Understanding CRON Expressions

Many Azure resources like web jobs, Azure functions, logic apps use CRON expression when they need to work with Time Triggers. A CRON expression lets you define a time trigger with the help of six fields. All the possible time triggers let it be 'Once an hour' to '14 times a day' can be defined using a CRON expression.

Understanding the expression

As mentioned earlier, a CRON expression includes six fields:

{second} {minute} {hour} {day} {month} {day-of-week}

And each field can have one of the following values:

Type	Example	Triggered
Specific Value	0 5 * * * *	At every hour at that hour past 5 minutes
All Values (*)	0 * 5 * * *	At the 5th hour of day (05:mm:00), trigger it every minute of the hour
A range ( - )	8-10 * * * * *	At hh:mm:08,hh:mm:09,hh:mm:10 where hh:mm is every minute of every hour. 3 times a minute at 8,9,10th second
A set ( , )	1,5,25 * * * * *	At hh:mm:01, hh:mm:05, hh:mm:25 where hh:mm is every minute of every hour. 3 times a minute at 1, 5, 25th second
An Interval ( / )	0 /5 * * *	At hh:05:00, hh:10:00, hh:15:00 and so on till hh:55:00, hh:00:00 where hh is every hour. 12 times an hour

CRON Examples

Some of the frequently used CRON expression samples

Expression	Triggered
/10 * * * *	Every 15 seconds
0 * * * * *	Every minute
15 * * * * *	Every minute at 15th Second of that minute
0 /10 * * *	Once every 10 minutes
0 0 * * * *	Once every hour at 0th minute
0 0 /4 * *	Once every 4 hours
0 0 9-17 * * *	Once every hour from 9AM to 5pm
0 0 0 * * *	Every day at 00:00:00
0 30 10 * * *	at 10:30 am every day
0 30 10 2 * *	at 10:30 am every 2nd day of every month
0 30 10 * * 0,6	at 10:30 am every weekend (Sat, Sun)
0 30 10 * * 1-5	at 10:30 am every weekday
0 0 * * * WED,FRI	Every hour on Wednesday and Friday
0 0 0 * * SUN	Every Sunday at 00:00:00
0 0 0 1-7 * SUN	Every first Sunday of the month at 00:00:00
0 0 0 1 * *	Every 1st of month at 00:00:00
0 0 0 1 1 *	Every 1st of January every year
0 0 * * 1 *	Every hour in January
30 5 /6 * *	Every 6 hours at 5 minute 30 seconds

Common pitfalls:

The CRON expression 0 0 */5 * * * means every 5 hours. So it executes at 00:00:00, 05:00:00, 10:00:00, 15:00:00, 20:00:00, 00:00:00 ... So it's not exactly every 5 hours. If you want a regular frequency, the following expressions are good:

For minutes and seconds : /2, /3, /4, /5, /6, /10, /12, /15, /20 and /30 (60 is divisible by these numbers)
For hours : /2, /3, /4, /6, /8 and /12
For Months : /2, /3, /4 and /6
For days : Nothing actually because there are leap years and months with 28, 29, 30 or 31 days.

If you want a cyclic approach, all other values can lead to "wrong" executions at the end of the cycle. e.g. Every 10 hours executes at 00:00:00, 10:00:00, 20:00:00, 00:00:00 (only 4 hours, not 10).

You can find CRON expression examples online, but many of them omit the {second} field. In case you copy one of them, remember to add the missing {second} field. Also, you will want a zero in that field and not an asterisk