Perl Basics

Programs

Perl file has extension ".pl". Start file with "shebang":

#!/usr/bin/perl -w

On unix systems, this tells the compiler which binary to use and with what options.

End lines with semicolon. Comment with #. Example: "hello_world.pl"

#!/usr/bin/perl -w
# Here is a comment.
print "Hello World!"

Execute from command line with

perl hello_world.pl

Pragmas

use strict;   # tells the compiler to be stricter in checking for problems
use integer;  # all arithmetic uses integers only

List Literals

(2, 5, 8)        # a list with three numerical elements
(1..10)          # a list with ten numerical elements
($n+3, $m)       # a list with two elements
("a", "b", "c")  # a list with three string elements
qw( a b c )      # same list

Scalar Variables

Scalar variables are initialized as undef, which translates to 0 for numbers and ´´ for strings.

$age = 12;
$name = "Bill";
($name, $age) = ("Bill", 12);

Arrays and Lists

@dogs = ("Lassie", "Biff", "Bruno");
@ages = (3, 4, 5);
$Lassie_age = $ages[0];
$last_element_index = $#dogs     # gives index of last element = size-1
$last_element = $dogs[$#dogs]    # last element
$last_element = $dogs[-1]        # also refers to last element
# shift, unshift, push, pop, reverse, sort
@array = qw$ a b c $;            # qw operates on any punctuation pair
$first = shift @array;           # $first is "a", @array becomes ("b", "c")
unshift @array, $first;          # @array is back to ("a", "b", "c")
$last = pop @array;              # $last is "c", @array becomes ("a", "b")
push @array, $last;              # @array is back to ("a", "b", "c")
push @array, 1..3;               # @array becomes ("a", "b", "c", 1, 2, 3)
@array = reverse @array;         # @array becomes (3, 2, 1, "c", "b", "a")
@array = sort @array;            # @array becomes (1, 2, 3, "a", "b", "c")
@array = reverse sort @array;    # @array becomes ("c", "b", "a", 3, 2, 1)
# iterate over an array
foreach $element (@my_array){
    # do something with $element
    # the elements of @my_array can be altered this way
}
# scalar vs list context
@array = qw( a b c );
$number = @array;                # $number is 3
$number = scalar @array;         # more explicit
@array = "hi, there";            # becomes a list with one string element

Hash Variables

%list = ("Lassie", 3, "Biff", 4, "Bruno", 5);
%same_list = (
    Lassie   => 3,
    Biff     => 4,
    Bruno    => 5,  # final trailing comma is legal but optional
); #strings implicitly quoted
$same_list_again{Lassie} = 3;
$same_list_again{Biff} = 4;
$same_list_again{Bruno} = 5;
$Lassie_age = $list{Lassie};
# extract keys and values
my @keys = keys %list;      # @keys is ("Lassie", "Biff", "Bruno")
my @values = values %list;  # @values is (3, 4, 5)
# check for a given key
if (exists $list{"Rover"}){...}
# delete a given key
delete $list{"Lassie"})
# iterate over a hash:
while ( ($key, $value) = each %my_hash ){
    # do something with $key and $value
}
# iterate over a hash in key-sorted order
foreach $key (sort keys %hash) {
    $value = $hash{$key};
    # do something with $key and $value
}
# flip a hash (values must be unique)
%ip_addresses = reverse %host_names;

Variable Scope

Variables are global by default. Loop control variables are local to the loop. Use the "my" command to declare locally.

Strings

Use double quotes to allow variable interpolation.

$name = "Barney";
print 'My name is $name';  # prints My name is $name
print "My name is $name";  # prints My name is Barney

Concatenate with period

$temp = "another "."variable";   # temp is another variable

Substring

print substr $name, 2, 3;        # returns ott
print substr $name, 2, 3, "at";  # returns ott
print $name;                     # returns Scat
my $suffix = substr($file, -3);  # last 3 characters

String comparison

if ($name eq "Barney")...   # equals
# also ne, gt, ge, lt, le

Tokenizer

use Text::ParseWords;
my @list = quotewords(":", 1, $line); # specific delimiter
OR
my @list = shellwords($line);  # split by whitespace

Length

my $size = length($line)

Dates

Represented as seconds since Jan 1, 1970 using long integers.

Date::Parse - Parse date strings into time values

use Date::Parse;
$time = str2time($date);

Timestamp

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)=localtime(time);
printf "%4d-%02d-%02d %02d:%02d:%02d\n",$year+1900,$mon+1,$mday,$hour,$min,$sec;

gives

2007-07-19 15:33:29

Default and Special Variables

# scalar default variable is $_
foreach (1..10) {  # Uses $_ by default
    print "I can count to $_!\n";
}
# array default variable is @_
sub is_greater{
    my ($first, $second) = @_;
    return ($first > $second);
}

• $0 is the name of the called program
• $1 - $99 are regular expression capture variables
• @ARGV is the array of arguments from the command line (not including program name)
• $! is the error message given by the last failed command
• $| set this to 1 to turn off buffering for the current output filehandle
• $` is everything before a regular expression match
• $& is the regular expression match itself
• $' is everthing after the regular expression match

Conditionals and Loops

Perl interprets the value 0 as false for numbers. The strings and '0' are interpreted as false.

Compound

if (expression) {block}
else {block}

unless (expression) {block} 
else {block}

if (expression) {block}
else {block}

if (expression1) {block} 
elsif (expression2) {block} 
else {block}

while (expression) {block}

until (expression) {block}

for ($i = 0; $i < $max; $i++) {block}

foreach var (list) {block}

One-liners

statement if expression;
statement unless expression;
statement while expression;
statement until expression;

System Calls

If you aren't collecting input from the call:

my $status = system("test -e $file");

If you are collecting input:

my @p_file_params = `$p_external_programs{pfprint} -f $p_file`
    or die("pfprint failed!\nAre you sure \"$p_file\" is a valid P-file?\n", usage());

Errors and Warnings

die aborts the program and gives a non-zero exit status:

open(FH, $fileName) or die "open $fileName failed: $!";
# add a newline to the die argument to suppress line numbers
if (@ARGV != 2){
    die "There should be two arguments!\n";
}

warn is just like die except that the program does not abort