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What Is cURL And How Does It Relate To APIs?

What Is cURL And How Does It Relate To APIs

Better public APIs and tools are being released for web development constantly, and cURL is one such tool. Even if you haven’t heard of cURL before, or just have a basic understanding of what an API is, this article will provide you with a thorough grasp of ‘what is cURL’.

What Is cURL?

What is curl?

cURL stands for client URL. Developers carry out data transfer to and from servers using cURL. At the most fundamental level, cURL enables you to interact with a server by specifying the location and the data you wish to send. It has access to a wide range of protocols, such as HTTP, SCP, IMAP, FTP server, SFTP, POP3, SMTP, LDAP, SMB, and many others.

It’s a helpful command line tool for the typical system administrator, whether you use it to script automatic updates or as a quick way to get a file that you need from the Internet. cURL is a valuable tool for evaluating remote APIs as well. Making web calls is done differently in each programming language. You may just demonstrate the call using cURL, rather than expending all of your energy attempting to demonstrate in every language.

cURL’s most fundamental command is curl https://apilayer.com. The URL from which we want to retrieve some form of data follows the curl command. In this instance, it would retrieve apilayer’s HTML source.

What Can I Do With cURL? 

There are hundreds of commands we can use with cURL. However, before we get into details of what we can do with cURL, we need to understand the fundamentals of sending an API request. 

Each request typically consists of the following four components: endpoint, HTTP method, header, and body. An endpoint is a location (URL) to which the request is being sent. Headers include information about the request, such as the content type and user agent. The body is the actual message, and any data that you send contains it. The body is typically used with PUT and POST requests.

Get An API Endpoint

The basic cURL commands can technically be used to query an API endpoint. You should ideally receive the API’s default response when you direct cURL at the API gateway URL. Not all APIs offer a response; however, the following is a decent example:

$ curl “https://apilayer.com/api/v4/projects” | less

This request retrieves a JSON data dump of a list of recently completed projects from APILayer, which you can then pass through a pager or parser like less.

Get HTML Headers

The initial data sent over HTTP includes headers, which are components. Although they are not visible during normal web interactions, headers frequently contain crucial data, such as the server response code.

With curl, you can view everything:

$ curl –head –show-error “https://apilayer.com”
HTTP/2 200
accept-ranges: bytes
age: 485487
cache-control: max-age=604800
content-type: text/HTML; charset=UTF-8
date: Wed, 12 Oct 2022 09:05:07 GMT
etag: “3147526947”
expires: Wed, 16 Oct 2022 09:05:07 GMT
last-modified: Wed, 16 Oct 2022 09:05:07 GMT
server: ECS (sjc/4E76)
x-cache: HIT
content-length: 1256

Use —fail-early to force cURL to fail in order to achieve faster response times, rather than wasting cycles trying to contact unresponsive servers.

Adding An HTTP Header

cURL HTTP can also be helpful when we have to add a new HTTP header to GET requests. To do so, just add an -H to the header name and its value within quotes.

curl -H “X-Header: Value” https://www.apilayer.com/

Resume A Download

We can continue to download files using the curl command. To do this, add a -c to the curl command, and the system will do the rest for you.

curl -C – -O https://apilayer.com/img/api-stats.png

Make API Calls To A Service

Due to the frequent URL structuring of web API interactions, complex requests can be sent as either a lengthy URL or by adding raw data strings to the target URL. This typically means that after a —data flag, you should enclose the text in quotes, rather than the “?” delimiter in complicated URLs. 

$ curl –request PUT \
–header “PRIVATE-TOKEN: your_access_token_here” \
–data “long=true” \
–data “per_page=10” \

Send An HTTP Header 

In most cases, an API gives you an authorization token, rather than requiring login credentials for regular use. In that situation, you must identify your token while submitting requests. An API frequently checks the header data of what you deliver for authorization:

$ curl –header “Authorization: Bearer F66eE5faXEp2Y” \

The Verbose Command

With a verbose command, you may see every step of the cURL process, from connection through headers to any data returned.

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

What Are The Benefits Of Using cURL?

  • cURL is compatible with every OS and connected device.
  • The error logging mechanism is highly beneficial.
  • If you need to test your endpoints, cURL is the man you are looking for.

When To Use cURL?

Suppose that a script requires access to data from a remote system, as if it were a local file or data stream. In that case, we can use file cURL because it supports connectivity to remote networks over HTTP, HTTPS, TELNET, and a wide range of additional supported protocols. This is crucial for developers who want to get data from a distant system and display it exactly as they did it locally. You can use cURL at a terminal or command prompt. However, you must first make sure your system has it installed before attempting to use it.

What Are The Alternatives To cURL?


Everything we require for API testing can be provided by Postman, including basic HTTP requests, authentication methods, system testing, and custom scripts. If you need to know all the levels your request and answer are passing through, their request analysis capabilities might be helpful.

Postman is an all-encompassing API testing tool, thanks to capabilities like cloud synchronization and sharing requests between users.

VSCode Rest Client

The extension plugin for Microsoft’s VSCode code editor is called VSCode Rest Client. It essentially enables HTTP call testing by adding just a few lines of curl-like syntax to a http file.

The main benefit of this plugin is that you can test all endpoints and connections without ever leaving the code editor. You most likely won’t need to have an external HTTP client program installed on your laptop if you employ VSCode to develop your apps and APIs.


An API designer tool and an API client are the two major components of insomnia.

Due to its versatility with custom plugins, compatibility for multiple response formats like PDF and pictures, and certificate management tools for SSL, Insomnia API Client is quite promising.

Why Should You Use APIs From APILayer?

APILayer is a tightly controlled API marketplace with a focus on dependability, scalability, and quality. Additionally, it enables API developers to monetize their APIs with just one API key. Learn more about what is an API key.

With more than 15 years of experience in the design and implementation of various APIs, APILayer is a leading participant in the API market. In addition to having access to exclusive premium private APIs through the APILayer marketplace, developers may promote and make money from their own unique APIs. The APILayer marketplace aims to improve service and data APIs’ usability and accessibility. Developers of all levels can create scalable apps and incorporate API features in as little as 10 minutes.

Start using APILayer by creating a free account!

Frequently Asked Questions (FAQs)

What is cURL used for?

It can transfer data to and from servers within the URL syntax.

Why is it called cURL?

cURL stands for Client URL

What is cURL in REST API?

Use the cURL commands from this article to execute your REST APIs. If you’re feeling more ambitious, check out some REST API interview questions to ace your next interview!

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