Add content in 108-posix-basics (#1333)

* Add content in 108-posix-basics

* Update content/roadmaps/101-backend/content/102-os-general-knowledge/108-posix-basics.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

content/roadmaps/101-backend/content/102-os-general-knowledge/108-posix-basics.md

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-# Posix basics
+# POSIX Basics
+
+POSIX (Portable Operating System Interface) is a family of standards for maintaining compatibility between operating systems. It describes utilities, APIs, and services that a compliant OS should provide to software, thus making it easier to port programs from one system to another.
+
+A practical example: in a Unix-like operating system, there are three *standard streams*, `stdin`, `stdout` and `stderr` - they are I/O connections that you will probably come across when using a terminal, as they manage the flow from the **standard input** (stdin), **standard output** (stdout) and **standard error** (stderr).
+
+So, in this case, when we want to interact with any of these streams (through a process, for example), the POSIX operating system API makes it easier - for example, in the `<unistd.h>` C header where the stdin, stderr, and stdout are defined as `STDIN_FILENO`, `STDERR_FILENO` and `STDOUT_FILENO`.
+
+POSIX also adds a standard for exit codes, filesystem semantics, and several other command line utility API conventions.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='blue' badgeText='Read' href='https://unix.stackexchange.com/a/220877'>Summary of some POSIX implementations</BadgeLink>
+<BadgeLink colorScheme='blue' badgeText='Read' href='https://www.baeldung.com/linux/posix'>A guide to POSIX</BadgeLink>
+<BadgeLink colorScheme='blue' badgeText='Documentation' href='https://pubs.opengroup.org/onlinepubs/9699919799/'>POSIX standard by IEEE</BadgeLink>