Nginx rewrite URL only if file exists

Question

I need to write a rewrite rule for Nginx so that if a user tries to go to an old image url:

/images/path/to/image.png

and the file doesnt exist, try to redirect to:

/website_images/path/to/image.png

ONLY if the image exists in the new URL, otherwise continue with the 404. The version of Nginx on our host doesn't have try_files yet.

Answer 1

location /images/ {
    if (-f $request_filename) {
        break;
    }

    rewrite ^/images/(.*) /new_images/$1 permanent;
}

Though, you might want to bug your host to upgrade or find a better host.

Answer 2

You could use something like this (untested for your specific case):

location ^/images/(?<imgpath>.*)$ {

    set $no_old  0;
    set $yes_new 0;

    if (!-f $request_filename)
    {
        set $no_old 1;
    }

    if (-f ~* "^/new_path/$imgpath")
    {
        set $yes_new 1$no_old;
    }

    # replacement exists in the new path
    if ($yes_new = 11)
    {
        rewrite ^/images/(.*)$ /new_path/$1 permanent;
    }

    # no image in the new path!
    if ($yes_new = 01)
    {
        return 404;
    }
}

Which is basically an alternative way of writing nested if statements, since you cannot nest in Nginx. See here for official reference on this "hack".