Re Linux-like userland: OS X's userland is closer to FreeBSD. Examples: OS X uses BSD find which requires the directory (it doesn't default to . as GNU find does); BSD commands lack --long-options; GNU commands have more options than in BSD; OS X lacks /proc; OS X's primary GUI is almost entirely different from Linux's (X11 is a mere sidecar on OS X); OS X's dynamic linkage system is entirely ... More @Wikipedia
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