You don't say why you want to access REMOTE_USER, and I'm guessing from context that you might misunderstand what REMOTE_USER is, and how it ordinarily gets set.

A web server will set REMOTE_USER on behalf of a CGI script if the script is password protected, or if the directory in which the script resides is password protected, and if the user has successfully logged in. You won't see REMOTE_USER in a shell's environment, nor should you expect to.

The only reason you would want to set REMOTE_USER yourself is if you're debugging a CGI script that depends up on it.