How to Run a Cron Job Inside a Docker Container

Need to schedule tasks inside a Docker container? Whether it’s a backup script, data sync, or a periodic cleanup, cron jobs are a simple and reliable tool. But running them inside a Docker container requires a few extra steps.

In this post, you’ll learn how to:

Set up and run cron inside a container
Keep it running properly
Debug common issues with scheduled jobs

Table of Contents

🧠 Why Cron in Docker?

By default, Docker containers run a single foreground process. Cron is a background daemon, so you can’t just RUN crontab and expect it to keep going. You’ll need to explicitly:

Install cron
Add your cron jobs
Ensure the container keeps running

✅ Step-by-Step: Running Cron in Docker

🏗️ 1. Create a Dockerfile

Here’s an example for a Debian-based image:

FROM ubuntu:22.04

# Install cron and any other dependencies
RUN apt-get update && \
    apt-get install -y cron && \
    apt-get clean

# Add your cron job file
COPY my-cron-job /etc/cron.d/my-cron-job

# Give execution rights on the cron job
RUN chmod 0644 /etc/cron.d/my-cron-job

# Apply the cron job
RUN crontab /etc/cron.d/my-cron-job

# Create the log file to be able to run tail
RUN touch /var/log/cron.log

# Start cron and keep container running
CMD cron && tail -f /var/log/cron.log

📄 2. Create the Cron Job File

my-cron-job:

* * * * * root echo "Cron job ran at $(date)" >> /var/log/cron.log 2>&1

⏰ This job runs every minute and logs the timestamp. Adjust timing and command as needed.

Make sure:

The file ends with a newline
It has appropriate permissions (0644)
The user (e.g., root) is specified in the cron line

🔁 3. Build and Run the Container

docker build -t cron-container .
docker run -d --name cron-job cron-container

Check logs:

docker logs cron-job

You should see entries from /var/log/cron.log appearing every minute.

🧪 Debugging Tips

🧵 Job not running? Make sure your cron file ends with a newline and has the correct syntax
🔒 Permission denied? Ensure the cron job is owned by root and has 0644 permissions
📦 Cron not installed? Double-check your base image and installation steps
🛑 Container exits? Ensure CMD keeps the container running (e.g., with tail -f)

🧰 Alternative: Use Host Cron to Call Docker

If you don’t want to run cron inside the container, a cleaner approach may be:

Run cron on the host and call Docker from it:

* * * * * docker exec my-container /path/to/script.sh

✅ Easier to manage, monitor, and avoids running multiple daemons in a container.

📝 Conclusion

While Docker wasn’t designed to run background daemons like cron, it’s possible with a bit of setup. Whether you run cron inside a container or use the host’s scheduler to invoke containers, you have flexible options to automate tasks in a containerized environment.

🔑 Summary

Task	Command/Step
Install cron	`apt install cron`
Add cron job	Copy file to `/etc/cron.d/`
Run container with cron	`CMD cron && tail -f /var/log/cron.log`
Build and run	`docker build` → `docker run -d`
Alternative (host cron)	`* * * * * docker exec ...`