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.