Why Your Cron Job Failed Silently (And How to Prevent It)

Published March 2026 · 4 min read

You scheduled a backup. It ran fine for months. Then one day you need the backup and discover it stopped running three weeks ago. No error. No notification. Nothing.

This is the most common cron failure mode, and it happens because cron has no built-in success confirmation. Here are the most common causes and how to catch each one.

1. The Job Errors Out Silently

Your script throws an error, but cron doesn't care — it ran the command, its job is done. Unless you've set MAILTO, the error output goes nowhere.

# Bad: errors vanish
0 2 * * * /usr/local/bin/backup.sh

# Better: capture output
0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

But logging only helps if you check the logs. Which you won't — not at 2 AM.

2. The Job Never Runs

Common causes:

Server rebooted and cron daemon didn't start
Crontab syntax error — one wrong character and the entire line is silently ignored
PATH issues — cron runs with a minimal PATH, so /usr/local/bin/backup.sh might not find the tools it needs
Permission change — someone chmod'd the script and now cron can't execute it

In all these cases, MAILTO won't help because the job produces no output — it simply doesn't execute.

3. The Job Runs But Does Nothing Useful

The script runs, exits 0, but the database it's backing up was already down. Or the disk is full so the backup file is 0 bytes. Exit code 0 doesn't mean success.

The Fix: Heartbeat Monitoring

Instead of watching for failures (which are invisible), confirm successes. Add a ping at the end of your job that only fires after the job completes successfully:

0 2 * * * /usr/local/bin/backup.sh && curl -fsS https://ping.trebben.dk/YOUR_SLUG

The && ensures the ping only fires if the script exits 0. A monitoring service watches for the ping. If it doesn't arrive on time, you get an alert.

This catches every failure mode — errors, non-execution, hangs, and silent failures.

For Paranoid Operators: Validate Before Pinging

#!/bin/bash
# backup-and-verify.sh
set -e

/usr/local/bin/backup.sh

# Verify the backup is actually there and non-empty
BACKUP_FILE="/backups/db-$(date +%Y%m%d).sql.gz"
if [ -s "$BACKUP_FILE" ]; then
  curl -fsS https://ping.trebben.dk/YOUR_SLUG
else
  echo "Backup file missing or empty!" >&2
  exit 1
fi

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