Run a Cron Job Inside of a Docker Container

External

• https://www.ekito.fr/people/run-a-cron-job-with-docker/

Internal

• cron
• Docker Recipes

Overview

The procedure consists in building an image that contains cron, and that runs the cron daemon in foreground as shown below:

/usr/sbin/crond -n -s

The daemon is configured to run in foreground, and also to log into syslog instead of sending e-mails (not tested).

Dockerfile

FROM centos:latest
RUN yum install -y crontabs logrotate
RUN echo "* * * * * root /opt/activity >> /tmp/cron.log 2>&1" >> /etc/crontab
COPY ./activity /opt/activity
ENTRYPOINT ["/usr/sbin/crond", "-n"]

cron Job Output

We do not want crond to e-mail stdout/stderr of the jobs it runs, but rather to log them locally, as shown here.

Note that this will write into the writable layer of the container, so either we should design the cron jobs to output nothing on success, and only use this mechanism for debugging.