cronparse

Parse and calculate different times related to Unix cron expressions

Parse Unix cron expressions and calculate things like:

• whether the expression is valid, as a bool
• the next time the expression will run, as a DateTime
• the last time the expression would have run, as a DateTime
• the duration until the next time the expression will run, as a Duration
• the duration since the last time the expression would have run, as a negative Duration

Usage

void main() {
    // validate a cron expression
    final valid = isValid("54 2-3,4-9 */3 FEB MON-FRI");
    print(valid); // true

    // calculate times based on a cron expression
    final time = DateTime.parse("2019-11-23 16:00:00");

    var cron = Cron("0 22 * * *");
    print(cron.nextRelativeTo(time)); // "2019-11-23 22:00:00.000"
    print(cron.untilNextRelativeTo(time) == Duration(hours: 6)); // true

    cron = Cron("*/15 * * * *");
    print(cron.previousRelativeTo(time)); // "2019-11-23 15:45:00.000"
    print(cron.sincePreviousRelativeTo(time) == Duration(minutes: -15)); // true
}

Cron expression matching strategy

High-level explanation on matching

• if the expression is a predefined nickname, translate it to the relevant actual expression. note: this library does not parse the @reboot nickname.
• tokenize the expression to each part (minute, hour, day of month, month, day of week)
• test each of these tokens to match
  • token values can be in a few forms:
    • an asterisk (*): this always matches
    • an asterisk with skips (*/15): iterate through the valid range for the field, incrementing by the skip. if any of these match, the expression matches.
    • exact value (48, feb, WED): match these exactly
    • range of values without skips (5-9): iterate through the range, starting at the lower bound and ending at the higher bound, incrementing by 1. if any of these match, the value matches
    • range of values with skip (20-30/2): iterate through the range, starting at the lower bound and ending at the higher bound, incrementing by the skip value. if any of these match, the value matches.
    • set of values or ranges (1,2,3, 5-10,45-50): test each value in the set using the above strategy. if any of the member values match, the value matches.
• iff all tokens match, the expression matches

@reboot

This library is focused on temporal calculation for cron expressions. The @reboot nickname is shorthand for "exactly once after reboot." Since the @reboot nickname is not a temporal representation, it is not handled by this library.

Links

• cron man page
• A cron-like job scheduler for Dart
• A Node.js library with similar functionality