Create linux-architecture-notes.md

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

@@ -0,0 +1,39 @@
+# Linux Architecture, Processes, and systemd
+
+## Core Components
+- **Kernel**: The core of Linux. Manages hardware, memory, CPU scheduling, and system calls.  
+- **User Space**: Where applications, shells, and user processes run. Communicates with the kernel via system calls.  
+- **Init/systemd**: The very first process started by the kernel. Initializes the system, starts services, and manages dependencies.
+
+---
+
+## Process Management
+- Processes are created using `fork()` (to duplicate) and `exec()` (to replace with new program).  
+- Each process has a **PID (Process ID)**.  
+- Parent and child processes are linked.  
+- The kernel schedules processes based on priority and resources.
+
+### Process States
+- **Running** → Actively using CPU.  
+- **Sleeping** → Waiting for an event or resource.  
+- **Zombie** → Finished execution but not cleaned up by parent.  
+- **Stopped** → Suspended, waiting to be resumed.  
+
+---
+
+## systemd
+- Default init system in most modern Linux distributions.  
+- Manages services, sockets, devices, and timers.  
+- Provides **parallel startup** → faster boot times.  
+- Unified commands (`systemctl`) for starting, stopping, enabling, and checking services.  
+- Centralized logging via `journalctl`.  
+- Critical for troubleshooting and service management in production.
+
+---
+
+## Daily Commands
+1. `ps aux` → List all processes with details.  
+2. `top` → Monitor CPU/memory usage in real time.  
+3. `systemctl status <service>` → Check service health.  
+4. `systemctl restart <service>` → Restart a service.  
+5. `journalctl -xe` → View detailed logs for debugging.