The subroutines used by VMS in reading data from a file have a small quirk associated with them, relative to VMS, which we have discovered. VMS seems to treat many, but not necessarily all, nulls as spaces. Similar routines in UNIX on the other hand treat them for the most part as not there and just skip them. This seemingly silly distinction is very annoyingly apparent when reading, on a UNIX system, what VMS claims as fixed length records from a file with nulls in it. Since the nulls are often skipped, the file can seem as if the fixed length records are now variable length.
Variables seem to be in different locations in different records for no easily discernible reason.
A plethora of warnings and errors can be the result. The vertical bar pipes the output of od in to grep. Grep is equivalent to VMS search. The argument nul to grep causes grep to search for the name nul which represents null in the od output. It's simpler than it seems at first glance.