Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :

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Chapter 2: Perl Overview

Version 0.85

2.1. Hello World in Perl

2.2. Overview of Perl Lexical Structure, Syntax and Operators

2.3. Perl Variables  

2.4. Basic Control Structures

2.5. Typical errors and pitfalls

2.6. Exercises

This chapter provide a general overview of Perl. The problem here is that it's impossible to construct a very logical sequence of chapters in such a way that material of the next would be completely based on previous. So here we will try to present something like view of Perl kingdom from a helicopter :-).

You should use Perl 5.10-5.16 documentation a reference, because there were several major changes in language constructs  since 5.8.5. This book covers version 5.10.  

Both Perl and C/C++ share the same basic rules and layout:

• Scripts are executed sequentially, on a statement by statement basis.

• Statements must be terminated by a semicolon, statements can be separated by new lines.

• Blocks are delimited by braces.

• Functions can be defined before or after you use them.

There are also several notable differences

• Comments are "Shell-style" they start with the letter # and span till the end of the line

• Variables do not need to be declared. Also they are weakly typed: the same variable can hold numeric value, string value and pointer.

• Memory allocation for all the variables is automatic.

• Operators are typed. So there is two comparison operators, one for strings (>eq==).

• Conversion from string value to numeric value are performed automatically based of the type of the operator.

• Language allow operation with undefined variables, in this case their initial value is assumed to be empty string in case of string operations and zero in case of numeric operations.

• As language is interpreted statements can be constructed and executed dynamically.

• "Dangling "else" problem is solved by not allowing it at all. Only block of statements is allowed after if (condition) construct.

• Control structures are a little bit more flexible and a use a little bit different keywords. 

Perl is also very close to bash (and Unix shells in general).  Generally sysadmins who know bash scripting well can be productive in Perl after a couple of days of training.

Here are major similarities

• Lexical structure is close. In both languages comments start with # and there are two types of sting literals -- single quotes and double quoted.
• Variable names are prefixes ($ for scalar variable, line is shell with $ means dereferencing)
• Double quoted strings are "interpolated" -- process to expand varibles contains in them
• Similar "duality" in comparison -- different sets of symbols are used to compare string and numbers. Using particular set convert particular variable to the target type (string of number)
• Unsuccessful conversion to numeric result in value zero, not in the error.
• You can execute arbitrary Unix commands or pipes using backquotes
• The same syntax for functions sub <name>  Like in shell subroutines and functions can be assed arbitrary number of arguments which are untyped.

There are also some differences. Among them:

• In Perl you use $ also on the left sigh of the assignment. Perl has two additional prefixes (@ and % ) for arrays and hashes correspondingly
• Perl uses C-style syntax for conditional and loops with  curvy brackets denoting a block
• Unlike shell arguments to function are passed by reference, not by value.
• Functions can be declared anywhere in the program not before the first invocation like in shell
• Unlike shell Perl behave more like a regular language without shell style quirks like textual macrosubstitution of variable before syntax analysis.
• Perl has an excellent built-in debugger
• Perl has an excellent  ecosystem of reusable modules (CPAN)

Here is the script  that replaces each line which contain a sting passed as the first argument with the replacement string supplied as the second argument for the file supplied as the third argument, coded in both language (such script might be useful for making small changes in configuration files)

#!/bin/bash
# find lines with the search string and replace them with the replace string
# for the whole file

search_string=$1
replace_string=$2
file=$3
cat $3 | while read line ; do 
     if [[ $line =~ $search_string ]] ; then 
        print "$replace_string\n"
     else 
        print $line
     fi
done

#!/usr/bin/perl
# find lines with the search string and replace them with the replace string
# for the whole file

   $search_string=$ARGV[0];
   $replace_string=$ARGV[1];
   $file=$ARGV[2];
   open (SYSIN, "<$file");
   while ( $line=<SYSIN> ) {
      if ( $line =~ /$search_string/) {
         print "$replace_string\n";
       } else {
         print $line;
       }
   }

Automatic conversion of bash scripts to shell is possible in only in simple cases.  but there are tools

1. a2p - awk-to-perl
2. s2p - sed-to-perl

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Etc

Society

Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy

Quotes

War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotes :  Somerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose Bierce :  Bernard Shaw : Mark Twain Quotes

Bulletin:

Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law

History:

Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOS :  Programming Languages History : PL/1 : Simula 67 : C : History of GCC development :  Scripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-Month :  How to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Hater�s Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D

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Last modified: March 12, 2019