The Definitive Cron Jobs Syntax Cheat Sheet: Master Scheduling Tasks in Linux

Introduction to Cron Jobs

Cron is a time-based job scheduler in Unix-like operating systems that enables users to schedule commands or scripts to run automatically at specified intervals. Cron jobs are defined in a crontab (cron table) file, which contains instructions for the cron daemon about when and how often to run specific tasks. Mastering cron syntax allows system administrators, developers, and power users to automate routine tasks, schedule backups, run maintenance scripts, and ensure critical processes execute reliably without manual intervention.

Cron Syntax Fundamentals

Basic Crontab Format

* * * * * command_to_execute
↑ ↑ ↑ ↑ ↑
│ │ │ │ │
│ │ │ │ └── Day of the week (0-6 or SUN-SAT)
│ │ │ └──── Month (1-12 or JAN-DEC)
│ │ └────── Day of the month (1-31)
│ └──────── Hour (0-23)
└────────── Minute (0-59)

Understanding Time Fields

Special Characters Explained

Common Cron Job Examples

Time-Based Schedules

Special Schedules

System-Wide vs. User Crontabs

User Crontabs

• Location: Generally stored in /var/spool/cron/crontabs/
• Access Command: crontab -e (edit), crontab -l (list), crontab -r (remove)
• Permissions: Runs with the permissions of the specified user
• Format: Does not include a username field

System-Wide Crontabs

• Main System Crontab: /etc/crontab
• Cron Directories:
  • /etc/cron.d/ – System job fragments
  • /etc/cron.daily/ – Daily jobs
  • /etc/cron.hourly/ – Hourly jobs
  • /etc/cron.monthly/ – Monthly jobs
  • /etc/cron.weekly/ – Weekly jobs
• Format: Includes a username field to specify which user should run the command

  * * * * * username command_to_execute
  

Crontab Commands and Management

Essential Crontab Commands

Environment Variables in Crontabs

Cron jobs run with a minimal environment. Add these variables to your crontab for proper execution:

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

• SHELL: Defines which shell to use
• PATH: Sets the search path for commands
• MAILTO: Where to send output (empty string means no mail)
• HOME: Sets the home directory

Advanced Cron Techniques

Redirecting Output

Multiple Commands in One Line

# Run multiple commands sequentially
0 5 * * * command1 && command2 && command3

# Run commands regardless of previous command success
0 5 * * * command1; command2; command3

# Run second command only if first fails
0 5 * * * command1 || command2

Using Predefined Schedules

# Create reusable variables
DAILY="0 0 * * *"
WEEKLY="0 0 * * 0"

# Use them in crontab
$DAILY /scripts/daily-backup.sh
$WEEKLY /scripts/weekly-report.sh

Common Cron Job Use Cases

System Maintenance

# Clear temp files daily at 2:30 AM
30 2 * * * find /tmp -type f -atime +7 -delete

# Update package lists weekly on Sunday
0 0 * * 0 apt-get update

# Rotate logs monthly
0 0 1 * * /usr/sbin/logrotate /etc/logrotate.conf

Backups

# Daily database backup at 1 AM
0 1 * * * mysqldump -u user -p'password' database > /backups/db-$(date +\%Y\%m\%d).sql

# Weekly file backup on Saturday at 2 AM
0 2 * * 6 tar -czf /backups/home-$(date +\%Y\%m\%d).tar.gz /home/user

# Monthly cloud sync on the 1st at 3 AM
0 3 1 * * rsync -avz /backups/ user@remote:/backups/

Website & Application Maintenance

# Check website availability every 5 minutes
*/5 * * * * curl -s http://example.com > /dev/null || mail -s "Website Down" admin@example.com

# Clean sessions daily
0 1 * * * find /var/www/sessions -type f -mtime +1 -delete

# Generate sitemap weekly
0 3 * * 0 /var/www/html/generate-sitemap.php

Report Generation

# Generate daily traffic report
0 7 * * * /scripts/traffic-report.sh > /reports/traffic-$(date +\%Y\%m\%d).txt

# Email monthly financial summary
0 8 1 * * /scripts/financial-summary.sh | mail -s "Monthly Financial Report" finance@example.com

Troubleshooting Cron Jobs

Common Issues and Solutions

Debugging Tips

• Add timestamp output to help track execution:

  * * * * * echo "Job ran at $(date)" >> /tmp/cronjob.log && /path/to/actual/script
  

• Test scripts separately before adding to crontab:

  $ sh -x /path/to/script.sh
  

• Check cron logs for errors:

  $ grep CRON /var/log/syslog
  

• Verify cron service is running:

  $ systemctl status cron
  

• Use a crontab validator to check syntax:

  $ crontab -e && echo "Syntax OK"
  

Cron Security Best Practices

• Use the principle of least privilege (run jobs as non-root users when possible)
• Avoid placing sensitive information like passwords directly in crontab
• Use restricted shell or specific commands to limit what cron can execute
• Implement proper file permissions on scripts run by cron
• Review crontabs regularly to remove unnecessary jobs
• Log and monitor cron job activity
• Consider using secret management tools instead of hardcoded credentials

Cron Alternatives and Extensions

Resources for Further Learning

Documentation

• Man pages: man cron, man crontab, man 5 crontab
• GNU Coreutils documentation
• Linux System Administrator’s Guide

Online Tools

• Crontab.guru – Interactive cron expression editor
• Cronitor – Cron job monitoring service
• Crontab Generator – Visual crontab creator

Books

• “Linux Administration: A Beginner’s Guide” by Wale Soyinka
• “UNIX and Linux System Administration Handbook” by Evi Nemeth et al.
• “Pro Linux System Administration” by James Turnbull

Websites & Blogs

• The Linux Documentation Project (TLDP)
• DigitalOcean Community Tutorials
• LinuxJournal
• ServerFault Q&A

By mastering the cron syntax and best practices outlined in this cheat sheet, you’ll be able to efficiently automate tasks, maintain system health, and ensure critical operations run smoothly and reliably on your Linux systems.