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TCL/TK Programming for NS2 - NS2 Tutorial 4

Tcl/Tk Programming

In this post, you may learn

·       To know the basics of Tcl/TK Programming

·       To create a Tcl script 

Tcl Programming

Over the span of 25 years a simple modular scripting language was developed and modified or improved by corporates, academia, open source community and web developers. The code developed and the binary source is free with no strings attached. This is one of the oldest scripting language still being used for testing the network equipment like routers, switches, etc. And used in electronic IC designs and used as a front end tool (testing tool) for Network Simulator 2 (NS2).

Tcl was created by John Ousterhout mainly for embedded system applications for its ability for rapid prototyping, scripted applications and also fir its small or bigger footprints.

3.1 Features of Tcl/Tk

Rapid Development:  Tcl almost runs 5 to 10X faster than other scripting languages if it involves GUI, string handling or integration.

Graphical User Interface  Using its Tk toolkit, Tcl provides facilities for creating GUIs that are incredibly simple yet remarkably powerful.

Cross Platform Tcl/Tk runs almost same on all operating systems like Windows, Macintosh, and Linux/Unix platform.

Continuous upgrades: Since 1990s Tcl and Tk have been under continuous and rapid upgrades which makes Tcl/Tk really a reliable and solid scripting language that can be relied upon.

Deployment Tcl can be deployed as a single application on a target machine very easily whereas other dynamic languages makes deployment harder.

Network-aware applications: Tcl has a wide number of event driven codes that are helpful in generating events for networked devices like routers, switches, routing extensions, etc.

Apart from these features, Tcl also has a vibrant Community that helps developers with provision of free extensions, plugins and updates. Also Tcl is easy to learn and all these come free. Tcl/Tk is completely free with even the binary source can also be modified or upgraded.


3.2 Tcl interpreter and Execution

Tcl scripts are usually having extension of .tcl and can be executed using tclsh or wish. In Linux/Unix families, following is the way to interpret the tcl scripts.

# tclsh filename.tcl

The command wish will be working for GUI and text mode. Sometimes tclsh can be coming along with version number for example tclsh86.exe and can be executed as follows

C:/> tclsh86 filename.tcl

C:/> source filename.tcl

Here is the screenshot of tclsh.

tclsh interpreter

tclsh interpreter

Since this book is related to Network Simulator 2 (NS2), the commands and the syntax in this chapter will be related to NS2 and its simulation libraries. However for the understanding of new developers, bits and pieces of the tcl scripts are presented.

3.3 Getting Started

Let us go through some basic syntax of Tcl Programming.  The following examples will give you a general introduction for Tcl Programming.

The syntax of Tcl is not similar to C or C++, rather it is simple and straightforward. This chapter will give you a glimpse of the Tcl syntax used in NS2

Example 1:  “Hello World”

# This is an example to demonstrate the Hello World

puts “Hello World!”;  #printing Hello World! to the screen

To run this file, tclsh ex1.tcl, if the operating system is windows then tclsh.exe ex1.tcl.

      puts is the command to print anything to screen

      if comments are to be placed at the end of the statement, then a trailing semi colon is a must, as shown in the example above.

      A comment always starts with a preceding # symbol.

 3.3.1 Basic syntax

Tcl scripts are full of commands separated by newlines. A typical example of Command is given below.

 expr 3 + 10

This command computes the sum of 3 and 10 and returns the result, 13. You can try out this example using tclsh.

There are different words that are separated by space. There are four words in this example: expr, 3, +, and 10. The expr command is an arithmetic expression, computes the result of that expression, and returns the result as a string. Sometimes the same above command can also be represented as

expr 3+10

All commands returns results, even if a command has no results to display, it displays a empty string.

3.3.2 Assigning values to Variables

In Tcl, everything is represented as string, but internally it also represents integers, double, list or some other types. This enables faster interpretation.

For this, the assignment command is used in Tcl which is set. When the set command is dealing with two arguments as in:

set name TclTk

the (name) in the first command references which stores TclTk in the memory. Set command always returns the contents of the variable named in the first argument. So in this command, TclTk without quotes is being stored in the memory which is referenced by the (name) in the first command.

Set can also be invoked with only one argument. When called with just one argument, it will return the contents of that argument.

Here's a summary of the set command.

set variableName valueOfVariable

If valueOfVariable is specified, then the contents of the variable variableName are set equal to valueOfVariable. (Like the example we have seen above)

If variableName consists only of alphanumeric characters, and no parentheses, it is a scalar variable.

If variableName has the form variableName (index), it is a member of an associative array.

Example 2: “Variable and Values”

set Name “I am Tcl”

set A 10

puts $A

puts $Name

set label "The value in A is: "

puts "$label $A"

3.3.3 Command substitution

Command substitution is a important concept in Tcl where result of one command argument can be used for another command. Here is an example

set a 25
set b [expr $a*4]

When a [ appears in a command, Tcl treats everything between it and the matching ] as a nested Tcl command. Tcl evaluates the entire expr $a*4 as a single command and substitutes as a separate command and another command set b will be holding the value 100. For future reference, b can used as $b.

