CRON Expression

Schedule of job execution is defined as CRON style expression . cron is a classical job scheduler in UNIX like OSes utilized to run periodical tasks. CRON expressions define schedule in calendar style. For instance, “at 8:00 each day from Monday to Friday” or “at 13:30 each last Friday of month”. CRON expressions is a powerful and simple tool which require not much of efforts to get accustomed to.

CRON Expression Syntax

CRON expression is a single line consists of 6 or 7 parts divided by spaces. Each part is a condition for schedule and all of them get joined in accordance by logical multiplication (AND). The scheme below shows the structure of CRON expression. Expression parts are marked with * and lines show descriptions.

 ┌───────────── Second
 │ ┌────────────── Minute
 │ │ ┌─────────────── Hour
 │ │ │ ┌──────────────── Day of month
 │ │ │ │ ┌───────────────── Month
 │ │ │ │ │ ┌───────────────── Day of week
 │ │ │ │ │ │ ┌───────────────── Year
 │ │ │ │ │ │ │
 * * * * * * *

CRON expression parts can contain any of the allowed values, along with various combinations of the allowed special characters for that part. Table below describes allowed values and allowed special characters for each part.

Part Name	Mandatory	Allowed Values	Allowed Special Characters
Second	Yes	0-59	, - * /
Minute	Yes	0-59	, - * /
Hour	Yes	0-23	, - * /
Day of Month	Yes	1-31	, - * / ? L W
Month	Yes	1-12 (1 - January)	, - * /
Days of week	Yes	1-7 (1 - Sunday)	, - * / ? L #
Year	No	1970-2099	, - * /

CRON Special Characters

*:Used to define all possible values. For example, * for minutes means “each minute”. ,:Used to enumerate values. For example, 2,4,6 for week days means “Monday, Wednesday and Friday”. -:Used to define values range. For example, 10-12 for hours means “10, 11 and 12 hours”. /:Used to define reiteration periods. For example, 0/15 for seconds means “0, 15, 30 и 45 seconds” while 5/15 means “5, 20, 35 and 50 seconds”; 1/3 for month days means “every 3 days from 1st day of month”. ?:Represents lack of specific value. Using of this character is allowed in one of the two parts - day of month or day of week, but not in both at once. For example, to plan a job execution on 10th day of each month and each week then day of month is defined as 10 while day of week is defined as ?. L:Has different meaning in each of the two parts - day of month and day of week. For example, value L for day of month means “the last day of the month” (31 for January, 29 for February in leap year). The same value for day of week means “the last day of week” - 7 (Saturday). However if used in the day of week parts after another value, it means “the last defined week day of month”. For example, 6L means “the last Friday of month”. You can also specify an offset from the last day of the month, such as L-3. For example, L-3 for day of month means “before 3 days until last day of the month”. W:Used to specify the weekday (Monday-Friday) nearest the given day of month. For example, 15W for day of month means “the nearest weekday to the 15th of the month”. For example, if 15th is Saturday, job will be executed on 14th on Friday. If 15th is Sunday, job will be executed on 16th on Monday. If 15th is Thursday, job will be execute on 15th on Thursday. However if value is equal 1W and 1st is Saturday then job will be executed on 3rd on Monday, because this rule defines jobs to be executed within one month. Combination LW is allowed and means “last weekday day of the month”. #:Used to define the n-th day of week in a month. For example, 6#3 for day of week means “3rd Friday of month”, 2#1 - “the 1st Monday of the month”, 4#5 - “the 5th Wednesday of the month”.

Defining CRON Expression

CRON expression can be used when job info is created. In this case one of CronExpression() overloads can be used.

 IJobInfoFactory factory;

 ...

 // Job "MyJob" will be executed daily
 // at 10:35 by MyJobHandler handler
 factory.CreateJobInfo<MyJobHandler>("MyJob",
     b => b.CronExpression("0 35 10 * * ?"))

As you can see CRON expressions are simple and main principle of building expressions is quite clear. But it is quite easy to forget meaning of parts CRON expression or some rules of building expressions. So the CronExpression() method has a few overloads which uses DSL (Domain Specific Language) concept. DSL is represented as fluent interface. Next example shows recently reviewed example but with using DSL-version of the CronExpression() method.

 IJobInfoFactory factory;

 ...

