Schedule recurring tasks with cron and understand crontab syntax and management.

Crontab Syntax

#  Minute  Hour  Day-of-Month  Month  Day-of-Week  Command
#  0-59   0-23      1-31       1-12     0-6 (Sun=0)
   *       *         *           *        *          /path/to/command

Common shortcuts: @reboot, @daily, @weekly, @monthly, @hourly

Manage Your Crontab

# Edit your cron jobs
crontab -e

# List your cron jobs
crontab -l

# Remove all your cron jobs
crontab -r

# Edit another user's crontab (as root)
crontab -u www-data -e

Common Examples

# Run backup daily at 2:30 AM
30 2 * * * /home/craig/scripts/backup.sh >> /var/log/backup.log 2>&1

# Clear temp files every Sunday at midnight
0 0 * * 0 find /tmp -type f -atime +7 -delete

# Renew SSL cert twice daily (certbot)
0 */12 * * * certbot renew --quiet

# Send a heartbeat every 5 minutes
*/5 * * * * curl -s https://hc-ping.com/your-uuid > /dev/null

# Run a Python script at 9 AM on weekdays
0 9 * * 1-5 /usr/bin/python3 /home/craig/scripts/report.py

# Run on the 1st of every month
0 6 1 * * /home/craig/scripts/monthly_report.sh

System Cron Directories

# Drop scripts into these directories:
/etc/cron.hourly/   # runs every hour
/etc/cron.daily/    # runs daily
/etc/cron.weekly/   # runs weekly
/etc/cron.monthly/  # runs monthly

# System-wide crontab
sudo nano /etc/crontab

Debug Cron Jobs

# Log all cron activity
grep CRON /var/log/syslog

# Redirect output in the cron job itself
0 3 * * * /path/to/script.sh >> /var/log/myjob.log 2>&1

# Test the script manually with the same environment
env -i /bin/bash -c 'export HOME=/root; /path/to/script.sh'