Docker Cronjob: How to Run Cron Jobs Inside a Docker Container (Step-by-Step Guide)

What is a Docker Cronjob?

A Docker cronjob is simply a cron job running inside a Docker container. Instead of scheduling the job on the host machine, you package the cron daemon and the scripts inside a container for:

• Portability: Runs the same in dev, staging, or production
• Isolation: Avoids conflicts with host cron or other apps
• Version Control: Your cron job configuration lives with your code
• Scalability: Easy to deploy multiple scheduled jobs with Docker or Kubernetes

Why Run Cron Jobs in Docker Containers?

Running cron jobs in Docker brings several benefits for DevOps and SRE teams:

1. Portability: Cronjobs become part of your Docker image, ensuring consistent execution across all environments.
2. Isolation: Keeps your scheduled tasks separated from the host system.
3. Dependency Management: All scripts, libraries, and environment variables are inside the container.
4. Version Control & CI/CD Friendly: Track and deploy cronjob changes easily with Git.
5. Scalability: Works seamlessly with Docker Compose or Kubernetes CronJobs for larger workloads.

How to Create a Docker Cronjob (Step-by-Step)

We’ll create a Docker cronjob that logs a message every minute.

1. Create the Cron Script

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
# This script logs a timestamped message
echo "The Docker cronjob ran at $(date)" >> /var/log/container_cron.log 2>&1

Make it executable:

chmod +x log_message.sh

2. Create a Cron Job Configuration

Create a file named container_cronjob:

* * * * * root /log_message.sh

This runs the script every minute.

3. Create a Dockerfile

Create a Dockerfile to install the necessary tools and copy the files. Here’s a sample:

FROM ubuntu:22.04
MAINTAINER YourName <your.email@example.com>

# Install cron
RUN apt-get update && \
    apt-get install -y cron && \
    rm -rf /var/lib/apt/lists/*

# Copy script and cron job
COPY ./files/container_cronjob /etc/cron.d/container_cronjob
COPY ./files/log_message.sh /log_message.sh

# Set permissions
RUN chmod +x /log_message.sh
RUN chmod 0644 /etc/cron.d/container_cronjob

# Start cron in foreground & keep container alive
CMD cron -f

4. Build and Run the Docker Cronjob

Build the image:

docker build -t docker-cronjob-example .

5. Run the container:

Spin up the container using:

docker run -itd --name my-cronjob docker-cronjob-example

6. Check logs:

To confirm if your cron job is running, use the following command:

docker logs -f my-cronjob

You should see:

The Docker cronjob ran at Mon Aug 4 10:00:01 UTC 2025
The Docker cronjob ran at Mon Aug 4 10:01:01 UTC 2025

How to Monitor Docker Cronjobs

• Logs: Use docker logs for quick checks
• Log Aggregation: Send logs to ELK, Loki, or CloudWatch
• Health Checks: Use Docker health checks or Kubernetes probes
• Cron Monitoring Tools: Cronitor or Healthchecks.io for alerts on job failures

Common Issues with Docker Cronjobs

1. Container Exits Immediately
   • Use cron -f to keep cron in the foreground
2. Cron File Not Executing
   • Ensure /etc/cron.d files have 0644 permissions
   • File must end with a newline
3. Logs Not Updating
   1. Redirect output to /var/log and check with docker logs

Conclusion

A Docker cronjob is an easy and reliable way to schedule tasks inside a container without depending on the host system.

You’ve learned how to:

1. Create a cron script
2. Configure cron inside Docker
3. Build and run the Docker container
4. Monitor and troubleshoot your cronjob

For scalable cronjobs in production, consider Kubernetes CronJobs and CI/CD tools like Devtron to automate scheduling, monitoring, and deployment.

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Nivesh Goyal

Nivesh got inspiration from his elder sister and brother to pursue a career in technology. He is working as DevOps Engineer in Devtron and writes technical blogs as well. He loves to do photography i