An application program interface (API) is a set of definitions and protocols that allows software programs to communicate with each other.

The term REST stands for representational state transfer. It is an architectural style that consists of a set of constraints to be used when creating web services.

RESTful API is an API that follows the REST architecture. Typically REST APIs use the HTTP protocol for sending and retrieving data and JSON formatted responses. You can use the standard HTTP methods to create, view, update, or delete resources through the API.

To test and interact with the RESTful APIs, you can use any library or tool that can make HTTP requests.

API requests are made up of four different parts:

• The endpoint. This is the URL that the client uses to communicate with the server.
• The HTTP method. It tells the server what action the client wants to perform. The most common methods are GET POST PUT DELETE and PATCH
• The headers. Used to pass additional information between the server and the client, such as authorization.
• The body. The data sent to the server.

In this article, we’re going to discuss how to use curl to interact with RESTful APIs. curl is a command-line utility for transferring data from or to a remote server. It is installed by default on macOS and most Linux distributions.

Curl Options #

The syntax for the curl command is as follows:

curl [options] [URL...]

Here are the options that we’ll use when making requests:

• -X, --request – The HTTP method to be used.
• -i, --include – Include the response headers.
• -d, --data – The data to be sent.
• -H, --header – Additional header to be sent.

HTTP GET #

The GET method requests a specific resource from the server.

GET is the default method when making HTTP requests with curl. Here is an example of making a GET request to the JSONPlaceholder API to a JSON representation of all posts:

curl https://jsonplaceholder.typicode.com/posts

To filter the results use query params:

curl https://jsonplaceholder.typicode.com/posts?userId=1

HTTP POST #

The POST method is used to create a resource on the server. If the resource exists, it is overridden.

The following command will create a new post using the data specified with the -d option:

curl -X POST -d "userId=5&title=Hello World&body=Post body." https://jsonplaceholder.typicode.com/posts

The type of the request body is specified using the Content-Type header. By default when this header is not given curl uses Content-Type: application/x-www-form-urlencoded

To send a JSON formatted data set the body type to application/json:

curl -X POST -H "Content-Type: application/json" \    -d '{"userId": 5, "title": "Hello World", "body": "Post body."}' \    https://jsonplaceholder.typicode.com/posts

HTTP PUT #

The PUT method is used to update or replace a resource on the server. It replaces all data of the specified resource with the request data.

curl -X PUT -d "userId=5&title=Hello World&body=Post body." https://jsonplaceholder.typicode.com/posts/5

HTTP PATCH #

The PUT method is used to make partial updates to the resource on the server.

curl -X PUT -d "title=Hello Universe" https://jsonplaceholder.typicode.com/posts/5

HTTP DELETE #

The DELETE method removes the specified resource from the server.

curl -X DELETE https://jsonplaceholder.typicode.com/posts/5

Authentication #

If the API endpoint requires authentication, you’ll need to obtain an access key. Otherwise, the API server will respond with the “Access Forbidden” or “Unauthorized” response message.

The process of obtaining an access key depends on the API you’re using. Once you have your access token you can send it in the header:

curl -X GET -H "Authorization: Bearer {ACCESS_TOKEN}" "https://api.server.io/posts"

Conclusion #

We’ve shown you how to use curl to make test API requests. For more information about curl, visit the Curl Documentation page.

If you have any questions or feedback, feel free to leave a comment.