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

1. GNU and GPL

The GNU Project, launched by Richard Stallman on September 27, 1983, aims to create a completely free operating system and is also known as the Free Software Engineering Project.

The GPL (GNU General Public License) is a "copyleft" license that ensures GNU software can be freely used, copied, studied, modified, and distributed, while requiring that the software be released in source‑code form.

Combining the GNU system with the Linux kernel yields a complete operating system commonly referred to as "GNU/Linux" or simply 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 belongs to the Unix family, supporting multiple users, multitasking, multithreading, and various CPU architectures. It inherits Unix's network‑centric design, making it a stable, multi‑user network operating system.

4. Swap Partition

A swap partition provides virtual memory when physical RAM is insufficient. Data from inactive programs is moved to disk space, freeing RAM for active processes. When needed, the data is swapped back into memory.

Swap size should be at least equal to physical memory, not less than 64 MB, and typically no more than twice the amount of RAM.

5. GRUB

GNU GRUB (GRand Unified Bootloader) is a multi‑OS boot manager from the GNU project.

It allows users to select which operating system to boot on machines with multiple OSes and can pass kernel parameters (e.g., to enter single‑user mode).

6. Buffer and Cache

Cache resides between the CPU and memory, offering fast temporary storage for frequently accessed data, thereby accelerating CPU‑memory data exchange.

Buffer is a high‑speed area that temporarily holds data destined for disk I/O, improving read/write performance. Buffers contain data to be written to disk, while cache holds data read from disk.

7. TCP Three‑Way Handshake

The client sends a SYN packet and waits for a response.

The server replies with a SYN‑ACK (SYN+1 and its ACK).

The client acknowledges with an ACK, completing the connection.

After this handshake, data transmission can begin.

8. Linux Directory Structure

The Linux file system is a single‑rooted tree with "/" as the top directory. Key directories include:

/ – root of the hierarchy.

/boot – kernel and boot loader 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. ln source new-link Symbolic (soft) links store a pathname to the target file, similar to Windows shortcuts; they can span directories and file‑system boundaries. Deleting the target renders the link broken.

ln -s source new-link

10. RAID Technology

RAID (Redundant Arrays 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 maximum performance; no redundancy.

RAID 1 – Mirroring for high reliability; writes to two disks.

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

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