1. Linux Operating System
Unit 6: More Scripting in Linux
4/22/2017
Linux Operating System
A Practical Guide to Fedora and Red Hat Enterprise Linux
Unit 6: More Scripting in Linux
Chapter 28: The Perl Scripting Language
By Fred R. McClurg
© Copyright 2013, All Rights Reserved
Chapter 28: The Perl Scripting Language

2. Perl History Written in 1987 by Larry Wall
Backronym: Practical Extraction and Report Language
Influenced By:
C Language
Bourne Shell
AWK
sed

3. Perl Strengths Purpose: Strengths: System Administration
Bioinformatics
Text Processing
General Purpose
Strengths:
Portable
Informal
Practical
Robust
Easy-to-use
Complete

4. Perl Distinctions
Nicknamed: The Swiss Army chainsaw of programming languages
Motto: TIMTOWDI (pronounced “Tim Toady”) acronym for “There is more than one way to do it”
Slogan: Easy things should be easy and hard things should be possible

5. More Perl Distinctions
Interpreted: Code does not need run through a compiler (compiled at run-time).
Dynamically Typed: Variable types do not have to be declared. Type is determined at run-time by the context.
Side effects are often beneficial!

6. Executing Perl First line of file: #!/usr/bin/perl
Via shell command (Win & Linux):
perl script.pl
After chmod +x (Linux only):
script.pl

7. Perl Documentation Online: perldoc: Man page-like docs Books:
Programming Perl (Camel book)
Learning Perl (Llama book)
Online:

8. Perl Terminology Variable: Block: Named storage for values.
Zero or more statements surrounded by curly braces “{}”.

9. Perl Terminology Function: Module:
Self Contained set of instructions that can be called repeatedly.
Module:
Encapsulated collection of functions, similar to a library in other languages.

10. More Perl Terminology Statement: Single instruction or command
Every statement must end with a semi-colon “;”
White space in a statement does not matter except within a quoted string.

11. Variable Types: Perl Variables Scalar Variables: “$”
Array Variables:
Hash Variables: “%”

12. Scalar Variable Defined
Defined: A named memory location containing a single value.
Begins with “$”

13. Storing Scalar Variables
Examples:
$bugs = "Wabbit";
$fudd = "Wascal $bugs";
$money = '$1,000,000';
$count = 70;
$times = "7";
$PI = ;
$forgive = $count * $times;

14. Displaying Scalar Variables
Examples:
print $bugs . "\n";
print "$fudd\n";
printf( "%s\n", $money );
printf( "%d\n", $times );
printf( "%02.3f\n", $PI );
printf( "%s\n", $forgive );
printf( "%d\n", $forgive );

15. Scalar Variable Output
Script Output:
Wabbit
Wascal Wabbit
$1,000,000 70
3.142
490

16. Array Variable Defined
Defined: A collection of variables referenced by a numeric index. Often called a list.
Begins with

17. Array Initialization by Element
Example:
$marx[0] = "Groucho"; $marx[1] = "Harpo"; $marx[2] = "Chico";

18. Array Initialization via List
Example:
@marx = ( "Groucho", "Harpo", "Chico" );

19. Defined: A collection of variables referenced by a numeric index
Hash Variable Defined
Defined: A collection of variables referenced by a numeric index
Begins with “%”

20. Hash Initialization by Element
Example:
$comic{'hulk'} = "David Banner";
$comic{'superman'} = "Clark Kent";
$comic{'batman'} = "Bruce Wayne";

21. Hash Initialization One Step
Example:
%comic = (
"hulk" => "David Banner",
"superman" => "Clark Kent",
"batman" => "Bruce Wayne" );

22. if : Flow Diagram if
statements
True
False

23. if: Control Structure
Definition: Evaluates the logical condition: if expression is true, statements are executed
Syntax:
if ( condition ) {
statements; }

24. if : Command Arg Example
Purpose: Display first 3 command line arguments if specified
if ( $#ARGV == 2 ) {
print "$ARGV[0]\n";
print "$ARGV[1]\n";
print "$ARGV[2]\n"; }

