Cron jobs using Whenever with VM, Docker & Dokku

Took me forever to work this one out - ended up call the rails command instead via crontabs. I've also got a rails app uploaded on dokku with ubuntu on the digital ocean server. Trying to get the Whenever gem to work... it just doesn't. whenever -i doesn't work.

Whenever doesn't actually create any new crontabs for the dokku environment. It's good for figuring out the Cron syntax though!

So this is how I got scheduled tasks to work in dokku:

1. Manually create your own crontab via sudo crontab -e which will open it up in vi/vim

You can use sudo crontab -r to remove it, or sudo crontab -l to view current crontabs

2. Add the following code to the new crontab

The below code will execute every 1 minute.

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
* * * * * /bin/bash -c 'dokku run appname rails r MyModel.run_something'

Make your environmental variables are equivalent to ones you have in the env command

3. grep CRON /var/log/syslog to see the output log for trouble shooting. You may have to install postfix via sudo apt-get install postfix in order for Cron to send mail notification of errors otherwise you may get “(CRON) info (No MTA installed, discarding output)” error from the syslog.

4. cat /var/mail/root to view the mail received from Cron - stating errors if a cronjob fails to work.

Hopefully that's helpful. That's what got me through at least!