Bash function to construct a request URL per the Metadata Query Protocol specification

construct_mdq_url

# Construct a request URL per the MDQ Protocol specification
# See: https://github.com/iay/md-query
# To construct a reference to ALL entities served by the 
# metadata query server, simply omit the second argument
construct_mdq_url () {
	# construct_mdq_url <base_url> <url_encoded_id>

	# make sure there are one or two command-line arguments
	if [ $# -lt 1 -o $# -gt 2 ]; then
		echo "ERROR: $FUNCNAME: incorrect number of arguments: $# (1 or 2 required)" >&2
		return 2
	fi
	local base_url=$1
	
	# strip the trailing slash from the base URL if necessary
	local length="${#1}"
	if [[ "${base_url:length-1:1}" == '/' ]]; then
		base_url="${base_url:0:length-1}"
	fi
	
	# append the identifier if there is one
	if [ $# -eq 2 ]; then
		echo "${base_url}/entities/$2"
	else
		echo "${base_url}/entities"
	fi
}