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2026/day-02/linux-architecture-notes.md

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+# Linux Architecture, Processes, and systemd
+
+## 1. Core Components of Linux
+
+### Kernel (known as "Linux Kernel")
+- The **core of the OS**(Heart)
+- Manages:
+  - CPU scheduling
+  - Memory management
+  - Hardware access (drivers)
+  - Process lifecycle
+- Runs in **kernel space** (full privileges)
+
+### User Space
+- Where **applications and users** operate
+- Includes:
+  - Shells (bash, zsh)
+  - Utilities (`ls`, `ps`, `top`)
+  - Services (nginx, docker)
+- Cannot access hardware directly → requests go via kernel
+
+### Init System (systemd)
+- First process started by the kernel (**PID 1**)
+- Responsible for:
+  - Starting services
+  - Restarting failed services
+  - Managing logs and dependencies
+
+---
+
+## 2. How Processes Work in Linux
+
+### Process Creation
+- A new process is created using:
+  - `fork()` → creates a copy
+  - `exec()` → loads a new program
+- Every process has:
+  - **PID** (Process ID)
+  - **PPID** (Parent PID)
+  - Owner (user)
+
+### Process States
+- **R (Running)** – Executing on CPU
+- **S (Sleeping)** – Waiting for I/O or event
+- **D (Uninterruptible Sleep)** – Waiting for disk/network (cannot be killed)
+- **T (Stopped)** – Paused (e.g., Ctrl+Z)
+- **Z (Zombie)** – Finished execution but parent hasn’t cleaned it up
+
+> Zombies don’t use CPU/memory, but many zombies indicate a bug.
+
+---
+
+## 3. What systemd Does (and Why It Matters)
+
+- Controls system services using **units**
+- Common unit types:
+  - `service` – background services
+  - `timer` – cron replacement
+  - `target` – system states (multi-user, graphical)
+
+### Why systemd is Important
+- Automatically restarts crashed services
+- Handles service dependencies
+- Centralized logging with `journalctl`
+- Faster boot and better observability
+
+---
+
+## 4. Daily Linux Commands (DevOps Essentials)
+
+1. `ps aux` – View running processes
+2. `top` / `htop` – Monitor CPU & memory usage
+3. `systemctl status <service>` – Check service health
+4. `journalctl -u <service>` – View service logs
+5. `kill -9 <PID>` – Force stop a stuck process
+
+---
+
+## 5. DevOps Takeaway
+
+- Kernel = control
+- Processes = workload
+- systemd = reliability
+
+If you understand these three:
+- You can debug crashes
+- Fix performance issues
+- Confidently manage production servers

