Interactive containers

running in the foreground

Running in the foreground means, that after the container has been created, it keeps running only until its task process is not completed. Console is often also attached to such containers.

Docker Docs: Docker run reference

This is the default mode of the docker run command.

Table of contents

1. Interactive containers
   1. Exercise 1
   2. Exercise 2

The containers running in the foreground are most often used in the following common scenarios.

Interactive session

A process is started in the container (e.g. bash) and the console is attached to it. The container keeps running until the process exits. As long as the standard input stream STDIN stays opened, you can work interactively inside the container from the console of your host computer.

Because all accetto containers include Xfce desktop environment, you don’t need to run them in the foreground mode to be able to work with them interactively.

One-time task

Container gets a task (e.g. some script), executes it and then exits.

Note that the container is not automatically removed, unless you ask for it explicitly by providing the parameter --rm to the docker run command.

The accetto containers are not intended for this scenario. It’s certainly possible to use them also this way, but sometimes it may be not so straightforward as expected (see the exercises below). However, you can always start an interactive session and execute the task that way.

Exercise 1

Let’s say that you want to create a randomly named temporary container for working inside it interactively with bash. The container should be automatically removed after it’s stopped.

You can create the container using the following command:

docker run -it --rm accetto/ubuntu-vnc-xfce-g3 --skip-vnc bash

The parameter -it attaches the host computer console to the container and keeps the standard input stream opened. It allows working inside the container interactively.

The parameter --rm ensures that the container is automatically removed after it is stopped.

After the image is pulled from Docker Hub and the container created, you can work inside it interactively until you exit the bash session by entering exit and then stop the container by pressing CTRL-c.

headless@7cb955144aeb:~$ id
uid=1000(headless) gid=1000(headless) groups=1000(headless)
headless@7cb955144aeb:~$ exit
exit
^C

Note that the whole graphical desktop subsystem is not used in this scenario and also the VNC server and noVNC client are not started. The container behaves as a basic Ubuntu container extended by some additional utilities.

However, if you would use the following command

docker run -it -P --rm accetto/ubuntu-vnc-xfce-g3 bash

then you would be able to connect to the running interactive container using a VNC viewer or a web browser until you stop it.

The parameter -it attaches the host computer console to the container and keeps the standard input stream opened. It allows working inside the container interactively.

The parameter -P binds all TCP ports exposed by the container to automatically selected free TCP ports of the host computer.

The parameter --rm ensures that the container is automatically removed after it is stopped.

While the interactive container is running, you can find out its name and ports by executing the following command in an another terminal on your host computer:

docker ps --format 'table {{.Names}}\t{{.Ports}}'

The output for the first case should look similar to this

NAMES                    PORTS
adoring_robinson         5901/tcp, 6901/tcp

or for the second case similar to this

NAMES                    PORTS
adoring_robinson         0.0.0.0:32771->5901/tcp, 0.0.0.0:32770->6901/tcp

The value adoring_robinson is the random container name assigned by Docker CLI.

The part 5901/tcp, 6901/tcp shows, that the container still exposes the VNC/noVNC ports (5901/6901), but they are not bound to any TCP ports of the host computer. Actually, the the VNC/noVNC servers inside the container have not been even started. See the startup help for more information.

The part 0.0.0.0:32771->5901/tcp, 0.0.0.0:32770->6901/tcp shows that the VNC/noVNC ports (5901/6901) have been bound to the ports 32771/32770 of the host computer.

Exercise 2

Let’s say that you want to execute a one-time task in a temporary container, that will be automatically removed after the task is completed.

Because of the startup script specifics, the container will not automatically stop after the task is completed unless you use the startup option --skip-startup. If you don’t, you’ll need to stop the container by pressing CTRL-c.

The following command creates a new container and lists the content of its /home/headless directory. The container will be automatically removed afterwards.

docker run --rm accetto/ubuntu-vnc-xfce-g3 --skip-startup ls -l /home/headless

The parameter --rm ensures that the container is removed after it is stopped.

The parameter --skip-startup is specific to accetto containers and if it’s not provided, then the container must be stopped by pressing CTRL-c.

The text ls -l //home/headless is the command to execute inside the container.

The command output should look like this:

total 12
drwxr-xr-x 1 root root 4096 Desktop
-rw-r--r-- 1 root root  185 readme.md
drwxr-xr-x 1 root root 4096 tests

You can see from the console output, that the container user’s home directory is not completely initialized. It is the consequence of using the startup option --skip-startup. This page explains why skipping the startup script could be a pitfall.

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