What is the difference between -d and --data in cURL? How to send different data formats?

In cURL, -d and --data are actually exactly the same parameter, just different short and long forms. However, cURL provides multiple ways to send data for different scenarios.

Difference Between -d and --data

bash
# Both are completely equivalent
curl -d "name=value" https://api.example.com
curl --data "name=value" https://api.example.com

Data Sending Methods Comparison

Parameter	Purpose	Content-Type	Example
-d / --data	Send data (default URL-encoded)	application/x-www-form-urlencoded	-d "name=value"
--data-ascii	Send ASCII text	application/x-www-form-urlencoded	--data-ascii "text"
--data-binary	Send binary data	Not set	--data-binary @file.bin
--data-urlencode	URL-encoded data	application/x-www-form-urlencoded	--data-urlencode "name=value"
--data-raw	Send as-is (no @ processing)	application/x-www-form-urlencoded	--data-raw "@username"

Detailed Usage Examples

bash
# 1. Basic data sending (URL-encoded)
curl -X POST https://api.example.com/login \
  -d "username=admin&password=123456"

# 2. Send JSON data (need to manually set Content-Type)
curl -X POST https://api.example.com/users \
  -H "Content-Type: application/json" \
  -d '{"name":"John","age":25}'

# 3. Read data from file
curl -X POST https://api.example.com/upload \
  -d @data.json

# 4. Send binary file
curl -X POST https://api.example.com/upload \
  -H "Content-Type: application/octet-stream" \
  --data-binary @/path/to/file.bin

# 5. URL-encode special characters
curl -X POST https://api.example.com/search \
  --data-urlencode "query=hello world!"
# Result: query=hello%20world%21

# 6. Send as-is (don't process @ symbol)
curl -X POST https://api.example.com/webhook \
  --data-raw "@channel message here"

Multiple Data Segments

bash
# Send multiple data segments
curl -X POST https://api.example.com/form \
  -d "name=John" \
  -d "email=john@example.com" \
  -d "age=25"

# Mixed usage
curl -X POST https://api.example.com/mixed \
  -d "title=Test" \
  --data-urlencode "content=Hello World!" \
  --data-binary @attachment.pdf

Different Data Format Examples

bash
# Form data (application/x-www-form-urlencoded)
curl -X POST https://api.example.com/form \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "field1=value1&field2=value2"

# JSON data (application/json)
curl -X POST https://api.example.com/api \
  -H "Content-Type: application/json" \
  -d '{"key":"value","array":[1,2,3]}'

# XML data (application/xml)
curl -X POST https://api.example.com/soap \
  -H "Content-Type: application/xml" \
  -d '<?xml version="1.0"?><root><item>value</item></root>'

# Plain text (text/plain)
curl -X POST https://api.example.com/log \
  -H "Content-Type: text/plain" \
  --data-binary @log.txt

Important Notes

1. Default behavior: -d automatically sets Content-Type: application/x-www-form-urlencoded
2. Override Content-Type: For JSON, must manually set -H "Content-Type: application/json"
3. File reading: -d @filename reads content from file
4. @ symbol handling: --data-raw prevents @ from being interpreted as file path
5. URL encoding: --data-urlencode automatically encodes special characters

Practical Tips

bash
# Send complex JSON (using heredoc)
curl -X POST https://api.example.com/users \
  -H "Content-Type: application/json" \
  -d @- <<'EOF'
{
  "user": {
    "name": "John",
    "email": "john@example.com",
    "roles": ["admin", "user"]
  },
  "settings": {
    "theme": "dark",
    "notifications": true
  }
}
EOF

# Send Base64-encoded data
curl -X POST https://api.example.com/upload \
  -H "Content-Type: application/json" \
  -d "{\"file\":\"$(base64 -w 0 image.png)\"}"

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