Run a Cron Job Inside of a Docker Container
External
Internal
Overview
The procedure consists in building an image that contains cron and that runs the cron daemon in foreground. If we want to use the standard pattern of configuring the application that runs in a container with environment variables, and assuming that the "application" is the job that is periodically run by cron, we need to launch cron with a script that collects required environment variables and places them in /etc/crontab, to be made available to the job. This is because cron controls the environment of the jobs that manage, and they do not have access to the standard environment.
Dockerfile
The Job Does Not Care about Environment Variables
FROM centos:latest RUN yum install -y crontabs RUN echo "* * * * * root /opt/activity >> /tmp/cron.log 2>&1" >> /etc/crontab COPY ./activity /opt/activity ENTRYPOINT ["/usr/sbin/crond", "-n"]
The Job Is Configured with Environment Variables
FROM centos:latest RUN yum install -y crontabs COPY ./cron-launcher /opt/cron-launcher ENTRYPOINT ["/opt/cron-launcher"]
where cron-launcher must be made executable, and should be similar to:
#!/usr/bin/env bash
echo "JAVA_HOME=${JAVA_HOME}" >> /etc/crontab
echo "ENV_VAR_1=${ENV_VAR_1}" >> /etc/crontab
echo "ENV_VAR_2=${ENV_VAR_2}" >> /etc/crontab
echo "* * * * * root /opt/my-periodical-job >> /var/log/cron.log 2>&1" >> /etc/crontab
exec /usr/sbin/crond -n
ENV_VAR_1, ENV_VAR_2 are picked up form the container's environment and written into /etc/crontab, thus will be made available in the job's environment when it runs.
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.