7.8 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 satisfies the cron expression.
In another project, I decided to use cron expression syntax to encode scheduling information. Thus this standalone library to parse and apply time stamps to cron expressions.
The time-matching algorithm in this implementation is efficient, it avoids as much as possible to guess the next matching time stamp, a common technique seen in a number of implementations out there.
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.
Predefined cron expressions
(Copied from https://en.wikipedia.org/wiki/Cron#Predefined_scheduling_definitions, with text modified according to this implementation)
Entry Description Equivalent to
@annually Run once a year at midnight in the morning of January 1 0 0 0 1 1 * *
@yearly Run once a year at midnight in the morning of January 1 0 0 0 1 1 * *
@monthly Run once a month at midnight in the morning of the first of the month 0 0 0 1 * * *
@weekly Run once a week at midnight in the morning of Sunday 0 0 0 * * 0 *
@daily Run once a day at midnight 0 0 0 * * * *
@hourly Run once an hour at the beginning of the hour 0 0 * * * * *
@reboot Not supported
Other details
- If only six fields are present, a
0
second field is prepended, that is,* * * * * 2013
internally become0 * * * * * 2013
. - If only five fields are present, a
0
second field is prepended and a wildcard year field is appended, that is,* * * * Mon
internally become0 * * * * Mon *
. - Domain for day-of-week field is [0-7] instead of [0-6], 7 being Sunday (like 0). This to comply with http://linux.die.net/man/5/crontab#.
- 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.NextTime("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 the same 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())
...
Use cronexpression.NoMatch()
to find out whether a valid time was returned. For example,
cronexpression.NoMatch(cronexpression.NextTime("* * * * * 1980", time.Now()))
will return true
, whereas
cronexpression.NoMatch(cronexpression.NextTime("* * * * * 2050", time.Now()))
will return false
(as of 2013-08-29...)
API
func NextTime
func NextTime(cronLine string, fromTime time.Time) time.Time
Given a time stamp fromTime
, return the closest following time stamp which matches the cron expression string cronLine
. The time.Location
of the returned time stamp is the same as fromTime
.
func NextTimeN
func NextTimeN(cronLine string, fromTime time.Time, n int) []time.Time
Given a time stamp fromTime
, return a slice of n
closest following time stamps which match the cron expression string cronLine
. The time stamps in the returned slice are in chronological ascending order. The time.Location
of the returned time stamps is the same as fromTime
.
Example:
cronexpression.NextTimeN("0 0 0 29 2 ? *", time.Now(), 5)
will result in the following time stamps being returned (as of 2013-08-30):
2016-02-29 00:00:00
2020-02-29 00:00:00
2024-02-29 00:00:00
2028-02-29 00:00:00
2032-02-29 00:00:00
func NewCronExpression
func NewCronExpression(cronLine string) *CronExpression
Return a new CronExpression
pointer which will interpret the cron expression string cronLine
.
func (*CronExpression) NextTime
func (cronexpr *CronExpression) NextTime(fromTime time.Time) time.Time
Given a time stamp fromTime
, return the closest following time stamp which matches the cron expression cronexpr
. The time.Location
of the returned time stamp is the same as fromTime
.
func (*CronExpression) NextTimeN
func (cronexpr *CronExpression) NextTimeN(fromTime time.Time, n int) []time.Time
Given a time stamp fromTime
, return a slice of n
closest following time stamps which match the cron expression cronexpr
. The time stamps in the returned slice are in chronological ascending order. The time.Location
of the returned time stamps is the same as fromTime
.
func NoMatch
func NoMatch(t time.Time) bool
Returns true
if time stamp t
is not a valid time stamp from CronExpression
point of view. An invalid time stamp is returned by this library whenever no matching time stamp is found given a specific cron expression.