Cron

Cron is a time-based task scheduler in Unix/Linux systems. Schedule recurring jobs with crontab or system cron files.

Cron Syntax

┌───────────── minute (0 - 59)
│ ┌───────────── hour (0 - 23)
│ │ ┌───────────── day of month (1 - 31)
│ │ │ ┌───────────── month (1 - 12)
│ │ │ │ ┌───────────── day of week (0 - 6) (Sunday is 0)
│ │ │ │ │
│ │ │ │ │
* * * * * command_to_run

Special Syntax

Expression	Description	Equivalent
`@yearly`	Once per year on January 1st	`0 0 1 1 *`
`@annually`	Same as @yearly	`0 0 1 1 *`
`@monthly`	Once per month on 1st	`0 0 1 * *`
`@weekly`	Once per week on Sunday	`0 0 * * 0`
`@daily`	Once per day at midnight	`0 0 * * *`
`@midnight`	Same as @daily	`0 0 * * *`
`@hourly`	Once per hour at minute 0	`0 * * * *`
`@reboot`	Run once at startup	Special

Common Examples

Pattern	Description
`0 0 * * *`	Daily at midnight
`0 12 * * *`	Daily at noon
`0 9 * * 1-5`	Weekdays (Mon-Fri) at 9 AM
`0 9 * * 0,6`	Weekends (Sat-Sun) at 9 AM
`/15 * * *`	Every 15 minutes
`0 /6 * *`	Every 6 hours
`0 0 1 * *`	First day of month at midnight
`0 0 1 1 *`	January 1st at midnight (New Year)
`30 2 * * *`	Daily at 2:30 AM
`0 22 * * 1-5`	Weekdays at 10 PM

Minute & Hour Patterns

# Every minute
* * * * *

# Every 5 minutes
*/5 * * * *

# Every 30 minutes
*/30 * * * *

# Every hour
0 * * * *

# Every 4 hours
0 */4 * * *

# Every 2 hours, Monday-Friday
0 */2 * * 1-5

# 9 AM - 5 PM every hour, weekdays
0 9-17 * * 1-5

Day & Date Patterns

# Every day
0 0 * * *

# Every other day
0 0 */2 * *

# 1st and 15th of month
0 0 1,15 * *

# Last day of month (approximation)
0 0 28-31 * *

# Every Sunday
0 0 * * 0

# Every Monday
0 0 * * 1

# Monday and Friday
0 0 * * 1,5

# Weekdays only (Monday-Friday)
0 0 * * 1-5

# Weekends (Saturday-Sunday)
0 0 * * 0,6

Real-World Examples

Backups

# Daily backup at 2 AM
0 2 * * * /home/user/backup.sh

# Weekly backup on Sunday at 3 AM
0 3 * * 0 /usr/local/bin/weekly-backup.sh

# Monthly backup on 1st at 1 AM
0 1 1 * * tar czf /backups/monthly-$(date +%Y%m%d).tar.gz /important/data

Maintenance Tasks

# Clear cache every day at 1 AM
0 1 * * * rm -rf /var/cache/app/*

# Log rotation every Sunday at 4 AM
0 4 * * 0 /usr/sbin/logrotate /etc/logrotate.conf

# Database optimization first Tuesday at 2 AM
0 2 * * 2 /usr/bin/mysqlcheck -Aao

Monitoring & Alerts

# Check disk space every 6 hours
0 */6 * * * /usr/local/bin/check-disk-space.sh

# Send health report daily at 9 AM
0 9 * * * /opt/monitoring/health-report.sh | mail -s "Daily Report" admin@example.com

# Verify SSL certificates expiry weekly
0 10 * * 1 /usr/local/bin/check-ssl-certs.sh

Data Processing

# Process daily logs at 1 AM
0 1 * * * /opt/scripts/process-logs.py

# Generate reports every Monday at 8 AM
0 8 * * 1 /usr/local/bin/generate-reports.sh

# Sync data every 30 minutes
*/30 * * * * /home/user/sync-data.sh

Cron Environment

Variables available in cron jobs:

SHELL=/bin/bash
PATH=/usr/bin:/bin
MAILTO=admin@example.com
HOME=/home/user

# Set in crontab before jobs
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
MAILTO=alerts@example.com

# Then define jobs
0 9 * * * /usr/local/bin/backup.sh

Logging & Redirection

# Log output to file
0 9 * * * /backup.sh >> /var/log/backup.log 2>&1

# Log to syslog
0 9 * * * /usr/local/bin/task.sh 2>&1 | logger -t mytask

# Append to log with timestamp
0 9 * * * echo "$(date): Running task" >> /var/log/task.log && /task.sh

# Suppress output
0 9 * * * /backup.sh > /dev/null 2>&1

# Email output if command fails
0 9 * * * /backup.sh || echo "Backup failed" | mail admin@example.com

System Cron Files

Location of system-wide cron jobs:

/etc/cron.d/          # System cron jobs (run as specified user)
/etc/cron.daily/      # Scripts run daily by run-parts
/etc/cron.hourly/     # Scripts run hourly
/etc/cron.weekly/     # Scripts run weekly
/etc/cron.monthly/    # Scripts run monthly

System Cron Format

# /etc/cron.d/example
# Format: minute hour day month weekday user command

# Run daily backup as root
0 2 * * * root /usr/local/bin/backup.sh

# Run hourly check as www-data user
0 * * * * www-data /usr/local/bin/health-check.sh

# Run weekly report as reports user
0 3 * * 0 reports /opt/reports/generate.sh

@reboot (Startup)

# Run script once at system startup
@reboot /usr/local/bin/startup.sh

# Delay before running at startup
@reboot sleep 60 && /usr/local/bin/delayed-start.sh

# Start service on reboot with logging
@reboot /etc/init.d/myservice start >> /var/log/startup.log 2>&1

Troubleshooting

Check Cron Status

# Service status (systemd systems)
systemctl status cron

# Service status (older systems)
service cron status

# Start cron service
sudo systemctl start cron

# Enable cron on boot
sudo systemctl enable cron

View Cron Logs

# Ubuntu/Debian
sudo tail -f /var/log/syslog | grep CRON

# CentOS/RHEL
sudo tail -f /var/log/cron

# Using journalctl (newer systems)
journalctl -u cron -f

# Search for specific cron job
grep CRON /var/log/syslog | grep "myjob"

Debug Cron Issues

# Run cron job manually to test
bash /home/user/script.sh

# Check script permissions (must be executable)
chmod +x /home/user/script.sh

# Verify crontab syntax
crontab -l

# Test environment variables
env > /tmp/cron-env.log

# Add verbose logging in script
#!/bin/bash
echo "$(date): Job started" >> /var/log/myjob.log
# ... rest of script

Best Practices

Always use absolute paths in cron jobs
Redirect output to log files for debugging
Use meaningful MAILTO variable for alerts
Test scripts manually before adding to cron
Use specific times to avoid resource conflicts
Add comments in crontab for clarity
Monitor cron logs regularly
Use version control for cron configurations
Document why each job exists
Include error handling in scripts

Resources

man 5 crontab - Crontab file format
man 8 cron - Cron daemon
Crontab Generator
Cron Expression Explainer

Last updated: 2026-03-30