Setting up schedules for scheduled tasks via a CRON expression

Introduction

cron is a UNIX tool that has been around for a long time, so its scheduling capabilities are powerful and proven.

ReflexBlue uses “cron expressions”, which are able to create firing schedules such as: “At 8:00am every Monday through Friday” or “At 1:30am every last Friday of the month”.

Cron expressions are powerful, but can be pretty confusing. This tutorial aims to take some of the mystery out of creating a cron expression, giving users a resource which they can visit before having to ask in a forum or mailing list.

Format

A cron expression is a string comprised of 6 or 7 fields separated by white space. Fields can contain any of the allowed values, along with various combinations of the allowed special characters for that field. The fields are as follows:

So cron expressions can be as simple as this: * * * * ? *

or more complex, like this: 0/5 14,18,3-39,52 * ? JAN,MAR,SEP MON-FRI 2002-2010

Special characters

• * (“all values”) - used to select all values within a field. For example, * in the minute field means “every minute”.
• ? (“no specific value”) - useful when you need to specify something in one of the two fields in which the character is allowed, but not the other. For example, if I want my trigger to fire on a particular day of the month (say, the 10th), but don’t care what day of the week that happens to be, I would put 10 in the day-of-month field, and ? in the day-of-week field. See the examples below for clarification.
• - - used to specify ranges. For example, 10-12 in the hour field means “the hours 10, 11 and 12”.
• , - used to specify additional values. For example, MON,WED,FRI in the day-of-week field means “the days Monday, Wednesday, and Friday”.
• / - used to specify increments. For example, 0/15 in the seconds field means “the seconds 0, 15, 30, and 45”. And 5/15 in the seconds field means “the seconds 5, 20, 35, and 50”. You can also specify / after the “character - in this case” is equivalent to having ‘0’ before the ‘/’. 1/3 in the day-of-month field means “fire every 3 days starting on the first day of the month”.
• L (“last”) - has different meaning in each of the two fields in which it is allowed. For example, the value L in the day-of-month field means “the last day of the month” - day 31 for January, day 28 for February on non-leap years. If used in the day-of-week field by itself, it simply means “7” or “SAT”. But if used in the day-of-week field after another value, it means “the last xxx day of the month” - for example 6L means “the last Friday of the month”. You can also specify an offset from the last day of the month, such as L-3 which would mean the third-to-last day of the calendar month. When using the L option, it is important not to specify lists, or ranges of values, as you’ll get confusing/unexpected results.
• W (“weekday”) - used to specify the weekday (Monday-Friday) nearest the given day. As an example, if you were to specify 15W as the value for the day-of-month field, the meaning is: “the nearest weekday to the 15th of the month”. So if the 15th is a Saturday, the trigger will fire on Friday the 14th. If the 15th is a Sunday, the trigger will fire on Monday the 16th. If the 15th is a Tuesday, then it will fire on Tuesday the 15th. However if you specify 1W as the value for day-of-month, and the 1st is a Saturday, the trigger will fire on Monday the 3rd, as it will not ‘jump’ over the boundary of a month’s days. The W character can only be specified when the day-of-month is a single day, not a range or list of days.

  The L and W characters can also be combined in the day-of-month field to yield LW, which translates to *“last weekday of the month”.

• # - used to specify “the nth” XXX day of the month. For example, the value of 6#3 in the day-of-week field means “the third Friday of the month” (day 6 = Friday and “#3” = the 3rd one in the month). Other examples: 2#1 = the first Monday of the month and 4#5 = the fifth Wednesday of the month. Note that if you specify #5 and there is not 5 of the given day-of-week in the month, then no firing will occur that month.

  The legal characters and the names of months and days of the week are not case sensitive. MON is the same as mon.

Examples

Here are some full examples:

Notes