Removing subdomain with bash

Question

Suppose i have a variable called $subdomainthat contain test.google.com .How can i strip the subdomain to get google.comand store it in a variable called $maindomain.

Answer 1

One subdomain

maindomain=${subdomain#*.}  

with more subdomains

maindomain=$(sed 's/.*\.\(.*\..*\)/\1/' <<< $subdomain)

Remove URI if there is one

maindomain=$(sed 's/.*\.\(.*\..*\)/\1/' <<< ${subdomain%/*})

Answer 2

maindomain=${subdomain#[[:alpha:]]*.}

will remove "test." or any other similar prefix, before the first point.

Better answer, as we can have multiple subsubdomains:

maindomain=$(expr match "$subdomain" '.*\.\(.*\..*\)')

It keeps only the trailing xxxxxx.xxx part of a domain.