I told Casey I wanted to do this a while back, and tonight I finally got
the time for it. The plan is to put a big notice at the top of perl.pod
which says "New to Perl? New to the Perl documentation? Read perlintro
FIRST for an overview."
K.
=head1 NAME
perlintro -- a brief introduction and overview of Perl
=head1 DESCRIPTION
This document is intended to give you a quick overview of the Perl
programming language, along with pointers to further documentation. It
is intended as a "bootstrap" guide for those who are new to the
language, and provides just enough information for you to be able to
read other peoples' Perl and understand roughly what it's doing, or
write your own simple scripts.
This introductory document does not aim to be complete. It does not
even aim to be entirely accurate. In some cases perfection has been
sacrificed in the goal of getting the general idea across. You are
B<strongly> advised to follow this introduction with more information
from the full Perl manual, the table of contents to which can be found
in L<perltoc>.
=head2 What is Perl?
Perl is a general-purpose programming language optimized for
scanning arbitrary text files, extracting information from those text
files, and printing reports based on that information. It's also a
good language for many system management tasks. The language is
intended to be practical (easy to use, efficient, complete) rather
than beautiful (tiny, elegant, minimal).
=head2 Running Perl programs
To run a Perl program from the Unix command line:
perl progname.pl
Alternatively, put this as the first line of your script:
#!/usr/bin/perl
... and run the script as C</path/to/script.pl>
For more information, including instructions for other platforms such as
Windows and MacOS, read L<perlrun>.
=head2 Basic syntax overview
Perl statements end in a semi-colon:
print "Hello, world";
Comments start with a hash symbol and run to the end of the line
# This is a comment
Whitespace is irrelevant:
print
"Hello, world"
;
... except inside quoted strings:
# this would print with a linebreak in the middle
print "Hello
world";
Double quotes or single quotes may be used around literal strings:
print "Hello, world";
print 'Hello, world';
However, only double quotes "interpolate" variables and special
characters such as newlines (C<\n>):
print "Hello, $name\n"; # works fine
print 'Hello, $name\n'; # prints $name\n literally
Numbers don't need quotes around them:
print 42;
You can use parentheses for functions' arguments or omit them
according to your personal taste. They are only required
occasionally to clarify issues of precedence.
print("Hello, world\n");
print "Hello, world\n";
More detailed information about Perl syntax can be found in L<perlsyn>.
=head2 Perl variable types
Perl has three main variable types: scalars, arrays, and hashes.
=over 4
=item Scalars
A scalar represents a single value:
my $animal = "camel";
my $answer = 42;
Scalar values can be used in various ways:
print $animal;
print "The animal is $animal\n";
print "The square of $answer is ", $answer * $answer, "\n";
There are a number of "magic" scalars with names that look like
punctuation or line noise. These special variables are used for all
kinds of purposes, and are documented in L<perlvar>. The only one you
need to know about for now is C<$_> which is the "default variable".
It's used as the default argument to a number of functions in Perl, and
it's set implicitly by certain looping constructs.
print; # prints contents of $_ by default
=item Arrays
An array represents a list of values:
my @animals = ("camel", "llama", "owl");
my @numbers = (23, 42, 69);
my @mixed = ("camel", 42, 1.23);
Arrays are zero-indexed. Here's how you get at elements in an array:
print $animals[0]; # prints "camel"
print $animals[1]; # prints "llama"
print $numbers[$#number]; # last element, prints 1.23
These start with a C<$> because you're getting just a single value out
of the array -- you ask for a scalar, you get a scalar.
To get multiple values from a array:
@animals[0,1]; # gives ("camel", "llama");
@animals[0..2]; # gives ("camel", "llama", "owl");
@animals[1..$#animals]; # gives all except the first element
This is called an "array slice".
You can do various useful things to lists:
my @sorted = sort @animals;
my @backwards = reverse @numbers;
There are a couple of special arrays too, such as C<@ARGV> (the command
line arguments to your script) and C<@_> (the arguments passed to a
subroutine). These are documented in L<perlvar>.
=item Hashes
A hash represents a set of key/value pairs:
my %animals = ("camel", "Programming Perl", "llama", "Learning Perl");
You can use whitespace and the C<< => >> operator to lay them out more
nicely:
my %animals = (
camel => "Programming Perl",
llama => "Learning Perl",
);
To get at hash elements:
$animals{"camel"}; # gives "Programming Perl"
You can get at lists of keys and values with C<keys()> and
C<values()>.
my @animals = keys %animals;
my @books = values %animals;
Hashes have no particular internal order, though you can sort the keys
and loop through them.