3.3.4 Quotes and braces

For example, consider the following script:

set x 10
set y 20
set z "$x + $y is [expr $x + $y]"

The value of z after the evaluation is 10+ 20 is 30. Everything command between the double quotes is passed to the set command as a single word. Note that a command and variable substitutions are performed on the text between the quotes; and the quotes themselves are not passed to the command. If the quotes in the above example were absent, the set command would have received 6 arguments, which would be an error.

Curly braces provide another way of grouping information into words. They are different from quotes in that no substitutions are performed on the text between the curly braces:

set z {$x + $y is [expr $x + $y]}
This command sets variable z to the value "$x + $y is [expr $x + $y]".

3.3.5 Looping and Control structures

Tcl provides almost all the control structures that were available with other programming languages. Tcl control structures are commands that take Tcl scripts as arguments.

if statements

if { test1 } {


} elseif { test2 } {


} else {



Example 3: Simple Example using if

puts "What’s your mark in Scripting Language"

gets stdin marks

if {$marks > 90} {

    puts "Great, you got S Grade"

} elseif {$marks > 50 && $marks < 90} {

    puts "Hey You passed the Exam"

} else {

    puts "You need to improve your marks next time"


This code gives a simple example of if statement with conditional statements. This example also helps understanding that how to get the input from the user using gets stdin

For Loop

The for loop in Tcl takes the same format as of other languages like

for { initialization }{ condition } { increment or decrement }{



Example 4: Sum to N Terms

set x 10

set sum 0

for { set i 0 } { $i <= $x } { incr i } {

set sum [expr  $sum+ $i]


puts $sum

The above example displays the sum of first 10 elements which is 55.

While Loop

The while loop syntax is as follows:

while { condition } {

Body of the loop


For example, here is a small code that helps in understanding while loops in a greater way

Example 5 – Squaring a number using while loop

set x 10

while { $x < 10 } {

puts [expr $x * $x ]

incr x


Like for and while, there are foreach and for…in loops are also used in Tcl programming. Since this book is being written for Network simulator 2 and its simulation objects, the readers are advised to learn more about Tcl programming in these books [1].

3.3.6 Procedures

A procedure is a block that contains series of commands. Functions in other programming languages are preferred as Procedures in Tcl. The Modularity of the program is maintained when procedures are used. Here are some of the pros of the procedures:

  • Reducing the code size by avoiding duplication
  • Groups in to simpler tasks rather than complex task
  • Reuse of code

There are user defined procedures and built in procedures. The keyword proc is used to define the user defined keyword.

Example 6 – Tcl Procedure Example

proc display {} { ; #This is the line to create a procedure using the command proc

   set ns [expr 1 + 2] ; # evaluates the expression 1 + 2 and assign it to ns

    puts $ns ; # display the value to the screen


display; #calling the procedure

the display procedure is called using display as given in the last line of the code. It is understood that this procedure does not contain any arguments or parameters.

Procedure with arguments

Procedures can also take arguments. The following examples show the procedure with single argument.

proc hello{f} { ; # f is the argument here

    puts “Hello Mr. $f”


puts [hello “Pradeep”]

puts [hello “Amit”]


Hello Mr. Pradeep

Hello Mr. Amit

Next example with more than one argument, let us see an example to find out the biggest among three numbers. So there can be three arguments.

proc biggest { a b c } {

if {$a > $b && $a > $c }

return $a

} elseif {$b >  $a && $b > $c} {

return $b

} else {

return $c


set a 10

set b 20

set c 30

set val [biggest $a $b $c]

puts “The biggest of $a, $b and $c is $val”

Procedures are the powerful commands in Tcl and it is very much useful in producing reusable codes that also reduces the code size.

3.3.7 Working with Files

Like other languages, Tcl also has file handling facility and it can open, read and write to files.


set nf [open file.txt w]

puts $nf “This is a simple example to write to a file

The file attribute can be

r – Open the file for reading and file should exist.

r+ - Open the file for reading and writing.

w – Open the file for writing, and if the file exists already, it will be overwritten, else a new file will be created.

w+ - open the file for reading and writing.

a - open the file for writing. If the file already exists, the cursor will point to the end of the file, if the file not exists, a new file will be created.

a+ - Open the file for reading and writing. If the file already exists, the cursor will point to the end of the file, if the file not exists, a new file will be created.

It is always a good practice to close a file once the job is done. The following code shows how to close the file. In Tcl programming, usually all the open handles like files, pipes, any global variables, etc can be closed at the end of the program.

set tracef [open file.dat a]

puts $tracef “This is an example to show the file close”

close $tracef

 The file is opened in the append mode and if the file.dat is already existing, then the string “This is an example to show the file close” will be written to the end of the file.


This chapter informs the bits and pieces of Tcl programming to a limited level. Since this book is intended for Tcl files that are written for network simulation and hence limited information is provided for Tcl. The readers are advised to read the full-fledged tutorial for Tcl if they wanted to do extensive programming on Tcl.



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