Displaying response headers and pretty JSON with cURL

The Problem

I tend toward using cURL when testing HTTP services, but I had a difficult time parsing the result when these services spit back unformatted JSON responses. To make my life a little easier, I set out to find a way to bend cURL to my will.

The Solution

Let's get straight to it. To make cURL display both response headers and formatted JSON, we need to make cURL send the headers to STDERR, so we can then send STDOUT to a formatting utility.

We can accomplish this as so.

curl -s -D "/dev/stderr" -X POST 'http://example.org/some/url' --data-binary @some-file | python -m json.tool  

This will show a nicely formatted response like this.

HTTP/1.1 500 Internal Server Error  
Content-Type: application/json; charset=utf-8  
X-Powered-By: ASP.NET  
Date: Wed, 18 Sep 2019 22:39:24 GMT  
Content-Length: 114

    "Status": 3,
    "Timestamp": "2019-09-18T22:39:24.1659165Z",
    "Error": {
        "Code": 1,
        "Message": "An error has occurred."
What's Going On Here?

Let's break this down a bit.

After some initial research into how to send the HTTP headers to STDERR, I found this article that demonstrated the use of -D "/dev/stderr".

Breaking the command down:

  • -s will silence the statistics output of cURL
  • -D "/dev/stderr" will send the HTTP headers to STDERR
  • python -m json.tool receives STDOUT, which is the JSON response of my service
  • The other options are specific to my service request, and are not relevant to the formatting

Note: you can also pipe the cURL output to a utility like jq, I just happen to use python -m json.tool.

A Step Further

These commands and then be turned into Bash aliases and functions.

I have this in my ~/.bashrc.

alias curl-json='curl -s -D "/dev/stderr"'  
        curl-json "$@" | python -m json.tool

Now I can use this command!

curl-json-format -X POST 'http://example.org/some/url' --data-binary @some-file | python -m json.tool