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20 March 2017

Use 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

    As of 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.

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

     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 to the beginning of your config/schedule.rb file.

env :PATH, ENV['PATH']

In case you need to set the bundle command explicitly, run the command with a --set variable,

bundle exec whenever --write-crontab --set 'environment=development&bundle_command=bundle exec'
tags: Docker - Whenever - Cron