"Notes". Overview of available variables in .htaccess and nginx.conf.

Even though my search ability is not low, I still struggle to quickly find many things.

.htaccess	nginx.conf	Description
%{REQUEST_SCHEME}	$scheme	http /https
%{HTTP_HOST}	$host	domain / ip
%{REQUEST_URI}	$request_uri	part after host/
%{REQUEST_FILENAME}	$request_filename	seems to be the same as uri
%{QUERY_STRING}	$args	query string (part after `?`)
%{HTTP_USER_AGENT}	$http_user_agent	client UA
%{REMOTE_ADDR}	$remote_addr	visitor's IP

$scheme://$host/or%{REQUEST_SCHEME}://%{HTTP_HOST}/can directly concatenate the current URL.

mod_rewrite Reference_Apache Chinese Documentation:

https://www.apachehttpd.com/mod/mod_rewrite.html

Htaccess - THE Ultimate .htaccess tutorial with 100's of Examples:

https://www.askapache.com/htaccess/#Htaccess_Variables

I. Regular expression matching, where:

~ is for case-sensitive matching;
~* is for case-insensitive matching;
!~ and !~* are for case-sensitive and case-insensitive non-matching, respectively;

II. File and directory matching, where:

-f and !-f are used to check if a file exists;
-d and !-d are used to check if a directory exists;
-e and !-e are used to check if a file or directory exists;
-x and !-x are used to check if a file is executable;

III. The last parameter of the rewrite directive is the flag, which can be:

last initiates a new request for the rewritten URI, the final effect depends on whether the new request can match another rule; "similar to [L] flag in Apache"
break the current directive specifies the final URI;
redirect returns a 302 temporary redirect, the browser address will display the redirected URL;
permanent returns a 301 permanent redirect, the browser address will display the redirected URL;

In 2022, I just learned that you can't directly use "and" or "or" operations in Nginx configuration?

# Used to view the visiting IP, for testing only
location ~* ^/ip/[\d-]+ {
    # default_type cannot be used inside if
    default_type    text/plain;
    return 200      "${remote_addr}";
}

# Match a certain path
# if ($uri ~* ^/SomePath) {
    # ………………
# }

# -----------------

# Default is 0
set $pass 0;

# Set $pass to 1 if any condition is met;
# equivalent to "or" operation;

# Check UA
if ($http_user_agent ~* Edg/\d+) {
    set $pass 1;
}

# Check IP
if ($remote_addr ~* 172.*) {
    set $pass 1;
}

# Deny access if not equal to 1
if ($pass != 1) {
    return 403;
}

# -----------------

set $flag 0;

# All conditions must be met;
# equivalent to "and" operation;

# Check UA
if ($http_user_agent ~* Edg/\d+) {
    set $flag "${flag}1";
}

# Check IP
if ($remote_addr ~* 172.*) {
    set $flag "${flag}1";
}

# Deny access if the result does not match
if ($flag != "011") {
    return 403;
}

Nginx implementation to only allow search engines or specified IP addresses to access:

# Frozen Ice and Floating Water (wdssmq)
location / {
    set $pass 0;
    # Check UA
    if ($http_user_agent ~* (Baiduspider|YisouSpider|360Spider|HaosouSpider|bingbot|Sosospider|Sosoimagespider|Bytespider|Sogou web spider|Sogou inst spider|Sogou News Spider|Sogou Pic Spider|Sogou Video Spider|Googlebot|Googlebot-Image|AdsBot-Google-Mobile)) {
        set $pass 1;
    }
    # Check IP
    if ($remote_addr ~* (123.123.123.123|192.168.0.\d+)) {
        set $pass 1;
    }
    # Deny access if not equal to 1
    if ($pass != 1) {
        return 403;
    }
    # Other rules for pseudo-static and so on
}