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!
