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Calculate when a cron job will be executed then next time

Asked 2008-11-26T17:26:31.127
52

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 (every minute between 02:00 and 02:59 on fridays)

And I have an unix timestamp.

Is there an obvious way to find (calculate) the next time (after that given timestamp) the job is due to be executed?

I'm using PHP, but the problem should be fairly language-agnostic.

[Update]

The class "PHP Cron Parser" (suggested by Ray) calculates the LAST time the CRON job was supposed to be executed, not the next time.

To make it easier: In my case the cron time parameters are only absolute, single numbers or "*". There are no time-ranges and no "*/5" intervals.

algorithm language-agnostic cron
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1 Answer

7

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, $
answered 2011-04-20T08:15:57.953

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