Running A Cronjob Inside Docker Container In 5 Steps

by Nivesh Goyal, DevOps Engineer, Devtron

In this blog, we will discuss and understand the steps on how to set a Cron job inside the container.

Prerequisites:

• Docker should be installed on your system.
• You should have an account on GitHub.

What is a Cron job?

Cron is a job scheduler. A cron job is a command used for scheduling tasks to be executed automatically after a specific time period. Mostly cron jobs are used for executing scripts, taking backup of files, etc.

Steps to run cron job inside a container:

1. Create a script file(task file)
2. Create a cron job file
3. Create a docker file
4. Create a docker image from the docker file
5. Run the docker image inside the container

Create a script file:

First, create a script file as per your requirement, which commands or tasks you have to run, and save it into the “.sh” file. Here, I have created a script. sh file:

!/bin/bash
touch/var/log/cron.log
crontab/etc/cron.d/container_cronjob

1. Create a cron job file:

Now create a cron file and write cron job details inside the file, like when you want to run your task. Below is the sample cron file; in this, the script is running every minute.

* * * * * echo “The test cron ran at $(date)” > /proc/1/fd/1 2>/proc/1/fd/2

NOTE:

An empty line is required in the cron job file at the end of the file for a valid cron file.

1. Create a docker file:

Finally, create a docker file and build your docker image. Below is a sample Dockerfile.

# Pulling ubuntu image with a specific tag from the docker hub.
FROM ubuntu:18.04

# Adding maintainer name (Optional).
MAINTAINER Nivesh_goyal

# Updating the packages and installing cron and vim editor if you later want to edit your script from inside your container.
RUN apt-get update \
&& apt-get install cron -y && apt-get install vim -y

# Crontab file copied to cron.d directory.
COPY ./files/cronjob /etc/cron.d/container_cronjob

# Script file copied into container.
COPY ./files/script.sh /script.sh

# Giving executable permission to script file.
RUN chmod +x /script.sh

# Running commands for the startup of a container.
CMD [“/bin/bash”, “-c”, “/script.sh && chmod 644 /etc/cron.d/container_cronjob && cron && tail -f /var/log/cron.log”]

NOTE:

Put the cron job file and script file inside the same directory in your code repository where you are keeping your Dockerfile.

1. Create a Docker Image:

Now run the following command to build the Dockerfile. You can change the tag as per your need:

# docker build –tag cronjob:testing .

1. Run the docker image inside the container:

Now Spin up a container using the above image, run the following command:

# docker run -itd –name container_name cronjob:testing

Note:

If you want to check, your cron is running or not, run the below command from outside the container:

# docker logs –tail 100 -f container_name

That’s all about how you can run a cron job inside a Docker container. If you want to deploy it inside the Kubernetes cluster using Devtron’s CI/CD tool, follow this blog.