25. if: Command Prompt Example
Purpose: Guess the “mystery” word when prompted
$target = "secret";
print "Guess word: ";
$guess = <>;
chomp( $guess );
if ( $guess eq $target ) {
printf( "That's right!\n“ ); }

26. if: Expression Modifier
Definition: Evaluates a logical condition and controls whether or not statements are executed
Syntax:
statement if ( cond );

27. if : Modifier Example
Purpose: Display message if three arguments specified
print "Three args\n"
if ( $#ARGV == 2 );

28. unless : Flow Diagram
unless
statements
False
True

29. unless: Control Structure
Definition: Evaluates the logical condition: if expression is false, statements are executed
Syntax:
unless( condition ) {
statements; }

30. unless : Example
Purpose: Prompt user for age and display a cheerful word of encouragement.
$retirement = 65;
print( "Enter age: " );
$age = <>;
unless ( $age >= $retirement ) {
print( "Get to work!\n" ); }

31. if ... else: Structure
Definition: Evaluates a condition. If condition is true, control follows one branch. If the condition is not true (else), control follows a different branch.
Syntax:
if ( condition ) {
statements; }
else

32. if ... then ... else: Flow Diagram
statements
else
branch
True
False

33. if ... else: What is truth? if ( 0 ) { print( "Right!\n" ); } else
Purpose: Determine what is true and what is false. Question: What is printed here?
if ( 0 ) {
print( "Right!\n" ); }
else
print( "Wrong.\n" );

34. if ... else: Guess a number Purpose: Guess a “mystery” prompted number
$target = 4;
print( "Guess number: " );
$guess = <>;
if ( $guess == $target ) {
print( "Right!\n" ); }
else {
print( "Wrong.\n" );

35. if ... elsif ... else: Flow Diagram
False if
statements
True
False if
branch
elsif
statements
True
else
elsif
branch
else
fallback
statements

36. if ... elsif ... else: Example
Purpose: File type of command-line argument
$file = $ARGV[0];
if ( -l $file ) {
printf( "Link: %s\n", $file ); }
elsif ( -f $file ) {
printf( "File: %s\n", $file );
elsif ( -d $file ) {
printf( "Directory: %s\n", $file );
else {
print( "Not link, file, or directory\n" );

37. Comparison Operators == eq Equals != ne Not equals < lt Less than
Numeric
String
Description == eq
Equals != ne
Not equals
< lt
Less than
> gt
Greater than
<= ge
Greater or equal to
>= le
Less or equal to

38. foreach: Flow Diagram
False
foreach
true
foreach
True
statements

39. foreach: Looping Structures
Description: Structure that performs given number of iterations on a statement or statements
Syntax:
foreach $var ) {
statements; }

40. foreach: Looping Example
Purpose: Display all elements in an array
@dwarfs = ( "Doc", "Grumpy",
"Happy", "Sleepy",
"Bashful", "Sneezy",
"Dopey" );
foreach $dwarf ) {
print( "Dwarf: $dwarf\n" ); }

41. for: Looping Structure
Description: General purpose looping structure for a given iteration
Syntax:
for ( init; cond; incDec ) {
statements; }

42. for: Looping Example Purpose: Count down from 5 to 1
for ( $i = 5; $i > 0; $i-- ) {
printf( "Count: %d\n",
$i ); }
print( "Lift off!\n" );

43. while: Flow Diagram
False
while
true
while
True
statements

44. while: Looping Structures
Description: Structure that performs iterations while a condition is true (i.e. repeat until false)
Syntax:
while ( condition ) {
statements; }

45. while: Looping Example
Purpose: Count file lines (wc -l)
while ( $line = <> ) {
$count++; }
print( "Lines: $count\n" );

46. until: Flow Diagram
True
until
until
false
False
commands

47. until: Looping Structures
Description: Structure that performs iterations while a condition is false (i.e. repeat until true)
Syntax:
until ( condition ) {
statements; }

