Understand cURL more

cURL is a cross-platform way to show requests and responses

Before moving on, let's pause a bit and learn more about cURL.

One of the advantages of REST APIs is that you can use almost any programming language to call the endpoint. The endpoint is simply a resource located on a web server at a specific path.

Each programming language has a different way of making web calls. Rather than exhausting your energies trying to show how to make web calls in Java, Python, C++, JavaScript, Ruby, and so on, you can just show the call using cURL.

cURL provides a generic, language agnostic way to demonstrate HTTP requests and responses. Users can see the format of the request, including any headers and other parameters. Your users can translate this into the specific format for the language they're using.

Almost every API shows how to interact with the API using cURL.

REST APIs follow the same model of the web

One reason REST APIs are so familiar is because REST follows the same model as the web. When you type an http address into a browser address bar, you're telling the browser to make an HTTP request to a resource on a server. The server returns a response, and your browser converts the response to a more visual display. But you can also see the raw code.

Try using cURL to GET a web page

To see an example of how cURL retrieves a web resource, open a terminal and type the following:

curl http://example.com

You should see all the code behind the site example.com. The browser's job is to make that code visually readable. cURL shows you what you're really retrieving.

Requests and responses include headers too

When you type an address into a website, you see only the body of the response. But actually, there's more going on behind the scenes. When you make the request, you're sending a header that contains information about the request. The response also contains a header.

1. To see the response header in a cURL request, include -i in the cURL request:

   curl http://example.com -i
   

   The header will be included above the body in the response.

2. To limit the response to just the header, use -I:

   curl http://example.com -I
   

   The response header is as follows:

   HTTP/1.1 200 OK
   Accept-Ranges: bytes
   Cache-Control: max-age=604800
   Content-Type: text/html
   Date: Fri, 19 Jun 2015 05:58:50 GMT
   Etag: "359670651"
   Expires: Fri, 26 Jun 2015 05:58:50 GMT
   Last-Modified: Fri, 09 Aug 2013 23:54:35 GMT
   Server: ECS (rhv/818F)
   X-Cache: HIT
   x-ec-custom-error: 1
   Content-Length: 1270
   

   The header contains the metadata about the response. All of this information is transferred to the browser whenever you make requests to URLs (that is, when you go to web pages), but the browser doesn't show you this information. You can see it using the Chrome Developer Tools console if you look on the Network tab.

3. Now let's specify the method. The GET method is the default, but we'll make it explicit here:

   curl -X GET http://example.com -I
   

   When you go to a website, you submit the request using the GET HTTP method. There are other HTTP methods you can use when interacting with REST APIs. Here are the common methods used when working with REST endpoints:

   | HTTP Method | Description | |-----|------| | POST | Create a resource | | GET | Read a resource | | PUT | Update a resource | | DELETE | Delete a resource |

   GET is used by default with cURL requests. If you use cURL to make HTTP requests other than GET, you need to specify the HTTP method.

Unpacking the weather API cURL request

Let's look more closely at the request you submitted for the weather:

  curl --get --include 'https://simple-weather.p.mashape.com/aqi?lat=37.354108&lng=-121.955236' \
  -H 'X-Mashape-Key: APIKEY' \
  -H 'Accept: text/plain'

cURL has shorthand names for the various options that you include with your request. The \ just creates a break for a new line for readability. (Don't use \ in Windows.)

Here's what the commands mean:

Most cURL commands have a couple of different representations. --get can also be written as -X GET.

Query strings and parameters

The latitude (lat) and longitude (lng) parameters were passed to the endpoint using "query strings." The ? appended to the URL is the query string where the parameters are passed to the endpoint:

?lat=37.354108&lng=-121.955236

After the query string, each parameter is concatenated with other parameters through the & symbol. The order of the parameters doesn't matter. The order only matters if the parameters are part of the URL path itself (not listed after the query string).

Common cURL commands related to REST

cURL has a lot of possible commands, but the following are the most common when working with REST APIs.

See the cURL documentation for a comprehensive list of cURL commands you can use.

Example cURL command

Here's an example that combines some of these commands:

curl -i -H "Accept: application/json" -X POST -d "{status:MIA}" http://personsreport.com/status/person123

We could also format this with line breaks to make it more readable:

curl -i \
     -H "Accept: application/json" \
     -X POST \
     -d "{status:MIA}" \
     http://personsreport.com/status/person123 \

(Of course line breaks are problematic on Windows, so I don't recommend formatting cURL requests like this.)

The Accept header instructs the server to process the post body as JSON.

Test your memory

Fill in the blanks to see how much you remember:

• -i means...
• -H means...
• -X POST means...
• -d means...

See the cURL parameters on the answer page to check your responses.

When you use cURL, the terminal and iTerm on the Mac provide a much easier experience than using the command prompt in Windows. If you're going to get serious about API documentation but you're still on a PC, consider switching. There are a lot of utilities that you install through a terminal that just work on a Mac.

More Resources

To learn more about cURL with REST documentation, see REST-esting with cURL.