Getting Started with Perl

Jeffrey ROACH28 November

What is Perl? What is it good for?

• Perl is a scripting language• Perl is a prototyping language• Perl is designed for relatively short scripts• Perl programs are best written by a single

programmer• Perl is ideal for:– Test processing: System Administration, Back-end

web administration, Bioinformatics

Popularity of Perl

The decline of Perl

• Replaced by PHP and Python• Syntax is very different from other languages– Programming constructs programmers expect are non-

supported– Subroutines are available, but weak– Object-oriented techniques are available, but weak and

slow• Perl is not suitable for large scale, multi-developer

projects• Perl 6 has been coming next year for the last five years

Nevertheless

• Perl remains useful as a scripting language• Perl is installed in all Linux/Unix/Mac OS X

machines• Perl is easily installed on Windows (Active State)• Perl is free and a good place to start learning

good practice• Perl allows non-programmers to write small

programs that can do worthwhile things quickly

Learning Perl

• Perl is a big, messy language• Two hours is not sufficient to even crack the

surface• What we will do:– Learn seven (7) basic concepts– Use this foundation to build some small, useful (at

least a little) tools– Decide whether you want to learn more on your

own: Learning Perl The Hard Way

Getting Started on Kure or Killdevil

• Step 1: Get the course materialscd /netscr/<your_onyen>cp –r /netscr/roachjm/Perl .ls

• Step 2: Choose an editornano 01_helloworld.pl (OR)vi 01_helloworld.pl (OR)emacs 01_helloworld.pl

01_helloworld.pl

01_helloworld.pl#!/usr/bin/perl

use strict;use warnings;

print "Hello, World!\n";

print 'Hello, World';

print "<--- No New Line\n";

print 'Hello, World!\n';

print "\n";

Notes• Indicates which perl to use

– Allows ./01_helloworld.pl– Use chmod u+x <file.pl>

• Strict and warnings pretty much standard

• Note distinction between “” and ‘’

• Note the new line characer “\n”

02_variables.pl

02_variables.pl#!/usr/bin/perl

use strict;use warnings;

my $a = 1;print "a: $a\n";

my $b = 2;print "b: $b\n";

$b = $a + $b;print "Added a to b.\n";print "b: $b\n";…my $first_name = "Jeff";my $last_name = "ROACH";print "Full Name: $first_name $last_name\n";…

Notes• Usual start• Standard practice

• Variables hold values• Values may be numbers or

strings• Perl is pretty promiscuous

about this

03_arrays.pl

03_arrays.pl…my @a = (1,2,3,4,5);

print "@a\n";

print '$a[0] $a[1] $a[2] $a[3] $a[4]:';print "\t$a[0] $a[1] $a[2] $a[3] $a[4]\n";

print '$a[-1] $a[-2] $a[-3] $a[-4] $a[-5]:';print "\t$a[-1] $a[-2] $a[-3] $a[-4] $a[-5]\n";

my $len_a = scalar(@a);for (my $i=0; $i<$len_a; $i++) { print "$a[$i] ";}print "\n";…

Notes• Arrays are variables that

hold more than one value• Individual values are indexed

by an integer• Array is @a• First element is $a[0]• Arrays can store numbers

and strings• Index must always be integer

04_hashes.pl

04_hashes.pl…my %a = ('one'=>1, 'two'=>2, 'three'=>3, 'four'=>4,

'five'=>5);

print "%a\n";

print '$a{\'one\'} $a{\'two\'} $a{\'three\'} $a{\'four\'} $a{\'five\'}:';

print "\t$a{'one'} $a{'two'} $a{'three'} $a{'four'} $a{'five'}\n\n";

foreach (keys %a) { print "$_ => $a{$_} ";}print "\n\n";…

Notes• Same as arrays, but with

arbitrary indices• Hash is %a• Element at ‘f’ is $a{‘f’}• Also called dictionaries,

associative arrays, maps

05_subs.pl

05_subs.plsub greeting { my @param = @_;

print "Hello, $param[0], how are you?\n\n";

return 0;}

print "Round 1:\n";foreach (@names) { greeting($_);}

Notes• Subroutines were the basis

for Structured Programming circa 1970 – 1975

• Natural way to break larger programs into smaller blocks

• Improves readability, code re-use, and code quality

• Perl support is somewhat underwhelming

06_scope.pl

06_scope.plmy @names = ('Jeff', 'Jon', 'David',

'Sam');

sub scope1 { print "In Scope1:\n"; print "\tNames: @names\n";

my @new_names = ('Ffej', 'Noj', 'Divad', 'Mas');

print "\tNew Names: @new_names\n";

}

Notes• Scope is the value that subroutines

add• Scope restricts the value that

variables take to a particular subroutine

• Essentially a context• Prevents name conflicts and allows

larger programs to be composed of smaller parts

• Hierarchical• Concept expanded in object-

oriented programming

07_files.pl

07_files.plsub read_file { my @params = @_;

my $filename = $params[0]; open(FILE,'<',$filename) or die $!;…}…print "\nRead File\n";read_file("File1.txt");

print "\nWrite File\n";copy_file("File1.txt","File2.txt");