2026/day-03/linux-commands-cheatsheet.md

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+# Linux Commands Cheat Sheet
+**DevOps Essential Command Toolkit**
+
+--
+## 📊 Process Management
+
+| Command | Usage |
+|---------|-------|
+| `ps aux` | Display all running processes with detailed info |
+| `top` | Real-time process monitoring and system stats |
+| `htop` | Interactive process viewer (better than top) |
+| `kill <PID>` | Terminate process by Process ID |
+| `kill -9 <PID>` | Force kill a process (SIGKILL) |
+| `killall <name>` | Kill all processes by name |
+| `pgrep <name>` | Find process ID by name |
+| `bg` | Resume suspended job in background |
+| `fg` | Bring background job to foreground |
+| `jobs` | List all background jobs |
+| `nohup <command> &` | Run command immune to hangups |
+| `systemctl status <service>` | Check service status |
+| `systemctl start <service>` | Start a service |
+| `systemctl stop <service>` | Stop a service |
+| `systemctl restart <service>` | Restart a service |
+
+---
+
+## 📁 File System Commands
+
+| Command | Usage |
+|---------|-------|
+| `df -h` | Show disk usage in human-readable format |
+| `du -sh <directory>` | Show directory size summary |
+| `ls -lah` | List files with hidden items and human-readable sizes |
+| `find / -name <filename>` | Search for file by name from root |
+| `find . -type f -size +100M` | Find files larger than 100MB |
+| `tail -f <logfile>` | Follow log file in real-time |
+| `tail -n 50 <file>` | Display last 50 lines of file |
+| `head -n 20 <file>` | Display first 20 lines of file |
+| `grep -r "error" /var/log/` | Search for "error" recursively in logs |
+| `chmod 755 <file>` | Change file permissions (rwxr-xr-x) |
+| `chown user:group <file>` | Change file ownership |
+| `ln -s <source> <link>` | Create symbolic link |
+| `tar -czf archive.tar.gz <dir>` | Create compressed archive |
+| `tar -xzf archive.tar.gz` | Extract compressed archive |
+| `mount /dev/sdb1 /mnt` | Mount a disk partition |
+
+---
+
+## 🌐 Networking & Troubleshooting
+
+| Command | Usage |
+|---------|-------|
+| `ping <host>` | Test connectivity to host |
+| `ip addr` | Show all network interfaces and IP addresses |
+| `ip route` | Display routing table |
+| `curl <URL>` | Transfer data from/to servers |
+| `curl -I <URL>` | Fetch HTTP headers only |
+| `wget <URL>` | Download files from web |
+| `netstat -tulpn` | Show listening ports and services |
+| `ss -tulpn` | Modern alternative to netstat |
+| `dig <domain>` | DNS lookup and query details |
+| `nslookup <domain>` | Query DNS name servers |
+| `traceroute <host>` | Trace packet route to destination |
+| `ifconfig` | Display/configure network interfaces (legacy) |
+| `tcpdump -i eth0` | Capture network packets on interface |
+| `iptables -L` | List firewall rules |
+| `hostname -I` | Show all IP addresses of host |
+
+---
+
+## 🔍 System Information
+
+| Command | Usage |
+|---------|-------|
+| `uname -a` | Display system information |
+| `uptime` | Show system uptime and load average |
+| `free -h` | Display memory usage in human-readable format |
+| `lsblk` | List block devices (disks and partitions) |
+| `lscpu` | Display CPU architecture information |
+| `vmstat` | Report virtual memory statistics |
+
+---
+
+## 📝 Log Analysis Quick Commands
+
+```bash
+# Check system logs
+journalctl -xe                    # Recent logs with explanations
+journalctl -u nginx.service       # Logs for specific service
+journalctl --since "1 hour ago"   # Logs from last hour
+
+# Apache/Nginx logs
+tail -f /var/log/nginx/error.log
+tail -f /var/log/apache2/access.log
+
+# System logs
+tail -f /var/log/syslog
+grep -i "error" /var/log/syslog | tail -20
+```
+
+---
+
+## 🚨 Emergency Troubleshooting Workflow
+
+```bash
+# 1. Check system resources
+top                    # CPU and memory
+df -h                  # Disk space
+free -h               # Memory
+
+# 2. Check service status
+systemctl status <service>
+journalctl -u <service> -n 50
+
+# 3. Check network
+ip addr               # IP configuration
+ss -tulpn            # Open ports
+ping 8.8.8.8         # External connectivity
+
+# 4. Check logs
+tail -f /var/log/syslog
+grep -i error /var/log/syslog
+```
+
+---
+
+## 💡 Pro Tips
+
+- Use `Ctrl + R` for reverse search in command history
+- Combine commands with `|` (pipe) for powerful workflows
+- Use `&&` to chain commands (runs second only if first succeeds)
+- Press `Tab` for auto-completion
+- Use `man <command>` to read manual pages
+- Add `| less` to paginate long outputs
+- Use `watch -n 2 <command>` to run command every 2 seconds
+
+---
+
+## 🎯 Common Real-World Scenarios
+
+**Scenario 1: Server running slow**
+```bash
+top                                    # Check CPU/memory
+df -h                                  # Check disk space
+ps aux | grep <service>                # Find problematic process
+```
+
+**Scenario 2: Service not responding**
+```bash
+systemctl status <service>             # Check service status
+journalctl -u <service> -n 50         # Check recent logs
+ss -tulpn | grep <port>               # Check if port is listening
+```
+
+**Scenario 3: Network connectivity issues**
+```bash
+ping 8.8.8.8                          # Test internet
+ip addr                                # Check IP config
+dig google.com                         # Test DNS
+traceroute <destination>              # Find where packets drop
+```