Understanding UNIX/LINUX Programming: A Guide to Theory and Practice by Bruce Molay is a work which has the reader learn system programming by re-implementing UNIX commands.
Yeah wondering if this would be interesting to do as a recreation project. Is copyright 2003 and mostly unvailable -- Amazon dares to "rent" it to me for 45$.... give me a break.
I was messing with it a bit several months ago. You'd have to look up newer data structures, but just looking up the man pages gave me the necessary information. While the implementation may be different now, the fundamentals of UNIX are pretty much the same.