Understanding the nginx config to serve from a subdirectory

[jeanas]

This FAQ suggests the following nginx configuration to serve Lychee from a subdirectory:

location ^~ /lychee {
    alias /var/www/lychee/public;
    index index.php;
    try_files $uri $uri/ @lychee;
    location ~ \.php$ {
        if (!-e $request_filename) {
            rewrite ^/lychee/?(.*)$ /lychee/index.php?/$1 last;
            break;
        }
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $request_filename;
    }
}
location @lychee {
    rewrite /lychee/(.*)$ /lychee/index.php?/$1 last;
}

I'm new to nginx, and I'm not sure I fully understand how this works.

• Why the two separate rewrite rules, one in location @lychee invoked by try_files and one in if (!-e $request_filename)? I tried with the latter commented out and it still seemed to work.

• Why location ~ \.php$? AFAICS Lychee has only one PHP file, index.php.

• What is fastcgi_index index.php for? According to the documentation, it does something with $fastcgi_script_name. But I used log_format to view the value of $fastcgi_script_name and it looked correct (/lychee/index.php). Removing fastcgi_index didn't change anything.

• Why the location ^~ /lychee instead of location /lychee?

This seems to be working for me:

    location /lychee {
      alias /var/www/selim/lychee/public;
      index index.php;
      try_files $uri $uri/ @lychee;
      location /lychee/index.php {
        include fastcgi-include.conf;
        fastcgi_pass unix:/var/run/php/php8.2-fpm.sock;
        fastcgi_param SCRIPT_FILENAME $request_filename;
      }
    }
    location @lychee {
      rewrite /lychee/(.*)$ /lychee/index.php?/$1 last;
    }