NS-2 LAB: TCL

2005-2006 - Semester A

Miriam Allalouf

TCL Introduction

Why Scripting languages?•C,C++,Java Perl, Python, TCL•UNIX scripting languages: XTERM + shell, PERL:

–based around C type language constructs.• Scripting languages chars.:

–Configure–Glue different application– Flexible data structure easy porting– Slow but processor’s power grew!– Undefined variable types– Code and Data are the same : easy to add code!

TCL Introduction

Scripting languages•A script file is a list of commands which a shell interpreter reads and executes. •First line is a line which indicates what interpreter to run. •Rest of the file is just a sequence of commands, most often just like those you can run on your command line. •Script languages include some method for manipulating variables and data.

TCL Introduction

Script languages – TCL •The tcl interpreter gives a bridging solutions:

–TK - display extension for using X widgets (TCL/TK)–XML–CORBA–ORACLE, Sybase–DCOM,SAP,WEB, HTTP–SNMP–Test Equipment

•The tcl interpreter is called "tclsh". You can run: which tclsh •in order to find out where your tcl interpreter is located. •The first line of your tclsh script should be:

–#!/your.location/tclsh

TCL Introduction

TCL - Tool Command Language - History1980’s Tcl invented by John Ousterhout while working on design tools for integrated circuits at the University of California at Berkeley. Wanted a language that could be used over many tools. •To create a good interpreted language •To build it as a library package that could be reused in many different applications. •To provide a language interpreter with a set of generic facilities, such as variables, control structures, and procedures. •To enable the application that used the language to extend it and add its own features into the language, so that the language could be used to control the application.

TCL Introduction

TCL GoalsThe goals set for the language were:•The language must be extensible: The language must be very easy to work on, and it should be easy and convenient to add features to the language in order to adapt this language to the programmer’s specific goal.•The application should be very simple and generic: The application should be able to work easily with many different applications in such a way that it does not restrict the features that those applications have to provide.•Integration: The language should be able to glue together the extensions. Thus the language must have good facilities for integration.The first version of Tcl in a graphical text editor was ready by the spring of 1988.

TCL/Tk: Tk was ready by the end of 1990.

TCL Plan:• Review Tcl syntax

• Expressions

• Lists, arrays

• Strings and pattern matching

• Control structures

• Procedures

• Error handling

• File and network I/O and process management

• Getting info at runtime

TCL Introduction

TCL Basics• Tcl script (String based) =

– Sequence of commands.– Commands separated by newlines, semi-colons.

• Tcl command =– One or more words separated by white space.– First word is command name, others are arguments.– Returns string result (always returns – usually numbers)

• Examples:set a 22 set the variable ‘a’ to 22

puts "Hello, World!" world’s shortest program

expr 100 + 20 Calculates the sum of 100 and 20 and results the result, which is 120

TCL Introduction

Division Of Responsibility

Tcl Parser

Command Procedure

Command

Words

Result

• Interprets words.• Can invoke parser

recursively.• Produces string result.

• Chops commands into words.• Makes substitutions.• Does not interpret values of

words.• Single pass operation!

TCL Introduction

Tcl: Tool Command Language• Simple syntax (similar to sh, C, Lisp):

set a 47 í 47

• Substitutions: set b $a í 47 set b [expr $a+10] í 57

• Quoting: set b "a is $a" í a is 47 set b {[expr $a+10]} í [expr $a+10]

TCL Introduction

Variable Substitution • Syntax: $varName• Variable name is letters, digits, underscores.

– This is a little white lie, actually.• Tcl will promote integers to reals when needed• All values translated to the same type• Note that expr knows about types, not Tcl!• May occur anywhere in a word.

Sample command Result

set b 66 66set a b bset a $b 66set a $b+$b+$b 66+66+66set a $b.3 66.3set a $b4 no such variable

TCL Introduction

Command Substitution• Syntax: [script]

• Evaluate script, substitute result.

• May occur anywhere within a word.

Sample command Result

set b 8 8set a [expr $b+2] 10set a "b-3 is [expr $b-3]" b-3 is 5

TCL Introduction

TCL expression• What’s happening in these expressions?

expr $a * cos(2*$b) -5.03443

$a, $b substituted by scanner before expr is called

expr {$b * [fac 4]} 120

here, $b is substituted by expr itself• Therefore, expressions get substituted more than

once! (In fact, TCL once, and expr once)set b \$aset a 4expr $b * 2 8

TCL Introduction

Tcl String Expressions • Some Tcl operators work on strings too

set a Bill Billexpr {$a < "Anne"} 0

• <, >, <=, >=, ==, and != work on strings

• Beware when strings can look like numbers

• You can also use the string compare function

TCL Introduction

Controlling Word Structure• Words break at white space and semi-colons, except:

