Perl

Reference

• Perldoc on local computer

% perldoc -q duplicate
  "How can I remove duplicate elements from a list or array?"
% perldoc -f split
  split /PATTERN/,EXPR,LIMIT
  split /PATTERN/,EXPR
  split /PATTERN/
  ...

• Links
  • http://www.perl.org/learn.html
  • http://www.perl.org/docs.html
  • http://perldoc.perl.org/perl.html
  • http://perldoc.perl.org/perlintro.html
  • http://faq.perl.org/
  • http://perldoc.perl.org/index-tutorials.html
  • http://perldoc.perl.org/index-modules-A.html
• Most relevant perldoc manpages (from the FAQ):

Basics          perldata, perlvar, perlsyn, perlop, perlsub
Execution       perlrun, perldebug
Functions       perlfunc
Objects         perlref, perlmod, perlobj, perltie
Data            Structures   perlref, perllol, perldsc
Modules         perlmod, perlmodlib, perlsub
Regexes         perlre, perlfunc, perlop, perllocale
Moving to perl5 perltrap, perl
Linking w/C     perlxstut, perlxs, perlcall, perlguts, perlembed
Various         http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz
                (not a man-page but still useful, a collection of various essays on Perl techniques)

• Command-Line - Useful command-line options

-e expression

specififies perl expressions.

-p

loops over and prints input.

-n

loops over and does not print input.

-l

strip newlines on input, and adds them on output. Use this option by default, unless the newlines need special handling, or for efficiency reasons.

Quick Introduction

Program Structure

A Simple Hello World

#!/usr/bin/perl

use strict;                 # Immediately stops on potential problem
use warning;                # Warnings

print "Hello, World!\n";

exit 0;

TBC

Example with modules

TBC

Control Sequence Instructions

TBC

One-Liner

See [1], [2], [3], [4].

perl -ne 'print unless /^$/../^$/' input                              # print lines, unless blank
perl -ne 'print if ! /^$/../^$/' input                                # reduce runs of blank lines to a single blank line
perl -nle 'print $.; close ARGV if eof' input input                   # $. need to be reset (by closing ARGV) between 2 input files
perl -nle 'print for m/\b(\S+)\b/g' paragraphs                        # print words from file paragraphs
perl -nle 'while(m/(\S+)\s+the\s+(\S+)/g){print "$1 $2"}' paragraphs  # ... while loop needed when using multiple back-references
perl -lne 'print for /id is <(\d+)>/g'                                # match pattern and extract backreference
perl -lne 'print $2 for /id is <(\d+)> or <(\d+)>/g'                  # ... print 2nd matched backreference
cat oldfile | perl -pe 's/(\d+)_/sprintf("%2.2d_",$1)/e' > newfile    # evaluate regex substitutions

Script Examples

Miscellaneous examples

# Various examples in Perl
die "can't run this";

#-----------------------------------------------------------------------------------------------------------------------------------
#Split a multi-line variable/output in line components - method 1.
my @ArrayList = `$CT lsvob -short`;              #any command producing a multi-line output

foreach (@ArrayList)
{
	chop();                                  #remove the trailing newline
	print "Array List - The VOB is $_.\n";
};

#-----------------------------------------------------------------------------------------------------------------------------------
#Split a multi-line variable/output in line components - method 2.
my $ScalarList = `$CT lsvob -short`;    #any command producing a multi-line output
my @ArrayList2 = split/\n/,$ScalarList; #split the scalar into several lines

foreach (@ArrayList2)
{
	print "Scalar List - The VOB is $_.\n";
};

#-----------------------------------------------------------------------------------------------------------------------------------
# Use ${variable} to split scalar identifier from the rest of a text
my $variable;
print "$variable_temp\n";               # NOK! Print a variable named variable_temp
print "${variable}_temp\n";             # OK! Print a $variable, followed by "_temp"

#-----------------------------------------------------------------------------------------------------------------------------------
#Append an array (or single element) to another
push(@array1,@array_or_element);

#-----------------------------------------------------------------------------------------------------------------------------------
#Add use strict; at the beginning to improve compilation warnings.
use strict;

#-----------------------------------------------------------------------------------------------------------------------------------
#Arrays / Hashes
#

