Script Scheduling with Cron: The Complete Guide

Master Linux's built-in job scheduler to automate script execution at precise intervals. This comprehensive guide covers cron syntax, scheduling patterns, error handling, and best practices for production-ready automation.

Understanding Cron: Linux's Time-Based Job Scheduler

1. Cron Syntax: The Five Fields Explained

2. Practical Cron Examples

3. Managing Crontab Files

User Crontab Commands

# Edit your personal crontab
crontab -e

# View your current crontab
crontab -l

# Remove all your cron jobs
crontab -r

# List another user's crontab (requires sudo)
sudo crontab -u username -l

System Crontab Files

# System-wide crontab (requires root)
/etc/crontab

# Directory for scripts that run hourly, daily, weekly, monthly
/etc/cron.hourly/
/etc/cron.daily/
/etc/cron.weekly/
/etc/cron.monthly/

# Additional cron configuration files
/etc/cron.d/

4. Production-Ready Crontab Template

# ================= SYSTEM MAINTENANCE =================
# Backup important directories daily at 2 AM
0 2 * * * /usr/local/bin/backup-files.sh >> /var/log/backup.log 2>&1

# Update system packages every Sunday at 3 AM
0 3 * * 0 /usr/local/bin/update-system.sh >> /var/log/updates.log 2>&1

# Cleanup temporary files daily at 4 AM
0 4 * * * /usr/local/bin/cleanup-temp.sh >> /var/log/cleanup.log 2>&1

# ================= MONITORING & ALERTS =================
# System health check every 30 minutes
*/30 * * * * /usr/local/bin/health-check.sh >> /var/log/health.log 2>&1

# Disk space monitoring every hour
0 * * * * /usr/local/bin/check-disk.sh >> /var/log/disk.log 2>&1

# Service monitoring every 5 minutes
*/5 * * * * /usr/local/bin/monitor-services.sh >> /var/log/services.log 2>&1

# ================= APPLICATION TASKS =================
# Database backup daily at 1 AM
0 1 * * * /usr/local/bin/backup-db.sh >> /var/log/db-backup.log 2>&1

# Log rotation daily at midnight
0 0 * * * /usr/local/bin/rotate-logs.sh >> /var/log/rotation.log 2>&1

# Cache clearing every 6 hours
0 */6 * * * /usr/local/bin/clear-cache.sh >> /var/log/cache.log 2>&1

# ================= CUSTOM SCHEDULES =================
# Business hours only (9 AM - 5 PM, weekdays)
0 9-17 * * 1-5 /usr/local/bin/business-process.sh >> /var/log/business.log 2>&1

# Every 10 minutes during working hours
*/10 9-17 * * 1-5 /usr/local/bin/monitor-production.sh >> /var/log/prod-monitor.log 2>&1

# Weekend maintenance
0 2 * * 6 /usr/local/bin/weekend-maintenance.sh >> /var/log/weekend.log 2>&1

5. Advanced Cron Techniques

Preventing Overlapping Jobs

#!/bin/bash
# Using flock to prevent overlapping executions
# create-lock.sh

LOCK_FILE="/tmp/my-script.lock"
TIMEOUT=300  # 5 minutes

# Use flock to acquire lock with timeout
if flock -n -x -w $TIMEOUT 200; then
    echo "Lock acquired, starting execution..."
    
    # Your script logic here
    sleep 60  # Simulate work
    
    echo "Execution completed"
    
    # Lock is automatically released when script ends
else
    echo "Another instance is already running or lock timeout"
    exit 1
fi 200>$LOCK_FILE

Environment Variables in Cron

# Cron has minimal environment by default
# Always set PATH and other variables at the top of your crontab

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
HOME=/home/youruser
LOGNAME=youruser

# Also set variables in your scripts
#!/bin/bash
source /home/youruser/.bashrc  # If needed
export MY_VAR="value"

# Better: Set critical variables explicitly
BACKUP_DIR="/backups"
LOG_FILE="/var/log/my-script.log"

Error Handling and Notifications

#!/bin/bash
# error-handling.sh - Cron job with proper error handling

# Configuration
SCRIPT_NAME=$(basename "$0")
LOG_FILE="/var/log/${SCRIPT_NAME}.log"
ERROR_LOG="/var/log/${SCRIPT_NAME}_errors.log"
ADMIN_EMAIL="admin@example.com"

# Redirect all output
exec >> "$LOG_FILE" 2>> "$ERROR_LOG"

# Function to send alert email
send_alert() {
    local subject="$1"
    local message="$2"
    
    echo "$message" | mail -s "$subject" "$ADMIN_EMAIL"
    echo "[$(date)] Alert sent: $subject" >> "$LOG_FILE"
}

# Trap errors
trap 'send_alert "CRON ERROR: $SCRIPT_NAME failed" "Script failed at $(date)"; exit 1' ERR

# Start execution
echo "=== Starting $SCRIPT_NAME at $(date) ==="

# Your main logic here
# Example: Backup database
if ! mysqldump -u root database > /backups/db.sql; then
    echo "ERROR: Database backup failed"
    send_alert "Database Backup Failed" "Check $ERROR_LOG for details"
    exit 1
fi

echo "=== Completed successfully at $(date) ==="

6. Cron Best Practices

7. Common Cron Problems & Solutions

8. Alternative Scheduling Tools

Systemd Timer Example

# /etc/systemd/system/backup.service
[Unit]
Description=Daily Backup Service
After=network.target

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh
User=backupuser
Group=backupuser

# /etc/systemd/system/backup.timer
[Unit]
Description=Daily Backup Timer

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target

# Enable and start
sudo systemctl enable backup.timer
sudo systemctl start backup.timer
sudo systemctl list-timers  # View all timers

Getting Started with Cron