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Perl
Practical Extraction and Report Language
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Objectives
Introduction Basic features Variables Operators and functions Control structures Input and output
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Perl is a script language Elements of csh, sh, awk and C Used for:
Text manipulation CGI scripting System administration
Written and maintained by Larry Wall Websites
www.perl.com www.perl.org www.perldoc.com www.perlfoundation.org www.cpan.org
Introduction to Perl
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Example:
#! /usr/bin/perl
# example of perl script
print “hello world\n”;
print(“hello world\n”);
printf(“hello world\n”);
printf(“%s\n”, “hello world”);
Introduction to Perl
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Variables
Scalar Single valued
Array Indexed collection of values
Hash Collection of key, value pairs
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Scalar Variables
string$word = “test”;
print “here it is: $word \n”;
quotes around string optional, if clear$word = test;
print “here it is $word \n”;
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Scalar Variables
integer
$i = 2;print $i;print “$i\n”;
$j = 2 * $i + 4;
printf(“Result %d \n”, $j);print “Result $j \n”;
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Scalar Variables
floating point
$f = 1.234;
$g = 2 * $f + 4;
printf("Result %f\n", $g);
print "Result $g\n";
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Reading Input
from Standard input
print("enter a number: ");
$number = <>;
chop($number);
print("here it is: $number \n");
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Array Variables
Array holds list:
("one", "two", "three")
(1, 4, 5, 7, 8, 0)
(1.2, 4.3, 6.5)
(“one”, 2, 3.14159)
($word, $i, $f)
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Array Variables
array variable starts with @
@list=("one", "two", "three");
print “@list \n”;
array element starts with $
print("here it is: $list[0]\n");
print("here it is: $list[1]\n");
print("here it is: $list[2]\n");
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Array Variables
array slice
@list=("one", "two", "three”, 4);@a = @list[1..3];@b = @list[2,0];
or:
@c = ("one", "two", "three”, 4)[2,0]
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Array Variables
assignment
($first, $second) = (“one”, “two”);
@list = ($first, $second);
($o1, $o2) = @list;
array size
$size = @array; highest array index
$#array
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Array Variables
example: swap two variables
$temp = $first;
$first = $second;
$second = $temp
better:($first, $second) = ($second, $first);
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Array functions
to remove element from front shift @array;
to remove element from back pop @array;
to reverse elements reverse @array;
to sort elements sort @array;
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Variables
Hash: associative array
%animals = ("cat“ => 10,
"dog“ => 12,
"fish“ => 23);
print("1: $animals{'cat'}\n");
print("2: $animals{'dog'}\n");
print("3: $animals{'fish'}\n");
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Hash variable operation
%animals = ("cat“ => 10,
"dog“ => 12,
"fish“ => 23);
%index = reverse %animals;
print("1: $index{‘10'}\n");
print("2: $index{‘12'}\n");
print("3: $index{‘23'}\n");
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Hash variable operation
%animals = ("cat“ => 10,
"dog“ => 12,
"fish“ => 23);
@names = keys %animals;
@numbers = values %animals;
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Special Variables
Default parameter: $_
Used by many operations when no parameters are given
Example:$_ = “hello world\n”;
print;
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Command line arguments
$ARGV[0], $ARGV[1], …
print("$ARGV[0], $ARGV[1]\n");
With a loop:$size = @ARGV;
for ($i = 0; $i < $size; $i++)
{
print("$ARGV[$i]\n");
}
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Environment variables
%ENV hash
examples:
print $ENV{HOME};
print $ENV{PATH};
$ENV{EDITOR} = “emacs”;
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Operators numeric: + - * / % ++ -- boolean: && || ! and or not & | ^ string:
. x
examples:
$a = “foo” . “bar”;
$b = “number “ . 1;
$c = “1” . “2”;
$d = “ba” . (“na”x4);
$e = 1 x 3;
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String functions
remove last character: chop
remove last character, if it is “\n”: chomp
$a = “foobar\n”;
print chop(a);
print chomp(a);
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String function: split
split string into words needs word separator example:
$test = “one two three”;
@list = split “ “, $test;
print “@list\n”;
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String function: split
split with no parameters will split $_ on whitespace
example:
$_ = “one two three”;
print split;
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String function: split
word separator examples: split on string
“:”
split on regular expression /e.e/ /[abc]/
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String function: join
opposite of split:
@list = (“one”, “two”, “three”);
$text = join “?”, @list;
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String function: substr manipulate substring
$string="the glistening trophies"; print substr($string, 15); # trophies print substr($string, -3); # ies print substr($string, -18, 9); # listening print substr($string, 4, 4); # glis
substr($string, 7, 4, "tter"); # Returns "sten" print $string; # the glittering trophies
substr($string, 7, 4) ="tter"; # Functions as lvalue
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Control Structures
sequence: run until end of file exit and die
conditional: if-then-else, unless
loops: for while
functions
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Sequence
all statements are terminated with semicolon to end execution
exit;
to end with error message die(“it was the food”);
to just print a warning (without exiting) warn(“it is too warm in here”);
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Conditional
if
if ( $i > 10 ) {
print “yes\n”;
} else {
print “no\n”;
}
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Conditional
unless
unless ( $i > 10 ) {
print “no\n”;
} else
print “yes\n”;
}
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Conditional
one liners:
print “no\n” unless ( $i > 10 );
print “yes\n” if ( $i > 10 );
die(“unhappy”) unless ( $happy > 0);
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Loops
C-like for loop:
