I recently had to create a Dockerfile for a small application and since we had to run it periodically, I chose to integrate it with the native cron available on Linux distributions.
Unfortunately, it took a little longer than I initially expected, but I finally managed to get it working correctly with the help of an old Stack Overflow response. 📚
Since I think it won't be the last time that I need to do a similar task, I decide to write this quick post as a reminder for myself. 🧓
I hope it will helps you as well! 😎
Dockerfile
# https://hub.docker.com/_/debian?tab=tags
FROM debian:stable-20210311
# Copy the cron file into our image
COPY synchronization.cron /etc/cron.d/synchronization.cron
# https://packages.debian.org/search?suite=buster&keywords=cron
# Install packages for our application
RUN apt-get update && apt-get -y install \
# Cron to periodically run our application
cron=3.0pl1-134+deb10u1 \
# Give execution rights on the cron job
&& chmod 0644 /etc/cron.d/synchronization.cron \
# Apply cron job
&& crontab /etc/cron.d/synchronization.cron
# Copy the JAR file into our image
COPY build/libs/sherby.synchronization.example.jar /opt/service/sherby.synchronization.example.jar
# Start cron in the foreground
CMD ["cron", "-f"]
Cron file
Instead of a normal log file as /var/log/cron.log 2>&1, I redirect the output directly to stdout/stderr as they are automatically collected by another tool in our infrastructure. 🏭
# Execute our application every minute (https://crontab.guru/#*_*_*_*_*)
* * * * * /opt/java/openjdk/bin/java -jar /opt/service/sherby.synchronization.example.jar > /proc/1/fd/1 2>/proc/1/fd/2
# An empty line is required at the end of this file for a valid cron file.
synchronization.cron file
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