In Curl
What is the curl command? Learning and testing APIs with …
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. 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
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 ‘
This command will return header fields such as Date, Content-Type etc
-v is the verbose option
curl –request GET ‘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.
-o stores the output in a file
curl –request GET ‘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 “NASA_API_KEY&date=2020-01-01” -s | python3 -c “import sys, json; print(()[‘url’])” | xargs curl -o && open -a Preview
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.
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:
A box is displayed with the option to select from a number of languages, including curl. Select curl to see the generated curl command.
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.
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.
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:
API Fundamentals
What is a REST API
Acknowledgements
This article was originally authored by Amara Graham and published in April of 2019.
cURL – Vertica
cURL is a command-line tool and application library used to transfer data to or from a server. It uses URL syntax, such as HTTP and HTTPS. All API requests sent to a Vertica server must be made using HTTPS. A request made using HTTP will fail.
There are four HTTP requests that can be passed using cURL to call API methods:
Request
Description
GET
Retrieves data.
PUT
Updates data.
POST
Creates new data.
DELETE
Deletes data.
Syntaxcurl
Options
-h –help
Lists all available command-line options.
-H –header
Allows you to use custom headers in your command. This is useful for sending a request that requires a VerticaAPIKEY.
curl -H “VerticaApiKey: ValidAPIKey”
-k –insecure
Allows SSL connections without certificate validation.
curl -k
-X –request
Specifies the custom request used when communicating with a server.
curl -X REQUEST
Can be one of the following values:
If no request is specified, cURL automatically defaults to the GET request.
There are many more options available to add to your cURL query. For a comprehensive list with descriptions, visit the cURL Documentation Website.
What is the curl command? Learning and testing APIs with …
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. 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
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 ‘
This command will return header fields such as Date, Content-Type etc
-v is the verbose option
curl –request GET ‘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.
-o stores the output in a file
curl –request GET ‘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 “NASA_API_KEY&date=2020-01-01” -s | python3 -c “import sys, json; print(()[‘url’])” | xargs curl -o && open -a Preview
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.
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:
A box is displayed with the option to select from a number of languages, including curl. Select curl to see the generated curl command.
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.
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.
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:
API Fundamentals
What is a REST API
Acknowledgements
This article was originally authored by Amara Graham and published in April of 2019.
Frequently Asked Questions about in curl
What is in curl for?
cURL is a command-line tool and application library used to transfer data to or from a server. It uses URL syntax, such as HTTP and HTTPS.
What is meaning of in curl command?
Updated February 23, 2021 | Published April 4, 2019. 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.Feb 23, 2021
What is in curl request?
Client for URLs (or cURL) is a software project comprised of two development efforts – cURL and libcurl. libcurl is a free, client-side URL transfer library with support for a wide range of protocols. cURL is a command-line tool for getting or sending files using URL syntax. …
