Question

I’m building some software that needs a scheduling input, and I’d really like to re-use the design of crontab because it simply works.

CrontabExpressions can be really simple */5 * * * * “run every five minutes” or more complex 2-59/3 1,9,22 11-26 1-6 ? 2003 “In 2003 on the 11th to 26th of each month in January to June every third minute starting from 2 past 1am, 9am and 10pm”.

I am not looking to use the linux software called crontab, I’m seeking a way I can evaluate these expressions correctly (for instance, output the next 25 timestamps that match the crontab, or generate it based on some abstracted GUI for the users).

I can’t really find any libraries or functions that do this in JavaScript or PHP or even other languages. If they don’t exist, what would be a good method to do this? I already know an overly-complicated regular expression is likely to be the wrong answer. I’m having a hard time finding the C source code in crontab that does this task as well, which makes me believe it might not take place here?

Answer 1

To output the next 25 timestamps that match the crontab you could use crontab Python module:

from datetime import datetime, timedelta
import crontab

tab = crontab.CronTab('2-59/3 1,9,22 11-26 1-6 ? 2012')

dt = datetime.now()
for _ in xrange(25):
    delay = tab.next(dt) # seconds before this crontab entry can be executed.
    dt += timedelta(seconds=delay)
    print(dt)

Output

2012-01-11 22:41:00
2012-01-11 22:44:00
2012-01-11 22:47:00
2012-01-11 22:50:00
2012-01-11 22:53:00
2012-01-11 22:56:00
2012-01-11 22:59:00
2012-01-12 01:02:00
2012-01-12 01:05:00
2012-01-12 01:08:00
2012-01-12 01:11:00
2012-01-12 01:14:00
2012-01-12 01:17:00
2012-01-12 01:20:00
2012-01-12 01:23:00
2012-01-12 01:26:00
2012-01-12 01:29:00
2012-01-12 01:32:00
2012-01-12 01:35:00
2012-01-12 01:38:00
2012-01-12 01:41:00
2012-01-12 01:44:00
2012-01-12 01:47:00
2012-01-12 01:50:00
2012-01-12 01:53:00

There is also python-crontab that provides crontab module but with richer functionality (parse/generate).