htaccess rewrite only main domain to subfolder - .htaccess

Question

I'm using a cPanel account to host multiple sites. To have a clean public_html, I use .htaccess to redirect the main domain into one of the subfolders.
So when user types in www.example.com, it will be rewritten into www.example.com/mainsite/ but still showing www.example.com in the URL.
But when using the codes below, every domain (including sub domains) will get redirected to that folder:
RewriteCond %{REQUEST_URI} !^/example
RewriteRule ^(.*)$ /mainsite/$1 [NC,L]
How can I target only www.example.com (and www.example.com/files) to be rewritten to the /mainsite?
*Other subdomains should be pointing to their own Document_Root as created.

Answer 1

You need one more RewriteCond to check for main domain:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(?:www\.)?example\.com$ [NC]
RewriteCond %{REQUEST_URI} !^/mainsite/ [NC]
RewriteRule ^(.*)$ /mainsite/$1 [L]

Answer 2

Do this, This line helps me solve the issue.
# .htaccess main domain to subdirectory redirect
# Do not change this line.
RewriteEngine on
# Change example.com to be your main domain.
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
# Change 'subdirectory' to be the directory you will use for your main domain.
RewriteCond %{REQUEST_URI} !^/subdirectory/
# Don't change the following two lines.
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
# Change 'subdirectory' to be the directory you will use for your main domain.
RewriteRule ^(.*)$ /subdirectory/$1
# Change example.com to be your main domain again.
# Change 'subdirectory' to be the directory you will use for your main domain
# followed by / then the main file for your site, index.php, index.html, etc.
RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteRule ^(/)?$ subdirectory/index.html [L]