– Double quotation marks group words as a single argument to a command and prevent breaks.Dollar signs and square brackets are interpreted inside double quotation marks.

set a "x is $x; y is $y"– Curly braces also group words into a single argument. In this

case, however, elements within the braces are not interpreted or substituted.

set a {[expr $b*$c]}– Backslashes quote special characters: For example, \n

generates a newline. set a word\ with\ \$\ and\ space

– Backslashes can escape newline (continuation)

• Substitutions don't change word structure:set a "two words"set b $a

TCL Introduction

Notes on Substitution and Parsing • Tcl substitution rules are simple and absolute

– Example: commentsset a 22; set b 33 <- OK# this is a comment <- OKset a 22 # same thing? <- Wrong!set a 22 ;# same thing <- OK

• Parser looks at a command just once!• It’s OK to experiment

– Expressions exist that can’t be written in one command

– Sometimes things get hairy “{[“cmd”]}”

TCL Introduction

Tcl Commands:• Built-in commands: this group of commands is

provided by the Tcl interpreter itself. • All of the commands discussed so far. • Present in all Tcl applications.

• Tcl extension commands: created using the Tcl extension mechanism. – Tcl APIs to create a new command where the

command procedure is implemented in C or C++, command name be registered with the Tcl interpreter. TCL recognizes this particular name for a Tcl command and, will execute the new command.

• proc commands: consists of procedures created with the proc command.

TCL Introduction

Lists • Zero or more elements separated by white

space:red green blue

• Braces and backslashes for grouping:a b {c d e} f (4 words)one\ word two three (3 words)

• List-related commands:concat lindex llength lsearchforeach linsert lrange lsortlappend list lreplace

• Note: all indices start with 0. end means last element

TCL Introduction

Lists are Powerful• Examples:

lindex {a b {c d e} f} 2 > c d elsort {red green blue} > blue green red

• A list makes a handy stackSample command Resultset stack 1 1push stack red red 1push stack {a fish} {a fish} red 1pop stack a fish (stack contains: red 1)

• push and pop are very short and use list commands to do their work

TCL Introduction

抽考寫寫看

More about Lists• A true list’s meaning won’t change when (re)scanned

red $animal blue $animal <= not a listred fish blue fish <= list

red \$fish blue \$fish <= not a list, but

list red \$fish blue \$fish gives you…

red {$fish} blue {$fish} <= which is a list

• Commands and lists are closely related– A command is a list– Use eval to evaluate a list as a command

TCL Introduction

String Manipulation string subcommands

compare first last index length

match range toupper tolower trim

trimleft trimright• Note: all indexes start with 0. end means last char

TCL Introduction

Tcl Arrays• Tcl arrays are 'associative arrays': index is any string

set x(fred) 44

set x(2) [expr $x(fred) + 6]

array names x

=> fred 2

• You can 'fake' 2-D arrays:

set A(1,1) 10

set A(1,2) 11

array names A

=> 1,1 1,2 (commas included in names!)

TCL Introduction

Control Structures • C-like in appearance.• Just commands that take Tcl scripts as arguments.• Example: list reversal. Set list b to reverse of list a:

set b ""set i [expr [llength $a] - 1]while {$i >= 0} { lappend b [lindex $a $i] incr i -1}

• Commands:if for switch breakforeach while eval continuesource

TCL Introduction

Control Structure Examples • if expr script

• for script expr script script

for {set i 0} {$i<10} {incr i} {...}

• switch (opt) string {p1 s1 p2 s2...}

• foreach index [array names a] {

puts $a($index)

}

TCL Introduction

More on Control Structures

• Brackets are not required

set x 3

if $x>2 {... <= this is OK, eval’ed once

while $x>2 {... <= eval’ed many times!

set a {red blue green}

foreach i $a <= this is OK

foreach i red blue green {...

NOT OK!• foreach index [array names A] is a common idiom

TCL Introduction

Procedures • proc command defines a procedure:

proc sub1 x {expr $x-1}

• Procedures behave just like built-in commands:sub1 3 í 2

• Arguments can have default values:proc decr {x {y 1}} { expr $x-$y}– Return value of a procedure is implicitly the return value of

the last statement in the script. Unless the ‘return’ command exists.

name

list of argument namesbody

TCL Introduction

Procedures and Scope • Scoping: local and global variables.

