empty variable in access log

[Maxim Dounin]

Hello!

On Mon, Nov 30, 2020 at 03:26:59PM -0800, Frank Liu wrote:

> ok, for testing, I removed the variable from the map, and add one line in a
> 2-way SSL server config, to create a fresh new variable:
> 
> set $test_var "test";
> 
> For a request without client cert (400), I see neither "test", nor "-" in
> the access log for $test_var. I only see blank, as if the $test_var was set
> to "".

That's because variables defined due to "set" somewhere in the 
configuration default to "" with an optional warning 
(http://nginx.org/r/uninitialized_variable_warn).  And in your 
test the variable is uninitialized, as the set directive is not 
executed for the request in question.

The "-" special value can be seen for various builtin variables, 
such as non-existing headers ($http_*, $sent_http_*, 
$upstream_http_*), non-existing arguments ($arg_*), cookies 
($cookie_*), $content_length if not available, $remote_user if not 
provided or empty, and so on.

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