To pass a custom Host header with the curl command in Linux or Unix, you can use the -H option followed by the Host: header and the hostname.
For example, to pass the Host header www.example.com with a curl request to http://localhost, you can use the following command:
curl -H 'Host: www.example.com' http://localhost
This will send an HTTP request to http://localhost with the Host: www.example.com header, indicating that the request is intended for the host www.example.com.
You can also use the --header option to set the Host header manually:
curl --header 'Host: www.example.com' http://localhost
This will have the same effect as using the -H option.
Keep in mind that the Host header is required in HTTP/1.1 requests and is used to specify the hostname of the server that the request is intended for. Some servers may block or ignore requests that do not include a Host header, or may treat them differently based on the value of the header.