How to build a cron docker image properly?

Question

I am trying to build a Debian-based image to dockerize a cron process, but my cron jobs are never started. Here is my Dockerfile:

FROM debian:jessie
RUN apt-get update && apt-get install -y --no-install-recommends cron
COPY jobs.txt /etc/crontab
RUN touch /log.txt
CMD ["cron", "-f"]

...and the jobs.txt file:

* * * * * root echo "job done" >> /log.txt

I realized that there is something wrong with the COPY command, because when I replace

COPY jobs.txt /etc/crontab

with

RUN echo '* * * * * root echo "job done" >> /log.txt' > /etc/crontab

it works perfectly.

So is there a problem just with jobs.txt file and Docker unable to copy it the right way? Should I only fix that file, or use a completely different approach?

Solution

The only difference between using COPY and RUN are the permissions on the /etc/crontab file: with COPY this is 664 and with RUN 644.

I cannot find anything on permissions that /etc/crontab needs to have but if you add

RUN chmod 644 /etc/crontab

after the COPY line in your Dockerfile the cronjobs run (at least for me). So I think the permissions have to be 644

Comments

[jazgot]

Maybe try to add empty line at and of jobs.txt.

[Andrii Maletskyi]

@jazgot Yes, empty line at the end is a must, but yet it doesn't solve a problem.

[monty0]

It's because of the linking that happens in the overlay filesystem. See stackoverflow.com/questions/34962020/…