MirOS Manual: perlintro(1)


PERLINTRO(1)    Perl Programmers Reference Guide     PERLINTRO(1)

NAME

     perlintro -- a brief introduction and overview of Perl

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 strongly advised to follow
     this introduction with more information from the full Perl
     manual, the table of contents to which can be found in perl-
     toc.

     Throughout this document you'll see references to other
     parts of the Perl documentation.  You can read that documen-
     tation using the "perldoc" command or whatever method you're
     using to read this document.

     What is Perl?

     Perl is a general-purpose programming language originally
     developed for text manipulation and now used for a wide
     range of tasks including system administration, web develop-
     ment, network programming, GUI development, and more.

     The language is intended to be practical (easy to use, effi-
     cient, complete) rather than beautiful (tiny, elegant,
     minimal).  Its major features are that it's easy to use,
     supports both procedural and object-oriented (OO) program-
     ming, has powerful built-in support for text processing, and
     has one of the world's most impressive collections of
     third-party modules.

     Different definitions of Perl are given in perl, perlfaq1
     and no doubt other places.  From this we can determine that
     Perl is different things to different people, but that lots
     of people think it's at least worth writing about.

     Running Perl programs

     To run a Perl program from the Unix command line:

         perl progname.pl

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     Alternatively, put this as the first line of your script:

         #!/usr/bin/env perl

     ... and run the script as "/path/to/script.pl".  Of course,
     it'll need to be executable first, so "chmod 755 script.pl"
     (under Unix).

     For more information, including instructions for other plat-
     forms such as Windows and Mac OS, read perlrun.

     Basic syntax overview

     A Perl script or program consists of one or more statements.
     These statements are simply written in the script in a
     straightforward fashion.  There is no need to have a
     "main()" function or anything of that kind.

     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 spe-
     cial characters such as newlines ("\n"):

         print "Hello, $name\n";     # works fine
         print 'Hello, $name\n';     # prints $name\n literally

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     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
     perlsyn.

     Perl variable types

     Perl has three main variable types: scalars, arrays, and
     hashes.

     Scalars
         A scalar represents a single value:

             my $animal = "camel";
             my $answer = 42;

         Scalar values can be strings, integers or floating point
         numbers, and Perl will automatically convert between
         them as required.  There is no need to pre-declare your
         variable types.

         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 perlvar.  The only one you need to know
         about for now is $_ which is the "default variable".
         It's used as the default argument to a number of func-
         tions in Perl, and it's set implicitly by certain loop-
         ing constructs.

             print;          # prints contents of $_ by default

     Arrays
         An array represents a list of values:

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             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"

         The special variable $#array tells you the index of the
         last element of an array:

             print $mixed[$#mixed];       # last element, prints 1.23

         You might be tempted to use "$#array + 1" to tell you
         how many items there are in an array.  Don't bother.  As
         it happens, using @array where Perl expects to find a
         scalar value ("in scalar context") will give you the
         number of elements in the array:

             if (@animals < 5) { ... }

         The elements we're getting from the array start with a
         "$" because we're getting just a single value out of the
         array -- you ask for a scalar, you get a scalar.

         To get multiple values from an 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 @ARGV
         (the command line arguments to your script) and @_ (the
         arguments passed to a subroutine).  These are documented
         in perlvar.

     Hashes
         A hash represents a set of key/value pairs:

             my %fruit_color = ("apple", "red", "banana", "yellow");

         You can use whitespace and the "=>" operator to lay them
         out more nicely:

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             my %fruit_color = (
                 apple  => "red",
                 banana => "yellow",
             );

         To get at hash elements:

             $fruit_color{"apple"};           # gives "red"

         You can get at lists of keys and values with "keys()"
         and "values()".

             my @fruits = keys %fruit_colors;
             my @colors = values %fruit_colors;

         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 well known of these is %ENV
         which contains environment variables.  Read all about it
         (and other special variables) in perlvar.

     Scalars, arrays and hashes are documented more fully in
     perldata.

     More complex data types can be constructed using references,
     which allow you to build lists and hashes within lists and
     hashes.

     A reference is a scalar value and can refer to any other
     Perl data type. So by storing a reference as the value of an
     array or hash element, you can easily create lists and
     hashes within lists and hashes. The following example shows
     a 2 level hash of hash structure using anonymous hash refer-
     ences.

         my $variables = {
             scalar  =>  {
                          description => "single item",
                          sigil => '$',
                         },
             array   =>  {
                          description => "ordered list of items",
                          sigil => '@',
                         },
             hash    =>  {
                          description => "key/value pairs",
                          sigil => '%',
                         },
         };

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         print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";

     Exhaustive information on the topic of references can be
     found in perlreftut, perllol, perlref and perldsc.

