Cron - Time is Relative
Table of Contents
Motivation #
Looking to create a twitter bot - I settled on a simple Python script for scaping some data and publishing it to a twitter account twice a day. This bot would scrape NOAA and the OpenWeatherMap API for the combined air and water temperatures. Scrapy is a fantastic library for this task. Looking to create a twitter bot - I settled on a simple Python script for scaping some data and publishing it to a twitter account twice a day. This bot would scrape NOAA and the OpenWeatherMap API for the combined air and water temperatures. Scrapy is a fantastic library for this task.
Here’s the source code for the curious: Source
Ready to deploy #
After running some tests - our bot appears to be posting to Twitter manually. But we want to send out our tweets twice a day, not manually. Enter Crontab, the ubiquitous Unix task scheduler.
It was decided that we wanted to run at 7 AM and 4 PM respectively each day. Since cron runs on a 24-hour clock our expression looks something like this:
0 7,16 * * * cd /opt/python/lpbcbot && source setenv.sh && ./bin/python main.py
07 hours being 7 AM and 16 hours being 4 PM. Setting the crontab locally and waiting for the job to run we see our Twitter bot posting to Twitter automagickly:
*Apparently the timestamp you see on the tweet is a bit off sometimes*
Great! Lets deploy the bot to a server. The README.md found in the repo goes as to how to do this.
6 AM the next day #
My phone buzzes and I see my Twitter bot hard at work telling me the weather. I look at my watch.. 6 AM? We told our bot to tweet at 7 AM…
After thinking about it for a bit I remembered that the server is a Digital Ocean instance in New York.. an hour ahead of Chicago.
*map from [AM Charts](http://pixelmap.amcharts.com)*
To confirm, I ssh into the DO box and lo and behold:
> sudo dpkg-reconfigure tzdata
Current default time zone: 'America/New_York'
Local time is now: Wed Apr 13 01:28:18 EDT 2016.
Universal Time is now: Wed Apr 13 05:28:18 UTC 2016.
Little did I know, crontab runs jobs on the local time of the server, not universal time.
Lesson learned - to fix it I just ticked forward my crontab settings to read:
0 8,17 * * * cd /opt/python/lpbcbot && source setenv.sh && ./bin/python main.py
since the VPC does live in a New York datacenter and this box supports several other applications, I decided changing the system clock was a bit drastic.