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Publish Date: Jun 9, 2009

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Introduction to Scripting in Perl, Python and Tcl

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Overview

The 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 LabVIEW give 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 a focus 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 Contents

  1. Introduction
  2. Running Scripts
  3. Basic Syntax
  4. Next Steps

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 what constitutes 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 leads to 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 easier to 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 Language
Python: Named after the BBC show “Monty Python’s Flying Circus”
Tcl: Stands for Tool Command Language

 

Running Scripts

Installing an Interpreter

The 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:

Setting up the Interpreter

One 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. In addition, 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 directly from 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 association to the Tcl command line interpreter.

  1. Open Windows Explorer
  2. Click Tools»Folder Options
  3. Switch to the File Types tab
  4. On the Registered file types listbox, scroll down to the extension TCL and select it
  5. The Opens with details defaults to Wish Application
  6. Click the Change button
  7. On the Opens With dialog, click Browse and select the ActiveTcl command line interpreter, tclsh
    This defaults to:
    C:\Tcl\Bin\tclsh<version_number>.exe
  8. Click the OK button on the Open With dialog
  9. Make sure that the TCL extension is now associated with Tclsh Application. Click the Apply button.
    FolderOptions.jpg

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. For instructions on how to do this on Windows XP, refer to Microsoft KnowledgeBase 310519: How To Manage Environment Variables in Windows XP.

Executing a Script

The 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. 

Editing Scripts

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

Basic Syntax

The 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 variable
my $myNumber = 5;

#Create a string
my $myString = "Hello World";

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

 

Python

#Create a numeric variable
myNumber = 5;

#Create a string
myString = "Hello World";

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

 

Tcl

#Create a numeric variable
set myNumber 5;

#Create a string
set myString "Hello World";

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

 

Displaying output to the Standard Output (STDOUT)

Perl

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

#Print a string and a number
my $sum = 5;

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

 

Python

#Print a string to the screen
print "Hello World";

#Print a string and a number

sum = 5;

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


Tcl

#Print a string to the screen
puts "Hello World";

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

 

Declaring and Using Lists

Perl

#Declare a list with three elements
my @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 list
print "The second element in the list is: ", $myList[1], "\n\n";


Python

#Declare a list with three elements
myList = [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 list
print "The second element in the list is: ", myList[1], "\n";

 

Tcl

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

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

#Access a single element in the list
puts "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 arguments
print "Number of Command Line Arguments: ", $#ARGV + 1, "\n";

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

 

Python

import sys;

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

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

 

Tcl

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

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

print "\n";

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

print "\n";

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


Python

#if, elseif, else
if (myNumber == 5):
    print "My Number is Five";
elif (myNumber == 3.14):
    print "My Number is Pi";
else:
    print "My Number is neither Five nor Pi \n";

 

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

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

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

 

Tcl

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

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

puts "\n";

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

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

 

Next Steps

Now 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 the next article in the series.

Developer Zone Tutorial: Calling Scripting Languages from TestStand

Downloads

hello_world.zip

scripting_basics.zip

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This tutorial (this "tutorial") was developed by National Instruments ("NI"). Although technical support of this tutorial may be made available by National Instruments, the content in this tutorial may not be completely tested and verified, and NI does not guarantee its quality in any way or that NI will continue to support this content with each new revision of related products and drivers. THIS TUTORIAL IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND AND SUBJECT TO CERTAIN RESTRICTIONS AS MORE SPECIFICALLY SET FORTH IN NI.COM'S TERMS OF USE (http://ni.com/legal/termsofuse/unitedstates/us/).