Run Whenever & Cron in the Docker container

Whenever is a Ruby gem that provides a clear syntax for writing and deploying cron jobs.

This blog introduces how to run your whenever cron job in the Docker container.

1. Install Cron

   Since the docker image ruby:2.5.0 don’t have a Cron builtin. We have to install it first. Open the Dockfile with your favorite text editer, and add the following script under the FROM ruby:2.5.0:

    RUN apt-get update && apt-get install -y --no-install-recommends cron \
    && rm -r /var/lib/apt/lists/*
   

2. Add a new task to docker-compose.yml file

   This result in an image named <sample_app>_cron after you run docker-compose build:

    cron:
      depends_on:
        - db
        - redis
      build: .
      command: sh bin/start-cron.sh
      volumes:
        - .:/app
   

3. Create a script /the-path-to-your-app/bin/start-cron.sh to start your cron job:

    #!/bin/sh
    bundle exec whenever --write-crontab --set environment=development
    cron -f
   

But after you start it with docker-compose start command. You might got an exception failed to load command: rake. It was because of the PATH environment not set properly.

It take me a while to figure it out, but the solution is trivial, just add the following two lines at the beginning of your config/schedule.rb file:

env :PATH, ENV['PATH']
env :BUNDLE_PATH, ENV['BUNDLE_PATH']

Using --set to set the environment variable:

bundle exec whenever --write-crontab --set 'environment=development&bundle_command=bundle exec'