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What is cURL and how does it relate to APIs?

cURL, which stands for client URL, is a command line tool that developers use to transfer data to and from a server. At the most fundamental, cURL lets you talk to a server by specifying the location (in the form of a URL) and the data you want to send. cURL supports several different protocols, including HTTP and HTTPS, and runs on almost every platform. This makes cURL ideal for testing communication from almost any device (as long as it has a command line and network connectivity) from a local server to most edge devices.

The most basic command in curl is curl http://example.com. The curl command is followed by the URL, from which we would like to retrieve some kind of data. In this case, it would return the html source for example.com.

Underlying the curl command is the libcurl development library, which has bindings for almost any codebase.

cURL is also the name of the software project, which encompasses both the curl command-line tool and the libcurl development library.

Prerequisites

To try out the commands in this article, you need a command shell and internet access. We are going to be using the NASA APOD (Astronomy Picture Of the Day) API to create some examples. This API is open source but you will need to sign up for a developer key, which takes just a minute to get signed up.

Why use curl?

So, why should you use cURL? Consider these benefits of this software project:

  • It is highly portable. It is compatible with almost every operating system and connected device.
  • It is useful for testing endpoints, to check if they are working.
  • It can be verbose, providing details of exactly what has been sent/received, which is helpful for debugging.
  • It has good error logging.
  • It can be rate limited.

Sending API requests

We can use curl to send API requests. Each request is generally made up of four main parts:

  • An endpoint, which is the address (URL) to which we are sending the request.
  • An HTTP method. The most common methods used are GET, POST, PUT and DELETE.

    • GET is used to retrieve a resource from a server. This could be a file, information, or an image.
    • POST is used to send information to the server.
    • PUT can be used to create or update a resource. This could be used to create or update a record in a database or update the contents of a file.
    • DELETE is used to delete a resource such as a database entry.

      These actions for these methods are the recommended actions, but it’s up to the API specification and implementation to define what exactly happens.

  • Headers, which contain metadata about the request, such as content type, user agent, and so on.

  • Body, which is the message body and contains the data that we want to send, if any. Generally, the body is used with POST and PUT methods.

curl command options

There are over two hundred curl options. You can see some of them by typing curl -h in a terminal. The most commonly used command options include these:

  • -I returns only the HTTPS headers

    curl --request GET 'https://api.nasa.gov/planetary/apod?api_key=<myapikey>&date=2020-01-01' -I

    This command will return header fields such as Date, Content-Type etc

    Screen capture showing the result of a curl command for only HTTPS headers

  • -v is the verbose option

    curl --request GET 'https://api.nasa.gov/planetary/apod?api_key=$NASA_API_KEY&date=2020-01-01' -v

    This verbose command will show you everything that happens when you run the curl command, from connection to the headers and any data returned. Here we also get the description of the image that is being returned by the request, along with the image url.

    Screen capture showing the result of a curl command for only HTTPS headers

  • -o stores the output in a file

    curl --request GET 'https://api.nasa.gov/planetary/apod?api_key=$NASA_API_KEY&date=2020-01-01' --output curloutput

Combining curl with other CLI commands

Combining curl with other cli commands can be really handy in situations where you want to use the output of a command as the input to a curl command or vice versa.

As an example, you could see if a webpage contains a certain piece of text using curl and grep.

Here is an example of using curl and Python to extract the image link from a request to the NASA API and display it in the Preview app (MacOS only):

curl --request GET "https://api.nasa.gov/planetary/apod?api_key=$NASA_API_KEY&date=2020-01-01" -s | python3 -c "import sys, json; print(json.load(sys.stdin)['url'])" | xargs curl -o NASApicture.jpg && open -a Preview NASApicture.jpg

In this example we use curl to make a GET request on the Nasa API endpoint. This returns json data, which we use in a small Python script to extract the url of the image. We then use the curl command to get the image and open it using Preview on the mac.

Screen capture showing the image returned from a curl and python command combined

Useful tools for making API calls

You don’t have to use the command line curl to make API requests. You can use a number of different tools to interact with an API, such as HTTPie, Postman, and Rest Client in VS Code.

HTTPie

HTTPie is a command-line HTTP client that is touted as more friendly to users. It also includes a more expressive, color-coded UI. They have an online version, which is really neat.

Postman

Postman is a UI-based client for all things related to API development, and it’s arguably one of the most popular. Postman is very powerful.

You can generate and execute curl commands from within Postman. To generate curl commands, you can enter the request URL and parameters, and then click on the code option on the right-hand side:

Screen capture showing the Postman URL and code option highlighted

A box is displayed with the option to select from a number of languages, including curl. Select curl to see the generated curl command.

Screen capture showing generated curl command in Postman

Postman gives you a history of all the requests that you’ve built and even data-stamps them, which can be nice for collecting data points or references as you work with an API. Think of it as the client taking notes for you.

Screen capture showing postman request history

Rest Client in VS Code

Rest Client for VS Code is probably one of my favorite tools for executing curl commands. It’s lightweight and has good syntax highlighting. It’s a really useful add-on to do some quick curl requests from within VS Code.

You can simply type in your curl command and a ‘send request’ option will appear above.

Screen capture of a curl command request in VS Code

After you click send request, another tab opens with the response.

Summary and Next Steps

In this article, we introduced you to the basic curl command and its most useful options. We also mentioned only a handful of the tools that are available to help you get started with cURL. Now you can begin using cURL to test your endpoints and troubleshoot your applications.

To find out more about APIs and API Management, check out the following articles and videos available on IBM Developer:

Acknowledgements

This article was originally authored by Amara Graham and published in April of 2019.