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TL;DR

If you are having trouble with cron not working on Docker Ubuntu, you can make it work by making your Dockerfile like below.

You can stop using CMD at the end and change it to RUN cron -f & or something like that.

FROM ubuntu:18.04

RUN apt-get update && apt-get install cron

RUN echo '* * * * * echo Hello World > /var/log/cron.log' > /etc/cron.d/crontab

RUN chmod 0644 /etc/cron.d/crontab

RUN /usr/bin/crontab /etc/cron.d/crontab

CMD ["cron", "-f"]

Why I’m in trouble

This website is built and running in Docker using Hugo.

I decided to use cron to refresh the repo information obtained from the GitHub API on the main page.

If I store the json retrieved from the API in the static folder and make some change, Hugo will rebuild the entire website, so I can keep it up-to-date.

Therefore, I used cron like this:

# Save json retrieved from API every hour at 0 minute
0 * * * * wget -O - "https://api.github.com/users/nagataaaas/repos?type=owner&sort=updated&per_page=100" --no-check-certificate >  /static/github_repos.json

So you can refresh the repo information every hour.

However, (and I didn’t know this), you can’t use cron by default within Docker (at least ubuntu:18.04).

So I’ve tried everything, but cron is not running in any way.

After much searching, I finally found this repository.

Before I found this, I tried the following without success

FROM ubuntu:18.04

RUN apt-get update && apt-get install -y cron --no-install-recommends

# setting cron
RUN echo '0 * * * * wget -O - "https://api.github.com/users/nagataaaas/repos?type=owner&sort=updated&per_page=100" --no-check-certificate >  /static/github_repos.json' >> /etc/crontab
RUN cron -f &

or

FROM ubuntu:18.04

RUN apt-get update && apt-get install -y cron --no-install-recommends

RUN /etc/init.d/cron start
RUN crontab -l > /crontab.txt
RUN echo '0 * * * * wget -O - "https://api.github.com/users/nagataaaas/repos?type=owner&sort=updated&per_page=100" --no-check-certificate >  /static/github_repos.json' >> /crontab.txt
RUN crontab /crontab.txt
RUN rm /crontab.txt

I don’t know exactly which one is causing the problem, but possible issues are

  • The --no-install-recommends option is specified during cron installation.
  • Wrong permissions for /etc/cron.d/crontab?
  • Didn’t run /usr/bin/crontab

I’m new to Linux, so I don’t know much about it, but I wonder if the --no-install-recommends option might not work correctly.

Anyway, I had to do this to get cron to work.