1PERLINTRO(1)           Perl Programmers Reference Guide           PERLINTRO(1)
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3
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NAME

6       perlintro -- a brief introduction and overview of Perl
7

DESCRIPTION

9       This document is intended to give you a quick overview of the Perl
10       programming language, along with pointers to further documentation.  It
11       is intended as a "bootstrap" guide for those who are new to the
12       language, and provides just enough information for you to be able to
13       read other peoples' Perl and understand roughly what it's doing, or
14       write your own simple scripts.
15
16       This introductory document does not aim to be complete.  It does not
17       even aim to be entirely accurate.  In some cases perfection has been
18       sacrificed in the goal of getting the general idea across.  You are
19       strongly advised to follow this introduction with more information from
20       the full Perl manual, the table of contents to which can be found in
21       perltoc.
22
23       Throughout this document you'll see references to other parts of the
24       Perl documentation.  You can read that documentation using the
25       "perldoc" command or whatever method you're using to read this
26       document.
27
28       Throughout Perl's documentation, you'll find numerous examples intended
29       to help explain the discussed features.  Please keep in mind that many
30       of them are code fragments rather than complete programs.
31
32       These examples often reflect the style and preference of the author of
33       that piece of the documentation, and may be briefer than a
34       corresponding line of code in a real program.  Except where otherwise
35       noted, you should assume that "use strict" and "use warnings"
36       statements appear earlier in the "program", and that any variables used
37       have already been declared, even if those declarations have been
38       omitted to make the example easier to read.
39
40       Do note that the examples have been written by many different authors
41       over a period of several decades.  Styles and techniques will therefore
42       differ, although some effort has been made to not vary styles too
43       widely in the same sections.  Do not consider one style to be better
44       than others - "There's More Than One Way To Do It" is one of Perl's
45       mottos.  After all, in your journey as a programmer, you are likely to
46       encounter different styles.
47
48   What is Perl?
49       Perl is a general-purpose programming language originally developed for
50       text manipulation and now used for a wide range of tasks including
51       system administration, web development, network programming, GUI
52       development, and more.
53
54       The language is intended to be practical (easy to use, efficient,
55       complete) rather than beautiful (tiny, elegant, minimal).  Its major
56       features are that it's easy to use, supports both procedural and
57       object-oriented (OO) programming, has powerful built-in support for
58       text processing, and has one of the world's most impressive collections
59       of third-party modules.
60
61       Different definitions of Perl are given in perl, perlfaq1 and no doubt
62       other places.  From this we can determine that Perl is different things
63       to different people, but that lots of people think it's at least worth
64       writing about.
65
66   Running Perl programs
67       To run a Perl program from the Unix command line:
68
69        perl progname.pl
70
71       Alternatively, put this as the first line of your script:
72
73        #!/usr/bin/env perl
74
75       ... and run the script as /path/to/script.pl.  Of course, it'll need to
76       be executable first, so "chmod 755 script.pl" (under Unix).
77
78       (This start line assumes you have the env program.  You can also put
79       directly the path to your perl executable, like in "#!/usr/bin/perl").
80
81       For more information, including instructions for other platforms such
82       as Windows and Mac OS, read perlrun.
83
84   Safety net
85       Perl by default is very forgiving.  In order to make it more robust it
86       is recommended to start every program with the following lines:
87
88        #!/usr/bin/perl
89        use strict;
90        use warnings;
91
92       The two additional lines request from perl to catch various common
93       problems in your code.  They check different things so you need both.
94       A potential problem caught by "use strict;" will cause your code to
95       stop immediately when it is encountered, while "use warnings;" will
96       merely give a warning (like the command-line switch -w) and let your
97       code run.  To read more about them check their respective manual pages
98       at strict and warnings.
99
100   Basic syntax overview
101       A Perl script or program consists of one or more statements.  These
102       statements are simply written in the script in a straightforward
103       fashion.  There is no need to have a "main()" function or anything of
104       that kind.
