DevOps Katas: Mastering DevOps Practices/

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Docker Kata 2: Disconnected Containers

Docker Kata 2: Disconnected Containers

Learn how to run a disconnected container and execute commands on a running container.

We'll cover the following...

The first kata demonstrated containers that exit immediately after they’re run. This kata will teach us how to run a container that continues running in the background after executing the docker container run command.

Step 1: Run a disconnected container

The command to run a disconnected container in the background is given below.

Press + to interact
docker container run -d ubuntu /bin/sh -c "while true; do echo hello docker; sleep 1; done"

The output will be something like this:

Commands

Parameter

Description

docker container

This is the parent command.

run

This is the command that runs a container from an image.

-d

The -d parameter runs a disconnected container.

ubuntu

This is the name of the image to run; in this case, the Ubuntu Linux image.

/bin/sh

This is the command to run inside the container when it starts. This is the shell, which can run commands and scripts.

-c

The -c parameter indicates that the next parameter will be the script to run; without the -c parameter, the shell will expect a file name parameter.


"while true; do echo hello docker; sleep 1; done"

This is the script to be run by the shell:

  • while true;: This causes the script to run forever (true is always true!).
  • do echo hello docker;: This echoes the text “hello docker” to the console.
  • sleep 1;: This echoes the text “hello docker” to the console.
  • done: This ends the script.

This step demonstrates how Docker can run a disconnected container in the background. Running a disconnected container is essentially the opposite of running it interactively. A disconnected container will run without user input or interaction as long as its start process runs.

This command does the following:

  • Runs a disconnected Ubuntu container
  • Specifies the command to run after the image name
  • Uses /bin/sh to run the sh shell, which runs scripts in the bash command language
  • Executes the inline script: "while true; do echo hello docker; sleep 1; done"

When this command is run, there’s one output, which is the ID of the started container. The Unix shell program bin/sh acts as the start command. ...