89 lines
4.4 KiB
Markdown
89 lines
4.4 KiB
Markdown
cronexpression for Go
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=====================
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Cron expression parser in Go language (golang).
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Given a cron expression and a time stamp, you can get the next time stamp which satisfy the cron expression.
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In another project, I decided to use Cron syntax to encode scheduling information. Thus this standalone library to parse and execute cron expressions.
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Implementation
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--------------
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The reference documentation for this implementation is found at
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https://en.wikipedia.org/wiki/Cron#CRON_expression, which I copy/pasted here (laziness) with modifications where this implementation differs:
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Field name Mandatory? Allowed values Allowed special characters
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---------- ---------- -------------- --------------------------
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Seconds No 0-59 * / , -
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Minutes Yes 0-59 * / , -
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Hours Yes 0-23 * / , -
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Day of month Yes 1-31 * / , - ? L W
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Month Yes 1-12 or JAN-DEC * / , -
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Day of week Yes 0-6 or SUN-SAT * / , - ? L #
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Year No 1970–2099 * / , -
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Asterisk ( * )
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--------------
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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.
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Slash ( / )
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-----------
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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.
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Comma ( , )
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-----------
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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.
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Hyphen ( - )
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------------
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Hyphens define ranges. For example, 2000-2010 indicates every year between 2000 and 2010 AD, inclusive.
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L
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-
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`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.
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W
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-
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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.
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Hash ( # )
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----------
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`#` 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.
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Question mark ( ? )
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-------------------
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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.
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With the following differences:
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* Supports optional second field (before minute field)
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* If five fields are present, a wildcard year field is appended
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* If six field are present, `0` is prepended as second field, that is, `* * * * * *` internally become `0 * * * * * *`.
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* Domain for day-of-week field is [0-7] instead of [0-6], 7 being Sunday (like 0).
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* `@reboot` is not supported
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* As of now, the behavior of the code is undetermined if a malformed cron expression is supplied
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Install
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-------
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go get github.com/gorhill/cronexpression
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Usage
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-----
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Import the library:
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import "github.com/gorhill/cronexpression"
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import "time"
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Simplest way:
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nextTime := cronexpression.NextTimeFromCronString("0 0 29 2 *", time.Now())
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Assuming `time.Now()` is "2013-08-29 09:28:00", then `nextTime` will be "2016-02-29 00:00:00".
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If you need to reuse many times a cron expression in your code, it is more efficient
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to create a `CronExpression` object once and keep a copy of it for reuse:
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cronexpr := cronexpression.NewCronExpression("0 0 29 2 *")
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nextTime := cronexpr.NextTime(time.Now())
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