Inside Docker container, cronjobs are not getting executed

Question

I have made a Docker image, from a Dockerfile, and I want a cronjob executed periodically when a container based on this image is running. My Dockerfile is this (the relevant parts):

FROM l3iggs/archlinux:latest

COPY source /srv/visitor

WORKDIR /srv/visitor

RUN pacman -Syyu --needed --noconfirm \
        && pacman -S --needed --noconfirm make gcc cronie python2 nodejs phantomjs \
        && printf "*/2 * * * *       node /srv/visitor/visitor.js \n" >> cronJobs \
        && crontab cronJobs \
        && rm cronJobs \
        && npm install -g node-gyp \
        && PYTHON=/usr/sbin/python2 && export PYTHON \
        && npm install

EXPOSE 80

CMD ["/bin/sh", "-c"]

After creation of the image I run a container and verify that indeed the cronjob has been added:

crontab -l

*/2 * * * *     node /srv/visitor/visitor.js

Now, the problem is that the cronjob is never executed. I have, of course, tested that "node /srv/visitor/visitor.js" executes properly when run manually from the console.

Any ideas?

Answer 1

One option is to use the host's crontab in the following way:

0 5 * * * docker exec mysql mysqldump --databases myDatabase -u myUsername -pmyPassword > /backups/myDatabase.sql

The above will periodically take a daily backup of a MySQL database.

If you need to chain complicated commands you can also use this format:

0 5 * * * docker exec mysql sh -c 'mkdir -p /backups/`date +\%d` && for DB in myDB1 myDB2 myDB3; do mysqldump --databases $DB -u myUser -pmyPassword > /backups/`date +\%d`/$DB.sql; done'

The above takes a 30 day rolling backup of multiple databases and does a bash for loop in a single line rather than writing and calling a shell script to do the same. So it's pretty flexible.

Or you could also put complicated scripts inside the docker container and run them like so:

0 5 * * * docker exec mysql /dailyCron.sh