How to RSYNC a single file?

Basic syntax

rsync options source destination

Example

rsync -az /var/www/public_html/filename root@<remote-ip>:/var/www/public_html

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You do it the same way as you would a directory, but you specify the full path to the filename as the source. In your example:

rsync -avz   --progress  /var/www/public_html/.htaccess root@<remote-ip>:/var/www/public_html/

As mentioned in the comments: since -a includes recurse, one little typo can make it kick off a full directory tree transfer, so a more fool-proof approach might to just use -vz, or replace it with -lptgoD.