dmarell / docker-ubuntu

Simple docker tutorial, showing basic handling. Useful for experimenting with installations/configurations on ubuntu

Docker tutorial

Simple docker tutorial, showing basic handling.

by Daniel Marell, C.A.G

Life cycle

Starting an ubuntu machine in docker is simple:

$ docker run -it ubuntu bash
root@93e788c4f2ec:/#

In another command window you can inspect running docker instances:

$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
93e788c4f2ec        ubuntu              "bash"              22 seconds ago      Up 19 seconds                           vigorous_colden

As we did not give it a name, it was given one: vigorous_colden

In order to get a feeling for instances vs types/images, touch the disk of the running instance vigorous_colden:

root@93e788c4f2ec:/# echo date > kilroy-was-here.txt
root@93e788c4f2ec:/# ls -l k*
-rw-r--r-- 1 root root 5 Sep  8 08:37 kilroy-was-here.txt

Stop the instance:

$ docker stop vigorous_colden
vigorous_colden

It's gone:

$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

Start it again:

$ docker start vigorous_colden
vigorous_colden
$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
93e788c4f2ec        ubuntu              "bash"              3 minutes ago       Up 4 seconds                           vigorous_colden

Check that it is 'our' instance:

root@93e788c4f2ec:/# ls -l k*
-rw-r--r-- 1 root root 5 Sep  8 08:37 kilroy-was-here.txt

Now stop and remove the instance:

$ docker stop vigorous_colden
vigorous_colden
$ docker rm vigorous_colden
vigorous_colden

Now when it has been removed it is of course not possible to start it again:

$ docker start vigorous_colden
Error response from daemon: No such container: vigorous_colden
Error: failed to start containers: vigorous_colden

But we can create a new instance again:

$ docker run -it ubuntu bash
root@331de7d0fa71:/# 

We got a new name:

$ docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
331de7d0fa71        ubuntu              "bash"              32 seconds ago      Up 30 seconds                           determined_darwin

And we can verify that it is a new machine and not the one we had before:

root@331de7d0fa71:/# ls -l k*
ls: cannot access 'k*': No such file or directory

Build a customized image

The Dockerfile specifies how the new image should be built. In this example we start with the same ubuntu image as we used before but with curl and git installed:

Create the following Dockerfile in an empty directory somewhere:

FROM ubuntu
RUN apt-get update && apt-get install -y \
    curl \
    git

Build an image from this Dockerfile:

$ docker build -t myubuntu-image .
...
Successfully built 42243b430191
Successfully tagged myubuntu-image:latest

Start it, and give it a name this time:

$ docker run -it --name myubuntu-1 myubuntu-image bash
root@632e26f1fd8a:/# 

Verify that it has curl installed:

root@632e26f1fd8a:/# curl --version
curl 7.47.0 (x86_64-pc-linux-gnu) libcurl/7.47.0 GnuTLS/3.4.10 zlib/1.2.8 libidn/1.32 librtmp/2.3

Adding a user, run as a daemon and login to container

For security reasons it is not recommended to run as root in container. Add a user by adding these lines to Dockerfile:

...
RUN useradd -ms /bin/bash user
USER user
WORKDIR /home/user

Docker instances runs a command or a specific service. In the above examples we did run bash.

Add an ENTRYPOINT in Dockerfile simulating that we are running some kind of daemon:

FROM ubuntu

RUN apt-get update && apt-get install -y \
    curl \
    git \
    && rm -rf /var/lib/apt/lists/*

RUN useradd -ms /bin/bash user
USER user
WORKDIR /home/user

ENTRYPOINT ["tail", "-f", "/dev/null"]

Note that we added rm -rf /var/lib/apt/lists/* according to Dockerfile best practicies.

Build it and run it as a daemon (not specifying bash):

$ docker build -t myubuntu-image .
...
$ docker run -d --name myubuntu-1 myubuntu-image
76ea35fbf20fb32571f1abb6fbdbb8586665ac202de11819523bfcec2a71df0e
$ docker ps
CONTAINER ID        IMAGE               COMMAND               CREATED             STATUS              PORTS               NAMES
76ea35fbf20f        myubuntu-image      "tail -f /dev/null"   11 seconds ago      Up 11 seconds                           myubuntu-1

Login to the running instance:

$ docker exec -it myubuntu-1 bash
user@76ea35fbf20f:~$ 

Note that you get a prompt $ instead of #. You are not root anymore.

If you need to login as root, login as root instead using the option -u 0:

$ docker exec -it -u 0 myubuntu-1 bash
root@76ea35fbf20f:/home/user#