Just like special scalars and arrays, there are also special hashes.
The most famous of these is C<%ENV> which contains environment
variables. Read all about it (and other special variables) in
L<perlvar>.
=back
Scalars, arrays and hashes are documented more fully in L<perldata>.
More complex data types can be constructed using references, which allow
you to build lists and hashes within lists and hashes.
my $variables = {
scalar => {
description => "single item",
sigil => '$',
},
array => {
description => "ordered list of items",
sigil => '@',
},
hash => {
description => "key/value pairs",
sigil => '%',
},
};
print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";
Exhaustive information on the topic of references can be found in
L<perlreftut>, L<perllol> and L<perlref>.
=head2 Conditional and looping constructs
Perl has most of the usual conditional and looping constructs except for
case/switch (but you can find a Switch module on CPAN, if you really
want one).
The conditions can be any Perl expression. See the list of operators in
the next section for information on comparison and boolean logic operators,
which are commonly used in conditional statements.
=over 4
=item if
if ( condition ) {
...
} elsif ( other condition ) {
...
} else {
...
}
There's also a backwards version of it:
unless ( condition ) {
...
}
This is provided as a more readable version of C<if (! condition)>.
=item while
while ( condition ) {
...
}
There's also a backwards version, for the same reason we have C<unless>:
until ( condition ) {
...
}
=item for
Exactly like C:
for ($i=0; $i <= $max; $i++) {
...
}
=item foreach
foreach (@array) {
print "This element is $_\n";
}
foreach (keys %hash) {
print "The value of $_ is $hash{$_}\n";
}
=back
=head2 Builtin operators and functions
Perl comes with a wide selection of builtin functions. Some of the ones
we've already seen include C<print>, C<sort> and C<reverse>. A list of
them is given at the start of L<perlfunc> and you can easily read
about any given function by using C<perldoc -f functionname>.
Perl operators are documented in full in L<perlop>, but here are a few
of the most common ones:
=over 4
=item Arithmetic
+ addition
- subtraction
* multiplication
/ division
=item Numeric comparison
== equality
!= inequality
< less than
> greater than
<= less than or equal
>= greater than or equal
=item String comparison
eq equality
ne inequality
lt less than
gt greater than
le less than or equal
ge greater than or equal
=item Boolean logic
&& and
|| or
! not
(C<and>, C<or> and C<not> are also supported as operators, but have
different precedence to C<&&> and friends. Check L<perlop> for more
detail.)
=item Miscellaneous
= assignment
. string concatenation
x string multiplication
.. range operator (creates a list of numbers)
=back
Many operators can be combined with a C<=> as follows:
$a += 1; # same as $a = $a + 1
$a -= 1; # same as $a = $a - 1
$a .= "\n"; # same as $a = $a . "\n";
=head2 Files and I/O
You can open a file for input or output using the C<open()> function.
It's documented in extravagant detail in L<perlopentut>, but in short:
open(INFILE, "input.txt") or die "Can't open input.txt: $!";
open(OUTFILE, ">output.txt") or die "Can't open output.txt: $!";
open(LOGFILE, ">>my.log") or die "Can't open logfile: $!";
You can read from an open filehandle using the C<< <> >> operator. In
scalar context it reads a single line from the filehandle, and in list
context it reads the whole file in, assigning each line to an element of
the list:
my $line = <INFILE>;
my @lines = <INFILE>;
The C<< <> >> operator is most often seen in a C<while> loop:
while (<INFILE>) { # assigns each line in turn to $_
print "Just read in this line: $_";
}
The default filehandle for the C<< <> >> operator is C<STDIN>.
while (<>) {
print "Read this line from STDIN: $_";
}
We've already seen how to print to standard output using C<print()>.
However, C<print()> can also take an optional first argument specifying
which filehandle to print to:
print STDERR "This is your final warning.\n";
print OUTFILE $record;
print LOGFILE $logmessage;
When you're done with your filehandles, you should C<close()> them
(though to be honest, Perl will clean up after you if you forget):
close INFILE;
=head2 Regular expressions
Perl's regular expression support is both broad and deep, and is the
subject of lengthy documentation in L<perlretut>, L<perlre>, and
elsewhere. However, in short:
=over 4
=item Simple matching
if (/foo/) { ... } # true if $_ contains "foo"
if ($a =~ /foo/) { ... } # true if $a contains "foo"
The C<//> matching operator is documented in L<perlop>. It operates on
C<$_> by default, or can be bound to another variable using the C<=~>
binding operator (also documented in L<perlop>).
