Loop through list of trackers and test if they are alive using nc

loop.sh

#!/bin/bash

##
# Loop through list of trackers and test if they are alive using nc
# nc -v -z -w 3 -u "$host" "$port"
##

usage () {
    printf '\n%s\n' "Loop through list of trackers and test if they are alive using nc"
	printf '\n%s\n\n' "Usage: $(basename "$0") [list-of-trackers.file]"
	exit
}

if [[ ! -f $1 ]]; then
	usage
fi


while read -r line; do

    FIELDS=($(echo $line | grep -o "[a-z0-9.-]*" | tr '\n' ' '))

	if [[ ${FIELDS[0]} == "udp" ]]; then
		# set udp flag if protocol is udp
		args="-z -v -w 1 -u"
	else
		args="-z -v -w 1"
	fi
	
	if [[ ! ${FIELDS[1]} ]]; then
		 echo '-'
	else
		if nc $( echo "$args ${FIELDS[1]} ${FIELDS[2]}"); then
		    echo "$line" >> alive.txt
		fi
	fi
	
done <"$1"