105
106       Perl statements end in a semi-colon:
107
108        print "Hello, world";
109
110       Comments start with a hash symbol and run to the end of the line
111
112        # This is a comment
113
114       Whitespace is irrelevant:
115
116        print
117            "Hello, world"
118            ;
119
120       ... except inside quoted strings:
121
122        # this would print with a linebreak in the middle
123        print "Hello
124        world";
125
126       Double quotes or single quotes may be used around literal strings:
127
128        print "Hello, world";
129        print 'Hello, world';
130
131       However, only double quotes "interpolate" variables and special
132       characters such as newlines ("\n"):
133
134        print "Hello, $name\n";     # works fine
135        print 'Hello, $name\n';     # prints $name\n literally
136
137       Numbers don't need quotes around them:
138
139        print 42;
140
141       You can use parentheses for functions' arguments or omit them according
142       to your personal taste.  They are only required occasionally to clarify
143       issues of precedence.
144
145        print("Hello, world\n");
146        print "Hello, world\n";
147
148       More detailed information about Perl syntax can be found in perlsyn.
149
150   Perl variable types
151       Perl has three main variable types: scalars, arrays, and hashes.
152
153       Scalars
154           A scalar represents a single value:
155
156            my $animal = "camel";
157            my $answer = 42;
158
159           Scalar values can be strings, integers or floating point numbers,
160           and Perl will automatically convert between them as required.
161           There is no need to pre-declare your variable types, but you have
162           to declare them using the "my" keyword the first time you use them.
163           (This is one of the requirements of "use strict;".)
164
165           Scalar values can be used in various ways:
166
167            print $animal;
168            print "The animal is $animal\n";
169            print "The square of $answer is ", $answer * $answer, "\n";
170
171           There are a number of "magic" scalars with names that look like
172           punctuation or line noise.  These special variables are used for
173           all kinds of purposes, and are documented in perlvar.  The only one
174           you need to know about for now is $_ which is the "default
175           variable".  It's used as the default argument to a number of
176           functions in Perl, and it's set implicitly by certain looping
177           constructs.
178
179            print;          # prints contents of $_ by default
180
181       Arrays
182           An array represents a list of values:
183
184            my @animals = ("camel", "llama", "owl");
185            my @numbers = (23, 42, 69);
186            my @mixed   = ("camel", 42, 1.23);
187
188           Arrays are zero-indexed.  Here's how you get at elements in an
189           array:
190
191            print $animals[0];              # prints "camel"
192            print $animals[1];              # prints "llama"
193
194           The special variable $#array tells you the index of the last
195           element of an array:
196
197            print $mixed[$#mixed];       # last element, prints 1.23
198
199           You might be tempted to use "$#array + 1" to tell you how many
200           items there are in an array.  Don't bother.  As it happens, using
201           @array where Perl expects to find a scalar value ("in scalar
202           context") will give you the number of elements in the array:
203
204            if (@animals < 5) { ... }
205
206           The elements we're getting from the array start with a "$" because
207           we're getting just a single value out of the array; you ask for a
208           scalar, you get a scalar.
209
210           To get multiple values from an array:
211
212            @animals[0,1];                 # gives ("camel", "llama");
213            @animals[0..2];                # gives ("camel", "llama", "owl");
214            @animals[1..$#animals];        # gives all except the first element
215
216           This is called an "array slice".
217
218           You can do various useful things to lists:
219
220            my @sorted    = sort @animals;
221            my @backwards = reverse @numbers;
222
223           There are a couple of special arrays too, such as @ARGV (the
224           command line arguments to your script) and @_ (the arguments passed
225           to a subroutine).  These are documented in perlvar.
226
227       Hashes
228           A hash represents a set of key/value pairs:
229
230            my %fruit_color = ("apple", "red", "banana", "yellow");
231
232           You can use whitespace and the "=>" operator to lay them out more
233           nicely:
234
235            my %fruit_color = (
236                apple  => "red",
237                banana => "yellow",
238            );
239
240           To get at hash elements:
241
242            $fruit_color{"apple"};           # gives "red"
243
244           You can get at lists of keys and values with "keys()" and
245           "values()".
246
247            my @fruits = keys %fruit_colors;
248            my @colors = values %fruit_colors;
249
250           Hashes have no particular internal order, though you can sort the
251           keys and loop through them.
252
253           Just like special scalars and arrays, there are also special
254           hashes.  The most well known of these is %ENV which contains
255           environment variables.  Read all about it (and other special
256           variables) in perlvar.
257
258       Scalars, arrays and hashes are documented more fully in perldata.
259
260       More complex data types can be constructed using references, which
261       allow you to build lists and hashes within lists and hashes.
