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Welcome to the spoken tutorial on  BLOCKS in Perl.

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In this tutorial, we will learn about the various  BLOCKS available in Perl

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I am using  Ubuntu Linux 12.04 operating system and  Perl 5.14.2

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I will also be using the  gedit Text Editor.

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You can use any text editor of your choice.

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As a pre-requisite, you should have basic knowledge of Variables, Comments in  Perl

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Knowledge of  Data structures in PERL will be an added advantage.

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Please go through the relevant spoken tutorials on the  spoken tutorial  website.

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Perl provides 5 special blocks.

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These blocks get executed at various stages of a Perl program.

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These blocks are:

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BEGIN

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END

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UNITCHECK

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CHECK.

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INIT

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Let us start with understanding the  BEGIN block.

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BEGIN block get executed at the time of compilation.

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So, any code written inside this block gets executed first during compilation.

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We can have several  BEGIN  blocks inside a  Perl script.

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These blocks will get executed in the order of declaration.

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That is in the  First define First execute  pattern

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The syntax for  BEGIN block is as follows

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BEGIN in capital letters space open curly bracket

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Press  Enter.

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Piece of code to be executed at the time of compilation

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Press Enter

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Close curly bracket

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Now, let us look at an example of BEGIN blocks.

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Open the Terminal and type

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gedit beginBlock dot pl  space ampersand

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and press  Enter.

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This will open the  beginBlock dot pl  file in gedit.

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Type the following piece of code as displayed on the screen.

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Let us look at what I have written inside the script.

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Here, we have printed some text before and after   BEGIN blocks.

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Similarly, I have written one  print statement in each BEGIN block.

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Please note, I have not given the semicolon after the  BEGIN blocks.

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Putting a semicolon, will result in a syntax error, on execution of the program.

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Now, press  Ctrl+s to save the file.

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Then switch to  terminal and execute the script by typing,

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perl beginBlock dot pl

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and press Enter.

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You will get the output as displayed on the terminal.

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Notice that The line written inside the first BEGIN block gets printed first and

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The first print statement in the script actually gets printed after the BEGIN block statements.

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BEGIN blocks gets executed in the order of their declaration.

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From this example, it is evident that:

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The code written inside the  BEGIN  blocks gets executed first.

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This is irrespective of the location of the  BEGIN block inside  PERL  script.

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BEGIN blocks  always get executed in the First In First Out manner.

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So one of the use of this block is to include files inside a Perl script, before actual execution starts.

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Now, let us understand the END block

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END block get executed at the end of PERL program

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Code written inside this block gets executed after PERL has finished executing the program.

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We can have several  END blocks  inside a Perl script.

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These blocks will get executed in reverse order of declaration

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That is, in Last define First execute pattern.

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The syntax for  END block is as follows

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END  in capital letters  open curly bracket

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Press  Enter

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Piece of code to be executed at the end of the  PERL  script

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Press  Enter

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Close curly bracket

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Now let us look at an example of END blocks.

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Open the Terminal and type

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gedit endBlock dot pl space ampersand

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and press Enter

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This will open the  endBlock dot pl file in gedit.

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Type the following piece of code as displayed on the screen.

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Let us look at what I have written inside this script.

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Here we have printed some text before and after  END blocks.

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Similarly, we have written one print statement in each  END block.

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Please note, I have not given the  semicolon after the END block.

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If we give the semicolon, there will be a syntax error on compilation.

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Now, press  Ctrl+s to save the file.

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Then switch to terminal and execute the script by typing,

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perl endBlock dot pl

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and press Enter.

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You will get the output as displayed on the terminal.

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Notice that: The line written inside the  END block is printed last.

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The last  print statement in the script actually gets printed before the END block statements and

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END blocks gets executed in the reverse order of their declaration.

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From the example, it is evident that

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The code written inside the  END blocks get executed at the end.

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This is irrespective of the location of the  END block inside the PERL script and

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END blocks gets executed in the  Last In First Out manner.

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So, one use of END block is to destroy objects created in the program, before exiting.

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Similarly,  PERL has  UNITCHECK, CHECK and  INIT blocks.

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These blocks are used rarely by developers and are a bit difficult to understand.

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So, I will be just briefing you about these blocks.

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UNITCHECK, CHECK and INIT blocks are useful-

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to catch the transition between compilation and execution phase of the main program and

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to perform some checks or initialisation, after compilation and before execution

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UNITCHECK and CHECK blocks runs in Last in First out manner

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whereas INIT  block runs in First In First Out manner.

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The syntax for UNITCHECK block is as follows

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UNITCHECK in capital letters space open curly bracket

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Press Enter

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Piece of code to be executed

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Press Enter

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Close curly bracket

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The syntax for CHECK block is as follows

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CHECK in capital letters space open curly bracket

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Press Enter

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Piece of code to be executed

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Press Enter

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Close curly bracket

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The syntax for INIT block is as follows

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INIT in capital letters space open curly bracket

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Press Enter

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Piece of code to be initialised

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Press Enter

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Close curly bracket

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For better understanding, I recommend that you experiment with these blocks in your Perl scripts.

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Let us summarize.

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In this tutorial, we have learnt -

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BEGIN and END blocks in detail and

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Introduction to  UNITCHECK, CHECK and INIT  blocks

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using sample programs

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Here is assignment for you -

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Type the below code inside a PERL script;

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Execute the script and observe the output.

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Watch the video available at the following link

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It summaries the Spoken Tutorial project

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If you do not have good bandwidth, you can download and watch it

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The Spoken Tutorial Project Team  Conducts workshops using spoken tutorials

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Gives certificates to those who pass an online  test

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For more details, please write to contact at spoken hyphen tutorial dot org

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Spoken Tutorial Project is a part of the Talk to a  Teacher project

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It is supported by the National Mission on Education through ICT, MHRD, Government of India.

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More information on this Mission is available at spoken hyphen tutorial dot org slash NMEICT hyphen Intro

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Hope you enjoyed this Perl tutorial.

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This is Amol signing off.

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Thanks for joining.