Understanding GNU/Linux: From GPL to RAID and System Basics

GNU Project and GPL

The GNU Project, launched by Richard Stallman in 1983, aims to create a completely free operating system. The GNU General Public License (GPL) protects the freedom to use, copy, study, modify, and redistribute software, while requiring that the source code remain publicly available.

Linux Distributions

A typical Linux distribution bundles the Linux kernel with GNU libraries and tools, a command‑line shell, the X Window graphical system, a desktop environment such as KDE or GNOME, and thousands of applications ranging from office suites to development tools.

Common distributions include Red Hat Enterprise Linux, CentOS, SUSE, Ubuntu, Debian, Fedora, and Gentoo.

Unix and Linux Relationship

Linux is a Unix‑like operating system that supports multiple users, multitasking, multithreading, and various CPU architectures. It inherits Unix’s network‑centric design, providing a stable, multi‑user network OS.

Swap Partition

Swap provides virtual memory when physical RAM is insufficient. The system moves inactive pages to a designated swap area on disk, freeing RAM for active processes. Recommended swap size is at least equal to physical memory, not less than 64 MiB, and up to twice the RAM size.

GRUB Bootloader

GNU GRUB (Grand Unified Bootloader) is a multi‑OS boot manager. It lets users select which operating system to start at power‑on and can pass kernel parameters (e.g., to enter single‑user mode) when loading Linux kernels.

Buffer vs Cache

Cache resides between CPU and main memory, storing frequently accessed data to speed up memory reads. Larger caches improve CPU performance. Buffering temporarily holds data destined for disk I/O, reducing read/write latency. Buffers contain data to be written; caches hold data read from disk.

TCP Three‑Way Handshake

Client sends a SYN packet (SYN=A) and waits for a response.

Server replies with SYN+ACK (A+1) and its own ACK (K).

Client acknowledges with an ACK (K+1), completing the connection.

After this exchange, the client and server can transmit data.

Linux Directory Structure

Linux uses a single‑rooted, linked tree hierarchy. The root directory "/" contains all other files and subdirectories. /: Root of the entire filesystem. /boot: Kernel images, initrd, and bootloader files (e.g., GRUB). /bin: Essential user commands (executable by all users). /sbin: System administration commands (executable by root). /etc: System configuration files. /dev: Device node files (terminals, disks, etc.). /var: Variable data such as logs and mail. /home: Home directories for regular users. /opt: Third‑party software installations. /lib: Shared libraries and kernel modules.

Hard Links and Symbolic Links

Hard links point to the same inode, allowing multiple filenames for a single file; they cannot reference directories or cross filesystem boundaries. Deleting one hard link does not affect the underlying file. ln source new-link Symbolic (soft) links store a pathname to the target, similar to Windows shortcuts. They can link to directories and cross filesystems. Removing the target renders the link broken.

ln -s source new-link

RAID Technology

RAID (Redundant Array of Independent Disks) combines multiple physical disks into a logical unit, enhancing performance, capacity, and fault tolerance. Users interact with the RAID array as a single disk for partitioning and formatting.

RAID Levels

RAID 0 : Striping across disks for maximum throughput; no redundancy—failure of any disk loses all data.

RAID 1 : Mirroring; data is duplicated on two disks, providing high reliability but lower write speed; supports hot‑swap.

RAID 0+1 (RAID 10) : Combines striping and mirroring; requires at least four disks, offering both performance and redundancy.

RAID 5 : Uses block-level striping with distributed parity; needs a minimum of three disks, balancing performance, storage efficiency, and fault tolerance.