Ruby CGI Programming

CGI (Common Gateway Interface) is a standard for interfacing external applications with information servers, such as HTTP or Web servers. In the context of web development with Ruby, you can use CGI to create dynamic web content.

Note: Ruby CGI is not commonly used in modern web development due to the rise of web frameworks like Ruby on Rails, Sinatra, etc., which provide a higher level of abstraction and more features. Nevertheless, understanding how to use Ruby CGI can be beneficial for learning purposes.

Here's a basic example of a CGI script in Ruby:

#!/usr/bin/env ruby

require 'cgi'

cgi = CGI.new

puts cgi.header
puts "<html><body>"
puts "<h1>Hello CGI</h1>"
puts "</body></html>"

Let's break down this script:

#!/usr/bin/env ruby: This is the shebang line, which tells the system to use Ruby to interpret this script.
require 'cgi': This loads the CGI library, which provides methods for handling CGI tasks.
cgi = CGI.new: This creates a new CGI object.
puts cgi.header: This prints out an HTTP header, which tells the browser that the server is returning an HTML document.
puts "<html><body>" and puts "</body></html>": These lines print out the start and end of an HTML document.
puts "<h1>Hello CGI</h1>": This prints out an HTML heading.

This script can be run from the command line or from a web server that supports CGI scripts. When run, it generates a simple HTML document with a single heading.

To use this with a web server, you'd typically put the script in the server's CGI directory, make sure the script has execute permissions (chmod +x scriptname.rb), and access it via a URL like http://yourserver.com/cgi-bin/scriptname.rb.

Remember that each server may have different requirements for running CGI scripts, and you may need to configure your server to allow running CGI scripts in the first place.

Important: When developing CGI scripts, ensure that your scripts are secure and do not expose any sensitive information or system resources. CGI scripts can be a potential security risk if not handled properly.

Getting started with CGI in Ruby:
- Description: CGI is a protocol for executing scripts on a web server. Ruby provides a CGI library to facilitate web programming.
- Code example (simple CGI script):
```
#!/usr/bin/env ruby

puts "Content-type: text/html\n\n"
puts "<html><body><h1>Hello, CGI in Ruby!</h1></body></html>"
```
Creating CGI scripts with Ruby:
- Description: CGI scripts in Ruby typically start with a shebang (#!/usr/bin/env ruby) and output headers followed by HTML content.
- Code example (basic CGI script):
```
#!/usr/bin/env ruby

puts "Content-type: text/html\n\n"
puts "<html><body><h1>Ruby CGI Script</h1></body></html>"
```
CGI environment variables in Ruby:
- Description: CGI scripts can access environment variables containing information about the request, such as query parameters.
- Code example (accessing query parameters):
```
#!/usr/bin/env ruby

puts "Content-type: text/html\n\n"
puts "<html><body>"
puts "<p>Query Parameter: #{ENV['QUERY_STRING']}</p>"
puts "</body></html>"
```

Form handling in Ruby CGI scripts:

Description: CGI scripts can handle HTML form submissions by accessing form data in the environment variables.

Code example (handling form submission):

#!/usr/bin/env ruby

puts "Content-type: text/html\n\n"
puts "<html><body>"
puts "<form method='post'>"
puts "<input type='text' name='user_input'>"
puts "<input type='submit' value='Submit'>"
puts "</form>"

user_input = ENV['REQUEST_METHOD'] == 'POST' ? gets : ''
puts "<p>User Input: #{user_input}</p>"
puts "</body></html>"

Working with cookies in Ruby CGI:

Description: CGI scripts can set and read cookies to maintain state between HTTP requests.

Code example (setting and reading cookies):

#!/usr/bin/env ruby

puts "Content-type: text/html"
puts "Set-Cookie: user_id=12345"
puts "\n\n"
puts "<html><body>"
puts "<p>Cookie Value: #{ENV['HTTP_COOKIE']}</p>"
puts "</body></html>"

Handling file uploads in Ruby CGI:

Description: CGI scripts can handle file uploads by accessing the uploaded file data in the environment variables.

Code example (handling file upload):

#!/usr/bin/env ruby

puts "Content-type: text/html\n\n"
puts "<html><body>"
puts "<form method='post' enctype='multipart/form-data'>"
puts "<input type='file' name='uploaded_file'>"
puts "<input type='submit' value='Upload'>"
puts "</form>"

uploaded_file = ENV['REQUEST_METHOD'] == 'POST' ? ENV['rack.input'].read : ''
puts "<p>File Content: #{uploaded_file}</p>"
puts "</body></html>"

Security considerations in Ruby CGI programming:
- Description: CGI scripts should be aware of security considerations such as input validation, avoiding code injection, and protecting against cross-site scripting (XSS).
- Code example (basic input validation):
```
#!/usr/bin/env ruby

user_input = ENV['QUERY_STRING']
sanitized_input = CGI.escapeHTML(user_input)

puts "Content-type: text/html\n\n"
puts "<html><body>"
puts "<p>Sanitized Input: #{sanitized_input}</p>"
puts "</body></html>"
```