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Introduction to Scripting in Perl, Python and TclPublish Date: Feb 02, 2012

OverviewThe ability to re-use existing code while migrating to a new test software framework can save developers both time and money. Both NI TestStand test management software and NI LabVIEWgive users the ability to directly call and integrate with code written in a variety of languages such as Perl, Python and Tcl.

This article examines the basics of three of the commonly used scripting languages – Perl, Python and Tcl. This is Part 1 of a three-part series on calling scripting languages from TestStand with afocus on the needs of engineers and scientists. In this first part we will discuss the basic syntax of Perl, Python and Tcl and illustrate how to write simple scripts in these languages.

Table of ContentsIntroduction

Running Scripts

Basic Syntax

Next Steps

1. Introduction

What is a scripting language?Scripting languages are programming languages that are typically written using high-level programming constructs, which makes them easy to learn. While there is no fixed definition of whatconstitutes a scripting language, some of the common distinguishing traits of these languages include:

Interpreted: Scripting languages are typically converted into machine level code during runtime by an interpreter, rather than being compiled into an executable before running. While this leadsto a performance hit as each line has to be interpreted on the fly, it makes for easier portability between systems.

Typeless: Variables can be used to hold any type of data without having to explicitly declare their type. While this can make it easy to run into typecasting errors, it makes the language easierto learn and can improve readability of the script.

Native Complex Types: Most scripting languages also natively provide certain complex data types like strings, arrays, lists and hashes.

Garbage Collection: Most scripting languages automate garbage collection (freeing of memory used by data). This can reduce the likelihood of memory leaks occurring.

What do scripting language names mean?Perl: Stands for Practical Extraction and Report LanguagePython: Named after the BBC show “Monty Python’s Flying Circus”Tcl: Stands for Tool Command Language

2. Running Scripts

Installing an InterpreterThe first step in running a script is downloading an interpreter that will take your script and convert it into machine level language.

For our tutorials and examples, we used ActiveState’s free interpreters for Perl, Python and Tcl:

Perl: (version 5.10)ActivePerl

Python: (version 2.6)ActivePython

Tcl: (version 8.5)ActiveTcl

Setting up the InterpreterOne of the benefits of using ActiveState’s interpreters is that the installers will automatically set up the computer’s Path environment variable so that you can call your scripts from any directory. Inaddition, they will also set up file associations with the file extension (.pl, .py, .tcl) with their interpreters so that you can run the script directly by calling its filename.

One caveat to be aware of is that, by default, ActiveTcl will associate .tcl files with a windows shell interpreter - wish - rather than a command line interpreter. In order to run your Tcl scripts directlyfrom the command line, you need to modify the .tcl file association to point to the Tcl command line interpreter - tclsh. The following direction will step you through pointing the .tcl file associationto the Tcl command line interpreter.

Open Windows Explorer

Click Tools»Folder Options

Switch to the tabFile Types

On the listbox, scroll down to the extension TCL and select itRegistered file types

The details defaults to Opens with Wish Application

Click the buttonChange

On the dialog, click and select the ActiveTcl command line interpreter, tclshOpens With BrowseThis defaults to:C:\Tcl\Bin\tclsh<version_number>.exe

Click the button on the dialogOK Open With

Make sure that the TCL extension is now associated with . Click the button.Tclsh Application Apply

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9. Make sure that the TCL extension is now associated with . Click the button.Tclsh Application Apply

NOTE: If you use an interpreter other than ActivePerl, ActivePython and ActiveTcl, you may have to manually set up your Path environment variable to point to the interpreter’s executable. Forinstructions on how to do this on Windows XP, refer to .Microsoft KnowledgeBase 310519: How To Manage Environment Variables in Windows XP

Executing a ScriptThe typical syntax for running a script from the command line is:<interpreter.exe> <script_filename > <command_line_arguments>

However, since we already associated the script file extensions with the interpreters, we can also directly run the script from the command line using:<script_filename> <command_line_arguments>

To make sure that your interpreter installed properly, you can try to run the following scripts from the command line.

Perl:Download hello_world.plRun using: perl.exe HelloWorld.pl

Python:Download hello_world.pyRun using: python.exe HelloWorld.py

Tcl:Download hello_world.tclRun using: tclsh<version_number>.exe HelloWorld.tcl

Editing ScriptsWhile you can edit a script file in any text editor like notepad, or any text based Development Environment like or Microsoft Visual Studio, we wrote our examples in ActiveState’sLabWindows/CVIfree Integrated Development Environment, (version 5.0). We chose Komodo Edit because it’s built in features (autocomplete, calltips, syntax coloring and checking) support theseKomodo Editthree scripting languages amongst others.

