Linux Fundamentals Explained: GNU, GPL, Distributions, Filesystems & RAID

1. GNU and GPL

GNU (GNU's Not Unix) is a free software project launched by Richard Stallman in 1983 with the goal of creating a completely free operating system. The GPL (GNU General Public License) protects the freedom to use, copy, study, modify, and distribute software, requiring source code distribution.

The combination of GNU tools with the Linux kernel forms a complete operating system often referred to as GNU/Linux.

2. Linux Distributions

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

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

3. Unix and Linux

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.

4. Swap Partition

Swap is a disk area used when physical memory is insufficient. The system moves inactive pages to swap, freeing RAM for active processes. Recommended swap size is at least equal to physical memory, not less than 64 MB, and at most twice the RAM size.

5. GRUB

GRUB (GRand Unified Bootloader) is a multi‑OS boot manager from the GNU project. It allows users to select which operating system to boot and can pass kernel parameters, such as entering single‑user mode.

6. Buffer and Cache

Cache is a small, fast memory between CPU and RAM that stores frequently accessed data blocks, speeding up CPU‑memory interactions. Buffer temporarily holds data being written to disk, improving I/O performance. Cache reads from disk; buffer holds data pending write.

7. TCP Three‑Way Handshake

Client sends a SYN packet and waits for a response.

Server replies with SYN‑ACK (its sequence number plus acknowledgment).

Client sends an ACK, completing the connection establishment.

After the handshake, data transmission can begin.

8. Linux Directory Structure

The Linux filesystem is a single‑rooted tree starting at "/". Key directories include:

/ – root of the hierarchy.

/boot – kernel and bootloader files.

/bin – essential user commands.

/sbin – system administration commands (root only).

/etc – configuration files.

/dev – device files.

/var – variable data such as logs and mail.

/home – users' home directories.

/opt – third‑party software.

/lib – shared libraries and kernel modules.

9. Hard Links and Symbolic Links

Hard links share the same inode, allowing multiple filenames to reference the same file; deleting one link does not affect the underlying data. Example command: ln source new-link Symbolic (soft) links are path‑based shortcuts that can span directories and filesystems. Deleting the target renders the link broken. Example command:

ln -s source new-link

10. RAID Technology

RAID (Redundant Array of Independent Disks) combines multiple physical disks into a logical unit to improve performance and provide data redundancy.

Common RAID levels:

RAID 0 – Striping for performance; no redundancy.

RAID 1 – Mirroring for redundancy; reads are fast, writes slower.

RAID 0+1 (RAID 10) – Combines striping and mirroring; requires at least four disks.

RAID 5 – Block-level striping with distributed parity; balances performance, redundancy, and storage efficiency.