## 1.3. The Core of Unix

In recent times, more attention has been paid on the newer and more lightweight varieties of Unix: FreeBSD, Linux, and now Darwin -- the version of BSD Unix that Apple used as the platform for the new Mac OS X. If you've worked with the larger Unix versions, you might be curious to see how it differs within these new environments.

For the most part, basic Unix functionality differs very little between implementations. For instance, I've not worked with a Unix box that doesn't have vi (Section 21.7) installed. Additionally, I've also not found any Unix system that doesn't have basic functionality, such as traversing directories with cd (Section 1.16) or getting additional help with man (Section 2.1).

However, what can differ between flavors of Unix is the behavior of some of the utilities and built-in commands, as well as the options. Even within a specific Unix flavor, such as FreeBSD, installations can differ because one installation uses the built-in version of a utility such as make (Section 40.3) and another installation has a GNU version of the same application.

An attempt was made to create some form of standardization with the POSIX effort. POSIX, which stands for Portable Operating System Interface, is an IEEE standard to work towards application interoperability. With this, C programs written on one flavor of Unix should work, with minimum modification, on another flavor of Unix.

Unfortunately, though the POSIX effort has had some impact on interoperability, there still are significant differences between Unix versions. In particular, something such as System V Unix can differ considerably from something such as Darwin.

However, there is stability in this seeming chaos: for the most part, the basic Unix utilities and commands behave the same in all Unix flavors, and aside from some optional differences, how a command works within one environment is exactly the same as in another environment. And if there are differences, using the facilities described in Chapter 2 should help you resolve these quickly.

-- SP

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