#!/bin/ksh
#This shellscript is udp_sender.
#Read input from the program named as a commmand line argument.
#The first character of the program's name is the program's "port number".
#Turn the input input into one "UDP packet" of at most 6 bytes.
#The first two bytes will be decimal digits.
#The first byte is the length of the packet (at most 6).
#The second byte is the port number, taken from the command line argument.
#This leaves at most 4 bytes for the data.
#Sample use: udp_sender 0_host/0_progname_udp

if [[ $# -ne 1 ]]
then
	echo $0: requires program name as command line argument 1>&2
	exit 1
fi

if [[ ! -x $1 ]]
then
	echo $0: sorry, I have no permission to execute $1 1>&2
	exit 2
fi

if [[ ! ${1##*/} == [0-9]* ]]
then
	echo $0: program name $1 must begin with a digit 1>&2
	exit 3
fi

port=`echo ${1##*/} | sed 's/\(.\).*/\1/'`
$1 > /tmp/$$
length=`wc -c < /tmp/$$ | awk '{print $1}'`

if [[ $length -gt 4 ]]
then
	echo $0: packet can hold only at most 4 bytes of data 1>&2
	exit 4
fi

#Output the two-byte UDP header.
echo $((length + 2))'\c'
echo $port'\c'

cat /tmp/$$
rm /tmp/$$
exit 0