[PHP] Re: Ensure only one instance of a script is running

[Jeremiah Fisher]

Test for a lock file when the script starts, and remove it when the 
script ends. This isn't 100% reliable, but it's the typical solution to 
this problem. If the script fails, the lock file may not be removed 
(ever have a Mozilla browser crash and tell you the default profile is 
locked?), so be careful with your error handling!

<?php
// test for lock file
if ( !$f = fopen( 'lock', 'r')) {
        // touch the lock file
        $f = fopen( 'lock', 'w');
        fwrite( $f, 'lock');
        fclose( $f);
} else {
        // lock file exists; another instance must be running (we hope)
        echo 'Error: An instance of this script is already running!';
        exit( 1);
}
/* script stuff */
// remove the lock file for the next instance
unlink( 'lock');
?>
Shaun wrote:
Hi,
I have a script that inserts data from files uploaded to our server. I need 
to make sure that only one instance of this script runs at anyone time, can 
anyone tell me how I can do this?

Many thanks 
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