You are tasked with developing a script to validate user input in a web application to prevent command injection attacks. How would you approach designing and implementing this script, considering the following requirements?

$$ The script should sanitise$[$TT$1]$ user input to remove any potentially malicious characters or commands.

$$ It should validate input against a whitelist of allowed characters or patterns to ensure that only safe input is accepted.

$$ The script should be implemented using a scripting language of your choice, such as Python, JavaScript, or Bash, depending on the application environment.

Provide a detailed explanation of your approach, including code snippets and examples, to demonstrate how you would address each requirement effectively.