Basics of TCL scripting. UDP agent is used as example. check http://csis.bits-pilani.ac.in/faculty/murali/resources/tutorials/ns2.htm

basic.tcl

#create ns 
set ns [ new Simulator ]

#output file
set outputfile [ open out.nam w ]
$ns namtrace-all $outputfile

#trace file
set tracefile [ open out.tr w ] 
$ns trace-all $tracefile

#node creation
set n0 [ $ns node ] 
set n1 [ $ns node ]

#link creation 
$ns duplex-link $n0 $n1 1mb 10ms DropTail

#agent creation 
set udp0 [ new Agent/UDP ] 
$ns attach-agent $n0 $udp0

#udp ends in a null agent
set null0 [ new Agent/Null ] 
$ns attach-agent $n1 $null0

#connect UDP n Null agent
$ns connect $udp0 $null0

#set color for UDP agent 
$udp0 set class_ 1 red

#simulating traffic over the connection 
set cbr0 [new Application/Traffic/CBR]
$cbr0 attach-agent $udp0

proc finish {} {
	global ns tracefile outputfile
	$ns flush-trace
	close $tracefile
	close $outputfile
	puts "running nam.... "
	exec nam out.nam &
	exit 0
}


#start cbr traffic at 1st second of simulation
$ns at 1.0 "$cbr0 start"

#finish the ns simulation at 60 seconds 
$ns at 60.0 "finish"

#run the ns simulator 
$ns run