I have a simple Dockerfile as follows

FROM ubuntu:latest
ADD crontab /etc/cron.d/test-cron
RUN chmod a+x /etc/cron.d/test-cron
RUN touch /var/log/cron.log
CMD cron && tail -f /var/log/cron.log

and the content of crontab file is as simple as

* * * * * root echo "Hello world" >> /var/log/cron.log 2>&1
# empty line

When I run this on my local OS X machine (with docker-machine running), it works fine ("Hello world" is printed to log file every minute). However, when I try to run it on an Ubuntu machine, the cron job does not run (empty log file).

Here's the command I use to run the container

docker build -t crontest .
docker run --name cron crontest

I am not sure why this would be the case. I wonder if something is wrong with the Ubuntu box that I have (wrong time setting?). I have tried to restart that machine to no effect. I currently do have other docker containers running on the Ubuntu box and they're running fine.

Any suggestion on what I could do to debug/ fix this would be hugely appreciated.

EDIT:

After going inside the container (docker exec -it cron /bin/bash), I can verify that cron is running there:

root@a2ad451af8d9:/# ps -ef | grep cron
root         1     0  0 20:15 ?        00:00:00 /bin/sh -c cron && tail -f /var/log/cron.log
root         6     1  0 20:15 ?        00:00:00 cron
root         7     1  0 20:15 ?        00:00:00 tail -f /var/log/cron.log
root        25    11  0 20:21 ?        00:00:00 grep --color=auto cron

Install rsyslog inside the container with apt-get install rsyslog and launch it with the command rsyslogd before starting cron with cron -L15 (maximum logging). Then watch the file /var/log/syslog inside the container to see the cron daemon's own log output. It will tell you if there was a problem parsing your crontab and it will, in your case, log an entry every minute similar to the below if it has registered and is trying to run your job.

CRON[16]: (root) CMD (echo "Hello world" >> /var/log/cron.log 2>&1)