botbotbot 's blog

Crontab on Docker

You can start crontab following Run a cron job with Docker just put crontab in /etc/cron.d/ and run cron for making it working.

However when you try something that complex than echo "hello world" sometimes it just not working and you don’t know why!!! You had alreay tried to run your script that working on your linux but no idea why not working on your docker.

rozkosz said:

 Several issues that I faced while trying to get a cron job running in a docker container were:
 1. time in the docker container is in UTC not local time;
 2. the docker environment is not passed to cron;
 3. as Thomas noted, cron logging leaves a lot to be desired and accessing it through docker requires a docker-based solution.

I think the problem that most people face is cron not run your script with same environment as your docker in terminal. You need to export your Docker environment to your cron that I found solution from postgres-s3-backup Dockerfile. You can following this for simple solution:

  1. In Dockerfile, export Docker environment to /root/env.txt
  CMD env > /root/env.txt && cron && your_init_script
  1. In your crontab file, You need to to import your Docker environment before run your script
  */5 * * * *  root env - `cat /root/env.txt`  your_scipt  >> /var/log/cron.log 2>&1
  # An empty line is required at the end of this file for a valid cron file.
  1. Test it. Your should put date in your script for debug and then cat /var/log/cron.log to see your results.

For the local timezone problem your can setup your Dockerfile following this

  • Debian/Linux Setting timezone

       # Set the time zone to the local time zone
       RUN echo "Asia/Bangkok" > /etc/timezone && dpkg-reconfigure --frontend noninteractive tzdata
  • Alpine Setting the timezone

      # Alpine Linux
      # set bangkok time
      USEo root
      RUN apk add --update tzdata \
      && cp /usr/share/zoneinfo/Asia/Bangkok /etc/localtime \
      && echo "Asia/Bangkok" >  /etc/timezone \
      && apk del tzdata \
      && mv /etc/localtime /usr/share/zoneinfo/Asia/Bangkok
      # or
      USER root
      RUN apk add --update tzdata
      ENV TZ=Asia/Bangkok
  • Will docker container auto sync time with the host machine?

      $ docker run -it -v /etc/localtime:/etc/localtime:ro < image >