curl -h | grep -e --header
-H, --header <header/@file> Pass custom header(s) to server
So --header is to pass custom header(s) to the server. Not to have the response-header be returned as JSON. With the right JSON tools you'd have to do it yourself. I'd suggest you give xidel a try.
Now it really depends on what you mean by "the json format". If all you want is the list of headers to be individual members in a JSON array, then this would do:
$ xidel -s --method=HEAD "https://github.com" -e 'array{$headers[.]}'
[
"HTTP/1.1 200 OK",
"Server: github.com",
"Date: Thu, 20 Mar 2025 12:49:30 GMT",
"Content-Type: text/html; charset=utf-8",
"Vary: X-PJAX, X-PJAX-Container, Turbo-Visit, Turbo-Frame, Accept-Language,Accept-Encoding, Accept, X-Requested-With",
"content-language: en-US",
"ETag: W/\"0c9499e9e08f25ec542a64ebf8d3631b\"",
"Cache-Control: max-age=0, private, must-revalidate",
"Strict-Transport-Security: max-age=31536000; includeSubdomains; preload",
"X-Frame-Options: deny",
"X-Content-Type-Options: nosniff",
"X-XSS-Protection: 0",
"Referrer-Policy: origin-when-cross-origin, strict-origin-when-cross-origin",
"Content-Security-Policy: default-src 'none'; [...] img-src 'self' data",
"Accept-Ranges: bytes",
"Set-Cookie: _gh_sess=[...]; Path=/; HttpOnly; Secure; SameSite=Lax",
"Set-Cookie: _octo=GH1.1.880431740.1742474976; Path=/; Domain=github.com; Expires=Fri, 20 Mar 2026 12:49:36 GMT; Secure; SameSite=Lax",
"Set-Cookie: logged_in=no; Path=/; Domain=github.com; Expires=Fri, 20 Mar 2026 12:49:36 GMT; HttpOnly; Secure; SameSite=Lax",
"X-GitHub-Request-Id: EE8A:62E88:2E4D37:2F3CCC:67DC0EE0"
]
[.] is to remove empty lines (or items strictly speaking, because $headers holds a sequence). Without it the array would end like this:
"X-GitHub-Request-Id: EE8A:62E88:2E4D37:2F3CCC:67DC0EE0"
""
]
If you still want to use curl, then you can pipe its output to xidel:
$ curl -sI "https://github.com" | xidel -se 'array{x:lines($raw)[.]}'
(x:lines($raw) is a shorthand for tokenize($raw,'\r\n?|\n') and turns $raw, the raw input, into a sequence where every new line is another item.)
If you want a JSON object instead of the array, then that's possible, but it requires a few extra steps:
$ xidel -s --method=HEAD "https://github.com" -e 'map:merge(for $item in $headers[.] let $kv:=tokenize($item,": ") return if (exists($kv[2])) then {$kv[1]:$kv[2]} else {"response":$kv[1]},{"duplicates":"combine"})'
$ xidel -s --method=HEAD "https://github.com" -e '
map:merge(
for $item in $headers[.]
let $kv:=tokenize($item,": ")
return
if (exists($kv[2]))
then {$kv[1]:$kv[2]}
else {"response":$kv[1]},
{"duplicates":"combine"}
)
'
{
"response": "HTTP/1.1 200 OK",
"Server": "github.com",
"Date": "Thu, 20 Mar 2025 12:49:30 GMT",
"Content-Type": "text/html; charset=utf-8",
"Vary": "X-PJAX, X-PJAX-Container, Turbo-Visit, Turbo-Frame, Accept-Language,Accept-Encoding, Accept, X-Requested-With",
"content-language": "en-US",
"ETag": "W/\"0c9499e9e08f25ec542a64ebf8d3631b\"",
"Cache-Control": "max-age=0, private, must-revalidate",
"Strict-Transport-Security": "max-age=31536000; includeSubdomains; preload",
"X-Frame-Options": "deny",
"X-Content-Type-Options": "nosniff",
"X-XSS-Protection": "0",
"Referrer-Policy": "origin-when-cross-origin, strict-origin-when-cross-origin",
"Content-Security-Policy": "default-src 'none'; [...] img-src 'self' data",
"Accept-Ranges": "bytes",
"Set-Cookie": ["_gh_sess=[...]; [...] SameSite=Lax", "_octo=GH1.1.880431740.1742474976; [...] SameSite=Lax", "logged_in=no; [...] SameSite=Lax"],
"X-GitHub-Request-Id": "EE8A:62E88:2E4D37:2F3CCC:67DC0EE0"
}
- The if-statement is required, because the very first item, "HTTP/1.1 200 OK", doesn't have a key-name, so you'd have to add one.
- In a JSON object duplicate keys are not allowed, so the 3 "Set-Cookie: [...]" items need to be reduced to just 1. The
$options map for map:merge() does just that. For all duplicates a sequence is created. The "Set-Cookie" attribute now holds a sequence of the previously 3 separate items.
