Understanding HTTP POST Requests for API Workflows

Whether you’re testing endpoints, automating workflows, or debugging integrations, cURL is the unsung hero of the command line. It’s deceptively simple, yet it can handle anything from a quick POST to complex JSON payloads and file uploads. Once you master it, your API interactions become faster, cleaner, and more reliable. Let’s dive in. We’ll give you actionable examples you can run immediately.

The Basics of HTTP POST Requests

POST differs from GET because the data goes in the request body, not the URL. That means cleaner URLs, more secure data, and often required by APIs.

The simplest POST request:

curl -d "param1=value1&param2=value2" http://example.com/

Notice there’s no -X POST? You don’t need it. -d automatically triggers POST.

Multiple parameters? Just chain multiple -d flags:

curl -d name=admin -d shoesize=12 http://example.com/

cURL merges them behind the scenes with ampersands.

For larger payloads, read from a file:

curl -d @filename http://example.com/

--data-binary preserves line breaks. If your data starts with @ but isn’t a file, use --data-raw.

How to Deliver Data in POST Requests

APIs expect data in different formats. Knowing how to match them saves headaches.

Form-encoded (default):

curl --data 'name=michael' http://example.com/my-form/

JSON requires headers:

curl --header "Content-Type: application/json" \
     --request POST \
     --data '{"name":"michael","value":"123"}' \
     https://example.com/api/

Windows command line? Escape those quotes:

curl --header "Content-Type: application/json" \
     --data "{\"emailAddress\":\"user@example.com\"}" \
     https://example.com/api/

Upload file contents:

curl --data @myfile.txt https://example.com/api/

Preserve formatting:

curl --data-binary @1.txt https://example.com/api/

Multipart forms? -F to the rescue:

curl -F files=@1.txt https://example.com/api/
curl -F 'file=@path/to/file.csv;type=text/csv' https://example.com/api/

How to Handle JSON and Custom Headers

JSON is the language of modern APIs. Getting headers right is critical.

Classic approach:

curl -H "Content-Type: application/json" \
     -X POST \
     -d '{"name":"value"}' \
     https://example.com/

cURL 7.82.0+ simplifies this with --json:

curl --json '{"tool": "curl"}' https://example.com/

One flag. All headers and formatting handled.

Read JSON from a file:

curl --json @data.json https://example.com/

Combine jo to generate JSON and jq to process responses:

jo name=value | curl --json @- https://example.com/ | jq

Special characters in JSON? Add -g to prevent URL globbing errors.

Wrapping Up

Once you get comfortable with cURL, working with APIs becomes a lot more straightforward. You can send data, handle JSON, and upload files without juggling multiple tools or interfaces.
The more you experiment, the easier it is to anticipate how APIs respond and structure your requests accordingly. Mistakes become learning opportunities, not roadblocks.
In practice, cURL becomes more than a command-line utility—it’s a way to simplify your workflow, reduce friction, and maintain consistency across projects. Start small, try different scenarios, and gradually, you’ll find that most API tasks are faster and more predictable than you thought.