– Interpreter knows variables by their name and scope– Each procedure introduces a new scope

• global procedure makes a global variable local> set x 10> proc deltax {d} { set x [expr $x-$d] }> deltax 1 => can't read x: no such variable> proc deltax {d} { global x set x [expr $x-$d] }> deltax 1 => 9

TCL Introduction

抽考Hanoi Tower

Procedures and Scope • Note that global is an ordinary command

proc tricky {varname} {global $varnameset $varname "passing by reference"

• upvar and uplevel let you do more complex things• level naming:

– #num: #0 is global, #1 is one call deep, #2 is 2…– num: 0 is current, 1 is caller, 2 is caller's caller…

proc incr {varname} {upvar 1 $varname varset var [expr $var+1]

}

TCL Introduction

Procedures and Scope• uplevel does for code what upvar does for variables

proc loop {from to script} {set i $fromwhile {$i <= $to} {

uplevel $scriptincr i

}}set s ""loop 1 5 {set s $s*}puts $s => *****

TCL Introduction

More about Procedures• Variable-length argument lists:

proc sum args { set s 0 foreach i $args { incr s $i } return $s}

sum 1 2 3 4 5>> 15sum>> 0

TCL Introduction

ERRORS• Errors normally abort commands in progress,

application displays error message:set n 0foreach i {1 2 3 4 5} { set n [expr {$n + i*i}]}í syntax error in expression "$n + i*i"

• Global variable errorInfo provides stack trace:set errorInfoí syntax error in expression "$n + i*i" while executing"expr {$n + i*i}" invoked from within"set n [expr {$n + i*i}]..." ("foreach" body line 2) ...

TCL Introduction

Advanced Error Handling

• Global variable errorCode holds machine-readable information about errors (e.g. UNIX errno value).

NONE (in this case)• Can intercept errors (like exception handling):

catch {expr {2 +}} msgí 1 (catch returns 0=OK, 1=err, other values...)set msgí syntax error in expression "2 +"

• You can generate errors yourself (style question:)error "bad argument"return -code error "bad argument"

TCL Introduction

Tcl file I/O

• Tcl file I/O commands:open gets seek flush globclose read tell cdfconfigure fblocked fileeventputs sourceeof pwd filename

• File commands use 'tokens' to refer to filesset f [open "myfile.txt" "r"] file4puts $f "Write this text into file"close $f

TCL Introduction

TCP, Ports, and Sockets : Tcl Network I/O• Tcl provides a simplified Socket library• socket creates a network connection

set f [socket www.sun.com 80]

fconfigure $f -buffering line

puts $f "GET /"

puts [read $f]

=> loads of HTML from Sun's home page

• Network looks just like a file!• To create a server socket, just use

socket -server accept portno

TCL Introduction

I/O and Processes

• exec starts processes, and can use '&'

set FAVORITE_EDITOR emacs

exec $FAVORITE_EDITOR &• no filename expansion; use glob instead

eval exec "ls [glob *.c]"• you can open pipes using open

set f [open "|grep foo bar.tcl" "r"]while {[eof $f] != 0} { puts [gets $f]}

TCL Introduction

Runtime Information Facilities

• The info commandwhat variables are there?–info vars, info globals, info locals, info exists

what procedures have I defined, and how?–info procs, info args, info default, info body, info commands

• the rename command–can rename any command, even built-in–can therefore replace any built-in command

TCL Introduction

NS-2 lab TCL Introduction

Resources•Bibliography: The Tcl/Tk track is based  on Ousterhout and Welch's book (Practical Programming in Tcl/Tk).

•Online resources, some of the  most useful Tcl/Tk links:  http://dev.scriptics.com/man/tcl8.3/contents.htm •Online Tcl/Tk manual pages: http://stage.caldera.com/Technology/tcl/Tcl.html•A broad  list of Tcl/Tk resources. •The Scriptics company (http://www.scriptics.com/)that commercializes Tcl and its developper web site. •Tutorials:

–A  Tcl/Tk introduction –The  TclTutor –An online overview with examples.

•Tcl/Tk course material in hebrew (see the lecture notes section

NS-2 lab Introduction

More Refernces•http://www.ixiacom.com/products/paa/customappdev/•http://www.cetus-links.org/oo_tcl_tk.html#oo_tcl_tk_start_here•http://www.pconline.com/~erc/tcl.htm•http://www.scriptics.com/doc/scriptHistory.html•http://www.itworld.com/AppDev/1243/UIR000804tk/•http://www.softpanorama.org/Scripting/tcl_tk.shtml•http://www.tcl.tk/advocacy/choose.html•http://www.cs.utah.edu/dept/old/texinfo/dejagnu/dejagnu_8.html•http://www.usenix.org/publications/library/proceedings/tcl95/•http://216.239.51.100/search?q=cache:wTF3fij-gGQC:www.usenix.org/publications/library/proceedings/tcl97/full_papers/poster_bergeman/bergeman-abstract.pdf+tcl/tk+application+area&hl=en&ie=UTF-8