Chris's coding blog

Running cron jobs inside a Docker container

September 07, 2016

Probably the most basic use of a Docker image is to run it, and let it exist - like docker run hello-world. If you want to do this on a regular schedule, you have several options but the cron one is narrowed down to running cron on the Docker host or in the container.

I wanted to run a dotnet core console app that parses Reddit posts and posts itself, inside a Docker container, every 30 minutes on set days of the week.

Running the Docker image as a daemon with cron running inside it makes it a lot easier to update the Docker image and also bring up on other machines.

There’s quite a few gotchas to doing this however, which I discovered and spent hours Googling the answers for. Most of the answers were dredged from Github repositories which luckily Google managed to find the keywords for. This included:

  • Cron doesn’t run as an interactive shell - you don’t get any environmental variables  - a well known restriction but nobody seemed to know the answer. I found my solution in this repository.
  • I’m running a .NET Core console app. It writes a temp file to its folder on the filesystem, but you don’t have permission to do this without CHMOD’ing the app directory.
  • Cron inside a Docker container can’t write to a cron.log so you end up having to map a volume. Without this you get the error “tail: unrecognized file system type 0x794c7630 for ‘/var/log/cron.log’“. This makes the Docker image rely on the host volume for storage, but it’s only a log file. The fix came from here.

Below is the Docker file and the scripts are with the Gist.

env - `cat /root/` /bin/bash -c $1 >> /var/log/cron.log 2>&1
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# Run every 30 minutes on Wednesday
*/30 * * * 3 /root/ /root/
# cron needs an empty line at the end, leave this here.
view raw crontab hosted with ❤ by GitHub
FROM microsoft/dotnet
# Fix from:
# Put cron logfiles into a volume. This also works around bug
# caused by base image using old version of coreutils
# which causes "tail: unrecognized file system type 0x794c7630 for '/var/log/cron.log'"
# when using docker with overlay storage driver.
VOLUME /var/log/
RUN mkdir /app
COPY ./src/MyApp/bin/Debug/netcoreapp1.0/publish /app
RUN apt-get update
RUN apt-get install -y cron
ADD *.sh /root/
RUN chmod 750 /root/
RUN chmod 750 /root/
RUN chmod 750 /app/
ADD crontab /var/spool/cron/crontabs/root
RUN chmod 0600 /var/spool/cron/crontabs/root
CMD touch /var/log/cron.log && cron && env > /root/ && tail -f /var/log/cron.log
dotnet /app/MyApp.dll
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Some relevant Stackoverflow posts that helped:

I'm Chris Small, a software engineer working in London. This is my tech blog. Find out more about me via GithubStackoverflowResume