3. Basic SyntaxThe following section walks you through the basic syntax used in Perl, Python and Tcl. You can download the entire sample here:scripting_basics.zip

Declaring and Using Scalar Variables

Perl

#Create a numeric variablemy $myNumber = 5;

#Create a stringmy $myString = "Hello World";

#Create a new numeric and assign the original to itmy $myNewNumber = $myNumber;

Python

#Create a numeric variablemyNumber = 5;

#Create a stringmyString = "Hello World";

#Create a new numeric and assign the original to itmyNewNumber = myNumber;

Tcl

#Create a numeric variableset myNumber 5;

#Create a stringset myString "Hello World";

#Create a new numeric and assign the original to itset myNewNumber $myNumber;

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Displaying output to the Standard Output (STDOUT)

Perl

#Print a string to the screenprint "Hello World\n";

#Print a string and a numbermy $sum = 5;

print "The sum is: ", $sum, "\n\n";

Python

#Print a string to the screenprint "Hello World";

#Print a string and a number

sum = 5;

print "The sum is: ", sum, "\n";

Tcl

#Print a string to the screenputs "Hello World";

#Print a string and a numberset sum 5;puts "The sum is: $sum \n";

Declaring and Using Lists

Perl

#Declare a list with three elementsmy @myList = (5, "foo", 3.14);

#The index of the last element is accessed by $#<listname>print "Size of myList: ", $#myList + 1, "\n";

#Access a single element in the listprint "The second element in the list is: ", $myList[1], "\n\n";

Python

#Declare a list with three elementsmyList = [5, "foo", 3.14];

#The length of the list is accessed by len(<listname>)print "Size of myList: ", len(myList);

#Access a single element in the listprint "The second element in the list is: ", myList[1], "\n";

Tcl

#Declare a list with three elementsset myList {5 "foo" 3.14};

#The index of the last element is accessed by llengthputs "Size of myList: [llength $myList]";

#Access a single element in the listputs "The second element in the list is: [lindex $myList 1] \n";

Reading User Input from the Command Line (Command Line Arguments)

Perl

#Command line arguments are stored in list ARGV#Get number of argumentsprint "Number of Command Line Arguments: ", $#ARGV + 1, "\n";

#Access individual argumentprint "The first Command Line Argument is: ", $ARGV[0], "\n\n";

Python

import sys;

#Command line arguments are stored in list sys.argv

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#Command line arguments are stored in list sys.argv#Get number of argumentsprint "Number of Command Line Arguments: ", len(sys.argv) - 1;

#Access individual argumentprint "The first Command Line Argument is: ", sys.argv[1], "\n";

Tcl

#Command line arguments are stored in list $argv and $argc#Get number of argumentsputs "Number of Command Line Arguments: $argc";

#Access individual argumentputs "The first Command Line Argument is: [lindex $argv 0] \n";

Reading User Input from the Standard Input (STDIN)

Perl

#Read a user input from the keyboard (terminates on return key)print "Enter value, and then press Enter: ";my $myUserInput = <STDIN>;

Python

#Read a user input from the keyboard (terminates on return key)myUserInput = raw_input("Enter value, and then press Enter: ");

Tcl

#Read a user input from the keyboard (terminates on return key)puts "Enter value, and then press Enter: ";gets stdin myUserInput;

Syntax of Common Conditional Statements

Perl

#if, elseif, else

if ($myNumber == 5){ print "My Number is Five \n";}

elsif ($myNumber == 3.14){ print "My Number is Pi \n";}

else{ print "My Number is neither Five nor Pi \n\n"}

#while loopwhile ($myNumber != 0) #could do: until ($myNumber == 0){ print "My Number is: ", $myNumber, "\n"; $myNumber -= 1;}

print "\n";

#for loopfor ($myNumber = 0; $myNumber < 5; $myNumber++){ print "My Number is: ", $myNumber, "\n";}

print "\n";

#foreach loopforeach my $currentElement (@myList){ print "The current element is: ", $currentElement, "\n";}

Python

#if, elseif, elseif (myNumber == 5): print "My Number is Five";elif (myNumber == 3.14):

print "My Number is Pi";

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print "My Number is Pi";else: print "My Number is neither Five nor Pi \n";

#while loopwhile (myNumber != 0): print "My Number is: ", myNumber; myNumber -= 1;print "\n";

#for loopfor myNumber in range (0, 5): print "My Number is: ", myNumber;print "\n";

#foreach loopfor currentElement in myList: print "The current element is: ", currentElement;

Tcl

#if, elseif, elseif {$myNumber == 5} { puts "My Number is Five \n";} elseif {$myNumber == 3.14} { puts "My Number is Pi \n";} else { puts "My Number is neither Five nor Pi \n\n";}

#while loopwhile {$myNumber != 0} { puts "My Number is: $myNumber"; incr myNumber -1;}

puts "\n";

#for loopfor {set myNumber 0} {$myNumber < 5} {incr myNumber} { puts "My Number is: $myNumber";}puts "\n";

#foreach loopforeach currentElement $myList { puts "The current element is: $currentElement";}

4. Next StepsNow that we have looked at the basics of these three scripting languages, we are ready to start integrating our scripts into our test sequences in TestStand. Click on the following link to go to thenext article in the series.

Developer Zone Tutorial: Calling Scripting Languages from TestStand