Day 25: Backup Architecture, Rsync Mirrors & Recovery

Day 25 · Lunch — Backup Architecture, Rsync Mirrors, Cron & Disaster Recovery

⚡ 60-Second Mental Model (Student Digest)

╔══════════════════════════════════════════════════════════════════════════════╗
║  ZOTHOS × 1NC0GN30 // PRODUCTION TELEMETRY, SYSTEMD & RECOVERY MATRIX        ║
╠══════════════════════════════════════════════════════════════════════════════╣
║                                                                              ║
║   ┌──────────────┐   ┌──────────────┐   ┌──────────────┐   ┌──────────────┐  ║
║   │ CPU & SCHED  │   │ MEMORY / VM  │   │ DISK & VFS   │   │ NET SOCKETS  │  ║
║   │ top / mpstat │   │ free / vmstat│   │ iostat / df  │   │ ss -tulpen   │  ║
║   └──────┬───────┘   └──────┬───────┘   └──────┬───────┘   └──────┬───────┘  ║
║          └──────────────────┴─────────┬────────┴──────────────────┘          ║
║                                       ▼                                      ║
║                  [ Automated Remediation: systemd + cron + rsync ]           ║
╚══════════════════════════════════════════════════════════════════════════════╝

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🕐 Day 25 Lunch: Advanced Cron Usage

🎯 Objective

Dive deeper into how cron works behind the scenes. Learn about timing syntax, using environment variables, and debugging cron jobs with logs.

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⏰ Cron Timing Syntax

Cron uses a specific 5-field format to schedule jobs:

* * * * * command_to_run
| | | | |
| | | | +----- Day of the week (0 - 7) (Sunday = 0 or 7)
| | | +------- Month (1 - 12)
| | +--------- Day of month (1 - 31)
| +----------- Hour (0 - 23)
+------------- Minute (0 - 59)

🔍 Examples

0 0 – every day at midnight
/15 – every 15 minutes
0 9 * 1-5 – every weekday at 9 AM

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🌍 Environment Variables in Cron

Cron runs in a limited environment, so it's best to define any variables or paths needed:

📄 Example

PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
MAILTO="your@email.com"
SHELL=/bin/bash

0 2 * * * /home/zoth/scripts/backup.sh

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🧾 Cron Logs and Troubleshooting

🔧 View Logs

* Debian/Ubuntu: /var/log/syslog

grep CRON /var/log/syslog

* RedHat/CentOS: /var/log/cron

⚠️ Common Pitfalls

* Commands fail silently → redirect output:

0 1 * * * /path/to/script.sh > /tmp/cron.log 2>&1

* Script lacks execute permissions:

chmod +x /path/to/script.sh

* Missing full paths to commands inside script:

#!/bin/bash
/usr/bin/tar -czf backup.tar.gz /etc

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🧠 Review Questions

1. What does */10 mean in the minute field of a cron expression?
2. Where can you check cron job execution logs on Ubuntu?
3. What variable should you set if you want cron job output sent via email?
4. Why should full paths to binaries be used in cron scripts?

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📚 Additional Resources

* Crontab Guru
* Linux CronHowto
* man 5 crontab – Manual for crontab syntax

✅ Next Step: Try modifying your existing cron jobs to include logging, or test a custom schedule with a non-critical script.

Day 25 · Dinner — Backup Architecture, Rsync Mirrors, Cron & Disaster Recovery

⚡ 60-Second Mental Model (Student Digest)

╔══════════════════════════════════════════════════════════════════════════════╗
║  ZOTHOS × 1NC0GN30 // PRODUCTION TELEMETRY, SYSTEMD & RECOVERY MATRIX        ║
╠══════════════════════════════════════════════════════════════════════════════╣
║                                                                              ║
║   ┌──────────────┐   ┌──────────────┐   ┌──────────────┐   ┌──────────────┐  ║
║   │ CPU & SCHED  │   │ MEMORY / VM  │   │ DISK & VFS   │   │ NET SOCKETS  │  ║
║   │ top / mpstat │   │ free / vmstat│   │ iostat / df  │   │ ss -tulpen   │  ║
║   └──────┬───────┘   └──────┬───────┘   └──────┬───────┘   └──────┬───────┘  ║
║          └──────────────────┴─────────┬────────┴──────────────────┘          ║
║                                       ▼                                      ║
║                  [ Automated Remediation: systemd + cron + rsync ]           ║
╚══════════════════════════════════════════════════════════════════════════════╝

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🍽️ Day 25 Dinner: Cron Job Automation Labs

🎯 Objective

Practice writing and troubleshooting cron jobs to automate Linux system tasks efficiently.

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🧪 Lab Task 1: Schedule Backups

✨ Goal

Set up a cron job to create a daily backup of the /etc directory.

🔧 Sample Script

#!/bin/bash
DATE=$(date +"%F")
BACKUP_FILE="/backups/etc-$DATE.tar.gz"
tar -czf "$BACKUP_FILE" /etc

⏰ Cron Entry

To schedule this at 2:00 AM daily:

0 2 * * * /usr/local/bin/backup.sh

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🧪 Lab Task 2: Automate Log Cleanup

✨ Goal

Clear out log files older than 7 days every Sunday at 3:00 AM.

🔧 Cron Entry

0 3 * * 0 find /var/log -type f -name '*.log' -mtime +7 -exec rm {} \;

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🧪 Lab Task 3: Troubleshoot a Failing Cron Job

✨ Goal

Debug a script that is not running as expected via cron.

🔧 Tips for Debugging

* Use full paths to all binaries (e.g., /bin/echo instead of echo)
* Redirect stdout and stderr:

  * * * * * /path/to/script.sh > /tmp/output.log 2>&1

* Check user-specific logs:

  grep CRON /var/log/syslog  # Debian/Ubuntu
  journalctl -u cron         # systemd systems

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🧠 Review Questions

1. What does 0 2 * mean in a cron expression?
2. How do you ensure a script runs properly when executed by cron?
3. Why is it important to use full paths in cron jobs?
4. How can you log output from a cron job for debugging?

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📚 Resources

* crontab.guru — Cron schedule editor and reference
* GNU Cron Manual
* An Introduction to Cron

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✅ Next Step: Apply what you've learned by automating something on your own system, like syncing files, pruning logs, or restarting a service!