Trying to understand launcher as a series of Docker commands

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I am trying to understand the Docker commands issued by launcher.

This also requires understanding the other bash commands in the script. As I am no bash expert I have to do some research to figure some parts. Normally I read the code and manually figure out the command and then test run it to make sure, e.g.

docker info 2> /dev/null

Sometimes some of the lines are more complicated and I run an equivalent, e.g.

test=$(($(stat -f --format="%a*%S" /var/lib/docker)/1024**3 < 5)); echo $test

The current subroutine I am working on is host_run() for which I have no idea where the input to the routine is coming from.

https://github.com/discourse/discourse_docker/blob/master/launcher#L259-#L287.

Now I could ask many questions for the next few days or sprinkle echo statements in a modified launcher but there has to be something better.

Is there a means like the command script that will capture the internal Docker commands being run and log them to a file?

EDIT

This seems to work. See: Debugging Bash scripts

Simply use the bash -x option, e.g.

groot@galaxy:/var/discourse$ sudo bash -x launcher start app

To capture this to file

  1. Create directory to hold traces. This is created under the ~ path to avoid permission problems.
groot@galaxy:/var/discourse$ mkdir ~/traces
  1. Run bash command of interest. The trick is that the trace output does not go to stdout, but stderr so use 2> instead of > which is really 1>.
groot@galaxy:/var/discourse$ sudo bash -x launcher start app 2> ~/traces/start_01
  1. Filter the file showing lines with the docker command.
groot@galaxy:/var/discourse$ grep /usr/bin/docker ~/traces/start_01

Example output:

+ docker_path=/usr/bin/docker
+ '[' -z /usr/bin/docker ']'
++ /usr/bin/docker info
+ /usr/bin/docker info
++ /usr/bin/docker --version
++ /usr/bin/docker images
++ /usr/bin/docker run -i --rm -a stdout -a stderr discourse/base:2.0.20200512-1735 echo working
++ /usr/bin/docker info --format '{{.DockerRootDir}}'
+ '[' -z /usr/bin/docker ']'
++ /usr/bin/docker --version
++ /usr/bin/docker ps
++ /usr/bin/docker ps -a
+ /usr/bin/docker start app

That may look like a lot more Docker commands than expected but launcher is doing a lot of integrity checking that you would have to do by hand. This convinces me to use launcher for the regular tasks for which it was created.