How to ensure only one cron job running?

Need Help
1

This question maybe is not about cakephp problem but linux etc.

In src/Shell/MyShell.php

public function foo(){
    ...
    // In this function, i get some records and then run foreach
    $query = $bar->find()->where($condition);
    foreach ($query as $q) {
        ...
    }
    ...
}

crontab

*/5 * * * * cd /var/www/html/myapp && bin/cake my foo > /dev/null 2>&1

I need to run MyShell::foo per 5 minutes. But this job might last for above 5 minutes.
That causes this problem: the query in the next cron job may find duplicate records to run but that’s wrong with my business :confused:

I google and add the following code in my foo function:

    exec("ps -ef | grep 'foo' | grep -v grep | wc -l", $ret);
    if(intval($ret[0]) > 2){
        // Too many processes running
        return;
    }

But it did not work always.

Do you guys have any idea to solve it. Thanks.