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Welcome to the Spoken Tutorial on special variables in Perl.

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In this tutorial we will learn about

 Global special variables

 Special command line variables

 Global special constants

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For this tutorial, I am using

 Ubuntu Linux 12.04 operating system

 Perl 5.14.2

 and the gedit Text Editor


You can use any text editor of your choice.

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As a prerequisite, you should have working knowledge of Perl Programming.

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If not, then go through the relevant Perl spoken tutorials on the spoken tutorial website.

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What are special variables?

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Special variables are predefined variables that have a special meaning in Perl.

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These do not need to be initialised before use.

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These are used to hold the results of searches, environment variables and flags to control debugging.

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First, we will learn about Global special variables.

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$_: (Dollar Underscore). This is a widely used special variable.

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$_ - Dollar Underscore is the default parameter for lot of functions and pattern-searching strings.

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Let us understand the usage of $_ variable using a sample program.

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I will open the special dot pl file which I have already created.

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Go to the terminal and type gedit special.pl ampersand and press Enter.

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special.pl file is now open in gedit. Type the code as displayed on the screen.
Let me explain the code now.

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There are 2 foreach loops here. Both these foreach loops will execute the same result.

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In each iteration of the loop, the current string is placed in $_.

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And it is used by the print statement, by default. $_ saves the use of one extra variable $color.

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Press Ctrl+S to save the file.

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Then switch to the terminal and execute the Perl script by typing perl special dot pl and press Enter.

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Here both foreach loops give the same output.

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Now, let us see another example to demonstrate how $_ variable is implicit.
Go back to the special dot pl file.

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Type the piece of code shown on the screen.

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This program reads a text file first.txt line by line. Then it loops through the DATA file, till all lines are read.

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print $_ variable prints the contents of the current line from the first.txt file.
In the 'while' loop, the use of $_ is implicit.

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We will see more about this in future tutorials.

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At the rate underscore is the special variable used to store subroutine parameters.

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Arguments for a subroutine are stored in this array variable.

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Array operations like pop/shift can be done on this variable, as we do in normal arrays.

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I will show an example for this. Let us switch back to special dot pl file once again.

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Type the code as displayed on the screen.

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This program will return the maximum value between two numbers.
@_ - At the rate underscore is a local array which stores the two arguments dollar a comma dollar b.

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That is, it is stored under  dollar underscore index of zero   and dollar underscore index of one

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The print statement prints the maximum of the two given numbers.

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Press Ctrl+S to save the file.

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Switch to the terminal and execute the Perl script by typing perl special dot pl and press Enter.

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The maximum value is displayed as output. Let’s move on.

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Environment variables are represented by percentage followed by capital ENV.

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Environment variables contain a copy of the current environment variables, such as the following.

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Let us understand%ENV variable using a sample program.

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We will switch back to the special dot pl file.

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Type the following code as displayed on the screen.

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Press Ctrl+S to save the file. Switch to the terminal and execute the Perl script.

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Type perl special dot pl and press Enter.

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We can see the current environment details such as  PWD (present working directory), username, language etc.

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Next we will see about another special variable dollar zero

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The special variable dollar zero ($0) contains name of the current Perl program that is being executed.

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This is generally used for logging purpose.

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For example: I have a file named First.pl within which I am using $0 variable as shown here.

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On executing, it will print the filename First.pl.

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Perl has a built-in function called sort that sorts an array.

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A comparison function will compare its parameters using the numerical comparison operator.

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This operator is represented by lesser than equal to greater than symbols, as shown here.

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Let us see an example for this.

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Open the terminal and type gedit sort.pl ampersand and press Enter.

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sort.pl file is now open in gedit Text Editor. Type the following code as displayed on the screen.

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Let me explain the code. The first line declares an array of numbers.

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The numerical comparison operator will compare the two values as numbers.

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'Dollar a and dollar b are special package local variables in which the values to be compared are loaded.

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And this sort function will sort the numbers in ascending order.

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Let us now save and execute the program.

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Switch back to the terminal and type, perl sort.pl and press Enter.

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We can see that the numbers are sorted in ascending order.

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Let’s see another special variable dollar exclamation.

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'dollar exclamation if used in string context, returns the system error string.
Here is an example of its usage.

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If the file hello.txt doesn't exist, it will print the error message,like:
Cannot open file for reading: No such file or directory.

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Let’s now look at another special variable namely, dollar at the rate.

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'This is another widely used variable. It returns an error message, returned from eval or require command.

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'This example will print: could not divide Illegal division by zero.

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dollar dollar is yet another special variable. This holds the process ID of the Perl interpreter running this script.

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The diamond operator is used to read every line, from the files specified on the command line.

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Let us see an example for this.

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Open the terminal and type gedit commandline.pl ampersand and press Enter.

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commandline.pl file is now open in gedit.

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Type the code as displayed on the screen.

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Save the file.

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Let me show you the text that I have in a file named sample.txt file.

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Now, run the program from the command line by typing: perl commandline.pl space sample.txt and press Enter.

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This is the text we had in sample.txt file.

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If no files are specified, it reads from the standard input i.e. from the keyboard.

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Perl has an array at the rate capital A R G V special variable.
This holds all the values from the command line.

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When using array at the rate capital A R G V, there is no need to declare the variables.

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'The values from the command line are automatically placed in this variable.

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Let’s now move on to Global Special Constants.

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underscore underscore E N D (in capital letters)underscore underscore.  indicates the logical end of the program.

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Any text following this special variable is ignored after this statement.

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underscore underscore FILE (in capital letters) underscore underscore represents the filename of the program at the point where it is used.

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underscore underscore LINE (in capital letters) underscore underscore represents the current line number

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underscore underscore PACKAGE (in capital letters) underscore underscore represents the current package name at compile time, or undefined if there is no current package.

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We will see a sample program on how Global Special Constants are used.

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Open the terminal and type gedit specialconstant.pl ampersand  and press Enter.

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specialconstant.pl file is now open in gedit.

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'Type the following code as displayed on the screen. Let me explain the code now.

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The special literals  PACKAGE, FILE, LINE  represent the  package name, current filename and
line number respectively at that point in the program.

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Let us execute the program.

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Switch back to the terminal and type: perl specialconstant.pl and press Enter.

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We can see the current package name, filename and line number of our program.

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This brings us to the end of this tutorial. Let us summarise.

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In this tutorial, we learnt about some commonly used special variables in Perl.

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As an assignment do the following. Write a Perl script to sort the following array of numbers in ascending and descending order.

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Note: For descending order, use the below code for comparison.

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Print the sorted result using ‘while loop’ and special variable $_.

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Save and execute the program.

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Now check the result.

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The video at the following link summarises the Spoken Tutorial project.
Please download and watch it

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The Spoken Tutorial Project Team
conducts workshops and gives certificates on passing online tests.

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For more details, please write to us.

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Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.

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More information on this mission is available at this link.

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This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.