The If-Modified-Since header is used in HTTP requests to tell the server that the client only wants a response if the requested resource has been modified since the specified date. This can be useful for caching purposes, as it allows the client to use a cached copy of the resource if it has not been modified since the specified date.
To use the If-Modified-Since header with the curl command on Linux or Unix, you can use the -H option to include the header in the request.
For example, to send a request to https://www.example.com with the If-Modified-Since header set to January 1, 2020, you can use the following command:
curl -H 'If-Modified-Since: Tue, 01 Jan 2020 00:00:00 GMT' https://www.example.com
This will send a request to https://www.example.com with the If-Modified-Since header set to January 1, 2020. If the resource has not been modified since that date, the server will return a 304 Not Modified response, and the response body will be empty. If the resource has been modified since that date, the server will return a 200 OK response, and the response body will contain the updated resource.
You can also use the --http1.1 option to force curl to use HTTP/1.1, which is required to use the If-Modified-Since header.
For example:
curl --http1.1 -H 'If-Modified-Since: Tue, 01 Jan 2020 00:00:00 GMT' https://www.example.com
This will send a request to https://www.example.com with the If-Modified-Since header set to January 1, 2020, and use HTTP/1.1 to send the request.
For more information about how to use the If-Modified-Since header with the curl command on Linux or Unix, you can consult the curl documentation or seek assistance from a qualified Linux or Unix administrator.