A pure Bash script to return a random *unused* ephemeral TCP port

ephemeral-port

#!/usr/bin/env bash

#
# ephemeral-port
#
# A script that returns a random *unused* TCP port within the IANA-suggested
# ephemeral port range of 49152 to 65535. Defaults to checking the localhost,
# but takes an optional hostname as an argument.
#
# See:
# - https://en.wikipedia.org/wiki/Ephemeral_port
# - http://mywiki.wooledge.org/BashFAQ/026
# - http://tldp.org/LDP/abs/html/devref1.html#DEVTCP
#

readonly HOST=${1:-127.0.0.1}

readonly FLOOR=49152
readonly RANGE=$((65535 - FLOOR + 1))

# Returns random number from 0 to ($1-1) in global var 'r'.
rand() {
	local max=$((32768 / $1 * $1))
	while (((r = RANDOM) >= max)); do :; done
	r=$((r % $1))
}

# Check if TCP connections to $HOST on port $1 are refused.
connection_refused() {
	! (echo "" >"/dev/tcp/${HOST}/${1}") &>/dev/null
}

while true; do
	rand $RANGE
	candidate=$((FLOOR + r))
	if connection_refused $candidate; then
		echo $candidate
		break
	fi
done