Squeaky clean pattern for Bash script with positional arguments

bestargs.sh

#!/bin/bash

HELP="bestargs.sh <arg1> <arg2> [ <arg3> ...]

Bash script positional arguments can be handled 
with simple, one-line-per-argument statements. No 
ifs, whiles, exits, or getopts required.

This makes use of some lesser-known parameter expansion
options in bash, :, ?, :-. The examples below are just a 
start. In their current form, an empty argument ('') is 
treated as if it were not set at all. They could be tweaked
to allow null arguments, or require arguments of minimum 
length, for example.
More details found in the bash manual:
http://tldp.org/LDP/abs/html/parameter-substitution.html

"

# Start of example:
ARG1=${1:?"${HELP}Error: arg1 is required"}    # Required args will show help, error msg,
ARG2=${2:?"${HELP}Error: arg2 also required"}  # and also exit 1 if they are not given.
ARG3=${3:-defaultvalue}                        # Optional args can have default values
ARG4=${4:-$(whoami)}                           # Optional arg with a subshell returning the default
ARG57=${@:5:3}                                 # Select a range of args (next three args starting at 5)
ARG8=${8:-arg8defaultvalue}                    # Another option arg with a default

echo "ARG1: ${ARG1}"
echo "ARG2: ${ARG2}"
echo "ARG3: ${ARG3}"
echo "ARG4: ${ARG4}"
echo "ARG5-7: ${ARG57}"
echo "ARG8: ${ARG8}"