Explaining Unix, Posix and Linux

Most people think about apps and websites, but underneath all of that runs a kernel that makes everything move. The kernel is the core part of an operating system that turns software instructions into hardware actions. Without it, nothing happens.

A kernel is the translator between software and hardware. Programs speak a high level language. Chips speak signals. The kernel sits in the middle and makes both sides understand each other.

Unix started the idea

Unix began at Bell Labs in the 1970s. It was proprietary, portable and clean in design. Universities and companies adopted it fast. Over time it shaped how we think about processes, files and tools. Even macOS today is Unix certified.

What POSIX really is

POSIX stands for Portable Operating System Interface. It is a common rule book for Unix like systems so software does not break when you move between them. POSIX defines system calls, process behavior, shells and the basic userland tools.

The result is simple. If a system follows POSIX you can compile and run the same source with little change. You also get a standard toolbelt so the basics feel the same across systems.

Core POSIX commands you already know

• ls list files and directories
• cd change directory
• pwd print current path
• cat output a file
• cp copy files and directories
• mv move or rename
• rm remove files
• mkdir create directories
• rmdir remove empty directories
• echo print text
• grep search text
• head and tail read file start or end
• chmod and chown change permissions and ownership

Because these tools and their flags are defined you can sit at macOS, FreeBSD or Linux and get work done the same way. That is the power of POSIX.

Linux took the torch and made it open

In 1991 Linus Torvalds released the Linux kernel. It was a free reimplementation inspired by Unix ideas. The community could read the code, improve it and share it. Distributions like Debian, Red Hat, Arch and Ubuntu formed around the kernel and the shared toolchain.

Why the GPL matters

The Linux kernel uses the GPL license. The simple rule is this. If you distribute a modified kernel you must share the source under the same license. That keeps improvements in the open. Every distro that ships the kernel follows this rule and publishes its kernel changes. This is why Linux stays free and why fixes flow back to everyone.

Where Linux runs in real life

• Servers that host most of the web
• All top supercomputers
• Android phones use the Linux kernel
• Routers, NAS devices and home gateways
• Cars, TVs, set top boxes and game consoles
• Cloud platforms and containers
• Small devices like Raspberry Pi and sensors

You probably touch Linux many times a day without noticing. It powers the internet and a huge part of consumer tech.

The simple connection

Unix shaped the model. POSIX defined the common rules so tools look and feel the same. Linux rebuilt the idea as free software and the GPL kept it open for everyone. Learn the POSIX basics and you are productive on any Unix like system.

CozyCLI.com is my small way to practice this stack. Short tasks, instant feedback and a focus on core commands. If you know the basics you can work anywhere.