Nginx HTTP Server - Fourth Edition

Variables

Modules also provide variables that can be used in the definition of directive values. For example, the Nginx HTTP core module defines the $nginx_version variable. Variables in Nginx always start with $—the dollar sign. When setting the log_format directive, you may include all kinds of variables in the format string:

[...] 
location ^~ /admin/ { 
    access_log logs/main.log; 
    log_format main '$pid - $nginx_version - $remote_addr'; 
} 
[...] 

Some directives do not allow you to use variables:

error_log logs/error-$nginx_version.log;

The preceding directive is valid, syntax-wise. However, it simply generates a file named error-$nginx_version.log, without parsing the variable.