262
263       A reference is a scalar value and can refer to any other Perl data
264       type.  So by storing a reference as the value of an array or hash
265       element, you can easily create lists and hashes within lists and
266       hashes.  The following example shows a 2 level hash of hash structure
267       using anonymous hash references.
268
269        my $variables = {
270            scalar  =>  {
271                         description => "single item",
272                         sigil => '$',
273                        },
274            array   =>  {
275                         description => "ordered list of items",
276                         sigil => '@',
277                        },
278            hash    =>  {
279                         description => "key/value pairs",
280                         sigil => '%',
281                        },
282        };
283
284        print "Scalars begin with a $variables->{'scalar'}->{'sigil'}\n";
285
286       Exhaustive information on the topic of references can be found in
287       perlreftut, perllol, perlref and perldsc.
288
289   Variable scoping
290       Throughout the previous section all the examples have used the syntax:
291
292        my $var = "value";
293
294       The "my" is actually not required; you could just use:
295
296        $var = "value";
297
298       However, the above usage will create global variables throughout your
299       program, which is bad programming practice.  "my" creates lexically
300       scoped variables instead.  The variables are scoped to the block (i.e.
301       a bunch of statements surrounded by curly-braces) in which they are
302       defined.
303
304        my $x = "foo";
305        my $some_condition = 1;
306        if ($some_condition) {
307            my $y = "bar";
308            print $x;           # prints "foo"
309            print $y;           # prints "bar"
310        }
311        print $x;               # prints "foo"
312        print $y;               # prints nothing; $y has fallen out of scope
313
314       Using "my" in combination with a "use strict;" at the top of your Perl
315       scripts means that the interpreter will pick up certain common
316       programming errors.  For instance, in the example above, the final
317       "print $y" would cause a compile-time error and prevent you from
318       running the program.  Using "strict" is highly recommended.
319
320   Conditional and looping constructs
321       Perl has most of the usual conditional and looping constructs.  As of
322       Perl 5.10, it even has a case/switch statement (spelled
323       "given"/"when").  See "Switch Statements" in perlsyn for more details.
324
325       The conditions can be any Perl expression.  See the list of operators
326       in the next section for information on comparison and boolean logic
327       operators, which are commonly used in conditional statements.
328
329       if
330            if ( condition ) {
331                ...
332            } elsif ( other condition ) {
333                ...
334            } else {
335                ...
336            }
337
338           There's also a negated version of it:
339
340            unless ( condition ) {
341                ...
342            }
343
344           This is provided as a more readable version of "if (!condition)".
345
346           Note that the braces are required in Perl, even if you've only got
347           one line in the block.  However, there is a clever way of making
348           your one-line conditional blocks more English like:
349
350            # the traditional way
351            if ($zippy) {
352                print "Yow!";
353            }
354
355            # the Perlish post-condition way
356            print "Yow!" if $zippy;
357            print "We have no bananas" unless $bananas;
358
359       while
360            while ( condition ) {
361                ...
362            }
363
364           There's also a negated version, for the same reason we have
365           "unless":
366
367            until ( condition ) {
368                ...
369            }
370
371           You can also use "while" in a post-condition:
372
373            print "LA LA LA\n" while 1;          # loops forever
374
375       for Exactly like C:
376
377            for ($i = 0; $i <= $max; $i++) {
378                ...
379            }
380
381           The C style for loop is rarely needed in Perl since Perl provides
382           the more friendly list scanning "foreach" loop.
383
384       foreach
385            foreach (@array) {
386                print "This element is $_\n";
387            }
388
389            print $list[$_] foreach 0 .. $max;
390
391            # you don't have to use the default $_ either...
392            foreach my $key (keys %hash) {
393                print "The value of $key is $hash{$key}\n";
394            }
395
396           The "foreach" keyword is actually a synonym for the "for" keyword.
397           See ""Foreach Loops" in perlsyn".
398
399       For more detail on looping constructs (and some that weren't mentioned
400       in this overview) see perlsyn.
401
402   Builtin operators and functions
403       Perl comes with a wide selection of builtin functions.  Some of the
404       ones we've already seen include "print", "sort" and "reverse".  A list
405       of them is given at the start of perlfunc and you can easily read about
406       any given function by using "perldoc -f functionname".
407
408       Perl operators are documented in full in perlop, but here are a few of
409       the most common ones:
410
411       Arithmetic
412            +   addition
413            -   subtraction
414            *   multiplication
415            /   division
416
417       Numeric comparison
418            ==  equality
419            !=  inequality
420            <   less than
421            >   greater than
422            <=  less than or equal
423            >=  greater than or equal
424
425       String comparison
426            eq  equality
427            ne  inequality
428            lt  less than
429            gt  greater than
430            le  less than or equal
431            ge  greater than or equal
432
433           (Why do we have separate numeric and string comparisons?  Because
434           we don't have special variable types, and Perl needs to know
435           whether to sort numerically (where 99 is less than 100) or
436           alphabetically (where 100 comes before 99).
437
438       Boolean logic
439            &&  and
440            ||  or
441            !   not
442
443           ("and", "or" and "not" aren't just in the above table as
444           descriptions of the operators.  They're also supported as operators
445           in their own right.  They're more readable than the C-style
446           operators, but have different precedence to "&&" and friends.
447           Check perlop for more detail.)
448
449       Miscellaneous
450            =   assignment
451            .   string concatenation
452            x   string multiplication
453            ..  range operator (creates a list of numbers or strings)
454
455       Many operators can be combined with a "=" as follows:
456
457        $a += 1;        # same as $a = $a + 1
458        $a -= 1;        # same as $a = $a - 1
459        $a .= "\n";     # same as $a = $a . "\n";
460
461   Files and I/O
462       You can open a file for input or output using the "open()" function.
463       It's documented in extravagant detail in perlfunc and perlopentut, but
464       in short:
465
466        open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";
467        open(my $out, ">",  "output.txt") or die "Can't open output.txt: $!";
468        open(my $log, ">>", "my.log")     or die "Can't open my.log: $!";
469
470       You can read from an open filehandle using the "<>" operator.  In
471       scalar context it reads a single line from the filehandle, and in list
472       context it reads the whole file in, assigning each line to an element
473       of the list:
474
475        my $line  = <$in>;
476        my @lines = <$in>;
477
478       Reading in the whole file at one time is called slurping.  It can be
479       useful but it may be a memory hog.  Most text file processing can be
480       done a line at a time with Perl's looping constructs.
481
482       The "<>" operator is most often seen in a "while" loop:
483
484        while (<$in>) {     # assigns each line in turn to $_
485            print "Just read in this line: $_";
486        }
487
488       We've already seen how to print to standard output using "print()".
489       However, "print()" can also take an optional first argument specifying
490       which filehandle to print to:
491
492        print STDERR "This is your final warning.\n";
493        print $out $record;
494        print $log $logmessage;
495
496       When you're done with your filehandles, you should "close()" them
497       (though to be honest, Perl will clean up after you if you forget):
498
499        close $in or die "$in: $!";
500
501   Regular expressions
502       Perl's regular expression support is both broad and deep, and is the
503       subject of lengthy documentation in perlrequick, perlretut, and
504       elsewhere.  However, in short:
505
506       Simple matching
507            if (/foo/)       { ... }  # true if $_ contains "foo"
508            if ($a =~ /foo/) { ... }  # true if $a contains "foo"
509
510           The "//" matching operator is documented in perlop.  It operates on
511           $_ by default, or can be bound to another variable using the "=~"
512           binding operator (also documented in perlop).
513
514       Simple substitution
515            s/foo/bar/;               # replaces foo with bar in $_
516            $a =~ s/foo/bar/;         # replaces foo with bar in $a
517            $a =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar
518                                      # in $a
519
520           The "s///" substitution operator is documented in perlop.
521
522       More complex regular expressions
523           You don't just have to match on fixed strings.  In fact, you can
524           match on just about anything you could dream of by using more
525           complex regular expressions.  These are documented at great length
526           in perlre, but for the meantime, here's a quick cheat sheet:
527
528            .                   a single character
529            \s                  a whitespace character (space, tab, newline,
530                                ...)
