How to run Cronjobs more often than once per minute?

Question

I created a Email-Queue database table. I will insert all Emails my PHP application needs to send into this table.

Another PHP script will then look for all unsent Emails and sends them.

I run this script using cronjobs. Unfortunately cronjobs can run only at a maximum of once per minute. So in the worst-case a user has to wait one minute until his Email is really going to be sent.

My current idea for a workaround is calling the script with an addtional sleep parameter and duplicating the cronjobs.

Example:

* * * * * curl emails.php?sleep=0 >/dev/null 2>&1
* * * * * curl emails.php?sleep=10 >/dev/null 2>&1
* * * * * curl emails.php?sleep=20 >/dev/null 2>&1
* * * * * curl emails.php?sleep=30 >/dev/null 2>&1
* * * * * curl emails.php?sleep=40 >/dev/null 2>&1
* * * * * curl emails.php?sleep=50 >/dev/null 2>&1

In the above example the script would run every 10 seconds. The first line of the emails.php Script would be:

sleep($_REQUEST['sleep']);

Answer 1

This has to be done at script level.

// cron.php running every 10 seconds

<?php

$expireTime = time() + 60;
while (time() < $expireTime) {
     // my php logic here

     sleep(10); 
     // sleep for 10 seconds
     // you may change the sleep time to change frequency
}

Answer 2

Here's a simple bash script I've written which can be used with crontab to run more frequently than 1 minute.

you can save it as ~/bin/runEvery.sh and then in crontab write something like this to run otherScript.sh every 5 seconds:

*/1 * * * * ~/bin/runEvery.sh 5 otherScript.sh

This is the script:

#!/bin/bash

inputPeriod=$1
runCommand=$2
RUN_TIME=60
error="no"

if [ 'x'"$runCommand" != 'x' ]
then
    if [ 'x'$inputPeriod != 'x' ]
    then
        loops=$(( $RUN_TIME / $inputPeriod ))
        if [ $loops -eq 0 ]
        then
            loops=1
        fi

        for i in $(eval echo {1..$loops})
        do
            $runCommand
            sleep $inputPeriod
        done

    else
        error="yes"
    fi
else
    error="yes"
fi

if [ $error = "yes" ]
then
    echo "runEvery - runs a command every X seconds for a minute"
    echo "Usage: runEvery.sh <# in seconds < 60> <command to run>"
fi