How to detect whether a PHP script is already running?

Question

I have a cron script that executes a PHP script every 10 minutes. The script checks a queue and processes the data in the queue. Sometimes the queue has enough data to last over 10 minutes of processing, creating the potential of two scripts trying to access the same data. I want to be able to detect whether the script is already running to prevent launching multiple copies of the script. I thought about creating a database flag that says that a script is processing, but if the script were ever to crash it would leave it in the positive state. Is there an easy way to tell if the PHP script is already running from withing a PHP or shell script?

Answer 1

You can just use a lock file. PHP's flock() function provides a simple wrapper for Unix's flock function, which provides advisory locks on files.

If you don't explicitly release them, the OS will automatically release these locks for you when the process holding them terminates, even if it terminates abnormally.

You can also follow the loose Unix convention of making your lock file a 'PID file' - that is, upon obtaining a lock on the file, have your script write its PID to it. Even if you never read this from within your script, it will be convenient for you if your script ever hangs or goes crazy and you want to find its PID in order to manually kill it.

Here's a copy/paste-ready implementation:

#!/usr/bin/php
<?php

$lock_file = fopen('path/to/yourlock.pid', 'c');
$got_lock = flock($lock_file, LOCK_EX | LOCK_NB, $wouldblock);
if ($lock_file === false || (!$got_lock && !$wouldblock)) {
    throw new Exception(
        "Unexpected error opening or locking lock file. Perhaps you " .
        "don't  have permission to write to the lock file or its " .
        "containing directory?"
    );
}
else if (!$got_lock && $wouldblock) {
    exit("Another instance is already running; terminating.\n");
}

// Lock acquired; let's write our PID to the lock file for the convenience
// of humans who may wish to terminate the script.
ftruncate($lock_file, 0);
fwrite($lock_file, getmypid() . "\n");

/*
    The main body of your script goes here.
*/
echo "Hello, world!";

// All done; we blank the PID file and explicitly release the lock 
// (although this should be unnecessary) before terminating.
ftruncate($lock_file, 0);
flock($lock_file, LOCK_UN);

Just set the path of your lock file to wherever you like and you're set.

Answer 2

If you are running Linux, this should work at the top of your script:

$running = exec("ps aux|grep ". basename(__FILE__) ."|grep -v grep|wc -l");
if($running > 1) {
   exit;
}

Answer 3

If you need it to be absolutely crash-proof, you should use semaphores, which are released automatically when php ends the specific request handling.

A simpler approach would be to create a DB record or a file at the beginning of the execution, and remove it at the end. You could always check the "age" of that record/file, and if it's older than say 3 times the normal script execution, suppose it crashed and remove it.

There's no "silver bullet", it just depends on your needs.

Answer 4

I know this is an old question but in case someone else is looking here I'll post some code. This is what I have done recently in a similar situation and it works well. Put put this code at the top of your file and if the same script is already running it will leave it be and end the new one.

I use it to keep a monitoring system running at all times. A cron job starts the script every 5 minutes but unless the other has stopped from some reason (usually if it has crashed, which is very rare!) the new one will just exit itself.

// The file to store our process file
define('PROCESS_FILE', 'process.pid');

// Check I am running from the command line
if (PHP_SAPI != 'cli') {
    log_message('Run me from the command line');
    exit;
}

// Check if I'm already running and kill myself off if I am
$pid_running = false;
if (file_exists(PROCESS_FILE)) {
    $data = file(PROCESS_FILE);
    foreach ($data as $pid) {
        $pid = (int)$pid;
        if ($pid > 0 && file_exists('/proc/' . $pid)) {
            $pid_running = $pid;
            break;
        }
    }
}
if ($pid_running && $pid_running != getmypid()) {
    if (file_exists(PROCESS_FILE)) {
        file_put_contents(PROCESS_FILE, $pid);
    }
    log_message('I am already running as pid ' . $pid . ' so stopping now');
    exit;
} else {
    // Make sure file has just me in it
    file_put_contents(PROCESS_FILE, getmypid());
    log_message('Written pid with id '.getmypid());
}

It will NOT work without modification on Windows, but should be fine in unix based systems.

Answer 5

You can use new Symfony 2.6 LockHandler. Source

$lock = new LockHandler('update:contents');
if (!$lock->lock()) {
    echo 'The command is already running in another process.';
}

Answer 6

A common way for *nix daemons (though not necessarily PHP scripts, but it will work) is to use a .pid file.

When the script starts check for the existence of a .pid file named for the script (generally stored in /var/run/). If it doesn't exist, create it setting its contents to the PID of the process running the script (using getmypid) then continue with normal execution. If it does exist read the PID from it and see if that process is still running, probably by running ps $pid. If it is running, exit. Otherwise, overwrite its contents with your PID (as above) and continue normal execution.

When execution finished, delete the file.

