1 0 * * * printf > /var/log/apache/error_log
The following line makes the user program
test.pl—ostensibly a Perl script—run every two hours, at midnight, 2am, 4am, 6am, 8am, and so on:
0 */2 * * * /home/username/test.pl
Predefined scheduling definitions
Several special predefined values can substitute in the CRON expression. Note that in some uses of the CRON format there is also a seconds field at the beginning of the pattern (e.g., Quartz).
||Run once a year at midnight in the morning of January 1||
||Run once a month at midnight in the morning of the first day of the month||
||Run once a week at midnight in the morning of Sunday||
||Run once a day at midnight||
||Run once an hour at the beginning of the hour||
||Run at startup||
# * * * * * command to execute # ? ? ? ? ? # ? ? ? ? ? # ? ? ? ? ? # ? ? ? ? ?????? day of week (0 - 6) (0 to 6 are Sunday to Saturday, or use names) # ? ? ? ??????????? month (1 - 12) # ? ? ???????????????? day of month (1 - 31) # ? ????????????????????? hour (0 - 23) # ?????????????????????????? min (0 - 59)
|Field name||Mandatory?||Allowed values||Allowed special characters||Remarks|
|Day of month||Yes||1-31||
|Month||Yes||1-12 or JAN-DEC||
|Day of week||Yes||0-6 or SUN-SAT||
||This field is not supported in standard/default implementations.|
In some uses of the CRON format there is also a seconds field at the beginning of the pattern. In that case, the CRON expression is a string comprising 6 or 7 fields.
Support for each special character depends on specific distributions and versions of cron
- Asterisk ( * )
- The asterisk indicates that the cron expression matches for all values of the field. E.g., using an asterisk in the 4th field (month) indicates every month.
- Slash ( / )
- Slashes describe increments of ranges. For example 3-59/15 in the 1st field (minutes) indicate the third minute of the hour and every 15 minutes thereafter. The form “*/…” is equivalent to the form “first-last/…”, that is, an increment over the largest possible range of the field.
- Percent ( % )
- Percent-signs (%) in the command, unless escaped with backslash (\), are changed into newline characters, and all data after the first % are sent to the command as standard input.
- Comma ( , )
- Commas are used to separate items of a list. For example, using “MON,WED,FRI” in the 5th field (day of week) means Mondays, Wednesdays and Fridays.
- Hyphen ( – )
- Hyphens define ranges. For example, 2000-2010 indicates every year between 2000 and 2010 AD, inclusive.
- ‘L’ stands for “last”. When used in the day-of-week field, it allows you to specify constructs such as “the last Friday” (“5L”) of a given month. In the day-of-month field, it specifies the last day of the month.
Note: L is a non-standard character and exists only in some cron implementations (Quartz java scheduler)
- The ‘W’ character is allowed for the day-of-month field. This character is 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 fires on Friday the 14th. If the 15th is a Sunday, the trigger fires on Monday the 16th. If the 15th is a Tuesday, then it fires on Tuesday the 15th. However if you specify “1W” as the value for day-of-month, and the 1st is a Saturday, the trigger fires on Monday the 3rd, as it does not ‘jump’ over the boundary of a month’s days. The ‘W’ character can be specified only when the day-of-month is a single day, not a range or list of days.
Note: W is a non-standard character and exists only in some cron implementations (Quartz java scheduler)
- Hash ( # )
- ‘#’ is allowed for the day-of-week field, and must be followed by a number between one and five. It allows you to specify constructs such as “the second Friday” of a given month.
- Question mark ( ? )
- Note: Question mark is a non-standard character and exists only in some cron implementations. It is used instead of ‘*’ for leaving either day-of-month or day-of-week blank.