Getting Started With cURL

It Starts With…A Server

Before we get into cUrl we need to understand about the server. A server is just another computer which is alwasy up and running. It waits for requests from your computer, laptops or mobile devices and its job is t reply to these requests. You computer or mobile devices are called clients.

When You open a website your browse rsends a message to the server and the sever sends the webpage back. When you try to log in to your account on some website or app the server checks your credentials and sends back the logged in page. THis happends all the time in the background (To knwo more about how the request response cycles work you can refer to this blog)

Your device (client) asks something and server asnwers. This is a simple cycle that is always followed and as programmers this conversation becomes reall yimportant as our apps entirely depend on it. And to build softwares we must undersatnd how to talk to servers. This is where cURL comes in the picture.

So What Is cURL?

cURL is a command line tool that enables you to talk to servers directly from your terminal. You can think of it as a messanger. You type in a command, cURL deliers your message to the server then the server replies and you see that reply. This does not requiere any browser or interface makin it the bset fit to play around and explore the wroking of the entire rquest and response system.

When you open a website in a browse, the browser sends a request to the server, with cURL you do the same thing but manually. This shows you the raw response exactly how the sever sends it. This you can imagine is very powerful for programmars. You are no longre guesiing what your app or browser is doing, you an see the commincation for yourself without any middleware. This way you can check serverse and test APIs and debug without writing any code.

Why Programmers Need cURL and How They Ue It

When you are writing code you would to keep testing the response of your server. Going back and forth between the rbowser and the edior can be tedious. So as someone wroking on the backend it is handy to have a tool that can let you do the testing in the command line itself. Here cURL will help you as it can send request to a server directly from your termnal window.'

Programmers frequently use cURL to test things quickly. Suppose you are building a login API. Instead to opening a browser and writing a frontend you can use cURL to send the rquest and check if the server is giving the desired response or not.

cURL is also very usefull if you want to debug problems. Say a website is not working properly then you can use cURL to see if the responses that are being sent by the server have any problems within them or not. This can help you narrow down if the problem is in the backend, frontend or the network itself.

Let us now help you write your ery firt cURL command.

$ curl https://www.google.com

This single line will send the entire HTML code for the google home page. Your terminal should print the entire response with the HTML tags and classes and what not. You just fetched an entire webpage without any browser or interactive user interface (Now that’s what a real prgrammer likes, NO UI AT ALL).

From here, you can start using cURL to test APIs, check server responses, and experiment with requests. It becomes a fast, reliable way to communicate with servers while building and testing software.

Understanding the Request and the Response

The entire web works on one basic rule, You send a request to the server, the server sends back a response.

When we write:

$ curl https://www.google.com

What did we send? Without writing anything extra in your command line, cURL by default sends a GET request to the server. It basically says “Hello Server! Can you please get me the webpage stored at that address?”

Then you may see something like this

Very intimidating write? But this is just an HTML page with not so goo formatting. But more importantly what does the server sends back? Any server replie with two main things status and data.

• Status: Did your request succeed or fail. In case of failiure what was the cause. 200 mean everything went well, anything else however means something went wrong.

• Data: The actual content you asked for.

The data that you ee printed on the terminal is the data returned by the server in this case the HTML of the webpage.

Introducing GET and POST

Before this point, you used cURL to fetch a webpage. That request was a GET request, even if you did not say it explicitly. GET is the most basic way to talk to a server. It simply says “Please give me this data.”

When you write a command like

$ curl https://www.google.com

You are writing a GET request by default. You are asking the server to return whatever lives at that URL. You are not sending any data to the server, you are only requesting some data. This is exactly how browsers get images and webpages and videos, becaue all of these assests live somewhere on a server physically whose adress you just provided unknowingly with a URL.

Now let’s look at the second important type of request, the POST request. POST means “I want to send some data to the server.” This is used when you fill out a form, log in to a website, or submit information. Here, you are not just asking for data rather you are sending data along with the request.

A POT request in cURL may look something like this

curl -X POST https://api.freeapi.app/api/v1/users/login \
-d '{"username":"doejohn","password":"test@123"}' \
-H "Content-Type: application/json"

In this command, three important things are happening:

• -X POST tells cURL that this is a POST request

• -d sends data to the server (your username and password)

• -H tells the server that you are sending JSON data

You are sending login details to the server, just like a login form on a website. The server reads this data and responds back, usually with a success message or an error. The response may look liek this, here you can claearly see the status code and the data.

Using cURL to Talk to APIs

What we did in the previous section while understanding POST was talking to an API. Now let us undertsand what an API is, why do we even care about it and how to use it.

An API (Application Programming Interface) is a way of communicating data from the frontend to the backend and vice-a-versa. It is just a server that sends back data instead of a webpage. When you open a website, you get HTML. When you call an API, you usually get JSON or structured data that we can see in the given image.

cURL lets you ask that API for data directly from your terminal. Let us now use another API with cURL to undersatnd this better. JSONPlaceholder is an API made for learning and testing purposes. follwoing command in cURL

$ curl https://jsonplaceholder.typicode.com/todos/1

You will see and output like this

You just fetched data from a server! The response you see is JSON data. This is what apps and websites read behind the scenes. This is a universal format that can be parsed or read bby any programming lanuguage. This means that you can write frontend in a language say JavaScript and backend in some other laguage say Python and yet both your frontend and backend will be talking in terms of JSON.

You can also send a POST request to this API as follows

$ curl -X POST https://jsonplaceholder.typicode.com/posts ^
-d "title=hello&body=fromcurl&userId=1"

This will return a response that looks something like

Common Mistakes Beginners Make with cURL

Most beginner issues with cURL come from small misunderstandings. A very common mistake is forgetting quotes around URLs that contain ?, &, or spaces, which makes the terminal split the command. Another frequent confusion is mixing up GET and POST. By default, cURL sends a GET request, so trying to send data without switching to POST leads to unexpected results. Many beginners also run a command, see a lot of text in the terminal, and assume something went wrong. In reality, that text is the server’s response, and learning to read it is part of using cURL.

When you are starting out try to avoid flag-heavy examples found online. This makes cURL look complicated when only a few basics are needed at the start. Sometimes the command is correct, but the server URL is wrong or the server is down, (I mean cloudfalre has been down a lot recently). Finally, cURL shows raw data like HTML or JSON, not a nicely formatted page like a browser, and this often surprises new users. Once these points are clear, cURL becomes much easier to use.

Conclusion

By now, you have seen that cURL is not a complex tool. It is simply a way to talk to servers from your terminal. You learned how requests and responses work, how to make a basic GET and POST request, and how cURL helps you test and understand APIs without writing extra code.

The real mastry of cURL witll come fomr integrating it in you development flow. You no longer have to guess what your application is sending or what the server is returning. You can see it yourself, clearly and directly. With just a few simple commands, you now have a practical skill that every programmer uses daily.