rated 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.  If FORMAT is the keyword 'auto', then the first
line of each file determines the number of fields output for each line.

Important: FILE1 and FILE2 must be sorted on the join fields.
E.g., use "sort -k 1b,1" if 'join' has no options,
or use "join -t ''" if 'sort' 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.
