When working with Linux system administration, you'll commonly encounter several cron-related directories:

• /etc/cron.d/ - For system-wide cron jobs
• /etc/cron.daily/ - Daily scripts
• /etc/cron.weekly/ - Weekly scripts
• /etc/cron.monthly/ - Monthly scripts

The standard crontab command might not be immediately available if you're not using a personal user account with cron privileges. Let's first check if cron is installed:

# Check cron service status
sudo systemctl status cron
# Or for newer systems:
sudo systemctl status crond

Since you have access to cron.d, we'll create a system-wide job. Here's how to schedule a reboot every 3rd Saturday at 23:30:

# Create a new cron job file
sudo nano /etc/cron.d/monthly-reboot

# Add this content:
30 23 * * 6 [ $(date +\%d) -le 21 ] && [ $(date +\%d) -ge 15 ] && /sbin/shutdown -r now

# Save and exit (Ctrl+X, Y, Enter)

The complex-looking command actually combines several elements:

• 30 23 * * 6 - Runs at 23:30 every Saturday
• [ $(date +\%d) -le 21 ] - Checks if day of month ≤ 21
• [ $(date +\%d) -ge 15 ] - Checks if day of month ≥ 15
• && /sbin/shutdown -r now - Reboots if both conditions are true

For systems that might be powered off during the scheduled time, consider using anacron:

# Create a custom anacron job
sudo nano /etc/anacrontab

# Add this line (runs with 7-day frequency):
@monthly 15 monthly-reboot /sbin/shutdown -r now

To confirm your cron job is set up correctly:

# List all cron jobs
sudo cat /etc/cron.d/monthly-reboot

# Check cron logs (timing may vary by distro)
sudo tail -f /var/log/cron.log
# Or alternatively:
sudo journalctl -u cron

When setting up automated reboots:

• Ensure critical services have auto-start configurations
• Consider notifying logged-in users before reboot
• Document the scheduled reboot in your system documentation
• For production systems, implement maintenance windows

If your reboot isn't executing as expected:

• Verify cron service is running: sudo systemctl start cron
• Check file permissions: sudo chmod 644 /etc/cron.d/monthly-reboot
• Ensure proper PATH settings in your cron job
• Test with a non-reboot command first

For more complex scheduling needs, consider creating a bash script in /usr/local/bin/ and calling that from your cron job instead.

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When administering Linux systems, scheduling regular reboots is a common maintenance task. In this case, we need to configure a reboot every 3rd Saturday of the month at 23:30. While this might sound specific, cron (the time-based job scheduler in Unix-like systems) can handle it perfectly.

Many new Linux users get confused about cron's file structure. You mentioned seeing these directories:

cron.d
cron.daily
cron.weekly
cron.monthly

These are system cron directories, but for user-specific cron jobs, we need to use crontab. Try running:

which crontab

If it's not found, you may need to install the cron package first with:

sudo apt-get install cron  # For Debian/Ubuntu
sudo yum install cronie    # For CentOS/RHEL

The magic happens in the crontab file. To edit your crontab:

crontab -e

For your specific requirement (3rd Saturday at 23:30), the entry would be:

30 23 * * 6 [ $(date +\%d) -le 21 ] && [ $(date +\%d) -gt 14 ] && /sbin/reboot

Let's break this down:

• 30 23 * * 6 - Runs at 23:30 every Saturday
• [ $(date +\%d) -le 21 ] - Day of month is 21 or less
• [ $(date +\%d) -gt 14 ] - Day of month is greater than 14
• && /sbin/reboot - If both conditions are true, execute reboot

Since you mentioned having cron.d, you could also create a file in /etc/cron.d/ (requires root):

sudo nano /etc/cron.d/monthly-reboot

Add this content:

30 23 * * 6 root [ $(date +\%d) -le 21 ] && [ $(date +\%d) -gt 14 ] && /sbin/reboot

After setting up, verify with:

crontab -l

For system-wide cron jobs in cron.d:

cat /etc/cron.d/monthly-reboot

Before deploying, test your logic with:

# Simulate 3rd Saturday:
date -d "2023-11-18" +"Day: %d, Weekday: %a"
[ $(date -d "2023-11-18" +\%d) -le 21 ] && [ $(date -d "2023-11-18" +\%d) -gt 14 ] && echo "Would reboot now"

# Simulate non-3rd Saturday:
date -d "2023-11-11" +"Day: %d, Weekday: %a"
[ $(date -d "2023-11-11" +\%d) -le 21 ] && [ $(date -d "2023-11-11" +\%d) -gt 14 ] && echo "Would reboot now" || echo "No reboot"

• Always use full paths to commands in cron jobs
• Consider logging: > /var/log/reboot.log 2>&1
• For systemd systems, you might alternatively use systemd timers
• Make sure cron service is running: sudo systemctl status cron