What does '--user' mean with curl

I'm working with an API and I have to send a POST request. I know how to set a header (-H) and (-d) is the body, but what is "--user".

If I submit this with Postman, or in a text editor with axios or just regular XMLRequest, where do I add this?

The docs say it is for regular http auth.

curl -X POST -H "Content-Type: application/json" \
     --user "<client_id>:<client_secret>" \
     -d '{"grant_type": "client_credentials", "scope": "public"}' \
     ...

Late to the party, but here goes...

You can use curl with the -v (verbose) parameter to see the headers sent. You will then see that the information provided with --user is transformed into a header, such as:

Authorization: Basic YWxhZGRpbjpvcGVuc2VzYW1l

The text after the Basic keyword is a base64 encoded text string of the username:password combination provided with the --user parameter

To manually generate the base64 encoded credentials on Linux, you can simply call:

echo -n "username:password" | base64 -w0

For windows, save the "username:password" to a file, then use certutil.exe to create a base64 encoded file:

certutil -encode credentials.txt credentials.asc

To test this end to end, you can remove --user username:password and substitute with --header Authorization: Basic YWxhZGRpbjpvcGVuc2VzYW1l and it will still authenticate just fine.

In summary, to do this manually without curl, you would need to base64 encode username:password combination. You would then need to set the HTTP Authorization header with the type as Basic along with the base64 encoded string.

--user parameter in curl used for server authentication. So if you don't define authentication type via other parameters like --digest or --negotiate, it means USER parameter for http basic authentication, it also could be combined with :PASSWORD chunk to set a password as well. The full answer on your question depends on what kind authentication is used behind API you are sending request to, and maybe curl would not be enough for it, as it support a limited set of authentication schemes ...

--user (or -u) in curl provides a basic auth to your request.

In Postman you can achieve the same result with a choice in Authorization tab.

--user "<client_id>:<client_secret>" becomes

• Type: Basic Auth
• Username: client_id
• Password: client_secret

Specify the user name and password to use for server authentication. If you simply specify the user name, curl will prompt for a password.

If your curl request does not have any -- user, then server that requires authentication sends back a 401 response code and an associated WWW-Authenticate: header that lists all the authentication methods that the server supports.

< HTTP/1.1 401 
< WWW-Authenticate: Basic realm="oauth2/client"

Then you will know the server is using Basic authentication

You can add --basic to explicitly tell it is Basic authentication

Please refer to HTTP authentication for more information

Sometimes (depending on server implementation) the --user will negotiate a digest authenticated session. The headers for digest users are a one-time use. I believe a request to the server will first fail with a 401, but include a WWW-Authenticate response, including the digest realm, and the nonce secret. With these, a second request can be made with a new header Authorization value.

example:

Authorization: Digest username="LXAIQKBC", realm="MMS Public API", nonce="rE3sYnLXEhVMbh72JyUK7kfLIb+bAbKj", uri="/api/atlas/v1.0/groups", cnonce="YTVhM4YwMDB3ZjZjMTkxbCNiODA1ODnxZDFjOGMyMzE=", nc=00000001, qop=auth, response="7a5fcb8e4f92a665315bf62cdd87a67d", algorithm="MD5"

As an addition to Jahmic's answer, Nodejs programmers can do this to convert to base64 string:

const cryptoJS = require("crypto-js");    
const base64Str = cryptoJS.enc.Base64.stringify(cryptoJS.enc.Utf8.parse(`${username}:${password}`))