4.4 KiB
cronexpression for Go
Cron expression parser in Go language (golang).
Given a cron expression and a time stamp, you can get the next time stamp which satisfy the cron expression.
In another project, I decided to use Cron syntax to encode scheduling information. Thus this standalone library to parse and execute cron expressions.
Implementation
The reference documentation for this implementation is found at https://en.wikipedia.org/wiki/Cron#CRON_expression, which I copy/pasted here (laziness) with modifications where this implementation differs:
Field name Mandatory? Allowed values Allowed special characters
---------- ---------- -------------- --------------------------
Seconds No 0-59 * / , -
Minutes Yes 0-59 * / , -
Hours Yes 0-23 * / , -
Day of month Yes 1-31 * / , - ? L W
Month Yes 1-12 or JAN-DEC * / , -
Day of week Yes 0-6 or SUN-SAT * / , - ? L #
Year No 1970–2099 * / , -
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 minute field 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.
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
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.
W
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.
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.
With the following differences:
- Supports optional second field (before minute field)
- If five fields are present, a wildcard year field is appended
- If six field are present,
0is prepended as second field, that is,* * * * * *internally become0 * * * * * *. - Domain for day-of-week field is [0-7] instead of [0-6], 7 being Sunday (like 0).
@rebootis not supported- As of now, the behavior of the code is undetermined if a malformed cron expression is supplied
Install
go get github.com/gorhill/cronexpression
Usage
Import the library:
import "github.com/gorhill/cronexpression"
import "time"
Simplest way:
nextTime := cronexpression.NextTimeFromCronString("0 0 29 2 *", time.Now())
Assuming time.Now() is "2013-08-29 09:28:00", then nextTime will be "2016-02-29 00:00:00".
If you need to reuse many times a cron expression in your code, it is more efficient
to create a CronExpression object once and keep a copy of it for reuse:
cronexpr := cronexpression.NewCronExpression("0 0 29 2 *")
nextTime := cronexpr.NextTime(time.Now())