How to perform a POST request using Curl

Key takeaways

cURL is a command-line tool used by developers to transfer data between clients and servers using various network protocols.
POST requests are used to send data to a server. They are commonly used for submitting forms’ data, APIs, and file uploads.
The basic cURL command looks like curl [options] -X POST [URL].
Some examples of how we can use cURL are as follows:
- We can use the curl -X POST [URL] command for simple post requests.
- We can send data with the curl command using the -d option: curl -X POST [URL] -H "Content-Type: application/json" -d '{"key": "value"}'
- We can upload files using -F flag, curl -X POST -F "file=@filepath.jpg".

Daniel Stenberg introduced cURL (curl), a command-line tool that enables users to make network requests. The word cURL stands for “Client for URL.” Developers choose cURL because it is available on the main operating systems, including Windows, Linux, and Mac. If not pre-installed, we can use operating system’s package manager to install it easily.

Utilizing cURL for HTTP POST requests—which are used to send data to a server—is one of its most popular applications. This can be useful for tasks such as submitting form data, interacting with APIs, or uploading files.

The `curl` command for the `POST` request

To perform a simple POST request, we can use the following command:

curl [options] -X POST [URL]

[options]: This option reprents various flags and parameters to customize the request.
-X POST: This specifies that the HTTP method should be POST.
[URL]: This is the URL to which the POST request is sent.

Here are some of the commonly used options with curl for making POST requests:

Posting a request body using cURL

Users can send data with the POST request using the -d flag. It post the request body using the curl command in the following way:

curl -X POST https://educative.com/api/login/\
     -H "Content-Type: application/json" \
     -d '{"name":"Arya", "age":16}'

Sending multiple fields with a `POST` request

The following POST request sends a user and a pass fields along with their corresponding values.

curl -d "user=user1&pass=abcd" -X POST https://educative.com/api/login

Posting with curl’s -d option will include a default header that looks like: Content-Type: value.

Specifying the `Content-Type` in `POST` request

A POST request’s Content-Type header inform the server of the type of data being sent. We can use the -H (or --header) option in curl command to specify the Content-Type header.

Posting JSON data with cURL

The -H flag can be used to send a specific data type or header with cURL. The following command sends a JSON object with the request.

curl -H 'Content-Type: application/json' \ 
     -d '{"name":"Arya", "age":16}'
     https://educative.com/api/login

Sending `multipart/form-data` in `POST` request

multipart/form-data is often used for uploading files along with other form data. Use the -F (or --form) option to specify form fields and files.

curl -H "Content-Type: multipart/form-data" \
     -F name=arya \
     -F season=2 \
     -F Rating="5" \
     -X POST \
     https://educative.com/api/data

Sending image data in the `POST` request

The -F flag is used to specify form fields (including images or files). In the code snippet below, the @ symbol is used to indicate that the data should be read from a file when uploading an image, rather than including the data directly in the request body.

curl -X POST https://educative.com/api/upload \
     -F "image=@/path/to/image.jpg"

Posting XML using cURL

Users can send XML data by specifying the Content-Type header to application/xml or text/xml and use the -d (or --data) option to include the XML content.

curl -X POST https://educative.com/api/data/xml
   -H "Content-Type: application/xml"
   -d " <author>O'Brien, Tim</author>
      <title>Microsoft .NET: The Programming Bible</title>"

Posting form data using cURL

Form data is frequently used in web-based applications and is usually encoded with a URL. Set the Content-Type to application/x-www-form-urlencoded and use the -d option for URL-encoded data.

curl -X POST https://educative.com/api/submit-form \
     -H "Content-Type: application/x-www-form-urlencoded" \
     -d "username=arya&password=got"

Uploading a file using cURL

We can also send complete files in the command-line using cURL. We can use the command given below:

curl -X POST --form "file=@/pathtoimg.jpg" https://educative.com/api/post

Here, we use --form (or -F) option, and the file=@/pathtoimg.jpg part inform cURL to include the contents of img.png as part of the form data, with the field name file.

Sending basic auth credentials with cURL `POST` request

To send basic auth credentials with cURL POST request includes the --user username:password option in the curl command. cURL automatically encodes these credentials into a Base64 string and includes them in the request header as Authorization: Basic [token].

curl -X POST https://educative.com/api/login \
     --user "username:password"

Frequently asked questions

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What is the difference between POST and GET cURL?

cURL POST is used to send data to the server, while cURL GET request is used to get data from the server.

Can we use cURL instead of Postman?

Yes, we can use cURL instead of Postman depending on our requirement. Postman provides a graphical interface for API testing, and cURL allows us to perform similar operations via the command line.

What is a cURL POST?

cURL POST request is used to send data to a server.

What are common key pitfalls when using the cURL POST request?

The following are some of the common key pitfalls when using the cURL POST request:

Case sensitivity of the options parameter
Missing or incorrect Content-Type Header
Handling line breaks incorrectly
Improper authentication handling