在php文件中调用shell_exec会进入无限循环

After asking many to help, even the support from where I'm hosting my website none of them could help me. So here I am trying my last bullet.

All I'm trying to do is to run this cron.php script from my browser:

<?php
    $output = shell_exec('bash startup.sh');
    die();
?>

This is startup.sh

#!/bin/sh

# Start tracker session
screen -S Track -d -m bash public-tracker
echo "Started tracker session!"

And this is public-tracker file

#!/bin/sh

`which php` ../../public-track.php something | tee -a tracker.log

And public-track.php is simply:

<?php
echo "Hello World";
?>

When I run cron.php, I getting data to tracker.log in a very fast way until I rename cron.php to something else. Then it will stop.

The data I'm infinitely receiving to tracker.log is something like that:

X-Powered-By: PHP/5.5.30
Content-type: text/html

X-Powered-By: PHP/5.5.30
Content-type: text/html

X-Powered-By: PHP/5.5.30
Content-type: text/html

X-Powered-By: PHP/5.5.30
Content-type: text/html

X-Powered-By: PHP/5.5.30
Content-type: text/html

X-Powered-By: PHP/5.5.30
Content-type: text/html

These lines keep spamming to tracker.log until I rename cron.php to something else.

What may be causing this issue ? Thanks