How to Set up Python Scripts with Crontab

Note: This method is applicable to all EG Series/RobustOS Pro devices

Author: Eldar Mustafin

1. Creating a Python Script

First, create your Python script with proper logging and error handling. Here's a template:

#!/usr/bin/env python3
import logging
from datetime import datetime
import sys
from pathlib import Path

# Set up logging
log_dir = Path('/path/to/your/logs')
log_dir.mkdir(parents=True, exist_ok=True)
log_file = log_dir / 'script.log'

logging.basicConfig(
    level=logging.INFO,
    format='%(asctime)s - %(levelname)s - %(message)s',
    handlers=[
        logging.FileHandler(log_file),
        logging.StreamHandler(sys.stdout)
    ]
)

def main():
    try:
        # Your main script logic here
        logging.info("Script started")
        
        # Example task
        current_time = datetime.now()
        logging.info(f"Current time: {current_time}")
        
        # More tasks...
        
        logging.info("Script completed successfully")
        
    except Exception as e:
        logging.error(f"An error occurred: {str(e)}", exc_info=True)
        sys.exit(1)

if __name__ == "__main__":
    main()

2. Making the Script Executable

Save your script (e.g., as task.py)

Make it executable:

chmod +x /path/to/your/task.py

3. Testing the Script

Before setting up crontab, test your script:

/path/to/your/task.py

4. Setting up Crontab

Basic Crontab Syntax

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

Common Crontab Examples

# Run every minute
* * * * * /path/to/your/task.py

# Run every hour at minute 0
0 * * * * /path/to/your/task.py

# Run at 2:30 AM daily
30 2 * * * /path/to/your/task.py

# Run every Monday at 9 AM
0 9 * * 1 /path/to/your/task.py

# Run first day of every month at midnight
0 0 1 * * /path/to/your/task.py

Setting Up Your Crontab

1. Open crontab editor:

crontab -e

2. Add your script (example):

# Add environment variables if needed
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PYTHONPATH=/path/to/python/packages

# Run script daily at 3 AM
0 3 * * * /path/to/your/task.py >> /path/to/your/logs/cron.log 2>&1

5. Best Practices

Use Absolute Paths: Always use full paths in your crontab entries
Set Environment Variables: Include necessary environment variables in crontab
Redirect Output: Capture both stdout and stderr in logs
Test Thoroughly: Test your script manually before adding to crontab
Monitor Logs: Regularly check your log files
Use Lock Files: Prevent multiple instances from running simultaneously:

import fcntl
import sys
from pathlib import Path

def obtain_lock():
    lock_file = Path("/tmp/my_script.lock")
    lock_fd = open(lock_file, 'w')
    try:
        fcntl.lockf(lock_fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
        return lock_fd
    except IOError:
        logging.warning("Another instance is running")
        sys.exit(0)

# Use in your script
lock_fd = obtain_lock()
# Your script logic here
# Lock file is automatically released when script ends

6. Troubleshooting

Check Logs: Review both script and cron logs
Test Permissions: Ensure script has proper execute permissions
Verify Paths: Confirm all paths are absolute and correct
Check Environment: Verify environment variables are set correctly
Monitor Resource Usage: Check CPU and memory usage

7. Viewing Crontab Status

# List your crontab entries
crontab -l

# View cron service status
sudo systemctl status cron

# Check cron logs
sudo grep CRON /var/log/syslog

Set up Python Scripts with Crontab | Robustel