Cron Jobs

Schedule recurring tasks with cron — backups, cleanup, reports, and maintenance

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Explanation

Cron is the standard Unix scheduler. It runs commands at specified times — automatically, without human intervention. Every production server uses cron for backups, log rotation, health checks, and cleanup tasks.

crontab — editing your cron jobs:

bash crontab -e # edit your crontab (opens in $EDITOR) crontab -l # list your current cron jobs crontab -r # remove all your cron jobs (careful!)

Cron syntax — 5 fields + command:

Common patterns:

# Every minute * * * * * /usr/bin/check-health.sh # Every 15 minutes */15 * * * * /usr/bin/sync-data.sh # Every hour at :30 30 * * * * /usr/bin/report.sh # Every day at 2:00 AM 0 2 * * * /usr/bin/backup.sh # Every day at 2:00 AM, Mon-Fri only 0 2 * * 1-5 /usr/bin/weekday-task.sh # Every Sunday at midnight 0 0 * * 0 /usr/bin/weekly-cleanup.sh # First day of each month at 3:00 AM 0 3 1 * * /usr/bin/monthly-report.sh

Shorthand aliases:

@reboot /usr/bin/start-app.sh # runs once at system startup @hourly /usr/bin/sync.sh # = 0 * * * * @daily /usr/bin/backup.sh # = 0 0 * * * @weekly /usr/bin/report.sh # = 0 0 * * 0 @monthly /usr/bin/archive.sh # = 0 0 1 * *

Critical: always use absolute paths in cron:

bash # WRONG — cron has a minimal PATH; "python" may not be found * * * * * python /home/bill/script.py # CORRECT — use the full path * * * * * /usr/bin/python3 /home/bill/script.py # Find the full path with: which python3

Capturing output:

bash # By default, cron emails output to the local user. Redirect to a file instead: 0 2 * * * /usr/bin/backup.sh >> /var/log/backup.log 2>&1 # >> appends, 2>&1 captures stderr into the same file # Suppress all output (silent cron) 0 2 * * * /usr/bin/cleanup.sh > /dev/null 2>&1

Examples

Setting up daily database backups

Always test your cron commands manually first — cron will not tell you if they are broken.

# Edit your crontab
crontab -e

# Add these lines:
# Daily backup at 2:00 AM
0 2 * * * /usr/bin/pg_dump mydb > /backups/mydb_$(date +\%Y\%m\%d).sql 2>> /var/log/backup.log

# Weekly cleanup: delete backups older than 30 days
0 3 * * 0 /usr/bin/find /backups/ -name "*.sql" -mtime +30 -delete >> /var/log/backup.log 2>&1

# Verify it was saved
crontab -l

# Test your backup script works before relying on cron
/usr/bin/pg_dump mydb > /backups/test_backup.sql
ls -lh /backups/test_backup.sql
# -rw-r--r-- 1 bill bill 4.2M Jun 27 test_backup.sql

Health check every 5 minutes

Health check scripts are one of the most practical uses of cron. Every production service should have one.

#!/bin/bash
# health_check.sh
set -e

URL="https://myapp.com/health"
LOG="/var/log/health-check.log"

# Timestamp
echo "$(date '+%Y-%m-%d %H:%M:%S') - Checking $URL" >> $LOG

# curl: -s silent, -o /dev/null discard body, -w print status code
STATUS=$(curl -s -o /dev/null -w "%{http_code}" "$URL")

if [ "$STATUS" != "200" ]; then
    echo "$(date) - ALERT: $URL returned $STATUS" >> $LOG
    exit 1
fi

echo "$(date) - OK: $STATUS" >> $LOG

# crontab entry:
# */5 * * * * /home/bill/health_check.sh >> /var/log/health-check.log 2>&1

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