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I've tried setting up cron to run in my Docker container, but without success thus far.

This is the cron-related parts of the Dockerfile:

FROM ruby:2.2.2

# Add crontab file in the cron directory
RUN apt-get install -y rsyslog
ADD crontab /etc/cron.d/hello-cron

# Give execution rights on the cron job
RUN chmod +x /etc/cron.d/hello-cron

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

# Run the command on container startup
RUN service cron start

When I log on to the container instance, cron appears to be running:

$ service cron status
cron is running.

And /etc/cron.d has my job:

$ cat /etc/cron.d/hello-cron
* * * * * root echo "Hello world" >> /var/log/cron.log 2>&1

But nothing is appended to /var/log/cron.log, so it doesn't appear to run.

If I then, from within the container, runs $ cron it registers my hello-cron file and the log file will have "Hello world" appended every minute.

Ronni Egeriis Persson
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1 Answers1

2

Your analysis is correct, the cron jobs are not running. This happens because normally, and by best practices, the container only runs a single process, such as Apache, NGINX, etc. - it does not run any of the normal operating system daemons such as crond.

No crond means, there is nothing that would read or execute your crontab.

There are several possibilities to solve this, but no perfect solution that I know of.

  • The worst one is to actually install crond, along with something like supervisord. It makes your container dramatically more complex.

  • You can create a separate container that runs nothing but cron. Mount whatever you need from the other containers as volumes. This is generally the recommended option, but it has limitations. The cron container needs to know a lot about the internals of your other containers, and the cron jobs don't execute in the same context as the rest of the containers.

  • You can create a cron job on the host, and have it execute scripts in the containers with docker exec. That works well, but creates a dependency between host and container. It may also not work at all if you don't have access to the host's operating system (for instance, in a hosted situation, or if a different team manages the host).

Kevin Keane
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