Running cron jobs inside Docker containers is a common DevOps practice, especially for automated backups, log rotations, or scheduled tasks in microservices environments. In this guide, you’ll learn how to configure and test a cron job step-by-step inside a Docker container using Ubuntu, along with a sample Dockerfile and script. This guide is ideal for developers, DevOps engineers, and SREs looking for a hands-on, repeatable setup.
Prerequisites
- Docker should be installed on your system.
- You should have an account on GitHub.
Steps to Run a Cron Job Inside a Container
Below is a step-by-step guide to create a Cron Job inside a Docker Container and execute the custom tasks periodically.
1. Create a Script File
First, create a script file as per your requirement, which commands or tasks you have to run, and save it as a .sh file. For example:
#!/bin/bash
touch /var/log/cron.log
crontab /etc/cron.d/container_cronjobSave the file as script.sh.
2. Create a Cron Job File
Now, create a cron file and write the cron job details inside it. Here’s a sample cron job that runs every minute:
* * * * * echo "The test cron ran at $(date)" > /proc/1/fd/1 2>/proc/1/fd/23. Create a Dockerfile
Create a Dockerfile to install the necessary tools and copy the files. Here’s a sample:
FROM ubuntu:18.04
MAINTAINER Nivesh_goyal
RUN apt-get update && \
apt-get install cron -y && \
apt-get install vim -y
COPY ./files/cronjob /etc/cron.d/container_cronjob
COPY ./files/script.sh /script.sh
RUN chmod +x /script.sh
CMD ["/bin/bash", "-c", "/script.sh && chmod 644 /etc/cron.d/container_cronjob && cron && tail -f /var/log/cron.log"]4. Build the Docker Image
Build the Docker image using the following command:
docker build --tag cronjob:testing .5. Run the Docker Container
Spin up the container using:
docker run -itd --name container_name cronjob:testingHow to Verify Cron Job Execution
To confirm if your cron job is running, use the following command:
docker logs --tail 100 -f container_nameConclusion: What’s Next
That’s all about how you can run a cron job inside a Docker container. Read the next blog to learn how to deploy a Cron Job inside a Kubernetes cluster using Devtron’s CI/CD tool.
Commonly Asked Questions
Can you run a cron job in a Docker container?
Yes. You can configure a cron job inside a Docker container by installing cron, writing a job file, and starting the cron daemon in your Dockerfile.
What is the best base image to use for cron jobs in Docker?
Ubuntu and Alpine are popular choices. Ubuntu is beginner-friendly, while Alpine is lightweight but may require extra packages for cron.
How do I check if my Docker cron job ran successfully?
Use docker logs <container_name> to view cron output. Make sure your cron job writes to stdout or a log file that you can tail.
Can I schedule Kubernetes cron jobs using Devtron?
Yes. Devtron supports Kubernetes-native cron jobs through its CI/CD pipeline configurations
