Docker container environment variable file during runtime

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I have a docker image that basically schedules a cron job at a frequency defined when building the image using the below.

COPY myjobtime /etc/cron.d/myjobtime
RUN chmod 0644 /etc/cron.d/myjobtime &&\
    crontab /etc/cron.d/myjobtime
CMD cron

I have the cron entry in the file myjobtime.

*/10 * * * * /usr/local/bin/sh /app/myscript.py

I would like to be able to pass the cron schedule during the runtime. Meaning, if someone wants to modify the cron schedule to a different frequency, they should be able to do that while running the container and passing an environment variable file with the new cron schedule in it. Can this be done?

docker containers cron
1 Answers

The important detail is that you need to create and install the crontab file when the container starts up. I find an entrypoint wrapper script to be a useful pattern for this: set the image's ENTRYPOINT to be a shell script that does whatever first-time setup is required, then have it exec "$@" to run the image's CMD.

If your image is ultimately based on a Linux distribution based on the GNU toolset, then envsubst is a really helpful program here. It reads in a text file, expands environment variable references, and writes out the result. I'll assume you have this available; on Alpine-based images you can do similar tricks with sed(1) (though escaping around the cron schedule will become tricky).

This makes the entrypoint wrapper script something like:

#!/bin/sh
# entrypoint.sh

# Set a default schedule, if the user didn't provide one
if [ -z "$CRON_SCHEDULE" ]; then
  export CRON_SCHEDULE='*/10 * * * *'
fi

# Run substitutions on the template file and inject the crontab
envsubst < /app/myjobtime.cron.tmpl | crontab

# Run the main container command
exec "$@"

Since the template isn't a "normal" crontab, it can't go in the "normal" crontab directory; putting it in the application directory is fine. That file has an environment variable reference where the schedule would go

# myjobtime.cron.tmpl
${CRON_SCHEDULE} /app/myscript.py

In your image, set the wrapper script to be the ENTRYPOINT, make sure the template file is in the right place, and leave the CMD unchanged.

# (assuming there's not a broad `COPY . .`)
COPY myjobtime.cron.tmpl .
COPY entrypoint.sh .

ENTRYPOINT ["/app/entrypoint.sh"] # must be JSON-array syntax
CMD cron                          # unchanged

This should allow you to override the cron schedule.

docker run -d --name hourly myappcron
docker run -d --name daily -e 'CRON_SCHEDULE=0 0 * * *' myappcron

Since the entrypoint wrapper script runs whatever command was provided, and you can override the command pretty easily, this also lets you double-check that the right schedule got set.

docker run --rm -e 'CRON_SCHEDULE=0 0 * * *' myappcron \
  crontab -l  # runs instead of the cron daemon
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