How to prevent the cron job execution, if it is already running

Question

I have one php script, and I am executing this script via cron every 10 minutes on CentOS.

The problem is that if the cron job will take more than 10 minutes, then another instance of the same cron job will start.

I tried one trick, that is:

1. Created one lock file with php code (same like pid files) when the cron job started.
2. Removed the lock file with php code when the job finished.
3. And when any new cron job started execution of script, I checked if lock file exists and if so, aborted the script.

But there can be one problem that, when the lock file is not deleted or removed by script because of any reason. The cron will never start again.

Is there any way I can stop the execution of a cron job again if it is already running, with Linux commands or similar to this?

Answer 1

Advisory locking is made for exactly this purpose.

You can accomplish advisory locking with flock(). Simply apply the function to a previously opened lock file to determine if another script has a lock on it.

$f = fopen('lock', 'w') or die ('Cannot create lock file');
if (flock($f, LOCK_EX | LOCK_NB)) {
    // yay
}

In this case I'm adding LOCK_NB to prevent the next script from waiting until the first has finished. Since you're using cron there will always be a next script.

If the current script prematurely terminates, any file locks will get released by the OS.

Answer 2

Maybe it is better to not write code if you can configure it:

https://serverfault.com/questions/82857/prevent-duplicate-cron-jobs-running

Answer 3

flock() worked out great for me - I have a cron job with database requests scheduled every 5 minutes, so not having several running at the same time is crucial. This is what I did:

$filehandle = fopen("lock.txt", "c+");

if (flock($filehandle, LOCK_EX | LOCK_NB)) {
    // code here to start the cron job
   flock($filehandle, LOCK_UN);  // don't forget to release the lock
} else {
    // throw an exception here to stop the next cron job
}

fclose($filehandle);

In case you don't want to kill the next scheduled cron job, but simply pause it till the running one is finished, then just omit the LOCK_NB:

if (flock($filehandle, LOCK_EX)) 

Answer 4

This is a very common problem with a very simple solution: cronjoblock a simple 8-lines shellscript wrapper applies locking using flock:

https://gist.github.com/coderofsalvation/1102e56d3d4dcbb1e36f

btw. cronjoblock also reverses cron's spammy emailbehaviour: only output something if stuff goes wrong. This is handy in respect to cron's MAILTO variable. The stdout/stderr output will be suppressed (so cron will not send mails) unless the given process has an exitcode > 0

Answer 5

flock will not work in php 5.3.3 as The automatic unlocking when the file's resource handle is closed was removed. Unlocking now always has to be done manually.

Answer 6

I use this ::

<?php
// Create a PID file
if (is_file (dirname ($_SERVER['SCRIPT_NAME']) . "/.processing")) { die (); }
file_put_contents (dirname ($_SERVER['SCRIPT_NAME']) . "/.processing", "processing");

// SCRIPT CONTENTS GOES HERE //

@unlink (dirname ($_SERVER['SCRIPT_NAME']) . "/.processing");
?>

Answer 7

#!/bin/bash
ps -ef | grep -v grep | grep capture_12hz_sampling_track.php
if [ $? -eq 1 ];
then
     nohup /usr/local/bin/php /opt/Apache/htdocs/cmsmusic_v2/script/Mp3DownloadProcessMp4/capture_12hz_sampling_track.php &
else
      echo "Already running"
fi

Answer 8

Another alternative:

<?php

/**
* Lock manager to ensure our cron doesn't run twice at the same time.
*
* Inspired by the lock mechanism in Mage_Index_Model_Process
*
* Usage:
* 
* $lock = Mage::getModel('stcore/cron_lock');
*
* if (!$lock->isLocked()) {
*      $lock->lock();
*      // Do your stuff
*      $lock->unlock();
* }
*/
class ST_Core_Model_Cron_Lock extends Varien_Object
{
    /**
     * Process lock properties
     */
    protected $_isLocked = null;
    protected $_lockFile = null;

    /**
     * Get lock file resource
     *
     * @return resource
     */
    protected function _getLockFile()
    {
        if ($this->_lockFile === null) {
            $varDir = Mage::getConfig()->getVarDir('locks');
            $file = $varDir . DS . 'stcore_cron.lock';
            if (is_file($file)) {
                $this->_lockFile = fopen($file, 'w');
            } else {
                $this->_lockFile = fopen($file, 'x');
            }
            fwrite($this->_lockFile, date('r'));
        }
        return $this->_lockFile;
    }

    /**
     * Lock process without blocking.
     * This method allow protect multiple process runing and fast lock validation.
     *
     * @return Mage_Index_Model_Process
     */
    public function lock()
    {
        $this->_isLocked = true;
        flock($this->_getLockFile(), LOCK_EX | LOCK_NB);
        return $this;
    }

    /**
     * Lock and block process.
     * If new instance of the process will try validate locking state
     * script will wait until process will be unlocked
     *
     * @return Mage_Index_Model_Process
     */
    public function lockAndBlock()
    {
        $this->_isLocked = true;
        flock($this->_getLockFile(), LOCK_EX);
        return $this;
    }

    /**
     * Unlock process
     *
     * @return Mage_Index_Model_Process
     */
    public function unlock()
    {
        $this->_isLocked = false;
        flock($this->_getLockFile(), LOCK_UN);
        return $this;
    }

    /**
     * Check if process is locked
     *
     * @return bool
     */
    public function isLocked()
    {
        if ($this->_isLocked !== null) {
            return $this->_isLocked;
        } else {
            $fp = $this->_getLockFile();
            if (flock($fp, LOCK_EX | LOCK_NB)) {
                flock($fp, LOCK_UN);
                return false;
            }
            return true;
        }
    }

    /**
     * Close file resource if it was opened
     */
    public function __destruct()
    {
        if ($this->_lockFile) {
            fclose($this->_lockFile);
        }
    }
}

Source: https://gist.github.com/wcurtis/9539178

Answer 9

I was running a php cron job script that dealt specifically with sending text messages using an existing API. On my local box the cron job was working fine, but on my customer's box it was sending double messages. Although this doesn't make sense to me, I double checked the permissions for the folder responsible for sending messages and the permission was set to root. Once I set the owner as www-data (Ubuntu) it started behaving normally.

This might mot be the issue for you, but if its a simple cron script I would double check the permissions.