Running a cron job in a docker conratiner

Question

I am trying to execute a cron job in a docker container. So, I checked my container and seems crontab, cron.d, cron.hourly files are absent from my container. cron.daily and cron.weekly are present. I'm running the container using /bin/bash.

Comments

How is running cron job on a host different from running it on a container?

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Running a cron job on a host is the most preferable method. It's very easy to set up. All you have to do is ssh into your host and if its Linux distro then cron is anyways available.

Only when the above is not possible it is advised to bring up a container with cron

Answer 1

This is the way I ran 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 file:

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

entry.sh file:

#!/bin/sh

# start cron
/usr/sbin/crond -f -l 8

script.sh file:

#!/bin/sh

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

to buiild:

docker build -t mycron .

and finally to run:

docker run -d mycron

add the scripts you want, edit the crontab.txt and build the image. It should run now.

Answer 2

Let’s create a new file called "crontab" to describe our job.

The following DockerFile describes all the steps to build your image

FROM ubuntu:latest
MAINTAINER docker@ekito.fr
RUN apt-get update && apt-get -y install cron
# Add crontab file in the cron directory
ADD crontab /etc/cron.d/hello-cron
# Give execution rights on the cron job
RUN chmod 0644 /etc/cron.d/hello-cron
# Apply cron job
RUN crontab /etc/cron.d/hello-cron
# Create the log file to be able to run tail
RUN touch /var/log/cron.log
# Run the command on container startup
CMD cron && tail -f /var/log/cron.log

Build and run it:

sudo docker build --rm -t ekito/cron-example .
sudo docker run -t -i ekito/cron-example