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:
*), which covers all values for the field. So a * in days means all days of a month (which varies with month and year).5. Naturally, the set of values that are valid for each field varies.1,2,3,4. The list can be unordered as in 3,4,2,6,1.1-10. You can also specify these in the wrong order and they will be fixed. So 10-5 will be treated as 5-10.*/4. This means every 4th value of the field. You can also use it in a range, as in 1-6/2.1-5,10,12,20-30/5The table below lists the valid values for each field:
Field Range CommentMINUTES 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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