What does the --data option do in Curl?

The --data option (or -d for short) in curl sends data in the body of an HTTP request. By default, it automatically converts the request method to POST and sets the Content-Type header to application/x-www-form-urlencoded.

Basic Syntax

curl --data "key=value" https://example.com/api
# or using the short form
curl -d "key=value" https://example.com/api

Form Data Examples

Simple Form Data

curl -d "username=john&password=secret" https://example.com/login

URL-encoded Special Characters

curl -d "message=Hello%20World%21&status=active" https://example.com/submit

Multiple Parameters

curl -d "name=John" -d "email=john@example.com" -d "age=30" https://example.com/users

JSON Data

When sending JSON, always set the Content-Type header:

curl -d '{"name":"John","email":"john@example.com","age":30}' \
     -H "Content-Type: application/json" \
     https://example.com/api/users

JSON from File

curl -d @data.json -H "Content-Type: application/json" https://example.com/api

Data Variations

--data-raw

Prevents curl from interpreting @ and < characters as file references:

curl --data-raw "email=user@domain.com" https://example.com/api

--data-urlencode

Automatically URL-encodes the data:

curl --data-urlencode "message=Hello World!" https://example.com/api

--data-binary

Sends data exactly as specified without processing:

curl --data-binary @image.jpg -H "Content-Type: image/jpeg" https://example.com/upload

Reading Data from Files

From Text File

curl -d @form-data.txt https://example.com/submit

From Standard Input

echo "username=admin&password=secret" | curl -d @- https://example.com/login

XML Data Example

curl -d '<?xml version="1.0"?><user><name>John</name><email>john@example.com</email></user>' \
     -H "Content-Type: application/xml" \
     https://example.com/api/users

Important Notes

• Automatic POST: Using --data automatically changes the HTTP method to POST
• Content-Type: Defaults to application/x-www-form-urlencoded unless overridden
• Special Characters: Use --data-urlencode for automatic URL encoding
• File Safety: Use --data-raw to prevent file interpretation of @ symbols
• Binary Data: Use --data-binary for exact data transmission without processing

Common Use Cases

• API Testing: Sending JSON payloads to REST APIs
• Form Submission: Automating web form submissions
• File Upload: Sending binary data or files to servers
• Authentication: Sending login credentials via POST requests