Add Linux architecture notes

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

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+# Core components of linux
+
+i) Hardware layer
+ii) Shell
+iii) Kernel
+iv) System libraries
+v) System utilities(like,GNU)
+
+# How processes are created in linux
+
+*fork() System Call: A running "parent" process initiates the fork() system call to create a new, nearly identical "child" process. 
+ The child process receives a copy of the parent's memory space, open file descriptors, and other resources
+*exec() System Call: After the fork(), the child process typically uses an exec() system call (e.g., execve()) to replace its entire memory space with a new program's code and data.
+*wait() System Call: The parent process often uses the wait() system call to pause its own execution until its child process finishes and exits,
+ allowing the parent to collect the child's exit status and prevent it from becoming a zombie process. 
+
+# Process states
+
+A process transitions through several states during its lifecycle: 
+
+1. Running (R): The process is either currently executing on the CPU or waiting in the run queue to be executed.
+2. Sleeping/Waiting (S or D): The process is waiting for some event to occur (e.g., I/O completion, a signal).
+3. Stopped (T): The process has been suspended by a job control signal (like Ctrl+Z).
+5. Zombie (Z): The process has terminated, but its parent process has not yet collected its exit status, so its entry still exists in the process table.
+
+# What systemd does
+
+1. Initializes the System: It is the first user-space process to run during boot (PID 1)
+2. Manages Services: It starts, stops, and restarts background services (daemons) efficiently using "unit files" which define how services should behave [2].
+3. Provides System Logging: It includes journald, a centralized logging management system [1].
+4. Manages Devices and Mount Points: It uses udev (as part of the suite) to manage device events and automatically handle device hot-plugging [1].
+5. Enables Parallelism: It uses socket and D-Bus activation to start services in parallel, significantly speeding up boot times [2].
+
+# Why does it matter
+
+1. Standardization: It provides a consistent, standardized framework across many different Linux distributions, making system administration and development more uniform [2].
+2. Faster Boot Times: Its design allows for aggressive parallelization during startup, which dramatically decreases the time it takes for a system to become usable [2].
+3. Modern Features: It offers robust features essential for modern computing, such as cgroup management for resource control, on-demand service activation, and better security isolation for services [1, 2].
+
+
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