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In most Raspberry Pi projects, sending data to a website (or in the website’s database) is very crucial. For example, in home automation, if we want to access the room’s sensor and consumption data in our devices through a website in real-time, we must first be able to do data transfer from RPi to website. In this tutorial, I am going to teach you a simple technique to do just that. We will be utilizing a curl bash command via Python in RPi, and we’ll build a simple PHP service in our website/client.

Here, curl sounds a very cute command when one hears it for the first time. For Linux users, this is nothing new for you. curl is a nifty command line tool for getting or sending files using URL syntax. To help you remember, you can write it this way: cURL. Meaning, just by changing the URL syntax, you can easily send data to your website!

In a sense, you will be having a URL that looks like this:

http://www.example.com/phpService.php?a1=val1&a2=val2&a3=val3

(If you try a Google Search, their URL will be somehow similar to the format above!)

We can denote a1, a2, a3 as three different variables, and val1, val2, val3 as their respective values. We will do the following tasks:

The cURL command

In order to use the curl command, we need to import the os module into Python. After which, we provide the necessary code to access our data (it may be an MySQL query, or another thing entirely). In this example, I will just provide arbitrary values for our data. We “package” the HTTP request nicely and then invoke the curl command. Here’s an example of how we can do it:

#Import necessary libraries
import os

#Write code here to access your data
val1 = 10
val2 = 20
val3 = 30

#Construct the cURL bash
http_address = "http://example.com/phpService.php?"
request_string = http_address + "a1=" + str(val1) + "&a2=" + str(val2) + "&a3=" + str(val3)

#Invoke the command
os.system('curl' + '"' + request_string + '"')

Including the os library is important so that the command will work. Your data may be stored inside your RPi’s local database, so just call the appropriate query and store those values inside your variables. Another note, you cannot concatenate integers and strings together, thus the reason for enclosing the val variables in an str() function.

Warning! My example here is a very simple one and is just used to demonstrate how the data transfer works. The problem with HTTP request is that your data is "exposed" in the transfer process, and may be prone to compromise. It is strongly recommended to encrypt one's data first before using the HTTP request.

Next we’ll be writing a PHP service that can understand the URL set by the curl command and thus give you an opportunity to do the necessary operations.

PHP Service to process the cURL command

<?php
print"<!DOCTYPE HTML PUBLIC \"-//W3C//DTD XHTML 1.0 Strict//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd\">
    <html xmlns="http://www.w3.org/1999/xhtml" xml:lang=\"en\" lang=\"en\">
        <head>
        </head>
        <body>&";

            if ($_SERVER['REQUEST_METHOD'] == "GET) {
               $storeVal1 = $_GET["a1"];
               $storeVal2 = $_GET["a2"];
               $storeVal3 = $_GET["a3"];
            //Now you have the values of val1, val2, val3 stored in storeVal1, storeVal2, storeVal3 respectively.
            }
        print "</body>;
?>

In web development jargon, what we just did is called an HTTP GET Request, if you’ve been scouring different programming forums, you might have read about the HTTP POST Request. Normally, I use POST requests on Forms (like in entering your Username and Password) and GET for simple services like this.

My own implementation of the curl bash command can be found here, while the PHP service can be found here. I first gathered the data from my RPI’s local database, and then send them to the web to be stored in the server database.