Unix-like

A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. These systems are known for their flexibility, stability, and ability to run on many different types of hardware. Many of the computers and servers used by businesses and individuals around the world rely on Unix-like systems to manage their tasks and store information.

A Unix-like application is designed to work in a way that resembles the tools and programs found on Unix systems. This includes using special commands and interfaces called Unix command and shell. While there are general philosophies guiding how these systems are built, there is no single rulebook that defines exactly what makes something "Unix-like," so different people might have slightly different ideas about it.

Some well-known examples of Unix-like operating systems include Linux, FreeBSD, and OpenBSD. These operating systems are often used on servers, which are powerful computers that provide services over networks, as well as on personal computers and many other devices. Because of their popularity, many important tools and programs, like the Apache web server and the Bash shell, are created to work best on Unix-like systems. This makes them an essential part of the modern computing world.

Definition

The Open Group owns the UNIX trademark and has specific rules for how it should be used. They don't like the term "Unix-like" because it doesn't follow their branding rules.

Many people use names like "Un*x" or "*nix" to talk about systems that work similarly to Unix. Examples include AIX, Linux, macOS, and Solaris. Even though these names don't all look the same, they are all recognized as being like Unix in how they operate.

History

Unix-like systems began appearing in the late 1970s and early 1980s. Early examples included Idris, UNOS, Coherent, and UniFlex, which aimed to give businesses features similar to the UNIX used by academics.

Later, when AT&T made UNIX available for commercial use, many new systems were created, such as AIX, HP-UX, IRIX, SunOS, Tru64, Ultrix, and Xenix. As these systems grew apart, standards like POSIX and the Single UNIX Specification were developed to help them work together.

In the 1980s and 1990s, free alternatives like 4.4BSD, Linux, and Minix appeared. Some of these formed the basis for commercial systems such as BSD/OS and macOS. While some macOS versions meet the Single UNIX Specification, BSD variants are called "Unix-like" because they are not certified under it.

Categories

Unix-like operating systems are systems that work in a similar way to the original Unix system. There are three main types:

The first type is called "genetic UNIX." These systems come from the original Unix code developed by AT&T. Many well-known Unix systems and the BSD systems, which started at the University of California, Berkeley, belong to this group.

The second type is "trademark or branded UNIX." These systems meet certain standards set by the Open Group and are allowed to use the Unix name. Most of these are commercial systems, but some, like Apple macOS and certain Linux distributions, are also included.

The third type includes systems like Linux and Minix. These act like Unix systems but do not come from the original AT&T code and are not officially certified as Unix. Many free and open-source systems fall into this category.

Compatibility layers

Some operating systems that are not like Unix can still act like Unix by using something called a compatibility layer. This helps them run programs made for Unix.

For example, IBM's z/OS has a part called UNIX System Services that works just like Unix. On Microsoft Windows, tools like Cygwin, MSYS, and MSYS2 let you use many Unix features. There are also special tools like the MKS Toolkit and UWIN that help run Unix programs on Windows. Windows also has a POSIX subsystem and something called the Subsystem for Unix-based Applications. Additionally, the Windows Subsystem for Linux lets you run a Linux environment, with Windows Subsystem for Linux version 2 (WSL2) offering a full Linux setup in a virtual machine. Systems like OpenHarmony and HarmonyOS NEXT also support Unix-like features using special libraries and ports.