 // Job "MyJob" will be executed daily
 // at 10:35 by MyJobHandler handler
 factory.CreateJobInfo<MyJobHandler>("MyJob",
     b => b.CronExpression(e => e.AtHourAndMinuteDaily(10, 35)))

CRON Expressions Examples

You can see examples of CRON expressions below: left - original CRON expression, right - lambda-expression to build the same expression with using ICronExpressionBuilder.

* * * * * ?:

// Each second.
b => { }

0 0 12 * * ?:

// Daily at 12:00.
b => b.AtHourAndMinuteDaily(12, 00)

0 15 10 * * ?:

// Daily at 10:15.
b => b.AtHourAndMinuteDaily(10, 15)

0 * 14 * * ?:

// Daily each minute from 14:00 to 14:59.
b => b.Hours(i => i.Each(14))
      .Minutes(i => i.Every())
      .Seconds(i => i.Each(0))

0 0/5 14 * * ?:

// Daily each 5 minute from 14:00 to 14:55.
b => b.Hours(i => i.Each(14))
      .Minutes(i => i.Each(0, 5))
      .Seconds(i => i.Each(0))

0 0/5 14,18 * * ?:  

// Daily each 5 minutes from 14:00 to 14:55 and from 18:00 to 18:55.
b => b.Hours(i => i.EachOfSet(14, 18))
      .Minutes(i => i.Each(0, 5))
      .Seconds(i => i.Each(0))

0 0-5 14 * * ?:

// Daily each minute с 14:00 по 14:05.
b => b.Hours(i => i.Each(14))
      .Minutes(i => i.EachOfRange(0, 5))
      .Seconds(i => i.Each(0))

0 10,44 14 ? 3 4:  

// Each Wednesday of March at 14:10 and 14:44.
b => b.Hours(i => i.Each(14))
      .Minutes(i => i.EachOfSet(10, 44))
      .Seconds(i => i.Each(0))
      .Month(i => i.Each(Month.March))
      .DayOfWeek(i => i.Each(DayOfWeek.Wednesday))

0 15 10 ? * 2-6:  

// Each day from Monday to Friday at 10:15.
b => b.AtHourAndMinuteDaily(10, 15)
      .DayOfWeek(i => i.EachOfRange(DayOfWeek.Monday, DayOfWeek.Friday))

0 15 10 15 * ?:

// 15th each month at 10:15.
b => b.AtHourAndMinuteDaily(10, 15)
      .DayOfMonth(i => i.Each(15))

0 15 10 L * ?:

// Last day of month each month at 10:15.
b => b.AtHourAndMinuteDaily(10, 15)
      .DayOfMonth(i => i.EachLast())

0 15 10 L-2 * ?:  

// Before 2 days until last day of every month at 10:15.
b => b.AtHourAndMinuteDaily(10, 15)
      .DayOfMonth(i => i.EachLast(2))

0 15 10 ? * 6L:

// Each last Friday of every month at 10:15.
b => b.AtHourAndMinuteDaily(10, 15)
      .DayOfWeek(i => i.EachLast(DayOfWeek.Friday))

0 15 10 ? * 6L 2016-2020:  

// Each last Friday of every month at 10:15 from 2016 to 2020 год.
b => b.AtHourAndMinuteDaily(10, 15)
      .DayOfWeek(i => i.EachLast(DayOfWeek.Friday))
      .Year(i => i.EachOfRange(2016, 2020))

0 15 10 ? * 6#3:  

// Each 3rd Friday of every month at 10:15.
b => b.AtHourAndMinuteDaily(10, 15)
      .DayOfWeek(i => i.EachNth(DayOfWeek.Friday, 3))

0 0 12 1/5 * ?:

// Each 5 days from 1st day of every month at 12:00.
b => b.AtHourAndMinuteDaily(12, 00)
      .DayOfMonth(i => i.Each(1, 5))

0 11 11 11 11 ?:  

// Every 11th November at 11:11.
b => b.AtHourAndMinuteDaily(11, 11)
      .DayOfMonth(i => i.Each(11))
      .Month(i => i.Each(Month.November))

0 15 10 ? * 2,4,6:  

// Each Monday, Wednesday and Friday at 10:15.
b => b.AtHourAndMinuteOnGivenDaysOfWeek(10, 15,
            DayOfWeek.Monday,
            DayOfWeek.Wednesday,
            DayOfWeek.Friday)

0 15 10 1,10,15 * ?:  

// 1th, 10th and 15th day at 10:15.
b => b.AtHourAndMinuteMonthly(10, 15,
            1, 10, 15)