Unix-like

A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that works in a similar way to a Unix system. These systems are known for being flexible and stable, and they can run on many different types of hardware. Many computers and servers around the world use Unix-like systems to help manage tasks and store information.

A Unix-like application is made to work like the tools and programs on Unix systems. This includes using special commands and interfaces called Unix command and shell. There are general philosophies that guide how these systems are built, but there is no single rulebook that defines exactly what makes something "Unix-like." Because of this, 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 they are popular, 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 important part of today's computing world.

Definition

The Open Group owns the UNIX trademark and has rules for how it should be used. They do not like the term "Unix-like" because it does not follow their rules.

Many people use names like "Un*x" or "*nix" to describe systems that work in a similar way to Unix. Examples include AIX, Linux, macOS, and Solaris. Even though these names look different, they all work in ways that are similar to Unix.

History

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

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

In the 1980s and 1990s, free alternatives like 4.4BSD, Linux, and Minix appeared. Some of these became 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.