Cron Scheduler with Docker Container in CentOS/RHEL 7

Using cron with the official CentOS Docker image requests activating systemd, keeping container running, and opening a Docker container in privileged mode. CentOS Docker Hub image includes the description of the setup which should be done to activate systemd and keep a container going. The missing is information how to install and set up cron. This article provides a summary of steps and a functioning sample of Dockerfile. Some customization is needed to implement your cron project.

Dockerfile

The Dockerfile can be used as a template to design your file. After line #15, you can add your commands to install any packages. You need to replace line #20 with your time zone. Finally, line #22 shows how to add a scheduled job to crontab file.

ROM centos:7

ENV container docker

RUN (cd /lib/systemd/system/sysinit.target.wants/; for i in *; do [ $i == \
systemd-tmpfiles-setup.service ] || rm -f $i; done); \
rm -f /lib/systemd/system/multi-user.target.wants/*;\
rm -f /etc/systemd/system/*.wants/*;\
rm -f /lib/systemd/system/local-fs.target.wants/*; \
rm -f /lib/systemd/system/sockets.target.wants/*udev*; \
rm -f /lib/systemd/system/sockets.target.wants/*initctl*; \
rm -f /lib/systemd/system/basic.target.wants/*;\
rm -f /lib/systemd/system/anaconda.target.wants/*;

VOLUME [ "/sys/fs/cgroup" ]

RUN yum install -y cronie && yum clean all

RUN rm -rf /etc/localtime
RUN ln -s /usr/share/zoneinfo/America/Toronto /etc/localtime

RUN crontab -l | { cat; echo "25 04 * * sun,mon,tue python3 /app/do_maintenance.py"; } | crontab -

CMD ["/usr/sbin/init"]

Dockerfile logical parts

• Activating systemd.

  RUN (cd /lib/systemd/system/sysinit.target.wants/; for i in *; do [ $i == \
  systemd-tmpfiles-setup.service ] || rm -f $i; done); \
  rm -f /lib/systemd/system/multi-user.target.wants/*;\
  rm -f /etc/systemd/system/*.wants/*;\
  rm -f /lib/systemd/system/local-fs.target.wants/*; \
  rm -f /lib/systemd/system/sockets.target.wants/*udev*; \
  rm -f /lib/systemd/system/sockets.target.wants/*initctl*; \
  rm -f /lib/systemd/system/basic.target.wants/*;\
  rm -f /lib/systemd/system/anaconda.target.wants/*;
  
  VOLUME [ "/sys/fs/cgroup" ]
  

• Install cron.

  RUN yum install -y cronie && yum clean all
  

• Set up your time zone.

  RUN rm -rf /etc/localtime
  RUN ln -s /usr/share/zoneinfo/America/Toronto /etc/localtime
  

• Add a job in crontab file.

  RUN crontab -l | { cat; echo "25 04 * * sun,mon,tue python3 /app/do_maintenance.py"; } | crontab -
  

• Keep container running.

  CMD ["/usr/sbin/init"]
  

Build conatiner

The image name is c7-cron and it's designated as local one.

docker build --rm -t local/c7-cron .

Run container

To create and start a container, use the command.

docker run --privileged --name=parking –v /sys/fs/cgroup:/sys/fs/cgroup:ro -d local/c7-cron

To access the container in a terminal, run the command.

docker exec -it parking /bin/bash