Understanding the Nginx map directive

When switching to Nginx I needed to have a variable that signified if the site was in HTTP or HTTPS mode. So I found this little bit of code. map $scheme $fastcgi_https { ## Detect when HTTPS is used default off; https on; }

This works great, but I really didn’t understand it until now so let’s take it line by line.

Line 1

map $scheme $fastcgi_https { ## Detect when HTTPS is used This is where all the glory happens. $scheme is an internal variable within Nginx and is either HTTP or HTTPS (maybe some other ones in the future like SPDY). Then you have $fastcgi_https, this is the name of the variable we’re creating for future use in our config files. Nothing but comments after that stuff.

Line 2 and 3

default off; https on; This is our map of values. When Nginx’s $sheme variable is equal to “HTTPS”, then the $fastcgi_https variable will equal “on”, otherwise the variable just equals “off”.

Not too complex, but I just didn’t pick up on the format until now. Read more at http://wiki.nginx.org/HttpMapModule