Complexity. Start with something where you can substantially wrap your head around the whole thing, like XV6 or Minix, and it's vastly easier to look at chucks

Complexity. Start with something where you can substantially wrap your head around the whole thing, like XV6 or Minix, and it's vastly easier to look at chucks of something like Linux and see "oh, this fits into the mental picture here". For me, in the mists of the old days, learning Unix V6 out of the Lyons books (very slim volumes of annotation...wonderful stuff) make it easy to understand what was going on in V7, which made 4.2BSD and progeny accessible, which made SVR[234] somewhat weird and different, but still conceptually understandable, which made most everything Unix-ish afterword (including Linux) something you still had the vernacular to understand. Now...to be fair...this leaves out a lot of really valuable OS knowledge. There's no IBM large system OS (e.g VM/370->z/OS, VM/CMS, TPF). No Seven Dwarfs OS (DEC VMS, Burroughs B-series, DG Nova, CDC NOS, etc.). Nothing for the really interesting Lisp or Smalltalk machines, the Perq or the Mesa/Cedar workstations. No interesting small systems OS: OS/2, classic MacOS, OS-9, much less the Windows NT family. Much less any of the embedded/RTOS world. But you can still learn alot.