Here, placed side-by-side for comparison, are GNU's implementation of
cat, Plan 9's implementation, Busybox's implementation, and NetBSD's
implementation, Seventh Edition Unix (1979), Tenth Edition Unix (1989),
and 4.3BSD.

For good measure (and because I suppose I am now committed to collecting
cats) also included are Second Edition Unix (in assembly) and Inferno's
implementation (in Limbo) for good measure.

All cat.c files (renamed by prefixing the name of the source)
are presented, unaltered and in their entirety.  Note how easy it is to
read and understand plan9-cat.c (it should take less than a couple of
minutes possibly even for coders that don't know C).  Other than that,
I think the files speak for themselves.

Keep in mind while reading that the cat utility's purpose is to
concatenate files.

Lastly, here are the line and character counts, sorted:
Lines  Chars  Filename
---------------------------
   35    531  plan9-cat.c
   48    955  busybox-cat.c
   48    986  inferno-cat.b
   63   1130  unix7-cat.c
   64    646  unix2-cat.s
   69   1241  unix10-cat.c
  222   3948  4.3bsd-cat.c
  316   6952  netbsd-cat.c
  782  22684  gnu-cat.c
 1647  39073  
--------------------------