Cron Job Fails Silently: How to Detect Hidden Failures

The Silent Failure Problem

Your cron job runs. It even shows up in the cron logs. But the job itself fails — and no one knows because:

• Output goes nowhere (not captured)
• The script doesn't check for errors
• There's no monitoring to detect missing runs
• Cron only logs that it started a job, not if it succeeded

You find out days or weeks later when a backup is missing, a report wasn't sent, or data is stale.

Step 1: Capture All Output

Redirect both stdout and stderr to a log file:

0 2 * * * /home/user/scripts/backup.sh >> /var/log/backup.log 2>&1

Breaking this down:

• >> appends stdout to the log file
• 2>&1 redirects stderr to stdout (so errors get logged too)

Now you can check /var/log/backup.log to see what happened.

Step 2: Make Your Script Exit on Errors

Add set -e at the start of your bash script:

#!/bin/bash
set -e  # Exit immediately if any command fails

# If pg_dump fails, the script stops here
pg_dump mydb > /backup/mydb.sql

# This won't run if pg_dump failed
aws s3 cp /backup/mydb.sql s3://my-bucket/

Without set -e, the script continues even if commands fail, which can cause cascading problems.

Step 3: Check Exit Codes Explicitly

For more control, check exit codes yourself:

#!/bin/bash

pg_dump mydb > /backup/mydb.sql
if [ $? -ne 0 ]; then
    echo "ERROR: Database dump failed" >&2
    exit 1
fi

aws s3 cp /backup/mydb.sql s3://my-bucket/
if [ $? -ne 0 ]; then
    echo "ERROR: S3 upload failed" >&2
    exit 1
fi

echo "Backup completed successfully"
exit 0

Step 4: Add Heartbeat Monitoring

The real solution: alert on the absence of success, not just on failure.

Add a ping that only fires if the job completes successfully:

0 2 * * * /home/user/scripts/backup.sh && curl -fsS https://api.cronsignal.io/ping/abc123

The && means curl only runs if backup.sh exits with code 0. If the script fails (or never runs at all), no ping is sent, and you get an alert.

This catches:

• Scripts that fail with an error
• Scripts that hang and never complete
• Jobs that don't run at all (cron daemon down, crontab deleted)
• Server crashes before the job runs

Step 5: Log Rotate Your Logs

If you're logging output, set up log rotation so logs don't fill your disk:

# /etc/logrotate.d/cron-backup
/var/log/backup.log {
    weekly
    rotate 4
    compress
    missingok
    notifempty
}

Putting It All Together

#!/bin/bash
set -e

# Log start time
echo "$(date): Starting backup"

# Your backup logic
pg_dump mydb > /backup/mydb_$(date +%Y%m%d).sql
aws s3 cp /backup/mydb_$(date +%Y%m%d).sql s3://my-bucket/

# Log success
echo "$(date): Backup completed successfully"

# Exit 0 for success
exit 0

And in crontab:

0 2 * * * /home/user/scripts/backup.sh >> /var/log/backup.log 2>&1 && curl -fsS https://api.cronsignal.io/ping/abc123

Now you have:

• Logged output for debugging
• Proper error handling
• External monitoring that alerts you when things go wrong