I want to redirect this subdomain:
http://abc.domain-name.com
to a folder on the root domain that uses the same name:
http:
Try this:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^subdomain\.yourdomain\.com
RewriteRule ^(.*)$ http://www\.yourdomain\.com/subdomain/$1 [L]
(from http://systembash.com/content/simple-redirect-subdomain-to-a-directory/)
You should consider about search engines and use permanent redirect (301), also its best practice to use NC flag at the end if second line to be matched in a case-insensitive manner.
RewriteEngine On
RewriteCond %{HTTP_HOST} ^blog\.example\.com [NC]
RewriteRule ^(.*) http://www.example.com/$1 [L,R=301]