$uri is not equivalent to $request_uri.
The $uri variable is set to the URI that nginx is currently processing – but it is also subject to normalisation, including:
- Removal of the
?and query string - Consecutive
/characters are replace by a single/ - URL encoded characters are decoded
The value of $request_uri is always the original URI and is not subject to any of the above normalisations.
Most of the time you would use $uri, because it is normalised. Using $request_uri in the wrong place can cause URL encoded characters to become doubly encoded.
Use $request_uri in a map directive, if you need to match the URI and its query string.