1. POST data with Curl#

The Content-Type header can be application/json, application/x-www-form-urlencoded, multipart/form-data, the first two is usually used with posting form data to server, the third is used to upload file to the server.

  • The format of application/json is '{"username":"davidzhu", "password":"778899a"}'.

  • The format of application/x-www-form-urlencoded is "username=davidzhu&password=778899a" .

e.g.,

POST x-www-form-urlencoded format form data to server:

# curl will set the request header to 'application/x-www-form-urlencoded' by default
curl localhost:8080/hello -d "username=davidzhu&password=778899a"

POST json format data with curl:

# you need specify the Content-Type header to application/json explictly
curl -X POST [URL]
     -H "Content-Type: application/json" 
     -d '{"username":"davidzhu", "password":"778899a"}'

Note that we use single quote to wrap the double quote which makes double quote lose its special meaning, it’s just all about shell scripting, learn more: Shell Script Basic Syntax - David’s Blog

Bad example:

# Bad example
curl localhost:8080/hello -d '{"username":"davidzhu", "password":"778899a"}'

The server will parse it wrong, because the data format is json obviously, but curl will set the Content-Type header to application/x-www-form-urlencoded by default. Therefore, don’t write it wrong, otherwise, your server won’t parse data from the request correctly.

The content below form the docs of curl, hope it will help:

To make a POST request with Curl, you can run the Curl command-line tool with the -d or --data command-line option and pass the data as the second argument. Curl will automatically select the HTTP POST method and application/x-www-form-urlencoded content type if the method and content type are not explicitly specified. Source

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 -H “Content-Type: application/json” command line parameter.

$ curl localhost:8080/hello 
    -H "Content-Type: application/json"
    -d '{"Id": 79, "status": 3}'

Learn more: application/x-www-form-urlencoded and multipart/form-data

Learn more about http request header: https://davidzhu.xyz/post/cs-basics/018-http-messages/

2. Query string with curl# 

$ curl -X POST "localhost:8080/postform?username=david&password=778899a"
# The code below won' work (you have to make it with double quote):
$ curl -X POST localhost:8080/postform?username=david&password=778899a # wrong