In an HTTP request, after the initial request-line, there will typically
follow a number of request headers. That is a set of name: value pairs that
ends with a blank line that separates the headers from the following request
body (that sometimes is empty).
curl will by default and on its own account pass a few headers in requests,
like for example Host:, Accept:, User-Agent: and a few others that may
depend on what the user asks curl to do.
All headers set by curl itself can be overridden, replaced if you will, by the
user. You just then tell curl's -H or --header the new header to use and
it will then replace the internal one if the header field matches one of those
headers, or it will add the specified header to the list of headers to send in
the request.
To change the Host: header, do this:
curl -H "Host: test.example" http://example.com/
To add a Elevator: floor-9 header, do this:
curl -H "Elevator: floor-9" http://example.com/
If you just want to delete an internally generated header, just give it to curl without a value, just nothing on the right side of the colon.
To switch off the User-Agent: header, do this:
curl -H "User-Agent:" http://example.com/
Finally, if you then truly want to add a header with no contents on the right side of the colon (which is a rare thing), the magic marker for that is to instead end the header field name with a semicolon. Like this:
curl -H "Empty;" http://example.com