DC Poor Cron – A Cron Without Cron

Since I’m just hosting my site and this blog on a free host – thanks to ByetHost for this; I don’t have the cron functionality. After several days of iterations, I have not come up with this cronless cron and I call this DC Poor Cron. For those who don’t have cron, you can checkout the code on my github acount.

Get the code here: DC Poor Cron on github

When to use?

• Your hosting does not have cron
• Free online cron is limited to a few seconds

Requirements

• Bash shell
• CURL – utility
• screen – utility or as long as you can have continous session 24/7

How does it works?

The poor cron script is a bash shell script that runs an inifinite loop while it triggers the cron workers. It uses a counter which ticks every MINUTE and resets after 1440 cycle which roughly equivalent to 24 hours ++ or so.

The default sleeps for a minute and cycles for 1440 rounds.

poor-cron.sh – you may not have to configure this file. This is the front end of the cron where the infinite loop/sleep happens.

poor-cron-worker.sh – this is where you put your curl commands that will in turn call your web based cron jobs of your application.

How to use?

For the scheduler to work, you need to think of the shedule you want. Currently, the cron does not work on exact time/date and will never be. It is designed for jobs on certain intervals such as every 5 minutes, every 30 minutes or every day (once a day).

The poor-cron.sh will pass a minute parameter to the poor-cron-worker.sh and using modulus we can achieve simple scheduling. For example:

To run jobs every 5 minutes: MINUTE % 5 = 0
To run jobs every 30 minutes: MINUTE % 30 = 0
To run jobs every hour: MINUTE % 60 = 0
To run jobs every day: MINUTE = 0

The example below calls the web applications mail queue to run every 5 minutes:

# For mail queue
# Check for divisible by 5 - every 5 minutes
let "dc_poor_cron_divfive = $1 % 5"

if [[ "$dc_poor_cron_divfive" = 0 ]]; then
	echo "$dc_poor_cron_now Calling mail queue" >> $dc_poor_cron_logfilename
	curl http://www.yoursite.com/cron/mail/run/token
fi

This assumes that your site has a web based cron job that needs to be triggered. Be sure to include security and validation to avoid abuse from malicious users.

How to set it up to run 24/7

If you invoke ~/path/to/poor-cron.sh & it will run the cron in the background. You should maintain it running and should not log out. If you can assure this, then you now have this poor cron running.

However, if you need to logout but the server will still be running 24/7, we can use the screen utility to achieve that.

• Invoke screen
• Invoke the script ~/path/to/poor-cron.sh &
• Press CTRL+a then press d to detach the screen session
• You can now logout freely yet your cron is running

To terminate the cron you have to re-attach the screen session.

• Invoke screen -list to view running screen sessions
• Invoke screen -r pid (pid is the screen session id)
• Invoke kill pid (pid here is the process id of the script) – kill %1 may work if there are no other background jobs running.