It defines an interface (POS I X) to a computing environment (POSI X) that is meant to be portable (P OSIX) across operating systems (P OS IX). This interface extends across both user (commands) and program (libraries) space, and allows both source code as well as shell scripts to work in any compliant environment.

Simply, if the program uses POSIX capabilities then it should work on any POSIX-compliant OS.

Programmers often overlook when it comes to putting efforts in understanding Operating System concepts in detail and they can't be blamed for it, considering their situation, there are always pressing needs to learn new frameworks and technologies on the go. But it doesn't make this fact any less important that Understanding of Operating System is equally needed. As a program, may it be written in C, Java, Python or any other programming language, after writing the syntactical pieces of code, it needs to be executed on an Operating System. When it comes to incorporating various technologies in one project, it becomes a necessity to understand how they are interacting with the operating system.

Good Knowledge of Operating System is a must and there are various flavours of OS readily available, RedHat Enterprise Linux is one such great flavour to start with. It is widely used in IT Industries and also provides well organised curriculum to learn and get yourself certified, and its certificates are widely recognised.

You can check thier website for all active certifications being offered currently: redhat.com/en/services/all-certifications-exams