Your question is vague, we can only assume that you are trying to publish the resource currently reachable via that second URL under a different, shorter URL.
Also assuming that this will not somehow collide with other resources this should point you into the right direction:
RewriteEngine on
RewriteRule ^/?users/login$ /sub-dir/users/login [QSA,END]
Note however tht this might cause issues with the internal references used in those applications you have, since you break the structure with such a single redirection.
In case you receive an internal server error (http status 500) using the rule above then chances are that you operate a very old version of the apache http server. You will see a definite hint to an unsupported [END] flag in your http servers error log file in that case. You can either try to upgrade or use the older [L] flag, it probably will work the same in this situation, though that depends a bit on your setup.
This implementation will work likewise in the http servers host configuration or inside a dynamic configuration file (".htaccess" file). Obviously the rewriting module needs to be loaded inside the http server and enabled in the http host. In case you use a dynamic configuration file you need to take care that it's interpretation is enabled at all in the host configuration and that it is located in the host's DOCUMENT_ROOT folder.
And a general remark: you should always prefer to place such rules in the http servers host configuration instead of using dynamic configuration files (".htaccess"). Those dynamic configuration files add complexity, are often a cause of unexpected behavior, hard to debug and they really slow down the http server. They are only provided as a last option for situations where you do not have access to the real http servers host configuration (read: really cheap service providers) or for applications insisting on writing their own rules (which is an obvious security nightmare).
Now I want to remove sub-dir only from second urlmore please? You mean you want to restrict someone from accessing this url OR you want to redirect it to some other url?sub-dirfrom all the instances of these URLs within your app? Otherwise, this is a Codeigniter question, not.htaccess?wordpress CMSand other two are ofcodeigniterapplication. So with change in.htaccessI want to access second url like example.com/users/login and other two should work as they are. Also I added my .htaccess file in question.Please let me know if it need more explanation. Thanks!wordpress CMSand other two are ofcodeigniterapplication. Now if I removesub-dirfrom url the server assumes that it's a wordpress url so gives404error. As it not even calling codeIgniter app so I think it can be handled via.htaccess./sub-dirfrom the URLs in your Codeigniter URLs - in your web application (HTML source), otherwise they are still going to be visible in the HTML of your page (and to your users). You don't use.htaccessfor this bit. You would then use.htaccessto internally rewrite the request back to/sub-dir(thus hiding/sub-dirfrom the user). Do you literally only have those 2 Codeigniter URLs.../users/loginand/merchant/login? Also, it depends how you have configured Codeigniter to route requests, ie. the value of theuri_protocolconfig.