Running a php script via ajax, but only if it is not already running

My intention is this.

My client.html calls a php script check.php via ajax. I want check.php to check if another script task.php is already being run. If it is, I do nothing. If it is not, I need to run it in the background.

I have an idea what I want to do, but am unsure how to do it.

Part A. I know how to call check.php via ajax.

Part B. In check.php I might need to run task.php. I think I need something like:

$PID = shell_exec("php task.php > /dev/null & echo $!");

I think the "> /dev/null &" bit tells it to run in the background, but am unsure what the "$!" does.

Part C. The $PID I need as a tag of the process. I need to write this number (or whatever) to a file in the same directory, and need to read it every call to check.php. I can't work out how to do that. Could someone give me a link of how to read/write a file with a single number in to the same directory?

Part D. Then to check if the last launched task.php is still running I am going to use the function:

function is_process_running($PID)
{
   exec("ps $PID", $ProcessState);
   return(count($ProcessState) >= 2);
}

I think that is all the bits I need, but as you can see I am unsure on how to do a few of them.

Solution

I would use an flock() based mechanism to make sure that task.php runs only once.

Use a code like this:

<?php

$fd = fopen('lock.file', 'w+');

// try to get an exclusive lock. LOCK_NB let the operation not blocking
// if a process instance is already running. In this case, the else 
// block will being entered.
if(flock($fd, LOCK_EX | LOCK_NB )) {
    // run your code
    sleep(10);
    // ...
    flock($fd, LOCK_UN);
} else {
    echo 'already running';
}

fclose($fd);

Also note that flock() is, as the PHP documentation points out, portable across all supported operating systems.

!$

gives you the pid of the last executed program in bash. Like this:

command &
pid=$!
echo pid

Note that you will have to make sure your php code runs on a system with bash support. (Not windows)

Update (after comment of opener).

flock() will work on all operating systems (As I mentioned). The problem I see in your code when working with windows is the !$ (As I mentioned ;) ..

To obtain the pid of the task.php you should use proc_open() to start task.php. I've prepared two example scripts:

task.php

$fd = fopen('lock.file', 'w+');

// try to get an exclusive lock. LOCK_NB let the operation not blocking
// if a process instance is already running. In this case, the else 
// block will being entered.
if(flock($fd, LOCK_EX | LOCK_NB )) {
    // your task's code comes here
    sleep(10);
    // ...
    flock($fd, LOCK_UN);
    echo 'success';
    $exitcode = 0;
} else {
    echo 'already running';
    // return 2 to let check.php know about that
    // task.php is already running
    $exitcode = 2; 
}

fclose($fd);

exit($exitcode);

check.php

$cmd = 'php task.php';
$descriptorspec = array(
   0 => array('pipe', 'r'),  // STDIN 
   1 => array('pipe', 'w'),  // STDOUT
   2 => array('pipe', 'w')   // STDERR
);

$pipes = array(); // will be set by proc_open()

// start task.php
$process = proc_open($cmd, $descriptorspec, $pipes);

if(!is_resource($process)) {
    die('failed to start task.php');
}

// get output (stdout and stderr)
$output = stream_get_contents($pipes[1]);
$errors = stream_get_contents($pipes[2]);

do {
    // get the pid of the child process and it's exit code
    $status = proc_get_status($process);
} while($status['running'] !== FALSE);

// close the process
proc_close($process);

// get pid and exitcode
$pid = $status['pid'];
$exitcode = $status['exitcode'];

// handle exit code
switch($exitcode) {
    case 0:
        echo 'Task.php has been executed with PID: ' . $pid
           . '. The output was: ' . $output;
        break;
    case 1:
        echo 'Task.php has been executed with errors: ' . $output;
        break;
    case 2:
        echo 'Cannot execute task.php. Another instance is running';
        break;
    default:
        echo 'Unknown error: ' . $stdout;
}

You asked me why my flock() solution is the best. It's just because the other answer will not reliably make sure that task.php runs once. This is because the race condition I've mentioned in the comments below that answer.