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Understanding WordPress Cron and a Reliable Alternative

How WordPress Handles Scheduled Tasks

Unlike traditional cron jobs, WordPress manages scheduled background tasks in a unique way. Instead of running at precise intervals, WordPress checks for pending tasks on every page load. If a scheduled task is due, it executes during that request.

Potential Issues with WP-Cron

While this system works for many sites, it has some drawbacks:

• Low-traffic sites may experience delays since tasks only run when someone visits.
• Irregular traffic patterns (e.g., few visitors overnight) can cause missed schedules.
• Coding errors in plugins/themes may prevent tasks from running properly.

Why Use WP-Cron?

Despite its quirks, WordPress relies on this system because:

• Shared hosting limitations: Many hosts restrict access to the system cron.
• Simplicity: The WordPress API is easier than configuring external cron jobs.
• Automatic retries: If a task misses its scheduled time, WP-Cron ensures it runs on the next page load.

The Problem: Unreliable Execution

Since WP-Cron depends on traffic, you can’t guarantee exact execution times. For mission-critical tasks, this uncertainty can be problematic.

A Better Solution: System Cron + WP-CLI

Instead of relying solely on WP-Cron, I wrote a bash script that triggers pending tasks across all WordPress sites on a server. By running this via the system cron every 10-15 minutes, you ensure tasks execute on time, regardless of traffic.

#!/usr/bin/env bash

# hold the primary WP CLI command
WP_CLI="wp --allow-root --path="

# hold the starting path
_PATH=/home/*/htdocs/*

# loop over our home directory for the htdocs folder...
for d in $_PATH; do
    
    # now hold the formatted command
    CMD=$WP_CLI$d/;

    # check if this site is indeed a wordpress website
    if $CMD core is-installed 2>/dev/null; then

        # check if the site is a network 
        if $CMD core is-installed --network 2>/dev/null; then

            # get the networks list of uri's
            _uris=$( $CMD site list --field=url )

            # loop over the sites
            for _uri in $_uris; do
                
                # fire off the sites cron
                $CMD cron event run --all --due-now --url="$_uri" & 

            done;

        # otherwise, we aren't in a network
        else
            
            # get the site's uri
            _uri=$( $CMD option get siteurl );
            
            # fire off the sites cron
            $CMD cron event run --all --due-now --url="$_uri" & 

        fi;

    fi;
    
done;

How It Works

1. Scans all WordPress installations in the specified path.
2. Checks for multisite networks and processes each subsite.
3. Runs all due cron tasks for every site, ensuring timely execution.

Setup Instructions

1. Save the script (e.g., wp-cron-runner.sh).
2. Make it executable:
   chmod +x wp-cron-runner.sh
3. Add a system cron job (e.g., every 15 minutes):
   */15 * * * * /path/to/wp-cron-runner.sh

Now, your WordPress tasks will run reliably, even during low-traffic periods.