How does try_files work?

Question

I looked at the nginx documentation and it still confuses me utterly.

How does try_files work? Here is what the documentation says:

From NginxHttpCoreModule

try_files

syntax: try_files path1 [path2] uri

default: none

context: server, location

availability: 0.7.27

Checks for the existence of files in order, and returns the first file that is found. A trailing slash indicates a directory - $uri /. In the event that no file is found, an internal redirect to the last parameter is invoked. The last parameter is the fallback URI and must exist, or else an internal error will be raised. Unlike rewrite, $args are not automatically preserved if the fallback is not a named location. If you need args preserved, you must do so explicitly:

I don't understand how it checks the paths and what if I don't want an internal error but have it resume the rest of the path in an effort to find another file?

If I want to try a cached file at /path/app/cache/url/index.html and if it fails to try /path/app/index.php how would I write that? If I wrote:

try_files /path/app/cache/ $uri
include /etc/nginx/fastcgi_params;
fastcgi_pass unix:/var/run/php-fastcgi/php-fastcgi.socket;
fastcgi_param SCRIPT_FILENAME $document_root/index.php;

I have index index.php index.html index.htm;. When I visit /urlname, will it try checking /path/app/cache/urlname/index.php then /path/app/cache/urlname/index.html? If we ignore everything after try_files is it possible for try_files to check the cache folder? I have been trying and have failed.

Answer 1

try_files tries the literal path you specify in relation to the defined root directive and sets the internal file pointer. If you use for instance try_files /app/cache/ $uri @fallback; with index index.php index.html; then it will test the paths in this order:

1. $document_root/app/cache/index.php
2. $document_root/app/cache/index.html
3. $document_root$uri

before finally internally redirecting to the @fallback named location. You can also use a file or a status code (=404) as your last parameter but if using a file it must exist.

You should note that try_files itself will not issue an internal redirect for anything but the last parameter. Meaning you cannot do the following: try_files $uri /cache.php @fallback; as that will cause nginx to set the internal file pointer to $document_root/cache.php and serve it, but since no internal redirect takes place the locations aren't re-evaluated and as such it will be served as plain text. (The reason it works with PHP files as the index is that the index directive will issue an internal redirect)

Answer 2

Here's another convenient use of try_files, as unconditional redirects to named locations. The named locations are effectively acting as subroutines, saving duplication of code. When the first argument to try_files is _ the fallback redirect is always taken (assuming that _ is not an existing filename). Because nginx needs a goto statement but doesn't have one.

    location =/wp-login.php { try_files _ @adminlock; }
    location ^~ /wp-admin/  { try_files _ @adminlock; }
    location @adminlock  {
            allow 544.23.310.198;
            deny all;
            try_files _ @backend;
            # wp-admin traffic is tiny so ok to send all reqs to backend 
    }
    location ~ \.php {  try_files _ @backend; }
    location / { try_files $uri $uri/ =403; }
    location @backend {
            fastcgi_pass 127.0.0.1:9000;
            include snippets/fastcgi-php.conf;
    }