empty variable in access log

[Maxim Dounin]

Hello!

On Mon, Nov 30, 2020 at 02:04:35PM -0800, Frank Liu wrote:

> I may have mixed this with special upstream variables, eg:
> $upstream_http_something. When upstream response header doesn't existing,
> the variable was logged - in the nginx access logs.

When a variable value is not found, it's logged as "-", that's 
expected behaviour.  That's documented in the log_format directive 
description (http://nginx.org/r/log_format):

: If the variable value is not found, a hyphen (“-”) will be 
: logged.

This doesn't apply to found but empty values as in your example 
though.

-- 
Maxim Dounin
http://mdounin.ru/