Learn the Process of Executing POST Requests via cURL

cURL, an open-source command-line tool, is widely recognized for its ability to transfer data over a network. POST is particularly significant among the various HTTP request methods it supports as it facilitates data transmission to a server. This guide will delve into the intricacies of utilizing cURL for POST requests.

Anatomy of a cURL POST Request

A POST request using cURL follows a specific format: curl -X POST [options] [URL]. The -X option specifies the HTTP method, in this case, POST. The [options] part includes various parameters that modify the request, and [URL] is the endpoint to which the request is sent.

Here’s an illustrative example of a POST request to submit a task to a REST API:

curl -H 'Content-Type: application/json' \
     -d '{ "title":"foo","body":"bar", "id": 1}' \
     -X POST \
     https://example.com/posts

In this command:

  • -X specifies the HTTP method, which is POST.
  • -d provides the data to be sent in the body of the HTTP request.
  • -H is used to define headers for the request.

These options have associated aliases. For -X, you can use --request, and for -H, you can use --headers.

Common POST Request Operations

Sending Form Data

Form data can be sent with POST requests in two common ways, each with different content types:

Using multipart/form-data

This method is often used to send binary data like images. Here’s an example:

curl -H "Content-type: multipart/form-data" \
     -F title=foo \
     -F body=bar \
     -F id="1" \
     -X POST \
     https://example.com/posts

In this command, -F (or --form option) is used to send data, and the Content-Type header is set to multipart/form-data.

Using application/x-www-form-urlencoded

This method is typically used to send text data. Here’s an example:

curl -H "Content-type: application/x-www-form-urlencoded" \
     -d "title=foo" \
     -d "body=bar" \
     -d "id=1" \
     -X POST \
     https://example.com/posts

In this command, -d (or --data option) is used to send data, and the Content-Type header is set to application/x-www-form-urlencoded.

By default, HTML forms use the application/x-www-form-urlencoded content type when submitted.

Sending Image Data

To send image data in a POST request, include the image in the form data by prefixing the filename with an @ symbol:

curl -F profile=@avatar.jpg \
     https://example.com/avatar.cgi

The @ symbol forces the content part of the form to be a file, which is then uploaded to the server.

Common Pitfalls in cURL POST Requests

  • Argument names are case-sensitive. For instance, -F corresponds to the argument for form data, while -f is a flag indicating whether we want to fail fast with no output.
  • When passing content of type application/json, ensure to wrap the JSON body in single quotes. For example, -d "{ "title":"foo" }" would be invalid.
  • Breaklines can only be used if you include \\, a backslash character. There should be no trailing spaces after the backslash.

cURL POST Request Arguments

The options field of the cURL POST format (curl -X POST [options] [URL]) supports various arguments:

Short OptionLong OptionArgument(s)Purpose
-X--request<method>Specify the request method to use when communicating with an HTTP server
-b--cookie`<datafilename>`
-c--cookie-jar<filename>
-d--data<data>Send data in POST request
-f--failFail fast with no output on server errors
-F--form<name=content>Form data for content-type application/x-www-form-urlencoded
-H--header<header/@file>Header to include in the request
-i--includeInclude HTTP headers in the output
-l--headFetch the headers only
-k--insecureSkip verification that connection is secure
-L--locationIf a redirect is received (3XX), force cURL to redo the request on the new address
-o--output<file>Write output to file instead of stdout
-O--remote-nameWrite output to a local file named like the remote file
-s--silentDo not show progress meter or error messages
-v--verboseMake cURL verbose during operation. Useful for debugging
-w--write-out<format>Make cURL display information on stdout after a completed transfer

Conclusion

In this comprehensive guide, we have explored the process of executing POST requests using the powerful command-line tool cURL. We have delved into the anatomy of a cURL POST request, discussed common operations such as sending form and image data, and highlighted potential pitfalls to avoid. Furthermore, we have provided a detailed list of arguments that can be used in the options field of a cURL POST request, enhancing your understanding and control over the data transmission process.

The ability to effectively use cURL for POST requests is an invaluable skill in network communication. Whether you’re interacting with web servers, APIs, or other network services, the knowledge gained from this guide will serve as a solid foundation.

Remember, practice is key to mastering any new skill. So, we encourage you to apply these concepts and techniques in your projects. As you continue to use cURL and understand its nuances, you’ll find it indispensable in your developer toolkit.