Docker Startup Script - Stay Interactive

It’s been a long time since I wrote my last article in this new blog. The only things that I update is the talks pages because I’m quite active at giving talks on this year and I’m very proud of it.

So Docker, when you want to run something in it you basically have three instructions to choose. There’s CMD, RUN and ENTRYPOINT. Every instructions can executing command like shell command

RUN apt-get install python3
CMD echo "Hello world"
ENTRYPOINT echo "Hello world"

You can find a better explanation for this in here

Startup

So when the container is created, it is a common use-case that you want to run something. Maybe you want to generate mock data to your database or create some files based on your environment variables. In this case you can use CMD or ENTRYPOINT. The problem is, when you want to go to interactive mode.

Usually what you have to do is just use this when you running your docker image

$ docker run -it ubuntu bash

This will create docker container and run interactive bash shell inside the container. But this won’t work if you have ENTRYPOINT in your dockerfile, or, the CMD will be overrided by the bash command (CMD is basically ENTRYPOINT that purposely easily overrided).

You can override the entrypoint as well with --entrypoint like this

$ docekr run -it --entrypoint "/bin/bash"

But it will also prevent your startup script from running. So how can we overcome this situation?

Startup Script

All you have to do is create the entrypoint script that will run your startup command but also run the command that you pass

#!/usr/bin/env bash
set -e

## Run startup command 
echo "Hello ENTRYPOINT" >> hello

## Running passed command
if [[ "$1" ]]; then
	eval "$@"
fi

And set this script as your entrypoint

## Copy the script to PATH, don't forget to add executable
COPY entrypoint.sh bin/entrypoint.sh

## Set it as your entrypoint
ENTRYPOINT [ "entrypoint.sh" ]

Tada ~ now you can run anything you passed on and still running your startup script!

For more examples please check the Dockerfile in here

Published 31 Dec 2019

Technical Stuff. Rants
Esa Firman on Twitter