     Variable scoping

     Throughout the previous section all the examples have used
     the syntax:

         my $var = "value";

     The "my" is actually not required; you could just use:

         $var = "value";

     However, the above usage will create global variables
     throughout your program, which is bad programming practice.
     "my" creates lexically scoped variables instead.  The vari-
     ables are scoped to the block (i.e. a bunch of statements
     surrounded by curly-braces) in which they are defined.

         my $a = "foo";
         if ($some_condition) {
             my $b = "bar";
             print $a;           # prints "foo"
             print $b;           # prints "bar"
         }
         print $a;               # prints "foo"
         print $b;               # prints nothing; $b has fallen out of scope

     Using "my" in combination with a "use strict;" at the top of
     your Perl scripts means that the interpreter will pick up
     certain common programming errors.  For instance, in the
     example above, the final "print $b" would cause a compile-
     time error and prevent you from running the program.  Using
     "strict" is highly recommended.

     Conditional and looping constructs

     Perl has most of the usual conditional and looping con-
     structs except for case/switch (but if you really want it,
     there is a Switch module in Perl 5.8 and newer, and on CPAN.
     See the section on modules, below, for more information
     about modules and CPAN).

     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 con-
     ditional statements.

     if

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             if ( condition ) {
                 ...
             } elsif ( other condition ) {
                 ...
             } else {
                 ...
             }

         There's also a negated version of it:

             unless ( condition ) {
                 ...
             }

         This is provided as a more readable version of "if
         (!condition)".

         Note that the braces are required in Perl, even if
         you've only got one line in the block.  However, there
         is a clever way of making your one-line conditional
         blocks more English like:

             # the traditional way
             if ($zippy) {
                 print "Yow!";
             }

             # the Perlish post-condition way
             print "Yow!" if $zippy;
             print "We have no bananas" unless $bananas;

     while
             while ( condition ) {
                 ...
             }

         There's also a negated version, for the same reason we
         have "unless":

             until ( condition ) {
                 ...
             }

         You can also use "while" in a post-condition:

             print "LA LA LA\n" while 1;          # loops forever

     for Exactly like C:

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

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         The C style for loop is rarely needed in Perl since Perl
         provides the more friendly list scanning "foreach" loop.

     foreach
             foreach (@array) {
                 print "This element is $_\n";
             }

             # you don't have to use the default $_ either...
             foreach my $key (keys %hash) {
                 print "The value of $key is $hash{$key}\n";
             }

     For more detail on looping constructs (and some that weren't
     mentioned in this overview) see perlsyn.

     Builtin operators and functions

     Perl comes with a wide selection of builtin functions.  Some
     of the ones we've already seen include "print", "sort" and
     "reverse".  A list of them is given at the start of perlfunc
     and you can easily read about any given function by using
     "perldoc -f functionname".

     Perl operators are documented in full in perlop, but here
     are a few of the most common ones:

     Arithmetic
             +   addition
             -   subtraction
             *   multiplication
             /   division

     Numeric comparison
             ==  equality
             !=  inequality
             <   less than
             >   greater than
             <=  less than or equal
             >=  greater than or equal

     String comparison
             eq  equality
             ne  inequality
             lt  less than
             gt  greater than
             le  less than or equal
             ge  greater than or equal

         (Why do we have separate numeric and string comparisons?
         Because we don't have special variable types, and Perl
         needs to know whether to sort numerically (where 99 is

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         less than 100) or alphabetically (where 100 comes before
         99).

     Boolean logic
             &&  and
             ||  or
             !   not

         ("and", "or" and "not" aren't just in the above table as
         descriptions of the operators -- they're also supported
         as operators in their own right.  They're more readable
         than the C-style operators, but have different pre-
         cedence to "&&" and friends.  Check perlop for more
         detail.)

     Miscellaneous
             =   assignment
             .   string concatenation
             x   string multiplication
             ..  range operator (creates a list of numbers)

     Many operators can be combined with a "=" as follows:

         $a += 1;        # same as $a = $a + 1
         $a -= 1;        # same as $a = $a - 1
         $a .= "\n";     # same as $a = $a . "\n";

     Files and I/O

     You can open a file for input or output using the "open()"
     function. It's documented in extravagant detail in perlfunc
     and 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 "<>" opera-
     tor.  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>;

     Reading in the whole file at one time is called slurping. It
     can be useful but it may be a memory hog. Most text file
     processing can be done a line at a time with Perl's looping
     constructs.

