Crontab Expression

Atif Aziz edited this page Nov 6, 2017 · 3 revisions

A crontab expression is a very compact way to express a recurring schedule. A single expression 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 1-31 -
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.

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