Master Linux Essentials: GNU, GPL, Distributions, Swap, GRUB, and More

1. GNU and GPL

GNU (the GNU Project) was launched by Richard Stallman on September 27, 1983, aiming to create 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 availability.

The combination of the GNU system 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 (e.g., KDE or GNOME), and thousands of applications ranging from office suites to development tools.

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

3. Unix and Linux

Linux is a Unix-like, multi‑user, multitasking, multithreaded operating system that supports various CPU architectures. It inherits Unix's network‑centric design, offering a stable, multi‑user network environment.

4. Swap Partition

Swap provides virtual memory when physical RAM is insufficient. Data from inactive programs is moved to swap space, freeing RAM for active processes. Recommended size is at least equal to physical memory, not less than 64 MB, and at most twice the RAM size.

5. GRUB Concept

GNU GRUB (GRand Unified Bootloader) is a multi‑OS boot manager that lets users select which operating system to start during boot and can pass kernel parameters (e.g., to enter 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 transferred between the disk (or other I/O devices) and memory, improving I/O performance.

7. TCP Three‑Way Handshake

Client sends a SYN packet.

Server replies with SYN‑ACK.

Client sends ACK, establishing the TCP connection.

Data transmission can then begin.

8. Linux Directory Structure

The Linux filesystem is a single‑rooted tree. Key directories include:

/ – root of the filesystem.

/boot – kernel and boot loader files.

/bin – essential user commands.

/sbin – system administration commands (root only).

/etc – configuration files.

/dev – device nodes.

/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 Soft Links

Hard Link : Multiple filenames point to the same inode; deleting one does not affect the underlying file. Example: ln source new-link Soft (Symbolic) Link : A pathname that points to another file or directory; if the target is removed, the link becomes broken. Example:

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.

RAID 0 (Striping): Parallel read/write across disks for high throughput; no redundancy.

RAID 1 (Mirroring): Exact copy of data on two disks; provides redundancy and read speed boost.

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

RAID 5: Striped disks with distributed parity; balances performance, storage efficiency, and fault tolerance; needs at least three disks.