Posted on Wednesday September 7, 2016

Running cron jobs inside a Docker container

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.

Some relevant Stackoverflow posts that helped: