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perlintro



DESCRIPTION

       This document is intended to give you a quick overview of
       the Perl programming language, along with pointers to fur­
       ther 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 peo­
       ples' 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 get­
       ting 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 perltoc.

       Throughout this document you'll see references to other
       parts of the Perl documentation.  You can read that docu­
       mentation 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 devel­
       opment, network programming, GUI development, and more.

       The language is intended to be practical (easy to use,
       efficient, 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

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

           #!/usr/bin/env perl

       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 ("\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 perlsyn.

       Perl variable types

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

               print;          # prints contents of $_ by default

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

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

               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 refer­
       ences, 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 references.

           my $variables = {
               scalar  =>  {
                            description => "single item",
                            sigil => '$',
                           },
               array   =>  {
                            description => "ordered list of items",
                            sigil => '@',
                           },
               hash    =>  {

           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 prac­
       tice.  "my" creates lexically scoped variables instead.
       The variables 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 com­
       pile-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 informa­
       tion about modules and CPAN).

       The conditions can be any Perl expression.  See the list
       of operators in the next section for information on com­
       parison and boolean logic operators, which are commonly
       used in conditional statements.

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

               # 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++) {
                   ...
               }

           The C style for loop is rarely needed in Perl since
           Perl provides the more friendly list scanning "fore­
           ach" 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",
               ==  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 compar­
           isons?  Because we don't have special variable types,
           and Perl needs to know whether to sort numerically
           (where 99 is 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 sup­
           ported as operators in their own right.  They're more
           readable than the C-style operators, but have differ­
           ent precedence 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 perl­
       func and perlopentut, but in short:

           open(INFILE,  "input.txt")   or die "Can't open input.txt: $!";

       looping constructs.

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

           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 perlre­
       quick, 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
           documented 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 meantime, here's a quick cheat sheet:

               [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
           previous thing you want to match on, where "thing"
           means either a literal character, one of the metachar­
           acters 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 pur­
           pose.  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

               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.

       lvar for more on that).  The default argument to the
       "shift" function just happens to be @_.  So "my $logmes­
       sage = shift;" shifts the first item off the list of argu­
       ments 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 ref­
       erences 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, perl­
       toot, 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.

       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 Mod­
       ule::Name".  Typically you will want to "use Mod­
       ule::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

perl v5.8.1                 2003-09-02               PERLINTRO(1)
  
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