Redirect HTTP requests to a certain domain using HTTPS

.htaccess

RewriteEngine On
RewriteBase /

# don't rewrite robots.txt
RewriteRule ^robots.txt$ - [L]

# redirect all requests to example.com with HTTPS
RewriteCond %{HTTP_HOST} !^example\.com$ [NC]
RewriteRule ^(.*)$ https://example.com/$1 [R=301,L]

# redirect non-HTTPS to HTTPS
RewriteCond %{HTTPS} Off [or]
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]