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/env.sh` /bin/bash -c $1 >> /var/log/cron.log 2>&1|
|# Run every 30 minutes on Wednesday|
|*/30 * * * 3 /root/cron_task.sh /root/run_app.sh|
|# cron needs an empty line at the end, leave this here.|
|# 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.|
|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/cron_task.sh|
|RUN chmod 750 /root/run_redditbot.sh|
|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/env.sh && tail -f /var/log/cron.log|
Some relevant Stackoverflow posts that helped: