Calculate when a cron job will be executed then next time

Question

I have a cron \"time definition\"

1 * * * * (every hour at xx:01)
2 5 * * * (every day at 05:02)
0 4 3 * * (every third day of the month at 04:00)
* 2 * * 5          


        

Answer 1

Thanks for posting this code. It definitely helped me out, even 6 years later.

Trying to implement I found a small bug.

date('i G j n w', $time) returns a 0 padded integer for the minutes.

Later in the code, it does a modulus on that 0 padded integer. PHP doesn't seem to handle this as expected.

$ php
<?php
print 8 % 5 . "\n";
print 08 % 5 . "\n";
?>
3
0

As you can see, 08 % 5 returns 0, whereas 8 % 5 returns the expected 3. I couldn't find a non padded option for the date command. I tried fiddling with the {$time[$k]} % $1 === 0 line (like changing {$time[$k]} to ({$time[$k]}+0), but couldn't get it to drop the 0 padding during the modulus.

So, I ended up just changing the original value returned by the date function and removed the 0 by running $time[0] = $time[0] + 0;.

Here is my test.

<?php

function parse_crontab($frequency='* * * * *', $time=false) {
    $time = is_string($time) ? strtotime($time) : time();
    $time = explode(' ', date('i G j n w', $time));
    $time[0] = $time[0] + 0;
    $crontab = explode(' ', $frequency);
    foreach ($crontab as $k => &$v) {
        $v = explode(',', $v);
        $regexps = array(
            '/^\*$/', # every 
            '/^\d+$/', # digit 
            '/^(\d+)\-(\d+)$/', # range
            '/^\*\/(\d+)$/' # every digit
        );
        $content = array(
            "true", # every
            "{$time[$k]} === $0", # digit
            "($1 <= {$time[$k]} && {$time[$k]} <= $2)", # range
            "{$time[$k]} % $1 === 0" # every digit
        );
        foreach ($v as &$v1)
            $v1 = preg_replace($regexps, $content, $v1);
            $v = '('.implode(' || ', $v).')';
    }
    $crontab = implode(' && ', $crontab);
    return eval("return {$crontab};");
}

for($i=0; $i<24; $i++) {
    for($j=0; $j<60; $j++) {
        $date=sprintf("%d:%02d",$i,$j);
        if (parse_crontab('*/5 * * * *',$date)) {
             print "$date yes\n";
        } else {
             print "$date no\n";
        }
    }
}

?>

Answer 2

Use this function:

function parse_crontab($time, $crontab)
         {$time=explode(' ', date('i G j n w', strtotime($time)));
          $crontab=explode(' ', $crontab);
          foreach ($crontab as $k=>&$v)
                  {$v=explode(',', $v);
                   foreach ($v as &$v1)
                           {$v1=preg_replace(array('/^\*$/', '/^\d+$/', '/^(\d+)\-(\d+)$/', '/^\*\/(\d+)$/'),
                                             array('true', '"'.$time[$k].'"==="\0"', '(\1<='.$time[$k].' and '.$time[$k].'<=\2)', $time[$k].'%\1===0'),
                                             $v1
                                            );
                           }
                   $v='('.implode(' or ', $v).')';
                  }
          $crontab=implode(' and ', $crontab);
          return eval('return '.$crontab.';');
         }
var_export(parse_crontab('2011-05-04 02:08:03', '*/2,3-5,9 2 3-5 */2 *'));
var_export(parse_crontab('2011-05-04 02:08:03', '*/8 */2 */4 */5 *'));

Edit Maybe this is more readable:

<?php

    function parse_crontab($frequency='* * * * *', $time=false) {
        $time = is_string($time) ? strtotime($time) : time();
        $time = explode(' ', date('i G j n w', $time));
        $crontab = explode(' ', $frequency);
        foreach ($crontab as $k => &$v) {
            $v = explode(',', $v);
            $regexps = array(
                '/^\*$/', # every 
                '/^\d+$/', # digit 
                '/^(\d+)\-(\d+)$/', # range
                '/^\*\/(\d+)$/' # every digit
            );
            $content = array(
                "true", # every
                "{$time[$k]} === 0", # digit
                "($1 <= {$time[$k]} && {$time[$k]} <= $2)", # range
                "{$time[$k]} % $1 === 0" # every digit
            );
            foreach ($v as &$v1)
                $v1 = preg_replace($regexps, $content, $v1);
            $v = '('.implode(' || ', $v).')';
        }
        $crontab = implode(' && ', $crontab);
        return eval("return {$crontab};");
    }

Usage:

<?php
if (parse_crontab('*/5 2 * * *')) {
    // should run cron
} else {
    // should not run cron
}

Answer 3

Created javascript API for calculating next run time based on @dlamblin idea. Supports seconds and years. Have not managed to test it fully yet so expect bugs but let me know if find any.

Repository link: https://bitbucket.org/nevity/cronner

Answer 4

Here's a PHP project that is based on dlamblin's psuedo code.

It can calculate the next run date of a CRON expression, the previous run date of a CRON expression, and determine if a CRON expression matches a given time. You can skip This CRON expression parser fully implements CRON:

1. Increments of ranges (e.g. */12, 3-59/15)
2. Intervals (e.g. 1-4, MON-FRI, JAN-MAR )
3. Lists (e.g. 1,2,3 | JAN,MAR,DEC)
4. Last day of a month (e.g. L)
5. Last given weekday of a month (e.g. 5L)
6. Nth given weekday of a month (e.g. 3#2, 1#1, MON#4)
7. Closest weekday to a given day of the month (e.g. 15W, 1W, 30W)

https://github.com/mtdowling/cron-expression

Usage (PHP 5.3+):

<?php

// Works with predefined scheduling definitions
$cron = Cron\CronExpression::factory('@daily');
$cron->isDue();
$cron->getNextRunDate();
$cron->getPreviousRunDate();

// Works with complex expressions
$cron = Cron\CronExpression::factory('15 2,6-12 */15 1 2-5');
$cron->getNextRunDate();

Answer 5

My answer is not unique. Just a replica of @BlaM answer written in java because PHP's date and time is a bit different from Java.

This program assumes that the CRON expression is simple. It can only contain digits or *.

Minute = 0-60
Hour = 0-23
Day = 1-31
MONTH = 1-12 where 1 = January.
WEEKDAY = 1-7 where 1 = Sunday.

Code:

package main;

import java.util.Calendar;
import java.util.Date;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class CronPredict
{
    public static void main(String[] args)
    {
        String cronExpression = "5 3 27 3 3 ls -la > a.txt";
        CronPredict cronPredict = new CronPredict();
        String[] parsed = cronPredict.parseCronExpression(cronExpression);
        System.out.println(cronPredict.getNextExecution(parsed).getTime().toString());
    }

    //This method takes a cron string and separates entities like minutes, hours, etc.
    public String[] parseCronExpression(String cronExpression)
    {
        String[] parsedExpression = null;
        String cronPattern = "^([0-9]|[1-5][0-9]|\\*)\\s([0-9]|1[0-9]|2[0-3]|\\*)\\s"
                        + "([1-9]|[1-2][0-9]|3[0-1]|\\*)\\s([1-9]|1[0-2]|\\*)\\s"
                        + "([1-7]|\\*)\\s(.*)$";
        Pattern cronRegex = Pattern.compile(cronPattern);