=item Simple substitution
s/foo/bar/; # substitutes foo for bar in $_
$a =~ s/foo/bar/; # substitutes foo for bar in $a
$a =~ s/foo/bar/g; # substitutes foo for bar GLOBALLY in $a
The C<s///> substitution operator is documented in L<perlop>.
=item More complex regular expressions
You don't just have to match on fixed strings. In fact, you can match
on just about anything you could dream of by using more complex regular
expressions. These are documented at great length in L<perlre>, but for
the meantime, here's a quick cheat sheet:
. matches a single character
\s matches a whitespace character
\S non-whitespace character
\d a digit (0-9)
\D a non-digit
\w a word character (a-z, A-Z, 0-9, _)
\W a non-word character
[aeiou] matches a single character in the given set
[^aeiou] matches a single character outside the given set
(foo|bar|baz) matches any of the alternatives specified
^ start of string
$ end of string
* zero or more of the previous thing
+ one or more of the previous thing
? zero or one of the previous thing
{3} matches exactly 3 of the previous thing
{3,6} matches between 3 and 6 of the previous thing
{3,} matches 3 or more of the previous thing
Some brief examples:
/^\d+/ string starts with one or more digits
/^$/ nothing in the string (start and end are adjacent)
/(a.)+/ matches a string in which every odd-numbered letter is a
# This loop reads from STDIN, and prints non-blank lines:
while (<>) {
next if /^$/;
print;
}
You can also capture the results of a regexp match by using parens. The
results end up in C<$1>, C<$2> and so on.
if ($email =~ /([^@]+@(.*)/) {
print "Username is $1\n";
print "Hostname is $2\n";
}
Perl regexps also support backreferences, lookaheads, and all kinds of
other complex details. Read all about them in L<perlre>.
=head2 Writing subroutines
Writing subroutines is easy:
sub log {
my $logmessage = shift;
print LOGFILE $logmessage;
}
What's that C<shift>? Well, the arguments to a subroutine are available
to us as a special array called C<@_> (see L<perlvar> for more on that).
The default argument to the C<shift> function just happens to be C<@_>.
So C<my $logmessage = shift;> shifts the first item off the list of
arguments and assigns it to C<$logmessage>.
We can manipulate C<@_> in other ways too:
my ($logmessage, $priority) = @_; # common
my $logmessage = $_[0]; # uncommon, and ugly
Subroutines can also return values:
sub square {
my $num = shift;
my $result = $num * $num;
return $result;
}
For more information on writing subroutines, see L<perlsub>.
=head2 OO Perl
OO Perl is implemented using references and some other trickery, and is
largely beyond the scope of this document. Read L<perlboot>,
L<perltoot>, L<perltootc> and L<perlobj>.
As a beginning Perl programmer, your most common use of OO Perl will be
in using third-party modules, which are documented below.
=head2 Using Perl modules
Perl modules provide a range of features to help you avoid reinventing
the wheel, and can be downloaded from CPAN (L<http://www.cpan.org>). A
number of popular modules are included with the Perl distribution
itself.
Categories of modules range from text manipulation to network protocols
to database integration to graphics. A categorised list of modules is
also available from CPAN.
To learn how to install modules you download from CPAN, read
L<perlmodinstall>
To learn how to use a particular module, use C<perldoc Module::Name>.
Typically you will want to C<use Module::Name>, which will then give you
access to exported functions or an OO interface to the module.
L<perlfaq> contains questions and answers related to many common
tasks, and often provides suggestions for good CPAN modules to use.
L<perlmod> describes Perl modules in general. L<perlmodlib> lists the
modules which came with your Perl installation.
If you feel the urge to write Perl modules, L<perlnewmod> will give you
good advice.
=head1 AUTHOR
Kirrily "Skud" Robert <[EMAIL PROTECTED]>
--
Kirrily 'Skud' Robert - [EMAIL PROTECTED] - http://infotrope.net/
"See, 'Garbage In Garbage Out' is what, in technical terms, is called an
Interface." -- Morgan (from the Netizen quotes file)
