How to run a cron job inside a docker container

Question

I tried to run a cron job inside a docker container but nothing works for me.
My container has only cron.daily and cron.weekly files.
crontab,cron.d,cron.hourly are absent in my container.
crontab -e is also not working.
My container runs with /bin/bash.

Answer 1

Here is how I run one of my cron containers.

Dockerfile:

FROM alpine:3.3  ADD crontab.txt /crontab.txt ADD script.sh /script.sh COPY entry.sh /entry.sh RUN chmod 755 /script.sh /entry.sh RUN /usr/bin/crontab /crontab.txt  CMD ["/entry.sh"] 

crontab.txt

*/30 * * * * /script.sh >> /var/log/script.log 

entry.sh

#!/bin/sh  # start cron /usr/sbin/crond -f -l 8 

script.sh

#!/bin/sh  # code goes here. echo "This is a script, run by cron!" 

Build like so

docker build -t mycron . 

Run like so

docker run -d mycron 

Add your own scripts and edit the crontab.txt and just build the image and run. Since it is based on alpine, the image is super small.

Answer 2

crond works well with tiny on Alpine

RUN apk add --no-cache tini  ENTRYPOINT ["/sbin/tini", "--"] CMD ["/usr/sbin/crond", "-f"] 

but should not be run as container main process (PID 1) because of zombie reaping problem and issues with signal handling. See this Docker PR and this blog post for details.