How to send JSON file as part of request body in CURL POST command

Question

I am using CURL command line to send HTTP POST to a web service. I want to include a file's contents as a PART of the body of the POST command. Is this possible? I know I can send a file as the entire body as answered here. But I only want a part of the body to be the content of the file.

For example

curl -d '{ "name": "rahul", "speed": "fast", "data": { "number": 1, "letter": "abd", "letter2": "efg"} }' 'http://...'

Here I only want data as the file's content. Not the entire body. How can I do this?

Answer 1

Set a variable to contain the file contents:

data=$(cat /path/to/file)

then substitute it into the JSON:

curl -d '{ "name": "rahul", "speed": "fast", "data": "'$data'" }' 'http://...'

Answer 2

You accepted @Barmar's answer, but for anyone reading this, @Barmar switched the double- and single-quotes, which will cause the command to not work as intended.

The following works:

data="$(cat filename)" && \
curl -d '{ "name": "rahul", "speed": "fast", "data": "'$data'" }' 'http://...'

Notice that the $data variable is surrounded by single-quotes first, then double-quotes.