        Matcher matcher = cronRegex.matcher(cronExpression);
        if(matcher.matches())
        {
            String minute = matcher.group(1);
            String hour = matcher.group(2);
            String day = matcher.group(3);
            String month = matcher.group(4);
            String weekday = matcher.group(5);
            String command = matcher.group(6);

            parsedExpression = new String[6];
            parsedExpression[0] = minute;
            parsedExpression[1] = hour;
            parsedExpression[2] = day;
            //since java's month start's from 0 as opposed to PHP which starts from 1.
            parsedExpression[3] = month.equals("*") ? month : (Integer.parseInt(month) - 1) + "";
            parsedExpression[4] = weekday;
            parsedExpression[5] = command;
        }

        return parsedExpression;
    }

    public Calendar getNextExecution(String[] job)
    {
        Calendar cron = Calendar.getInstance();
        cron.add(Calendar.MINUTE, 1);
        cron.set(Calendar.MILLISECOND, 0);
        cron.set(Calendar.SECOND, 0);

        int done = 0;
        //Loop because some dates are not valid.
        //e.g. March 29 which is a Friday may never come for atleast next 1000 years.
        //We do not want to keep looping. Also it protects against invalid dates such as feb 30.
        while(done < 100)
        {
            if(!job[0].equals("*") && cron.get(Calendar.MINUTE) != Integer.parseInt(job[0]))
            {
                if(cron.get(Calendar.MINUTE) > Integer.parseInt(job[0]))
                {
                    cron.add(Calendar.HOUR_OF_DAY, 1);
                }
                cron.set(Calendar.MINUTE, Integer.parseInt(job[0]));
            }

            if(!job[1].equals("*") && cron.get(Calendar.HOUR_OF_DAY) != Integer.parseInt(job[1]))
            {
                if(cron.get(Calendar.HOUR_OF_DAY) > Integer.parseInt(job[1]))
                {
                    cron.add(Calendar.DAY_OF_MONTH, 1);
                }
                cron.set(Calendar.HOUR_OF_DAY, Integer.parseInt(job[1]));
                cron.set(Calendar.MINUTE, 0);
            }

            if(!job[4].equals("*") && cron.get(Calendar.DAY_OF_WEEK) != Integer.parseInt(job[4]))
            {
                Date previousDate = cron.getTime();
                cron.set(Calendar.DAY_OF_WEEK, Integer.parseInt(job[4]));
                Date newDate = cron.getTime();

                if(newDate.before(previousDate))
                {
                    cron.add(Calendar.WEEK_OF_MONTH, 1);
                }

                cron.set(Calendar.HOUR_OF_DAY, 0);
                cron.set(Calendar.MINUTE, 0);
            }

            if(!job[2].equals("*") && cron.get(Calendar.DAY_OF_MONTH) != Integer.parseInt(job[2]))
            {
                if(cron.get(Calendar.DAY_OF_MONTH) > Integer.parseInt(job[2]))
                {
                    cron.add(Calendar.MONTH, 1);
                }
                cron.set(Calendar.DAY_OF_MONTH, Integer.parseInt(job[2]));
                cron.set(Calendar.HOUR_OF_DAY, 0);
                cron.set(Calendar.MINUTE, 0);
            }

            if(!job[3].equals("*") && cron.get(Calendar.MONTH) != Integer.parseInt(job[3]))
            {
                if(cron.get(Calendar.MONTH) > Integer.parseInt(job[3]))
                {
                    cron.add(Calendar.YEAR, 1);
                }
                cron.set(Calendar.MONTH, Integer.parseInt(job[3]));
                cron.set(Calendar.DAY_OF_MONTH, 1);
                cron.set(Calendar.HOUR_OF_DAY, 0);
                cron.set(Calendar.MINUTE, 0);
            }

            done =  (job[0].equals("*") || cron.get(Calendar.MINUTE) == Integer.parseInt(job[0])) &&
                    (job[1].equals("*") || cron.get(Calendar.HOUR_OF_DAY) == Integer.parseInt(job[1])) &&
                    (job[2].equals("*") || cron.get(Calendar.DAY_OF_MONTH) == Integer.parseInt(job[2])) &&
                    (job[3].equals("*") || cron.get(Calendar.MONTH) == Integer.parseInt(job[3])) &&
                    (job[4].equals("*") || cron.get(Calendar.DAY_OF_WEEK) == Integer.parseInt(job[4])) ? 100 : (done + 1);
        }

        return cron;
    }
}

Answer 6

Check this out:

It can calculate the next time a scheduled job is supposed to be run based on the given cron definitions.