How to debug a rails app in docker with pry?

I have a rails app running in a docker container in development environment.

When I try to debug it with placing binding.pry somewhere in the code and attaching to the container I can see the pry prompt in the output but it doesn't pause on it and I can't interact with it like it was without docker container.

So how do I debug a containerized app?

If you're using docker-compose, you can add these flags to docker-compose.yml:

app:
  tty: true
  stdin_open: true

And then attach to your process with docker attach project_app_1. pry-rails works here now. Ensure less is installed on your container for the optimal pry experience.

cf. https://github.com/docker/compose/issues/423#issuecomment-141995398

to use pry you have to run it differently:

docker-compose run --service-ports web

check out this article for more info:

http://blog.carbonfive.com/2015/03/17/docker-rails-docker-compose-together-in-your-development-workflow/

as Gabe Kopley answer, assume your rails container is called app, set stdin_open and tty to true :

app:
  stdin_open: true
  tty: true

and I wrote a bash script to make life easier, save it to bin/dev:

#!/bin/bash
docker-compose up -d && docker attach $(docker-compose ps -q app)

don't forget to make dev be executable by chmod +x bin/dev

In your terminal, type bin/dev, it will automatically run up containers and attach the app container. when binding.pry called, you can type to the terminal directly.

I had this same issue when I was running pry in Passenger. Try changing "pry-rails" in the Gemfile to gem "pry-remote", which will initiate a dRuby, or distributed protocol with no dependencies.

Where you want to stop the code in execution call "binding.remote_pry" as opposed to "binding.pry"

Then just call remote-pry in the console to access it. It should work the same. In your test environment just the usual binding.pry works fine.

If you don't use docker-compose though, you can simply run the container with -it option.

For example:

docker run -v /Users/adam/Documents/Rails/Blog/:/usr/src/app -p 3000:3000 -it blog

If you are running in under alpine linux aka ruby-*-alpine you need to install package ncurses wich includes infocmp.

If that's the reason you will have this in your error log:

bin/rails: No such file or directory - infocmp                                                              
Error: undefined method `split' for nil:NilClass 
...
bin/rails:4:in `<main>'                                                                                                                 
FATAL: Pry failed to get user input using `Readline`.                                                       
To fix this you may be able to pass input and output file descriptors to pry directly. e.g.                                    
  Pry.config.input = STDIN                                                                                   
  Pry.config.output = STDOUT                                                                           
  binding.pry    

To fix it, on your Dockerfile add:

RUN apk add ncurses

Although You probably already had an apk add line, so just add ncurses there.

then you can do