Correctly redirecting non-www and https (SSL) traffic is crucial for Search Engine Optimization (SEO). With just a few lines in your .htaccess file (Apache only) you can redirect all incoming links independent of the domain.
RewriteEngine On
RewriteCond %{HTTPS} off [NC]
RewriteCond %{HTTP_HOST} !^www\\.(.*)$ [NC]
RewriteRule (.*) https://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
RewriteCond %{HTTP_HOST} !^www\\.(.*)$ [NC]
RewriteRule (.*) https://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
What is this doing?
RewriteEngine On
Enable the Apache rewrite engine.
RewriteCond %{HTTPS} off [NC]
Execute the rewrite rule only if the current request is not on SSL. [NC] makes the definition case-insensitive.
RewriteCond %{HTTP_HOST} !^www\\.(.*)$ [NC]
Also redirect if the current hostname lacks www..
RewriteRule (.*) https://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
Execute the actual redirect with status code 301. Redirect to the current hostname including www. and any request paths and parameters that were used.
RewriteCond %{HTTP_HOST} !^www\\.(.*)$ [NC]
This is a second check if the current request lacks www.. It will check all requests over SSL as the previous rewrite already made sure our connections are over SSL.
RewriteRule (.*) https://www.%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
Again redirect any non-www requests to a request including www..
Test
Always test these kind of snippets on a test or staging environment. Different setups can behave differently so make sure it behaves like you expect and actually need it.