curl json post request via terminal to a rails app
First off, there is an extraneous " at the end of your command.
Try this
curl -v \ -H "Accept: application/json" \ -H "Content-type: application/json" \ -X POST \ -d ' {"user":{"first_name":"firstname","last_name":"lastname","email":"email@email.com","password":"app123","password_confirmation":"app123"}}' \ http://localhost:3000/api/1/users