Thanks Ygor, That was just what I was missing; -e requires -o. The -o option requires knowing the number of fields in files a.txt and b.txt, which ideally one need not know (join could count them),...
I am trying to use join with the -e option to join columns in two files, with the -e option to replace empty fields with periods. I use the command:
join -e "." -a1 -a2 a.txt b.txt
But the -e...