Test if nginx variable is set

Question

I need to set a variable to a default value if it is not already set. Following config does the job:

if ($foo ~ "^$") { set $foo default-value; } 

But it generates runtime warnings:

using uninitialized "foo" variable

Is there a way to test if a variable is set without generating the warning and without turning off the warning globally with:

uninitialized_variable_warn off; 

Answer 1

I'm using the following to check if a variable exists:

if ($dir = false) { set $dir ""; } 

If it's not set before it reaches this location block, then it will set it to a blank string. You can just as easily add a string between the quotes. I do not get any errors when doing a configuration test using this.

Please let me know if you're seeing otherwise.

Answer 2

The behaviour seems to have changed or at least be different for query parameter values using $arg_<name>. On nginx 1.13.6, using the following to check if a query value in the ngx variable exists:

if ($arg_test = false) { default_type 'text/plain'; return 404; } 

does not pass to the 404. Using:

if ($arg_test = '') { default_type 'text/plain'; return 404; } 

returns a 404, if the query parameter does not exist. Useful for not hitting upstream if a required query parameter does not exist.

Answer 3

I think the simple answer is that you ought to use a map, but none of the answers here or in similar questions were an approach that worked for my usage in the latest Nginx versions, so for those situations the below seems to work.

In my case I needed to overwrite the Host header, conditionally, in an included general configuration file shared by many sites, whilst only setting that for the sites that needed it.

In this case, in your http block (e.g. nginx.conf):

map '' $proxied_host { default ''; } map "$proxied_host" $resolved_host { default $host; ~. $proxied_host; } 

and in the included configuration file in every location:

proxy_set_header Host $resolved_host:$server_port; 

then in the sites that need to overwrite it, in the location block:

set $proxied_host your.example.com; 

This is the only way I found to avoid:

1. Floods of warnings in error.log saying using uninitialized "proxied_host" variable
2. Errors related to using uninitialized variables in the map like nginx: [emerg] unknown "proxied_host" variable
3. Errors relating to cycles in variables cycle while evaluating variable "proxied_host"

These last two errors appeared when following the answers in this question

Answer 4

If GET QUERY parameter detection

e.g： foo.com?proxy proxy variable exists Do not use $arg_proxy

if ($args ~ '(&|^)proxy([&=]|$)' ) { #exists set $port '8080'; } https://forum.nginx.org/read.php?2,224860,224860

Answer 5

An easy option, if you're optionally setting the variable in an included file, is simply to set the default value first.

set $value 'default'; include maybe-set-value.conf; 

The second set statement in the conf file will replace the default value.