This is a trivial example, but is meant to demonstrate how you can pipe arguments to a bash function.

#!/usr/bin/env bash

function lowercase()
{
	# Grab input.
	declare INPUT=${1:-$(</dev/stdin)};

	# Use that input to do anything.
	echo "$INPUT" | tr '[:upper:]' '[:lower:]'
}

echo "HELLO there, FRIEND!" | lowercase

Which outputs the following.

hello there, friend!