#!/bin/bash

Usage()
{
    echo -e "\nUsage: $(basename "$0") [-a|-b|-c]"
    echo "Anything else you'd like to say, explain arguments, etc."
}

# Somewhere soon we do argument parsing and hit -h|--help:
Usage           # Yes, it's that simple.

# Or find an invalid argument:
Usage           # Still easy, and now we've reused code.