What is Common Gateway Interface (CGI)?

CGI (Common Gateway Interface) is a protocol or technology used in web development to enable communication between a web server and external programs or scripts. Its primary purpose is to facilitate dynamic content generation and user interaction on websites. CGI serves as a bridge that allows web servers to execute these external programs in response to HTTP requests, leading to the creation of dynamic web pages and the ability to interact with databases and other resources.

Dissecting Common Gateway Interface (CGI)

The CGI emerged in the early 1990s, transforming the static web into an interactive platform. It was developed to introduce interactivity and dynamic content to web pages during a time when the web was primarily static HTML documents. Collaborative efforts by individuals and organizations, including Rob McCool, the author of the NCSA HTTPd web server, led to CGI's creation. The CGI specification was formalized by the NCSA and CERN, defining rules for web servers to communicate with external programs.

CGI's primary purpose was to enable web servers to execute external scripts in response to HTTP requests, ushering in dynamic web content and interactive applications. Prior to CGI, the web was static, lacking the ability to process user input, generate real-time content, or interact with databases.

How CGI works

To enable web servers to execute external scripts in response to HTTP requests, thereby allowing for the dynamic generation of web content and interaction with databases and other resources, CGI must perform the following:

1. Web Server Configuration: The web server (e.g., Apache, Nginx) is configured to recognize certain directories or file extensions (e.g., .cgi, .pl) as CGI scripts. A directory is designated as the CGI directory where CGI scripts are stored.

2. User Request: A user initiates an HTTP request by accessing a URL that corresponds to a CGI script. This request can be triggered by clicking a link, submitting a form, or directly accessing the URL in a web browser.

3. Web Server Recognition: The web server receives the HTTP request and checks whether the requested URL matches any configured CGI patterns or file extensions.

4. CGI Script Execution: If the server recognizes the request as a CGI request, it launches the corresponding CGI script. The web server sets up a series of environment variables containing information about the HTTP request. These environment variables include details such as the request method (GET or POST), query parameters, HTTP headers, and more. The CGI script can access these environment variables to gather information about the request.

5. Processing by CGI Script: The CGI script processes the information from the environment variables and performs actions based on the request. These actions can include:

• Generating dynamic content, such as HTML, based on user input or data retrieved from databases.

• Handling form submissions, processing user input, and validating data.

• Executing calculations or computations and returning the results.

6. Output Generation: After processing, the CGI script generates an HTTP response, which typically consists of two parts:

• HTTP headers: These headers include information such as the content type (e.g., Content-Type: text/html), cookies, and other relevant metadata.

• Content body: The content body contains the actual data or HTML content that is sent back to the client's web browser.

7. Response to Client: The web server sends the complete HTTP response, including the headers and content, back to the client's web browser.

8. Client Rendering: The client's web browser receives the response and renders the content accordingly. This may involve displaying the dynamic web page, processing form results, or presenting the computed data to the user.

9. Cleanup: After handling the request, the CGI script typically exits, and any resources it used are released. This ensures that subsequent requests can be handled efficiently.

CGI Scripting Languages

CGI scripts can be written in a variety of programming languages. The choice of language often depends on the specific requirements of the web application, the functionality needed, and the preferences or expertise of the developer. Some of the most commonly used languages for CGI scripting include:

• Perl: This language established itself as a go-to for CGI scripting in the web's nascent years, thanks to its exceptional text manipulation abilities. Its rich standard library, including the CGI.pm module, significantly streamlines tasks like form parsing and response generation. A notable feature is the Taint mode, enhancing security by scrutinizing data origins and mitigating vulnerabilities.

• Python: Renowned for its simplicity and readability, Python is a popular choice across various developer skill levels. The language includes CGI and HTTP modules in its standard library, facilitating efficient server interactions and content generation. Beyond basic scripts, Python's compatibility with web frameworks like Django and Flask enables the development of intricate web applications.

• PHP: Typically integrated directly into web servers, PHP also operates effectively in CGI mode. Designed with a focus on web development, it incorporates functions for managing HTTP requests and seamlessly generating HTML. The ease of embedding PHP code within HTML aids in the rapid creation of dynamic web pages, making it a favorite among web developers.

• Ruby: Celebrated for its elegant syntax, Ruby offers a user-friendly CGI scripting environment. The language's emphasis on readability and simplicity reduces web programming complexity. With CGI support in its standard library and the extension of capabilities through Ruby on Rails, Ruby is adept at developing advanced web applications.

• Shell Script: Bash and other shell scripting languages provide a minimalist approach to CGI scripting, especially in Unix/Linux settings. Ideal for basic CGI tasks like simple form handling and system automation, shell scripts are less appropriate for more complex web applications due to processing and security limitations.

• C and C++: Selected for performance-critical CGI scripting, C and C++ deliver unmatched speed and system-level access. These languages, while powerful, necessitate diligent management of security and memory, as they lack the inherent safeguards found in higher-level languages.

• Tcl: Tcl, or Tool Command Language, is valued for its straightforwardness and usability, particularly in smaller CGI applications. Its uncomplicated syntax and sufficient functionality for standard CGI operations like form processing make it a viable option. Tcl's ability to integrate smoothly with various programming languages and systems further enhances its utility.