#The following actually adds six elements to array VOBRecords. It doesn't create an extra element contains the hash.
my %VOBAttrib = ( VOBName => $VOBName,  IsProjectVOB => $IsProjectVOB, IsProjectAdminVOB => $IsProjectAdminVOB, ProjectName => $ProjectName );
push (@VOBRecords, %VOBAttrib);

#-----------------------------------------------------------------------------------------------------------------------------------
# Handling reference
my $VOBAttrRef = $VOBs{'AdminMask'};                #This return a reference to a Hash
my %VOBAttr = %$VOBAttrRef;                         #This dereference the reference above and return a Hash

print $VOBAttr{'ProjectName'},"\n";                 #We can use our new Hash variable
print $$VOBAttrRef{'ProjectName'},"\n";             # ... or we can dereference our reference variable using the $$ construct
print $VOBAttrRef->{'ProjectName'},"\n";            # ... but -> can also be used to dereference
print $VOBs{'AdminMask'}->{'ProjectName'},"\n";     #We can also skip altoghether the reference variable
print $VOBs{'AdminMask'}{'ProjectName'},"\n";       # ... This notation is also available as a shortcut, -> can be omitted

#-----------------------------------------------------------------------------------------------------------------------------------
#Autovivification - example on how to create a hash of array
#

my %Projects;
foreach my $VOBName (keys %VOBs)
{
	my $ProjectName = $VOBs{$VOBName}{'ProjectName'};
	push(@{$Projects{$ProjectName}}, $VOBName); # <-- we dereference value returned by $Projects{$ProjectName} as 
                                                    #     an array, hence creating automatically an array if undef
}

#-----------------------------------------------------------------------------------------------------------------------------------
#Read something from standard input
$line = <STDIN>;
$line = readline(*STDIN);       # same thing
chop($line = <STDIN>);          # remove trailing newline

#-----------------------------------------------------------------------------------------------------------------------------------
#Read one character}
#print "Press RETURN...";
#$key = getc();

#-----------------------------------------------------------------------------------------------------------------------------------
# Regex Matching

# Use /regex/, or m!regex! (where ! can be any quoting character)
# Use =~ to match a given variable, otherwise $_ is used. Use !~ to reverse the sense of the match.
# SCALAR CONTEXT: // return true/false if matching found
$myvar =~ /World/               #scalar context, returns true if $myvar contains World
/World/                         #scalar context, same as above except that now it is $_ that is matched
"Hello World" =~ /World/        #scalar contect, same as above, to show that left member doesn't need to be an L-Value

# Extracting matches
# The grouping metacharacters () also allow the extraction of the parts of a string that matched. For each grouping, the part that
# matched inside goes into the special variables $1 , $2 , etc. They can be used just as ordinary variables:

# extract hours, minutes, seconds
$time =~ /(\d\d):(\d\d):(\d\d)/;  # match hh:mm:ss format
$hours = $1;
$minutes = $2;
$seconds = $3;

# LIST CONTEXT, a match /regex/ with groupings will return the list of matched values ($1,$2,...) . So we could rewrite it as

($hours, $minutes, $second) = ($time =~ /(\d\d):(\d\d):(\d\d)/);

# If the groupings in a regex are nested, $1 gets the group with the leftmost opening parenthesis, $2 the next opening parenthesis,
# etc. For example, here is a complex regex and the matching variables indicated below it:

# /(ab(cd|ef)((gi)|j))/;
#  1  2      34

#Associated with the matching variables $1 , $2 , ... are the backreferences \1 , \2 , ... Backreferences are matching variables
# that can be used inside a regex:

    /(\w\w\w)\s\1/; # find sequences like 'the the' in string

# $1 , $2 , ... should only be used outside of a regex, and \1 , \2 , ... only inside a regex.

#-----------------------------------------------------------------------------------------------------------------------------------
# Regex Search & Replace

# Use s/regex/replacement/modifiers
# Use =~ to match a given variable, otherwise $_ is used.
# SCALAR CONTEXT: s/// returns the number of matches, or false if no match.

$x = "Time to feed the cat!";
$x =~ s/cat/hacker/;   # $x contains "Time to feed the hacker!"

# VARIABLES:
# $1,$2: matched variables are immediately available in the replacement string.

# MODIFIERS:
# - g: find all matches
# - e: wraps an eval{...} around the replacement string and the evaluated result is substituted for the matched substring.

