Test Whether a Server Variable is Empty in Popular Web Servers

Q: How to test whether a variable is empty in server configurations of popular web servers such as Apache and Nginx?

Problem
It is AMAZING how unfriendly server configuration syntax can be. Even tasks as small as testing whether a value is empty can be confusing. Since I've personally used Apache and Nginx for a long time allow me to unravel the mysteries of how to evaluate whether a servervariable (e.g. document root, query string, etc.) is empty in server's configuration file.

Solution for Apache
In your httpd.conf or .htaccess simply use ="" to evaluate whether an Apache server variable is empty. Use !="" to test whether an Apache server variable is NOT empty. Here's an example:

RewriteEngine on
...
RewriteCond %{DOCUMENT_ROOT}/cache%{REQUEST_URI} -f
RewriteCond %{QUERY_STRING} =""
RewriteRule (.+)$ /cache/$1 [L]
...
This block of rules basically checks whether %{DOCUMENT_ROOT}/cache%{REQUEST_URI} exists as a file and whether the HTTP request has no URL parameters. If both are true rewrite current request to /cache/{current request uri}.

Common Apache Server Variables: By the way the following is some comments that tell you how common server variables such as %{DOCUMENT_ROOT}, %{REQUEST_URI} and %{REQUEST_FILENAME} work as they are often confusing as hell:

#
# Suppose your website is http://www.mensfashionforless.com/ 
# and document root is set to /usr/repository/trunk (via DocumentRoot directive in httpd.conf).
#
# Now someone issues a request for 
# http://www.mensfashionforless.com/2010/10/g-by-guess-grey-low-boot-cut-jeans.html
# then the following is a list of common Apache server variables and their values:
#
# %{DOCUMENT_ROOT} = /usr/repository/trunk
# %{REQUEST_URI} = /2010/10/g-by-guess-grey-low-boot-cut-jeans.html
# %{REQUEST_FILENAME} = /usr/repository/trunk/2010/10/g-by-guess-grey-low-boot-cut-jeans.html
#

Questions? Let me know!

Solution for Nginx
Nginx is a popular web server for its speed and efficiency. Simply use ='' to check whether an Nginx server variable is empty. use !='' to check whether an Nginx server variable is not empty. Here's an example:

server {
  listen  80;
  server_name  www.mensfashionforless.com;
  rewrite_log on;
  ...
  location / {
    # test whether $document_root/cache$request_uri exists 
    # in the file system
    if (-f $document_root/cache$request_uri) {
      set $test P;
    }
    # test whether there's no URL argument to this request
    # = '' tests whether the value is empty!
    if ($args = ''){
      set $test  "${test}C";
    }
    # if both of the above tests are true, do the rewrite
    if ($test = PC){
      rewrite ^/(.+)$ /cache/$1 last;
      break;
    }
    ...
  }
  ...
}

This block of code checks whether $document_root/cache$request_uri exists as a file in the file system AND whether there is no URL parameter to this HTTP request. If both are true rewrite current request to /cache/{current request uri}. Refer to the post on How do I test multiple conditions in Nginx server configuration? if you are confused by the syntax of testing multiple conditions.

If you have any questions please let me know and I will do my best to help you!