48. until: Looping Example
Purpose: Guess a number
$secret = 4;
print "Number between 1 & 10\n";
print "Your guess: ";
until ( $secret ==
( $guess = <> ) ) { }
print( "Correct!\n" );

49. last: Looping Interrupt
Description: Terminates the current iteration and breaks out of the loop
Example:
for ( $i = 1; $i < 6; $i++ ) {
last if ( $i == 3 );
print( "$i\n" ); }

50. next: Looping Interrupt
Description: Terminates the current loop and continues the next iteration
Example:
for ( $i = 1; $i < 6; $i++ ) {
next if ( $i == 3 );
print( "$i\n" ); }

51. sort: Function Description: Returns a sorted list @reindeers =
( "Dancer", "Prancer",
"Vixen", "Comet",
"Cupid", "Donner",
"Blitzen", "Rudolph" );
@reindeers = );
foreach $reindeer ) {
print "$reindeer\n"; }

52. subroutine: Custom Function
Description: Encapsulates a set of commands
$first = 1;
$second = 2;
&add();
print "Answer: $ansr\n";
sub add() {
$ansr = $first + $second; }

53. Returning subroutine value
Description: Returning a value from sub
$first = 1;
$second = 2;
$ansr = &add();
print "Answer: $ansr\n";
sub add() {
return( $first + $second ); }

54. Passing subroutine params
Description: Passing parameters to a subroutine
$first = 1;
$second = 2;
$ansr = &add( $first, $second );
print "Answer: $ansr\n";
sub add() {
my( $one, $two )
return( $one + $two ); }

55. Regular Expression matching
Description: Few character wildcards can perform powerful pattern matching.
print( "Enter a number: " );
chomp( $digit = <> );
if ( $digit =~ /^[0-9]+$/ ) {
print( "Thanks!\n" ); }
else {
print( "Not a number\n" );

56. Regular Expressions Char Matches ^ Beginning of line anchor $
End of line anchor
(...)
Regular expression grouping .
Any character except newline “\n” \\
Backslash (escaped) *
Zero or more of previous ?
Zero or one of previous +
One of more of previous

57. More Regular Expressions
Char
Matches
[...]
Set of characters \d
Numeric digit [0-9] \D
Non-numeric digit [^0-9] \s
White space (space, tab, newline, etc) \S
Non-white space [^\s] \w
Word char (alpha or numeric)
[a-zA-Z0-9] \W
Non-word character [^a-zA-Z0-9]

58. Greedy & Non-greedy Matches
Description: Regular expressions usually match the longest string possible (greedy matches).
Non-greedy regular expressions match the shortest string possible.

59. REs are usually greedy Example:
$html = "<b>bold font</b>";
$text = $html;
$text =~ s/<.*>//g;
print "Text: \"$text\"\n";

60. Making a RE non-greedy Example:
$html = "<b>bold font</b>";
$text = $html;
$text =~ s/<.*?>//;
print "Text: \"$text\"\n";

61. Grouping Patterns Example: $firstLast = "Fred McClurg";
$lastFirst = $firstLast;
$lastFirst =~ s/([^\s]+)\s([^\s]+)/$2, $1/g;
print "$lastFirst\n";

62. split and other stuff
@passwd = `cat /etc/passwd`; foreach $line ) { chomp( $line = split( /:/, $line ); # frmcclurg:x:500:500:Fred R. McClurg:/home/frmcclurg:/bin/bash $shell = $parts[6]; $userCount++; $shells{$shell}++; } printf( "User count: %d\n\n", $userCount ); foreach $shell ( keys %shells ) { printf( "Shell: %s\t%d\n", $shell, $shells{$shell} );

63. Debugging Shell Scripts
Description: Print warning messages regarding possible problems
Example:
#!/usr/bin/perl -w

64. Debugging Shell Scripts
Description: Force declaration of variables.
Example:
use strict;
my( $variable ) = 0;
print "$variable\n";