Our first containers

Colorful plastic tubs
Figure 39 : Colorful plastic tubs

Objectives

At the end of this lesson, you will have:

  • Seen Docker in action.

  • Started your first containers.

Hello World

In your Docker environment, just run the following command:

$ docker run busybox echo hello world
hello world

(If your Docker install is brand new, you will also see a few extra lines, corresponding to the download of the busybox image.)

That was our first container!

  • We used one of the smallest, simplest images available: busybox.

  • busybox is typically used in embedded systems (phones, routers...)

  • We ran a single process and echo'ed hello world.

A more useful container

Let's run a more exciting container:

$ docker run -it ubuntu
root@04c0bb0a6c07:/#

  • This is a brand new container.

  • It runs a bare-bones, no-frills ubuntu system.

  • -it is shorthand for -i -t.

    • -i tells Docker to connect us to the container's stdin.

    • -t tells Docker that we want a pseudo-terminal.

Do something in our container

Try to run figlet in our container.

root@04c0bb0a6c07:/# figlet hello
bash: figlet: command not found

Alright, we need to install it.

Install a package in our container

We want figlet, so let's install it:

root@04c0bb0a6c07:/# apt-get update
...
Fetched 1514 kB in 14s (103 kB/s)
Reading package lists... Done
root@04c0bb0a6c07:/# apt-get install figlet
Reading package lists... Done
...

One minute later, figlet is installed!

Try to run our freshly installed program

The figlet program takes a message as parameter.

root@04c0bb0a6c07:/# figlet hello
 _          _ _       
| |__   ___| | | ___  
| '_ \ / _ \ | |/ _ \
| | | |  __/ | | (_) |
|_| |_|\___|_|_|\___/

Beautiful! 😍

Counting packages in the container

Let's check how many packages are installed there.

root@04c0bb0a6c07:/# dpkg -l | wc -l
190

  • dpkg -l lists the packages installed in our container

  • wc -l counts them

How many packages do we have on our host?

Counting packages on the host

Exit the container by logging out of the shell, like you would usually do.

(E.g. with ^D or exit)

root@04c0bb0a6c07:/# exit

Now, try to:

  • run dpkg -l | wc -l. How many packages are installed?

  • run figlet. Does that work?

Comparing the container and the host

Exit the container by logging out of the shell, with ^D or exit.

Now try to run figlet. Does that work?

(It shouldn't; except if, by coincidence, you are running on a machine where figlet was installed before.)

Host and containers are independent things

  • We ran an ubuntu container on an Linux/Windows/macOS host.

  • They have different, independent packages.

  • Installing something on the host doesn't expose it to the container.

  • And vice-versa.

  • Even if both the host and the container have the same Linux distro!

  • We can run any container on any host.

    (One exception: Windows containers cannot run on Linux machines; at least not yet.)

Where's our container?

  • Our container is now in a stopped state.

  • It still exists on disk, but all compute resources have been freed up.

  • We will see later how to get back to that container.

Starting another container

What if we start a new container, and try to run figlet again?

$ docker run -it ubuntu
root@b13c164401fb:/# figlet
bash: figlet: command not found

  • We started a brand new container.

  • The basic Ubuntu image was used, and figlet is not here.

  • We will see in the next chapters how to bake a custom image with figlet.