Why does the 10.x/11.x cksum command produce a different
checksum than the 9.x cksum command?
The 9.x cksum was defective - it did not correctly implement the POSIX
spec. That is understandable, because the POSIX cksum spec does not
supply a test suite to easily verify an implementation is correct.
The 10.x/11.x cksum is correct. Unfortunately that means it can not
match 9.x cksum.
Another implementation that did not match POSIX spec was in AIX 3.*; I
haven't checked AIX 4.* or Solaris.
Other good implementations (they give the same answer as 10.x/11.x
- GNU cksum
- Solaris 7/8
- HP Software Distributor swpackage program, on both 9.x and 10.x