Design a basic CGI script using a programming language (e.g., Python, Perl) to capture user input from a form and display the result. Explain the script's key functions.

Below is a simple example of a CGI (Common Gateway Interface) script written in Python. This script captures user input from an HTML form and then displays the result back to the user.

### Basic CGI Script Example (Python)

1. **Create the HTML Form**:
   Before implementing the CGI script, we need an HTML form to collect user input. Save this HTML code as `form.html`.

```html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>User Input Form</title>
</head>
<body>
    <h1>Please Enter Your Name</h1>
    <form action="/cgi-bin/display.py" method="post">
        <label for="name">Name:</label>
        <input type="text" id="name" name="name" required>
        <input type="submit" value="Submit">
    </form>
</body>
</html>
```

2. **Create the CGI Script**:
   Now create a file named `display.py` in the `/cgi-bin/` directory of your web server. Here’s the CGI script:

```python
#!/usr/bin/env python3

import cgi
import cgitb

# Enable debugging
cgitb.enable()

print("Content-Type: text/html")    # HTTP Header
print()                              # Blank line indicates end of headers

# Create instance of FieldStorage
form = cgi.FieldStorage()

# Retrieve the input value from the form
name = form.getvalue('name')

# Display the result
print(f'''
<html>
<head>
    <title>User Input Result</title>
</head>
<body>
    <h1>Your name is: {name}</h1>
</body>
</html>
''')
```

### Key Functions Explained:

1. **Shebang Line (`#!/usr/bin/env python3`)**:
   - This line indicates that the script should be run using the Python 3 interpreter.

2. **Import Statements**:
   - `import cgi`: The `cgi` module provides functionality to handle form data.
   - `import cgitb`: This module helps in debugging CGI scripts by providing detailed error messages in the browser.

3. **Print HTTP Headers**:
   - The line `print("Content-Type: text/html")` specifies that the content being returned is HTML.
   - The following `print()` statement outputs a blank line, which is needed to indicate the end of headers.

4. **FieldStorage Instance**:
   - `form = cgi.FieldStorage()`: This creates an instance of `FieldStorage`, which is used to parse form data submitted in either GET or POST methods.

5. **Getting Form Data**:
   - `name = form.getvalue('name')`: This retrieves the value of the input field with the name `name` that was submitted in the form.

6. **Generating HTML Output**:
   - The script concludes with an HTML block that displays the user's input. The variable `name` is inserted into the HTML using an f-string.

### Running the Script:
1. Place the `form.html` file in the directory accessible by your web server.
2. Configure your web server to use the CGI directory (usually `/cgi-bin/`).
3. Access the `form.html` file through your web browser, enter your name, and submit the form.
4. The `display.py` script will be executed, and the result will be returned to the browser showing the name entered by the user.

### Note:
Make sure your web server has CGI enabled, and the script file (`display.py`) has permission to be executed (using `chmod +x display.py` in a UNIX-like system).