for ($i=0; $i<5; $i++) {print “Counter is $i\n”;
}
simplified:
for $i (0..4) {print “Counter is $i\n”;
}
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Loops
for arrays:
@array = (“one”, “two”, “three”);
foreach (@array) {
print “value is $_ \n”;
}
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Loops
while:
$i=10;
while ($i > 5) {
print “value is $i \n”;
$i--;
}
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Loops
special keywords within while:
next skip to next iteration
last end loop
redo rerun iteration
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Pattern matching
Perl supports regular expressions basic and extended enclosed in forward slash “/”
match operator =~ return boolean true or false
$sea = “water sand jaws swimmers”;print “Shark Alert !” if $sea =~ /jaws/;
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Pattern matching
special characters:\w - word \W - non word\s - space \S - non space\d - digit \D - non digit\b - word boundary
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Pattern matching
pattern may include parenthesis () identifies matched junks as $1, $2, $3
$sea = “water sand jaws swimmers”;print “Shark Alert !” if $sea =~ /(j??s)/;print $1;
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Pattern substitution
$test = “he said she said”;$test =~ s/said/did/;
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Sub-functions
example:
sub greeting {
print “hello world\n”;
}
…
greeting();
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Sub-functions
parameters are passed via @_ array:
sub action {($one, $two, $three) = @_;warn “too few parameters”
unless $one && $two && $three;print “The $one $two on the $three\n”;
} …
action(“cat”, “sat”, “hat”);
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Sub-functions
return results via return:
sub action {($one $two $three) = @_;warn “too few parameters”
unless $one && $two && $three;return “The $one $two on the $three\n”;
} …
print action(“cat”, “sat”, “hat”);
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Sub-functions
local variables: visible only with scope of sub-function
sub getwords {
my pat = $_[0];
my ($line, @subwords);
$line = <>;
chop($line);
@subwords = split /$pat/, $line;
return @subwords;
}
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Reading Input
from Standard input
print("enter a number: ");
$number = <STDIN>;
print("here it is: $number");
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Reading Input
from Standard input
$i = 0;
while ($line = <STDIN>) {
print $line;
$i++;
}
print(“line count: $i \n");
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File I/O
based on file handle predefined file handles
STDIN
STDOUT
STDERR
print STDERR “…oohh, something went wrong !\n”;
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File I/O
to create new file handle open file open filehandle filename;
filename can be: “file” “<file” “>file” “>>file”
“ls|” “|wc”
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Reading from a File
if (!open(FILE, "$ARGV[0]")) { print "cannot open: $ARGV[0]\n“ ;}while ($line = <FILE>) { print "$line“ ;}
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Reading complete File
die “horribly” unless open FILE “t.txt”;@all = <FILE>;$linecount = @all;print “line count: $linecount”;
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Writing to a File
die "cannot open: $ARGV[0]” unless open FILE, “>$ARGV[0]“
@array = (“one”, “two”, “three”);foreach (@array) { print FILE “$_ \n”;}
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Running system commands
via backquotes ` `$result = `ls | wc –w`
via system function$result = system(“ls | wc –w”);
via file I/Oopen FILE, “ls | wc –w|”;$result = <FILE>;
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Running system commands
#!/usr/local/bin/perl# convert series of files from DOS to Unix format
if ($#ARGV < 0) { print "usage: fconvert files\n"; exit;}foreach (@ARGV) { $cmd = "dos2unix $_ $_.unix"; print "$cmd\n"; if(system($cmd)) { print "dos2unix failed\n"; }}
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Html via Perl#!/usr/local/bin/perl# create n html files linked together in slide showif ($#ARGV != 1) { print "usage: htmlslides base num\n"; exit;}$base = $ARGV[0];$num = $ARGV[1];for ($i=1; $i <= $num; $i++) { open(HTML, ">$base$i.html"); if($i==$num) { $next = 1; } else { $next = $i+1; } print HTML "<html>\n<head>\n<title>$base$i”; print HTML “</title>\n</head>\n<body>\n"; print HTML "<a href=\"$base$next.html\">”; print HTML “<img src=\"$base$i.jpg\"></a>\n"; print HTML "</body>\n</html>\n"; close(HTML);}
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Perl modules
contain large amounts of reusable code CPAN: Comprehensive Perl Archive Network
networking web related cgi processing database access spam protection …
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Web & CGI
HTML may include forms to invoke server side functions CGI (Common Gateway Interface) is a protocol
governing how browsers and servers communicate Scripts that send or receive information need to follow
the CGI protocol Perl is the most commonly used language for CGI
programming Perl scripts are written to get, process, and return
information through Web pages
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Html via Perl
#! /usr/local/bin/perl
print “Content-type: text/html\n\n”;
print “<html><head><title>Hello World</title>”;
print “</head><body>\n”;
print “<h2>Test Website</h2>";
print “</body></html>”;
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Html via Perl
CGI module:
use CGI qw(:standard);
print(header);
print(start_html("Hello World"));
print h2("Test Website");
print(end_html);
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Web Page
<html><head><title>COP 3344</title></head><body><h1>User Inquiry</h1><form method=get action=/cgi-bin/example.cgi>Enter your name: <input type="text" size=20 name="userid"><br><input type="submit" value="Submit"></form></body></html>
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Interactive Web Page
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Perl script: example.cgi
#! /usr/local/bin/perl# example perl scriptuse CGI qw(:standard);print(header);print(start_html("Anser Page"));print h2("Answer Page");print("Welcome: ", param('userid'));print(end_html);
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Interactive Web Page
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Chapter Summary
Perl is a powerful scripting language Perl scripts are used to create web pages CGI is a protocol or set of rules governing how
browsers and servers communicate
