker run -d –name mysql ctlc/mysql
$ docker run –link mysql:db ubuntu env

Thanks to Brian Morearty who has a hands-on training course for Docker.

(Edit: Brian removed this online course because it was too out-of-date, created in Jan 2014 and using Docker 0.7.6)