In 1985, individuals from companies throughout the computer industry joined together to develop the POSIX (Portable Operating System Interface for Computer Environments) standard, which is based largely on the UNIX System V Interface Definition (SVID) and other earlier standardization efforts. These efforts were spurred by the U.S. government, which needed a standard computing environment to minimize its training and procurement costs. Released in 1988, POSIX is a group of IEEE standards that define the API, shell, and utility interfaces for an operating system. Although aimed at UNIX-like systems, the standards can apply to any compatible operating system. Now that these stan- dards have gained acceptance, software developers are able to develop applications that run on all conforming versions of UNIX, Linux, and other operating systems.

摘自《Linux实用指南》

让我来给出粗鲁的“非官方”解释。

POSIX是一组标准，试图将“UNIX”和类UNIX系统与与它们不兼容的系统区分开来。它是由美国政府为采购目的而创建的。当时的想法是，美国联邦采购部需要一种方法来合法地规定各种投标和合同的要求，这种方法可以用来排除现有代码库或编程人员无法移植到的系统。

因为POSIX是事后写的……为了描述一组大致相似的竞争系统……它不是以一种可以实现的方式编写的。

因此，例如，微软的NT是用足够的POSIX一致性编写的，有资格参加一些投标…尽管POSIX子系统在实际可移植性和与UNIX系统的兼容性方面基本上是无用的。

在过去的几十年里，已经编写了各种UNIX标准。比如SPEC1170(指定了1170个函数调用，必须兼容地实现)和SUS(单一UNIX规范)的各种版本。

在大多数情况下，这些“标准”对任何实际技术应用都是不够的。它们大多是为了争论、法律纠纷和其他不正常的原因而存在的。

关于如何使操作系统与晚期UNIX操作系统兼容的规范(蓝图)(愿上帝保佑他!)这就是为什么macOS和GNU/Linux有非常相似的终端命令行、GUI、库等。因为它们都是根据POSIX蓝图设计的。

POSIX并不告诉工程师和程序员如何编码，而是告诉他们编码什么。

另一方面，POSIX代表“可移植操作系统接口”，POSIX是一组定义操作系统和应用软件之间接口的标准。POSIX类似于UNIX标准，它旨在确保为一个POSIX兼容的操作系统编写的软件可以在其他POSIX兼容的操作系统上运行而无需修改。

POSIX标准定义了一组在基于unix的操作系统中常见的系统调用、库和实用程序。

许多操作系统，包括Linux、BSD和macOS，都是POSIX兼容的，这意味着它们满足POSIX标准中定义的要求。

In 1985, individuals from companies throughout the computer industry joined together to develop the POSIX (Portable Operating System Interface for Computer Environments) standard, which is based largely on the UNIX System V Interface Definition (SVID) and other earlier standardization efforts. These efforts were spurred by the U.S. government, which needed a standard computing environment to minimize its training and procurement costs. Released in 1988, POSIX is a group of IEEE standards that define the API, shell, and utility interfaces for an operating system. Although aimed at UNIX-like systems, the standards can apply to any compatible operating system. Now that these stan- dards have gained acceptance, software developers are able to develop applications that run on all conforming versions of UNIX, Linux, and other operating systems.

摘自《Linux实用指南》

POSIX为操作系统或程序定义了一组标准。 目标是编写与类unix系统兼容的新软件。

例如，在Linux上运行的程序也可以在其他类unix系统上编译和运行，如Solaris、HP-UX和AIX等。

最流行的例子是GNU Bash，它100%符合POSIX和gawk实用程序。