135 lines
6.5 KiB
Markdown
135 lines
6.5 KiB
Markdown
Golang Cron expression parser
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=============================
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Given a cron expression and a time stamp, you can get the next time stamp which satisfies the cron expression.
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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.
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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.
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There is also a companion command-line utility to evaluate cron time expressions: <https://github.com/gorhill/cronexpr/tree/master/cronexpr> (which of course uses this library).
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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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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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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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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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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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`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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The `W` character is allowed for the day-of-month field. This character is used to specify the business day (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 business day to the 15th of the month."
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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.
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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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The `W` character can also be combined with `L`, i.e. `LW` to mean "the last business day of the month."
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#### Hash ( # )
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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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Predefined cron expressions
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---------------------------
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(Copied from <https://en.wikipedia.org/wiki/Cron#Predefined_scheduling_definitions>, with text modified according to this implementation)
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Entry Description Equivalent to
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@annually Run once a year at midnight in the morning of January 1 0 0 0 1 1 * *
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@yearly Run once a year at midnight in the morning of January 1 0 0 0 1 1 * *
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@monthly Run once a month at midnight in the morning of the first of the month 0 0 0 1 * * *
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@weekly Run once a week at midnight in the morning of Sunday 0 0 0 * * 0 *
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@daily Run once a day at midnight 0 0 0 * * * *
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@hourly Run once an hour at the beginning of the hour 0 0 * * * * *
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@reboot Not supported
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Other details
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-------------
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* If only six fields are present, a `0` second field is prepended, that is, `* * * * * 2013` internally become `0 * * * * * 2013`.
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* If only five fields are present, a `0` second field is prepended and a wildcard year field is appended, that is, `* * * * Mon` internally become `0 * * * * Mon *`.
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* 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#.
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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/cronexpr
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Usage
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-----
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Import the library:
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import "github.com/gorhill/cronexpr"
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import "time"
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Simplest way:
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nextTime := cronexpr.MustParse("0 0 29 2 *").Next(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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You can keep the returned Expression pointer around if you want to reuse it:
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expr := cronexpr.MustParse("0 0 29 2 *")
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nextTime := expr.Next(time.Now())
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...
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nextTime = expr.Next(nextTime)
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Use `time.IsZero()` to find out whether a valid time was returned. For example,
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cronexpr.MustParse("* * * * * 1980").Next(time.Now()).IsZero()
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will return `true`, whereas
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cronexpr.MustParse("* * * * * 2050").Next(time.Now()).IsZero()
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will return `false` (as of 2013-08-29...)
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You may also query for `n` next time stamps:
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cronexpr.MustParse("0 0 29 2 *").NextN(time.Now(), 5)
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which returns a slice of time.Time objects, containing the following time stamps (as of 2013-08-30):
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2016-02-29 00:00:00
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2020-02-29 00:00:00
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2024-02-29 00:00:00
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2028-02-29 00:00:00
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2032-02-29 00:00:00
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The time zone of time values returned by `Next` and `NextN` is always the
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time zone of the time value passed as argument, unless a zero time value is
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returned.
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API
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---
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<http://godoc.org/github.com/gorhill/cronexpr>
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License
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-------
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License: pick the one which suits you best:
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- GPL v3 see <https://www.gnu.org/licenses/gpl.html>
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- APL v2 see <http://www.apache.org/licenses/LICENSE-2.0>
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