docker - no crontab for root

Question

My Dockerfile appears to build correctly (it tells me so). When I run the container, I get the below error message. I have tried running the commands (CMD) with and without the service's directory.

crontab.sh basically writes a cron schedule to a text file (cron.jobs) and then imports the text file to crontab.

Dockerfile:

FROM node:0.10
MAINTAINER Tom

VOLUME /var/log/

RUN mkdir /pulse
ADD . /pulse
WORKDIR /pulse

RUN apt-get update && apt-get install -y cron

ADD *.sh /pulse/
RUN chmod 750 /pulse/crontab.sh && chmod 750 /pulse/

RUN chmod 644 /etc/crontab

CMD cron -f
CMD touch /var/log/cron.log && sh /pulse/crontab.sh && tail -f /var/log/cron.log
CMD cron /pulse/cron.jobs
CMD crontab -l

edited to add crontab.sh

crontab.sh (some crons have been removed):

#!/bin/bash

cat <<- 'EOF' > cron.jobs

0 * * * * node /pulse/scripts/awsPulseTest.js > /tmp/awsPulseTest.log 2>&1

EOF

crontab cron.jobs

Error:

no crontab for root

Side notes:

• Pulse is the name of the service.
• The version of node is old due to the service, this will be upgraded.
• The service is essentially for cron jobs in node

Answer 1

It's an issue with the dockerfile (rather than the commands in the file). Only one CMD is run (the last one) - see https://docs.docker.com/engine/reference/builder/#cmd

There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the last CMD will take effect.

Answer 2

As the other answers have already explained, only one CMD will be run per Dockerfile and the command you want to run is wrong.

But there is a more pressing issue with your setup IMO - Docker containers are not usually designed to work this way. What you should do instead is running the cron services from the host (or your orchestrator) as one-off processes (probably using something like docker run or docker-compose run, or, if for some reason you don't want to start a separate container for this, I guess you could use docker exec).

This is just my view on how containers should be used though, so obviously you should take it with a grain of salt.

Answer 3

If you add this to /etc/crontab, this wouldn't show up in root's personal crontab, as this contains only the user-specific crontab edited with crontab -e, not the system-wide one in /etc.

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More details:

My guess is that /pulse/crontab.sh (which you don't show, why?) adds the relevant crontab line to the system wide crontab file /etc/crontab. You later execute the command crontab -l, but this only shows an error because it lists roots personal crontab only (which happens to be empty), not the system-wide one in /etc/crontab. This is all perfectly normal and expected. To show the line your script added, you would replace CMD crontab -l with CMD cat /etc/crontab.

All of this has nothing to to with any dockerfile commands like ADD, RUN or CMD, it's just basic Linux stuff.