Curl is a popular command-line software for transferring data to or from the server using URL syntax. Curl supports many protocols, including HTTP, HTTPS, FTP, SFTP, and works on all modern platforms and hardware, including Linux, Windows, and macOS.
What is JSON?
What is HTTP POST?
POST is one of the most widely used methods of the HTTP protocol. The POST method requests the webserver to receive and process the data enclosed in the body of the POST message. The POST method is often used to upload files and submit HTML forms.
How to make a POST request with Curl?
There are two ways to send a POST request with Curl.
When you use command-line parameters such as --data or --form and do not explicitly specify the required HTTP method, Curl automatically selects the POST method and sends a POST request with the application/x-www-form-urlencoded content type (or multipart/form-data for --form).
Explicitly specify the required HTTP method using the -X command-line argument. For example, you need to use the -X POST-command-line parameter to send JSON using the POST method.
Curl POST Request Syntax
The general form of a Curl command for making a POST request with a JSON body is as follows:
Curl POST Request with JSON
curl -X POST [URL]
-H "Content-Type: application/json"
-d "[JSON data]"
-X, --request: HTTP method to use when communicating with the server.
-H, --header: HTTP headers to send to the server with a POST request.
-d, --data: Data to be sent to the server using a POST request.
Why do I need to explicitly specify the Content-Type when posting JSON using Curl?
If you submit data using Curl and do not explicitly specify the Content type, Curl uses the application/x-www-form-urlencoded content type for your data. Therefore, when sending JSON (or any other data type), you must specify the data type using the explicitly -H "Content-Type: application/json" command line parameter.
Why is it important to specify the correct Content-Type when submitting JSON?
In short, if you don't set the correct Content-Type, your application may not work. The Content-Type header field indicates the media type included in the message payload. The specified media type determines both the format of the data and the way the server handles that data.
For example, if the server can accept XML and JSON data on the same API endpoint, setting the Content-Type to application/json will let the server know that the client is sending JSON data, and application/xml will tell the server that the client is sending XML.
How to post JSON file with Curl?
You can post a JSON file using Curl if you pass the filename in the -d command line parameter after the "@" symbol:
Curl POST JSON File Example
curl -X POST https://reqbin.com/echo/post/json -d @filename