Redirect Main Site to Subsite in Multisite WordPress

I have multi-site wordpress, I want to redirect the main site to one of the sub-site but redirection in .htacecss gives error. Is there any way to redirect a main site to its sub-site. That looks not possible but I am not a wordpress expert so just verifying.

The redirect should be like

redirect 301 www.example.com/main-site www.example.com/main-site/sub-site

Or if there is any other way to do that?

You can use the parse_request action to accomplish this. Simply enable this plugin on your primary blog. Place the following code in a .php file and upload it to your plugins directory.

/*
Plugin Name: Redirect Main Site To Sub-Site
Description: Redirect 'main-site' to 'main-site/sub-site/'
Version: 0.1
Author: WPSE
Author URI: http://wordpress.stackexchange.com
License: GPL2
*/

add_action('parse_request', 'redirect_to_sub_site');
function redirect_to_sub_site(){
    global $wp;

    #Sniff requests for a specific slug
    if('main-site' === $wp->request){

        #The URL to redirect TO
        $url="http://www.example.com/main-site/sub-site/";

        #Let WordPress handle the redirect - the second parameter is obviously the status
        wp_redirect($url, 301);

        #It's important to exit, otherwise wp_redirect won't work properly
        exit;
    }
}

Let me know if you have any questions.