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How to build a Docker cron job Container easily [2020]

In this tutorial, we’ll be setting up Cron jobs in Docker and discuss how to avoid a common pitfall.

Let’s Code It!

We’ll build a docker cron job that runs while other services on docker are running. This is particularly useful if we want to backup our databases.

Setting things up

Directory structure.

Create a docker-cron root directory.

Container #1: Node

To start we’ll setup a node image that will act as a service that’s supposed to run while the cron job is running.

Generate a blank node app in docker-cron:

Create a Dockerfile in the node-app directory:

 Common Pitfall

Running cron in the same container as a service.

The Docker ubuntu image by default doesn’t run cron at runtime because it’s intended to run only one process at a time. Below is improperly attempting to run two processes simultaneously, something Docker doesn’t support:

The trick is splitting cron into its own separate container.

Container #2: Cron

We’ll setup an ubuntu image that will run our cron job in a separate container. This image doesn’t come with cron by default, so we’ll need to install it and do a general update. Create a cron-app directory in the docker-cron project folder.

Create mycron in cron-app that specifies your cron:

Make sure you have a blank line at the end of mycron

This cron will run every minute.

Create do.sh in cron-app that specifies what your cron does:

Create a Dockerfile in the cron-app directory with the following contents:

Docker Compose

Now we’ll bring the two images together with Docker Compose.

Create a docker-compose.yml file in the root docker-cron directory with the following contents:

Thank you for taking the time to read this tutorial. Check out our previous article about docker add vs copy. Also, Your feedback will be appreciated.

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