     The "<>" operator is most often seen in a "while" loop:

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         while (<INFILE>) {     # assigns each line in turn to $_
             print "Just read in this line: $_";
         }

     We've already seen how to print to standard output using
     "print()". However, "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 "close()"
     them (though to be honest, Perl will clean up after you if
     you forget):

         close INFILE;

     Regular expressions

     Perl's regular expression support is both broad and deep,
     and is the subject of lengthy documentation in perlrequick,
     perlretut, and elsewhere.  However, in short:

     Simple matching
             if (/foo/)       { ... }  # true if $_ contains "foo"
             if ($a =~ /foo/) { ... }  # true if $a contains "foo"

         The "//" matching operator is documented in perlop.  It
         operates on $_ by default, or can be bound to another
         variable using the "=~" binding operator (also docu-
         mented in perlop).

     Simple substitution
             s/foo/bar/;               # replaces foo with bar in $_
             $a =~ s/foo/bar/;         # replaces foo with bar in $a
             $a =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar in $a

         The "s///" substitution operator is documented in per-
         lop.

     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 perlre, but for the mean-
         time, here's a quick cheat sheet:

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             .                   a single character
             \s                  a whitespace character (space, tab, newline)
             \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

         Quantifiers can be used to specify how many of the pre-
         vious thing you want to match on, where "thing" means
         either a literal character, one of the metacharacters
         listed above, or a group of characters or metacharacters
         in parentheses.

             *                   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)
             /(\d\s){3}/         a three digits, each followed by a whitespace
                                 character (eg "3 4 5 ")
             /(a.)+/             matches a string in which every odd-numbered letter
                                 is a (eg "abacadaf")

             # This loop reads from STDIN, and prints non-blank lines:
             while (<>) {
                 next if /^$/;
                 print;
             }

     Parentheses for capturing
         As well as grouping, parentheses serve a second purpose.
         They can be used to capture the results of parts of the
         regexp match for later use. The results end up in $1, $2
         and so on.

             # a cheap and nasty way to break an email address up into parts

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             if ($email =~ /([^@]+)@(.+)/) {
                 print "Username is $1\n";
                 print "Hostname is $2\n";
             }

     Other regexp features
         Perl regexps also support backreferences, lookaheads,
         and all kinds of other complex details.  Read all about
         them in perlrequick, perlretut, and perlre.

     Writing subroutines

     Writing subroutines is easy:

         sub log {
             my $logmessage = shift;
             print LOGFILE $logmessage;
         }

     What's that "shift"?  Well, the arguments to a subroutine
     are available to us as a special array called @_ (see perl-
     var for more on that). The default argument to the "shift"
     function just happens to be @_. So "my $logmessage = shift;"
     shifts the first item off the list of arguments and assigns
     it to $logmessage.

     We can manipulate @_ 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 perlsub.

     OO Perl

     OO Perl is relatively simple and is implemented using refer-
     ences which know what sort of object they are based on
     Perl's concept of packages. However, OO Perl is largely
     beyond the scope of this document. Read perlboot, perltoot,
     perltooc and perlobj.

     As a beginning Perl programmer, your most common use of OO
     Perl will be in using third-party modules, which are docu-
     mented below.

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     Using Perl modules

     Perl modules provide a range of features to help you avoid
     reinventing the wheel, and can be downloaded from CPAN (
     http://www.cpan.org/ ).  A number of popular modules are
     included with the Perl distribution itself.

     Categories of modules range from text manipulation to net-
     work protocols to database integration to graphics.  A
     categorized list of modules is also available from CPAN.

     To learn how to install modules you download from CPAN, read
     perlmodinstall

     To learn how to use a particular module, use "perldoc
     Module::Name". Typically you will want to "use
     Module::Name", which will then give you access to exported
     functions or an OO interface to the module.

     perlfaq contains questions and answers related to many com-
     mon tasks, and often provides suggestions for good CPAN
     modules to use.

     perlmod describes Perl modules in general.  perlmodlib lists
     the modules which came with your Perl installation.

     If you feel the urge to write Perl modules, perlnewmod will
     give you good advice.

AUTHOR

     Kirrily "Skud" Robert <skud@cpan.org>

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