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

# List current crontab
crontab -l

# List for specific user
crontab -l -u username

# Edit crontab
crontab -e

# Add from command line
(crontab -l 2>/dev/null; echo "0 9 * * * /path/to/script.sh") | crontab -

# Remove all crontabs
crontab -r

# Remove for specific user
crontab -r -u username

# Run job immediately (test)
/path/to/script.sh

# Check cron service status
systemctl status cron  # or crond

# View cron logs
journalctl -u cron     # systemd
tail -f /var/log/cron # other systems

# Using Python
python3 -c "
from croniter import croniter
import sys
if len(sys.argv) > 1:
    expr = sys.argv[1]
    if croniter.is_valid(expr):
        print('Valid cron expression')
        # Show next 5 runs
        cron = croniter(expr)
        for i in range(5):
            print(cron.get_next())
    else:
        print('Invalid cron expression')
"

# Using online tools or crontab.guru
curl -s "https://crontab.guru/0+9+*+*+*" 

# Using python-crontab
python3 -c "
from crontab import CronSitemap
c = CronSitemap('0 9 * * *')
print(c.human_readable)
"

# Add daily backup at 2am
(crontab -l 2>/dev/null; echo "0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1") | crontab -

# Clean logs older than 7 days, daily at 3am
(crontab -l 2>/dev/null; echo "0 3 * * * find /var/log -name '*.log' -mtime +7 -delete") | crontab -

# Check every 5 minutes
(crontab -l 2>/dev/null; echo "*/5 * * * * curl -s https://example.com/health") | crontab -