[atomize]

FreeBSD is the reference for OS X, but they used different kernels. Many of the BSD flavored behaviors are/were retained, however.

[sigjuice]

What would be examples of the retained BSD flavored behaviors?

[spijdar]

A couple examples off the top of my head:

Userland tools tend to be the BSD ones with BSD style arguments. e.g. BSD du vs GNU du, GNU requires size suffixes in uppercase, BSD takes lowercase. MacOS tends to include the more useful GNU tools though, like grep. Find is still BSD, and the missing GNU extensions occasionally causes grief.

SIGINFO is preserved. On BSD, pressing Ctrl+t in the terminal will print process status. Some processes add extra info, but it always gives at least CPU load and running time. This is a distinctly BSD feature.