# reverse all the words in a string
$x = "the cat in the hat";
$x =~ s/(\w+)/reverse $1/ge;    # $x contains "eht tac ni eht tah"

#-----------------------------------------------------------------------------------------------------------------------------------
# The split operator
# split /regex/, string splits string into a list of substrings and returns that list. The regex determines the character sequence
# that string is split with respect to. For example, to extract a comma-delimited list of numbers, use

$x = "1.618,2.718,   3.142";
@const = split /,\s*/, $x;      # $const[0] = '1.618'
                                # $const[1] = '2.718'
                                # $const[2] = '3.142'

# If the empty regex // is used, the string is split into individual characters. If the regex has groupings, then the list produced
# contains the matched substrings from the groupings as well:

$x = "/usr/bin";
@parts = split m!(/)!, $x;      # $parts[0] = ''    Since the first character of $x matched the regex, an initial element was prepended.
                                # $parts[1] = '/'
                                # $parts[2] = 'usr'
                                # $parts[3] = '/'
                                # $parts[4] = 'bin'

#-----------------------------------------------------------------------------------------------------------------------------------
# The lookahead and lookbehind assertions are generalizations of the anchor concept. Lookahead and lookbehind are zero-width
# assertions that let us specify which characters we want to test for. The lookahead assertion is denoted by (?=regexp) and the
# lookbehind assertion is denoted by (?<=fixed-regexp). Some examples are

    $x = "I catch the housecat 'Tom-cat' with catnip";
    $x =~ /cat(?=\s+)/;  # matches 'cat' in 'housecat'
    @catwords = ($x =~ /(?<=\s)cat\w+/g);  # matches,
                                           # $catwords[0] = 'catch'
                                           # $catwords[1] = 'catnip'
    $x =~ /\bcat\b/;  # matches 'cat' in 'Tom-cat'
    $x =~ /(?<=\s)cat(?=\s)/; # doesn't match; no isolated 'cat' in
                              # middle of $x

#-----------------------------------------------------------------------------------------------------------------------------------
# Grep
my @array = ("el1","gel2","el3","gel1","gel2");
my @array2 = grep {s/(.*el)/reverse $1/e} @array;

$,="\n";
print @array2;

#-----------------------------------------------------------------------------------------------------------------------------------
# How to discard stderr on windows
# note: on windows, we use \nul instead of nul because each folder has it's own nul handler, and we want to reduce the number of
#       used handle
my $STDERRNULL = "2>\\nul";             #use this on windows
my $STDERRNULL = "2>/dev/null";         #use this on unix

my $AT="@";

my @CTResults =	qx($CT lstype -local -fmt "%n \\\"%[type_scope]p\\\"\\n" lbtype:$labelName$AT$vobName $STDERRNULL);

#-----------------------------------------------------------------------------------------------------------------------------------
# here's a file-private function as a closure,
# callable as &$priv_func;  it cannot be prototyped.
my $priv_func = sub {
    # stuff goes here.
};

#-----------------------------------------------------------------------------------------------------------------------------------
# Capture command output: use back-ticks ``, qx(), or system("") with redirection.
my @ouput = `ls`;
my @ouput = qx(ls);
system("ls >output.txt");

#-----------------------------------------------------------------------------------------------------------------------------------
# Capture command exit status: use system("")
my $exit_status = system("del file.txt");

#-----------------------------------------------------------------------------------------------------------------------------------
#Handling of @ in scalar / list context.
#                                            RESULT      CONTEXT  EXPLANATION
my @a = ("titi","tutu");
my $varnoquote=@a; print "$varnoquote\n"; # "2"         (SCALAR - @_ is evaluated in scalar context)
my $varquote="@a"; print "$varquote\n";   # "titi tutu" (EXPAND - @_ is quote-expanded, each item being separated by space)
print @a; print"\n";                      # "tititutu"  (LIST   - $, is empty)
print(@a); print"\n";                     # "tititutu"  (LIST   - $, is empty)
printf @a; print"\n";                     # "titi"      (LIST   - 1st element in list is interpreted as the format string)
printf(@a); print"\n";                    # "titi"      (LIST   - 1st element in list is interpreted as the format string)
print @a,"\n";                            # "tititutu"  (LIST   - $, is empty)
printf "%s\n",@a;                         # "titi"      (LIST   - only 1st element is read)