Answer 7

This worked for me. Set a database record with a lock flag and a time stamp. My script should complete well within 15min so added that as a last locked feild to check:

       $lockresult = mysql_query("
       SELECT *
       FROM queue_locks
       WHERE `lastlocked` > DATE_SUB(NOW() , INTERVAL 15 MINUTE)    
       AND `locked` = 'yes'
       AND `queid` = '1'
       LIMIT 1
        "); 
$LockedRowCount = mysql_num_rows($lockresult);

if($LockedRowCount>0){
    echo "this script is locked, try again later";
    exit;   
}else{
//Set the DB record to locked and carry on son
$result = mysql_query("
            UPDATE `queue_locks` SET `locked` = 'yes', `lastlocked` = CURRENT_TIMESTAMP WHERE `queid` = 1;
            ");

}

Then unlock it at the end of the script:

    $result = mysql_query("UPDATE `queue_locks` SET `locked` = 'no' WHERE `queid` = 1;");

Answer 8

I know this is an old question, but there's an approach which hasn't been mentioned before that I think is worth considering.

One of the problems with a lockfile or database flag solution, as already mentioned, is that if the script fails for some reason other than normal completion it won't release the lock. And therefore the next instance won't start until the lock is either manually cleared or cleared by a clean-up function.

If, though, you are certain that the script should only ever be running once, then it's relatively easy to check from within the script whether it is already running when you start it. Here's some code:

function checkrun() {
    exec("ps auxww",$ps);
    $r = 0;
    foreach ($ps as $p) {
        if (strpos($p,basename(__FILE__))) {
            $r++;
            if ($r > 1) {
                echo "too many instances, exiting\n";
                exit();
            }
        }
    }
}

Simply call this function at the start of the script, before you do anything else (such as open a database handler or process an import file), and if the same script is already running then it will appear twice in the process list - once for the previous instance, and once for this one. So, if it appears more than once, just exit.

A potential gotcha here: I'm assuming that you will never have two scripts with the same basename that may legitimately run simultaneously (eg, the same script running under two different users). If that is a possibility, then you'd need to extend the checking to something more sophisticated than a simple substring on the file's basename. But this works well enough if you have unique filenames for your scripts.

Answer 9

Assuming this is a linux server and you have cronjobs available

///Check for running script and run if non-exist///

#! /bin/bash

check=$(ps -fea | grep -v grep | grep script.php | wc -l)
date=$(date +%Y-%m%d" "%H:%M:%S)
if [ "$check" -lt 1 ]; then
echo "["$date"] Starting script" >> /path/to/script/log/
/sbin/script  ///Call the script here - see below///
fi

script file

#/usr/bin/php /path/to/your/php/script.php

Answer 10

Home / Check if a PHP script is already running

Check if a PHP script is already running If you have long running batch processes with PHP that are run by cron and you want to ensure there’s only ever one running copy of the script, you can use the functions getmypid() and posix_kill() to check to see if you already have a copy of the process running. This post has a PHP class for checking if the script is already running.

Each process running on a Linux/Unix computer has a pid, or process identifier. In PHP this can be retrieved using getmypid() which will return an integer number. This pid number can be saved to a file and each time the script is run a check made to see if the file exists. If it is the posix_kill() function can be used to see if a process is running with that pid number.

My PHP class for doing this is below. Please feel free to use this and modify to suit your individual requirements.

class pid {

    protected $filename;
    public $already_running = false;
   
    function __construct($directory) {
       
        $this->filename = $directory . '/' . basename($_SERVER['PHP_SELF']) . '.pid';
       
        if(is_writable($this->filename) || is_writable($directory)) {
           
            if(file_exists($this->filename)) {
                $pid = (int)trim(file_get_contents($this->filename));
                if(posix_kill($pid, 0)) {
                    $this->already_running = true;
                }
            }
           
        }
        else {
            die("Cannot write to pid file '$this->filename'. Program execution halted.n");
        }
       
        if(!$this->already_running) {
            $pid = getmypid();
            file_put_contents($this->filename, $pid);
        }
       
    }

    public function __destruct() {

        if(!$this->already_running && file_exists($this->filename) && is_writeable($this->filename)) {
            unlink($this->filename);
        }
   
    }
   
}

Use Class below

$pid = new pid('/tmp');
if($pid->already_running) {
    echo "Already running.n";
    exit;
}
else {
    echo "Running...n";
}

Answer 11

Inspired by Mark Amery's answer I created this class. This might help someone. Simply change the "temp/lockFile.pid" to where you want the file placed.

class ProcessLocker
{
    private $lockFile;
    private $gotLock;
    private $wouldBlock;

    function __construct()
    {
        $this->lockFile = fopen('temp/lockFile.pid', 'c');

        if ($this->lockFile === false) {
            throw new Exception("Unable to open the file.");
        }

        $this->gotLock = flock($this->lockFile, LOCK_EX | LOCK_NB, $this->wouldBlock);
    }

    function __destruct()
    {
        $this->unlockProcess();
    }

    public function isLocked()
    {
        if (!$this->gotLock && $this->wouldBlock) {
            return true;
        }

        return false;
    }

    public function lockProcess()
    {
        if (!$this->gotLock && !$this->wouldBlock) {
            throw new Exception("Unable to lock the file.");
        }

        ftruncate($this->lockFile, 0);
        fwrite($this->lockFile, getmypid() . "\n");
    }

    public function unlockProcess()
    {
        ftruncate($this->lockFile, 0);
        flock($this->lockFile, LOCK_UN);
    }
}

Simply use the class as such in the beginning of your script:

$locker = new ProcessLocker();
        
if(!$locker->isLocked()){
    $locker->lockProcess();
} else{
    // The process is locked
    exit();
}