A crontab expression is a very compact way to express a recurring schedule. 

It is composed of 5 space-delimited fields: MINUTES HOURS DAYS MONTHS DAYS-OF-WEEK

Each field is expressed as follows:

  • A single wildcard (*), which covers all values for the field. So a * in days means all days of a month (which varies with month and year).
  • A single value, e.g. 5. Naturally, the set of values that are valid for each field varies.
  • A comma-delimited list of values, e.g. 1,2,3,4. The list can be unordered as in 3,4,2,6,1.
  • A range where the minimum and maximum are separated by a dash, e.g. 1-10. You can also specify these in the wrong order and they will be fixed. So 10-5 will be treated as 5-10.
  • An interval specification using a slash, e.g. */4. This means every 4th value of the field. You can also use it in a range, as in 1-6/2.
  • You can also mix all of the above, as in: 1-5,10,12,20-30/5

The table below lists the valid values for each field:

Field

Range

Comment

MINUTES

0-59

 

HOURS

0-23

 

DAYS

0-31

 

MONTHS

1-12

Zero (0) is not valid. Month names also accepted.

DAYS-OF-WEEK

0-6

Where zero (0) means Sunday. Names of days also accepted.

 
Two fields also accept named values in English: MONTHS and DAYS-OF-WEEKS. So you can use names like January, February, March and so on for MONTHS and Monday, Tuesday, Wednesday and so on for DAYS-OF-WEEK. The names are not case-sensitive and you can even use short forms like Jan, Feb, Mar or Mon, Tue, Wed. In fact, at the moment, the parser in NCrontab will use the first match that it finds. Consequently, if you specify just the letter J for the month, then it will be interpreted as January since it occurs before the months June and July. If you specify Ju, then June will be assumed for the same reason. However, you should stick to either the 3 letter abbreviations or the full name since that is the norm among cron implementations. Finally, you can also mix numerical and named values, as in Jan,Feb,3,4,May,Jun,6.

Examples of crontab expressions

* * * * *
This pattern causes a task to be launched every minute.

5 * * * *
This pattern causes a task to be launched once every hour and at the fifth minute of the hour (00:05, 01:05, 02:05 etc.).

* 12 * * Mon
This pattern causes a task to be launched every minute during the 12th hour of Monday.

* 12 16 * Mon
This pattern causes a task to be launched every minute during the 12th hour of Monday, 16th, but only if the day is the 16th of the month.

59 11 * * 1,2,3,4,5
This pattern causes a task to be launched at 11:59AM on Monday, Tuesday, Wednesday, Thursday and Friday. Every sub-pattern can contain two or more comma separated values.

59 11 * * 1-5
This pattern is equivalent to the previous one. Value ranges are admitted and defined using the minus character.

*/15 9-17 * * *
This pattern causes a task to be launched every 15 minutes between the 9th and 17th hour of the day (9:00, 9:15, 9:30, 9:45 and so on... note that the last execution will be at 17:45). The slash character can be used to identify periodic values, in the form of a/b. A sub-pattern with the slash character is satisfied when the value on the left divided by the one on the right gives an integer result (a % b == 0).

* 12 10-16/2 * *
This pattern causes a task to be launched every minute during the 12th hour of the day, but only if the day is the 10th, the 12th, the 14th or the16th of the month.

* 12 1-15,17,20-25 * *
This pattern causes a task to be launched every minute during the 12th hour of the day, but the day of the month must be between the 1st and the 15th, the 20th and the 25, or at least it must be the 17th.

Frequently used CronTab expressions

0 6 * * 0-6
This pattern causes the task to be launched everyday of the week at 0600utc

0 4,16 * * 0-6
This pattern causes the task to be launched everyday of the week at both 0400utc and 1600utc

0 6 * * 1 
This pattern causes the task to be launched every Monday at 0600utc. Should you replace the 1 with a 3 it would be every Wednesday at 0600utc.  

15 18 * * 0,6
This pattern causes the task to be launched every Saturday & Sunday of each week at 1815utc

0 6 1 * *
This pattern causes the task to be launched every first day of the month at 0600utc

0 6 1,2,3,4,5,6,7 * 0
This pattern causes the task to be launched every first Sunday of the month at 0600utc

0 4 1 January,April,July,October *
This pattern causes the task to be launched at 0400utc on the 1st day of January, April, July and October

0 6 1,15 * *
This pattern causes the task to be launched at 0600utc on the 1st day and the 15th day of each month


See also
How to configure a scheduled report
Filter scheduling expressions