I run nginx. In a "server" section for one of websites I have this:

```
server {
  listen 443 ssl http2;
  listen [::]:443 ssl http2;

  set $app_name myapp;
  set $app_port 4000;

  ssl_certificate /etc/letsencrypt/live/$app_name.com/fullchain.pem;
  ssl_certificate_key /etc/letsencrypt/live/$app_name.com/privkey.pem;

  access_log /var/log/nginx/$app_name.com.access.log;
  error_log /var/log/nginx/$app_name.com.error.log;

  #.............

```

At least one of the custom variables won't get resolved - `app_name`; for instance, it'll create a file `/var/log/nginx/$app_name.com.access.log`

namely, with **$app\_name** in it.

Consequently, in all other places of the config file where it's used, it won't get resolved either.

In the "error.log" I see that value of the variable is empty.

Why not? How to fix it?