Overview

Teaching: 15 min
Exercises: 5 min

Questions

• How are containers run?

• How do you monitor containers?

• How are containers exited?

• How are containers restarted?

Objectives

• Run containers

• Understand container state

• Stop and restart containers

To use a Docker image as a particular instance on a host machine you run it as a container. You can run in either a detached or foreground (interactive) mode.

Run the image we pulled as an interactive container

docker run -ti matthewfeickert/intro-to-docker:latest /bin/bash

You are now inside the container in an interactive bash session. Check the file directory

pwd

/home/docker/data

and check the host to see that you are not in your local host system

hostname

<generated hostname>

Further, check the os-release to see that you are actually inside a release of Debian (given the Docker Library’s Python image Dockerfile choices)

cat /etc/os-release

PRETTY_NAME="Debian GNU/Linux 11 (bullseye)"
NAME="Debian GNU/Linux"
VERSION_ID="11"
VERSION="11 (bullseye)"
VERSION_CODENAME=bullseye
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"

Monitoring Containers

Open up a new terminal tab on the host machine and list the containers that are currently running

docker ps

CONTAINER ID        IMAGE         COMMAND             CREATED             STATUS              PORTS               NAMES
<generated id>      <image:tag>   "/bin/bash"         n minutes ago       Up n minutes                            <generated name>

Notice that the name of your container is some randomly generated name. To make the name more helpful, rename the running container

docker rename <CONTAINER ID> my-example

and then verify it has been renamed

docker ps

CONTAINER ID        IMAGE         COMMAND             CREATED             STATUS              PORTS               NAMES
<generated id>      <image:tag>   "/bin/bash"         n minutes ago       Up n minutes                            my-example

Renaming by name

You can also identify containers to rename by their current name

docker rename <NAME> my-example

Exiting and restarting containers

As a test, create a file in your container by printing the current datetime into a file

date > test.txt

In the container exit at the command line

exit

You are returned to your shell. If you list the containers you will notice that none are running

docker ps

CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

but you can see all containers that have been run and not removed with

docker ps -a

CONTAINER ID        IMAGE         COMMAND             CREATED            STATUS                     PORTS               NAMES
<generated id>      <image:tag>   "/bin/bash"         n minutes ago      Exited (0) t seconds ago                       my-example

To restart your exited Docker container start it again and then attach it to your shell

docker start <CONTAINER ID>
docker attach <CONTAINER ID>

Starting and attaching by name

You can also start and attach containers by their name

docker start <NAME>
docker attach <NAME>

Notice that your entry point is still /home/docker/data and then check that your test.txt still exists with the datetime you printed into it

ls *.txt

test.txt

cat test.txt

Sun Oct 17 16:51:51 UTC 2021

So this shows us that we can exit Docker containers for arbitrary lengths of time and then return to our working environment inside of them as desired.

Clean up a container

If you want a container to be cleaned up — that is deleted — after you exit it then run with the --rm option flag

docker run --rm -ti <IMAGE> /bin/bash

Key Points

• Run containers with docker run

• Monitor containers with docker ps

• Exit interactive sessions just as you would a shell

• Restart stopped containers with docker start