Sending JSON

Most APIs today want JSON, not form encoding. Two things have to change, and forgetting the second is the single most common mistake in this whole area.

curl -s -H 'Content-Type: application/json' \
     -d '{"name":"ada"}' http://127.0.0.1:9125/users

First the body becomes JSON. Second you must declare it with a Content-Type header, because curl still labels the body as form data otherwise. The server then tries to parse JSON as key=value pairs and rejects it, usually with a 400.

So when an API says your JSON is invalid and it looks perfectly valid to you, check the header before you check the JSON.

Mind the quoting too. JSON is full of double quotes, so wrap the whole body in single quotes and the shell will leave it alone.