Curl (programming language) – Wikipedia
Not to be confused with cURL, a software library and command-line tool with a similar name.
CurlParadigmmulti-paradigm: object-oriented, markupDesigned bySteve Ward, MITDeveloperCurl, Inc., Sumisho Computer Systems Corp., SCSK CorporationFirst appeared1998Stable release8. 0. 10
/ 30 August 2019; 2 years agoTyping disciplinestrongOSLinux, MacOS, WindowsWebsite byHTML, JavaScript, Lisp
Curl at Wikibooks
Curl is a reflective object-oriented programming language for interactive web applications whose goal is to provide a smoother transition between formatting and programming. It makes it possible to embed complex objects in simple documents without needing to switch between programming languages or development platforms. The Curl implementation initially consisted of just an interpreter, but a compiler was added later.
Curl combines text markup (as in HTML), scripting (as in JavaScript), and heavy-duty computing (as in Java, C#, or C++) within one unified framework. It is used in a range of internal enterprise, B2B, and B2C applications.
Curl programs may be compiled into Curl applets, that are viewed using the Curl RTE, a runtime environment with a plugin for web browsers. Currently, it is supported on Microsoft Windows. Linux, and macOS was dropped on March 25, 2019 (starting with version 8. 8). [1] Curl supports “detached applets”, which is a web deployed applet which runs on the user’s desktop independent of a browser window much as in Silverlight 3 and Adobe AIR.
Architecture[edit]
The Curl language attempts to address a long-standing problem: the different building blocks that make up any modern web document most often require wildly different methods of implementation: different languages, different tools, different frameworks, often completely different teams. The final — and often most difficult — hurdle has been getting all of these blocks to communicate with each other in a consistent manner. Curl attempts to side-step these problems by providing a consistent syntactic and semantic interface at all levels of web content creation: from simple HTML to complex object-oriented programming.
Curl is a markup language like HTML—that is, plain text is shown as text; at the same time, Curl includes an object-oriented programming language that supports multiple inheritance. Curl applications are not required to observe the separation of information, style, and behavior that HTML, Cascading Style Sheets (CSS), and JavaScript have imposed, although that style of programming can be used in Curl if desired.
While the Curl language can be used as an HTML replacement for presenting formatted text, its abilities range all the way to those of a compiled, strongly typed, object-oriented system programming language. [2] Both the authoring (HTML-level) and programming constructs of Curl can be extended in user code. The language is designed so Curl applications can be compiled to native code of the client machine by a just-in-time compiler and run at high speed. Curl applets can also be written so that they will run off-line when disconnected from the network (occasionally connected computing). In fact, the Curl IDE is an application written in Curl.
Syntax[edit]
A simple Curl applet for HelloWorld might be
{Curl 7. 0, 8. 0 applet}
{text
color = “blue”,
font-size = 16pt,
Hello World}
This code will run if the user has at least one of the Curl versions 7. 0 or 8. 0 installed.
Curl provides both macros and text-procedures in addition to anonymous procedures and named methods.
An alternative using the text-procedure paragraph would be:
{paragraph
paragraph-left-indent=0. 5in,
{text color = “red”, font-size = 12pt,
Hello}
{text color = “green”, font-size = 12pt,
World}}
Recently this style of layout has been adopted by “builders” in the Groovy language for the JVM, but is also familiar to users of CSS or Tcl/Tk. Most features for web applications now implemented through combinations of JavaScript libraries + HTML + CSS are already found within the Curl language, including features usually associated with Prototype + such as accordion panes.
Curl sets callbacks in the manner also adopted by Groovy:
{CommandButton width=100pt,
height = 50pt,
label = {center {bold Invokes an event handler when clicked}},
control-color = “orange”,
|| Attach the following event handler to this CommandButton
{on Action do
{popup-message
title = “Your Message”,
“This is a user message dialog. “}}}
Curl comments use the vertical bar in several variations. The simplest is as follows:
{text A comment can be on a line by itself,
|| A comment on a line by itself
or it can be at the end || A comment on the same line as code
of a line. }
Curl as lightweight markup[edit]
Because Curl provides for both user-defined text procedures and stylesheets, Curl can be used readily as domain-specific lightweight markup. A major advantage over plain text HTML markup is that the text encoding can be set to UTF-8, and text entered in a Unicode-enabled editor without any escaping of characters (like JavaScript, Curl is Unicode friendly). A poetry example would be:
{poem || wraps entire poem
{stanza || first verse here in any language}
{stanza || another verse here in any language}}
which can initially be implemented by defining the poem and stanza markup as paragraph text formats. Stanza could be further refined to include a hidden navigation anchor for page navigation using the Curl {destination} which is itself a text procedure.
