Unless -t CHAR is given, leading blanks separate fields and are ignored,
else fields are separated by CHAR. Any FIELD is a field number counted
from 1. FORMAT is one or more comma or blank separated specifications,
each being `FILENUM.FIELD' or `0'. Default FORMAT outputs the join field,
the remaining fields from FILE1, the remaining fields from FILE2, all
separated by CHAR.
Important: FILE1 and FILE2 must be sorted on the join fields.
E.g., use `sort -k 1b,1' if `join' has no options.
Note, comparisons honor the rules specified by `LC_COLLATE'.
If the input is not sorted and some lines cannot be joined, a
warning message will be given.