Notes./07_files.pl File1.txt File2.txtdiff File1.txt File2.txtcp File1.txt File2CP.txtdiff file2.txt File2CP.txt

• Read and write from files• Fundamental importance

08_echo.pl

08_echo.pl#!/usr/bin/perl

use strict;use warnings;

my $argc = scalar(@ARGV);

print "@ARGV\n\n";

for (my $i=0; $i<$argc; $i++) { print "$ARGV[$i] ";}print "\n\n";

foreach (@ARGV) { print "$_ ";}print "\n";

Notes./08_echo.pl This is a testecho This is a test

• Uses @ARGV builtin array

09_stats.pl

09_stats.pl

my $argc = scalar(@ARGV); # You can …

print "@ARGV\n";

my $sum = 0.0;my $sumsq = 0.0;for (my $i=0; $i<$argc; $i++) { $sum = $sum + $ARGV[$i]; $sumsq = $sumsq + $ARGV[$i]*$ARGV[$i]; }

my $mean = $sum / $argc;my $stddev = sqrt(($sumsq - $sum*$sum/$argc) / ($argc - 1));

print "n: $argc\n";print "mean: $mean\n";print "stddev: $stddev\n";

Notes./09_stats.pl 1 2 3 4 5

• # denotes comments• Uses @ARGV builtin array

• Single pass standard deviation

10_cat.pl

10_cat.plsub read_file { my @params = @_;

my $filename = $params[0]; open(FILE,'<',$filename) or die $!;

while (my $line = <FILE>) {print $line;

}

close(FILE); return 0;}

Notes./10_cat.pl File1.txt File2.txtcat File1.txt File2.txt

• Basic command line args• Basic file I/O

11_wc.pl

11_wc.plsub count_file { my @params = @_;

my $filename = $params[0]; open(FILE,'<',$filename) or die $!;

my $characters = 0; my $words = 0; my $lines = 0;

while (my $line = <FILE>) {

$characters = $characters + length($line);

my @word_array = split(' ',$line);

$words = $words + scalar(@word_array);

$lines = $lines + 1; }

close(FILE);

return "$characters $words $lines";}

Notes./11_wc.pl File1.plwc File1.pl

• Basic file I/O• Accumulator• String Split

• Character, word, line order reversed

12_cut.pl

12_cut.plsub cut_file { my @params = @_;

my $filename = $params[0]; open(FILE,'<',$filename) or die $!;

while (my $line = <FILE>) {

chomp($line);

my @line_array = split(',',$line);

my $column1 = $line_array[0];my $column4 = $line_array[3];

print "$column1,$column4\n"; }

close(FILE);

return 0;}

Notes./12_cut.pl File1.csvcut –d , -f 1,4 File1.csv

• Split on comma• Print selected columns

13_grep.pl

13_grep.pl

while (my $line = <FILE>) {

chomp($line);

if ($line =~ m/System Sleep/) {

$times = $times + 1;

my @line_array = split(' ',$line);

my $date1 = $line_array[0]; my $date2 = $line_array[1]; my $time = $line_array[2];

print "System Sleep at: $date1 $date2 $time\n";}

}

Notes./13_grep.pl File1.log

• Chomp procedure• If/then control structure• Regular expression

• Log file analysis

14_head.pl

14_head.plsub read_file { my @params = @_;

my $filename = $params[0]; open(FILE,'<',$filename) or die $!;

my $lines_written = 0; while ((my $line = <FILE>) and ($lines_written < 10)) {

print $line;$lines_written = $lines_written + 1;

#Not allowed with strict#if ($lines_written == 10) {# break;#}

}

close(FILE); return 0;}

Notes./14_head.pl File1.loghead File1.log

• Use of boolean and in while• Failed use of break• Common use of comments

15_awk.pl

15_awk.plsub hist_file { my @params = @_;

my $filename = $params[0]; open(FILE,'<',$filename) or die $!;

my %cities = ();

while (my $line = <FILE>) {

chomp($line);

my @line_array = split(',',$line);

if ($line_array[4] eq '"Active"') {

my $city = $line_array[0]; my $hours = $line_array[5];

Notes• ./15_awk.pl File2.csv

• AWK is actually is own programming

• String eq not =

15_awk.pl

15_awk.pl my $city_total = $cities{$city}; if ($city_total) { $cities{$city} = $city_total + $hours; } else {$cities{$city} = $hours; }}

}

close(FILE);

return %cities;}

Notes• If/Then/Else checking for

key• Returns cities hash

Conclusions

• Perl has tons of possibilities• Expressive: You can write things a million different

ways• Useful: You can make useful little tools relatively

easily• Organic development and prototyping• For further self-study:– Learning Perl The Hard Way

• There is also Python, Ruby, PHP, and Lua