Crying with NGINX Regex matching, IF statement and module http rewrite $1 $2 meaning

If Statement and common condition

Syntax:	if (condition) { ... }
Default:	—
Context:	server, location

A condition may be any of the following:

a variable name: false if the value of a variable is an empty string or “0”;
comparison of a variable with a string using the “=” and “!=” operators;

set $NAME ""; => setting var is "", and assign to $NAME
if ($NAME = "0") {} => auto-false
if ($NAME = "") {} => auto-fasle
if ($NAME != "TEST") {}

matching of a variable against a regular expression using the ~ (for case-sensitive matching)
~* (for case-insensitive matching) operators
Negative operators “!~~” and “!~~*” are also available
If a regular expression includes the “}” or “;” characters, the whole expressions should be enclosed in single or double quotes
checking of a file existence with the “-f” and “!-f” operators;
checking of a directory existence with the “-d” and “!-d” operators;
checking of a file, directory, or symbolic link existence with the “-e” and “!-e” operators;
checking for an executable file with the “-x” and “!-x” operators.

if ($http_user_agent ~ MSIE) {
    rewrite ^(.*)$ /msie/$1 break;
}

if ($http_cookie ~* "id=([^;]+)(?:;|$)") {
    set $id $1;
}

if ($request_method = POST) {
    return 405;
}

if ($slow) {
    limit_rate 10k;
}

if ($invalid_referer) {
    return 403;
}

Multiple condition

Syntax: set $variable value; Default: — Context: server, location, if

NGINX only has single condition, we need using variable stragety to make multiple condition:

  set $UD "";
  
  if ($request_method = 'GET'){
    set $UD "${UD}GET";
  }
  
  if ($request_filename !~* .(gif|svg|html|jpe?g|png|json|ico|js|css|flv|swf|pdf|xml|woff|woff2)$ ) {
    # detect static files request
    
    set $UD "${UD}RED";
  }
  
  if ($UD = "GETRED") {
    # add trailing slash for every url except static files, redirect to slash url
    # http://test.com => http://test.com/
    
    rewrite ^([^.\?]*[^/])$ $1/ permanent;
  }

$1..$9 Meaning?

Let check example:

rewrite ^(/data/.*)/geek/(\w+)\.?.*$ $1/linux/$2.html last;

=> url/data/distro/geek/test.php will get rewritten as url/data/distro/linux/test.html

$1 and $2 will capture the appropriate strings from the original URL that doesn’t change
$1 in the replacement string will match whatever is inside the 1st parenthesis 1st ( ) in the reg-ex.
In our example, $1 is /data/
Similarly $2 will match whatever is inside the 2nd parenthesis ( ) in the reg-ex.
So, $2 is (\w+), which is any word that comes after the /geek/ in the original URL.
In our example, $2 is test

Note: *$ – This indicates the extension in the original URL. even though you call .php in the original URL, it will only serve the .html file in the rewritten URL.

The last thing, How to output variable in nginx log for debugging ?

Way 1

You can send nginx variable values via headers. Handy for development.

Example:

add_header X-uri "$uri";

and you'll see in your browser's response headers:

X-uri:/index.php

location ~ \.php$ {
    add_header X-debug-message "A php file was used" always;
    ...
}

Way 2

You can set a custom access log format using the log_format directive which logs the variables you're interested in.

https://nginx.org/en/docs/http/ngx_http_log_module.html#log_format

Way 3

Using Echo https://github.com/openresty/echo-nginx-module

REF: https://serverfault.com/questions/404626/how-to-output-variable-in-nginx-log-for-debugging

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