531            \S                  non-whitespace character
532            \d                  a digit (0-9)
533            \D                  a non-digit
534            \w                  a word character (a-z, A-Z, 0-9, _)
535            \W                  a non-word character
536            [aeiou]             matches a single character in the given set
537            [^aeiou]            matches a single character outside the given
538                                set
539            (foo|bar|baz)       matches any of the alternatives specified
540
541            ^                   start of string
542            $                   end of string
543
544           Quantifiers can be used to specify how many of the previous thing
545           you want to match on, where "thing" means either a literal
546           character, one of the metacharacters listed above, or a group of
547           characters or metacharacters in parentheses.
548
549            *                   zero or more of the previous thing
550            +                   one or more of the previous thing
551            ?                   zero or one of the previous thing
552            {3}                 matches exactly 3 of the previous thing
553            {3,6}               matches between 3 and 6 of the previous thing
554            {3,}                matches 3 or more of the previous thing
555
556           Some brief examples:
557
558            /^\d+/              string starts with one or more digits
559            /^$/                nothing in the string (start and end are
560                                adjacent)
561            /(\d\s){3}/         three digits, each followed by a whitespace
562                                character (eg "3 4 5 ")
563            /(a.)+/             matches a string in which every odd-numbered
564                                letter is a (eg "abacadaf")
565
566            # This loop reads from STDIN, and prints non-blank lines:
567            while (<>) {
568                next if /^$/;
569                print;
570            }
571
572       Parentheses for capturing
573           As well as grouping, parentheses serve a second purpose.  They can
574           be used to capture the results of parts of the regexp match for
575           later use.  The results end up in $1, $2 and so on.
576
577            # a cheap and nasty way to break an email address up into parts
578
579            if ($email =~ /([^@]+)@(.+)/) {
580                print "Username is $1\n";
581                print "Hostname is $2\n";
582            }
583
584       Other regexp features
585           Perl regexps also support backreferences, lookaheads, and all kinds
586           of other complex details.  Read all about them in perlrequick,
587           perlretut, and perlre.
588
589   Writing subroutines
590       Writing subroutines is easy:
591
592        sub logger {
593           my $logmessage = shift;
594           open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
595           print $logfile $logmessage;
596        }
597
598       Now we can use the subroutine just as any other built-in function:
599
600        logger("We have a logger subroutine!");
601
602       What's that "shift"?  Well, the arguments to a subroutine are available
603       to us as a special array called @_ (see perlvar for more on that).  The
604       default argument to the "shift" function just happens to be @_.  So "my
605       $logmessage = shift;" shifts the first item off the list of arguments
606       and assigns it to $logmessage.
607
608       We can manipulate @_ in other ways too:
609
610        my ($logmessage, $priority) = @_;       # common
611        my $logmessage = $_[0];                 # uncommon, and ugly
612
613       Subroutines can also return values:
614
615        sub square {
616            my $num = shift;
617            my $result = $num * $num;
618            return $result;
619        }
620
621       Then use it like:
622
623        $sq = square(8);
624
625       For more information on writing subroutines, see perlsub.
626
627   OO Perl
628       OO Perl is relatively simple and is implemented using references which
629       know what sort of object they are based on Perl's concept of packages.
630       However, OO Perl is largely beyond the scope of this document.  Read
631       perlootut and perlobj.
632
633       As a beginning Perl programmer, your most common use of OO Perl will be
634       in using third-party modules, which are documented below.
635
636   Using Perl modules
637       Perl modules provide a range of features to help you avoid reinventing
638       the wheel, and can be downloaded from CPAN ( <http://www.cpan.org/> ).
639       A number of popular modules are included with the Perl distribution
640       itself.
641
642       Categories of modules range from text manipulation to network protocols
643       to database integration to graphics.  A categorized list of modules is
644       also available from CPAN.
645
646       To learn how to install modules you download from CPAN, read
647       perlmodinstall.
648
649       To learn how to use a particular module, use "perldoc Module::Name".
650       Typically you will want to "use Module::Name", which will then give you
651       access to exported functions or an OO interface to the module.
652
653       perlfaq contains questions and answers related to many common tasks,
654       and often provides suggestions for good CPAN modules to use.
655
656       perlmod describes Perl modules in general.  perlmodlib lists the
657       modules which came with your Perl installation.
658
659       If you feel the urge to write Perl modules, perlnewmod will give you
660       good advice.
661

AUTHOR

663       Kirrily "Skud" Robert <skud@cpan.org>
664
665
666
667perl v5.26.3                      2018-03-23                      PERLINTRO(1)
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