The same markup can be used for different results, as one can style text to be visible in one context and invisible in another. Curl also permits top-level file inclusion so that a source text in markup can be included in different parent files. In education, for example, one could create a source file of test questions, and include it in both a student and a teacher version of the text.
See also[edit]
Homoiconicity: Curl is both a programming language and a data format[citation needed]
References[edit]
^ “Changes to Supported Operating Systems of Curl RTE Products”. SCSK Corporation. Archived from the original on 2019-05-13. Retrieved 2020-04-01.
^ M. Hostetter; D. Kranz; C. Seed; C. Terman; S. Ward. “Curl: A Gentle Slope Language for the Web”. MIT Laboratory for Computer Science. Archived from the original on 2008-12-24. Retrieved 2005-12-20.
External links[edit]
Wikibooks has a book on the topic of: Curl
MIT Curl Project, the results of the Curl research project at MIT’s Laboratory for Computer Science.
Curl, Inc. and Curl International Corp. develop and distribute a suite of commercial products which are based on the results of the MIT research. Owned by SCSK Corporation a majority-owned unit of Sumitomo Corp.
InfoWorld review of Curl 6. 0 by Martin Heller. InfoWorld awarded Curl its 2008 Technology of the Year award in the Rich Internet Application category.
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 command in Linux with Examples – GeeksforGeeks
curl is a command line tool to transfer data to or from a server, using any of the supported protocols (HTTP, FTP, IMAP, POP3, SCP, SFTP, SMTP, TFTP, TELNET, LDAP or FILE). curl is powered by Libcurl. This tool is preferred for automation, since it is designed to work without user interaction. curl can transfer multiple file at [options] [URL… ]URL: The most basic uses of curl is typing the command followed by the This should display the content of the URL on the terminal. The URL syntax is protocol dependent and multiple URLs can be written as sets like:curl one, two, three}
URLs with numeric sequence series can be written as:curl 1-20]
Progress Meter: curl displays a progress meter during use to indicate the transfer rate, amount of data transferred, time left -# -o curl –silent If you like a progress bar instead of meter, you can use the -# option as in the example above, or –silent if you want to disable it completely. Example:Options:-o: saves the downloaded file on the local machine with the name provided in the -o [file_name] [URL… ]
Example:curl -o Output:The above example downloads the file from FTP server and saves it with the name: This option downloads the file and saves it with the same name as in the -O [URL… ]
Example:curl -O –: This option resumes download which has been stopped due to some reason. This is useful when downloading large files and was -C – [URL… ]
Example:curl -C – -O limit-rate: This option limits the upper bound of the rate of data transfer and keeps it around the given value in –limit-rate [value] [URL]
Example:curl –limit-rate 1000K -O Output:The command limits the download to 1000K bytes. -u: curl also provides options to download files from user authenticated FTP -u {username}:{password} [FTP_URL]
Example:curl -u demo:password -O: This option helps to upload a file to the FTP -u {username}:{password} -T {filename} {FTP_Location}
If you want to append a already existing FTP file you can use the -a or –append option. –libcurl:This option is very useful from a developers perspective. If this option is appended to any cURL command, it outputs the C source code that uses libcurl for the specified option. It is the code similar to the command line [URL… ] –libcurl [filename]
Example:curl > –libcurl code. c
Output:The above example downloads the HTML and saves it into and the code in code. c file. The next command shows the first 30 lines of the code. -x, –proxy: curl also lets us use a proxy to access the -x [proxy_name]:[port] [URL… ]
If the proxy requires authentication, it can be used with the command:curl -u [user]:[password] -x [proxy_name]:[port] [URL… ]
Sending mail: As curl can transfer data over different protocols, including SMTP, we can use curl to send –url [SMTP URL] –mail-from [sender_mail] –mail-rcpt [receiver_mail] -n –ssl-reqd -u {email}:{password} -T [Mail text file]DICT protocol: The Libcurl defines the DICT protocol which can be used to easily get the definition or meaning of any word directly from the command [protocol:[dictionary_URL]:[word]
Example:curl dict
Output:Note: There are a number of other options provided by cURL which can be checked on the man page. The Libcurl library has been ported into various programming languages. It’s advisable to visit the individual project site for documentation.
Frequently Asked Questions about curl programming
What is a cURL in programming?
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 does the cURL command do?
curl is a command line tool to transfer data to or from a server, using any of the supported protocols (HTTP, FTP, IMAP, POP3, SCP, SFTP, SMTP, TFTP, TELNET, LDAP or FILE).May 15, 2019
How do I run cURL commands?
Testing your cURL installationLaunch your command-line interface. In Windows, open the Start menu, type cmd in the search box, and press Enter. … Copy the cURL statement from your text file and paste it at the command prompt. … Press Enter to run the cURL statement.