#-----------------------------------------------------------------------------------------------------------------------------------
#Give default value if no parameter in sub
sub myfunc
{
	my($suffix) = @_ ? "@_" : "defaultvalue";
}

#-----------------------------------------------------------------------------------------------------------------------------------
#Report a complete error message when loading a script
#This hack allows for printing a custom error message + file not found-like error message (given by $!) + syntax error messages (@_)
do "your script.pl"
	or (print "Your error message\n$!\n" and die @_);

#-----------------------------------------------------------------------------------------------------------------------------------
#Example on how to embed perl into a w2K shell script.
#Notice how the first rem is actually a multiline assignment to perl array variable @rem, where the value is quoted with ' '.
@rem= 'PERL for Windows NT - ccperl must be in search path
@echo off
ccperl %0 %1 %2 %3 %4 %5 %6 %7 %8 %9
goto endofperl
@rem ';

# Your Perl code comes here

# End of Perl section

__END__
:endofperl

#-----------------------------------------------------------------------------------------------------------------------------------
#Example on use of qw (=''), join, etc to build complex line using variable and function calls (here vobName() is a function)
system "echo", $CT, qw(rmtype), join("",'trtype:MKELEM_POST_OWNER@',vobName($vobname));

#-----------------------------------------------------------------------------------------------------------------------------------
#Example on how to temporarily disable STDERR and restore it afterwards
open(SAVE_STDERR, '>&STDERR');
close(STDERR) unless $ENV{CLEARCASE_TRACE_TRIGGERS};
$exe = qx(file $ptmp) =~ /executable|bourne|commands text|\bscript/i;
open(STDERR, '>&SAVE_STDERR');
close(SAVE_STDERR);

#-----------------------------------------------------------------------------------------------------------------------------------
#REDIRECTION OF STDERR WITH system()
#
#It seems that STDERR can only be redirected if it occurs in the command, not in args!
system "echo hello world! 2>\\nul";             # OK
system qq(echo hello world! 2>\\nul);           # OK
system "echo", "hello world!"," 2>\\nul";       # OK
system "$CT hello world! 2>\\nul";              # OK
system qq(CT hello world! 2>\\nul);             # OK
system "$CT", "hello world!"," 2>\\nul";        # NOK (seems that external pgm can not be redirected like that)

#-----------------------------------------------------------------------------------------------------------------------------------
#Searching or modifying arrays: map / grep
@chars = map(chr, @nums);
@foo = grep(!/^#/, @bar);                       # weed out comments

#-----------------------------------------------------------------------------------------------------------------------------------
#Handling of various quoting
#
# qx ``
#  => $VAR is expanded
#  => "..." quote are conversed
#  => \ is processed (so use \\ for backslash in a windows path for instance)
qx($CTNDEBUG lstype \n -fmt "%n\\n" -kind brtype -invob $VOBAdminName);

#-----------------------------------------------------------------------------------------------------------------------------------
#Choping character: very useful to remove the trailing "\n", also on list!
chop( my $userinput=<STDIN> );                  #Chop the trailing "\n" in user input
chop( my @list=qx(ls);                          #Chop the trailing "\n" in the command output

#-----------------------------------------------------------------------------------------------------------------------------------
#CHOMPing character: safer version (in case for instance the last line doesn't have the \n character
chomp( my $userinput=<STDIN> );	                #Chomp the trailing "\n" in user input IF PRESENT
chomp( my @list=qx(ls);                         #Chomp the trailing "\n" in the command output IF PRESENT

my %VOBs = (
	"AdminMask"                  => { ProjectName => "\"\""          },
	"MaskADKSAM"                 => { ProjectName => "\"MaskADKSAM\""}
);

#-----------------------------------------------------------------------------------------------------------------------------
#passing filehandle as sub parameters and return values --> use reference
#
# First as return values:

sub openTimeOut($)
{
	my $filename = shift;
	my $timeout=15;
	while( !open(LOG,$filename) ) { sleep 1; --$timeout or die "Time out trying to open file $filename"; }
	return \*LOG;
}

sub printToFile($@)
{
	my $filename = shift;
	my $fh = openTimeOut(">$filename");
	print $fh @_;
	close($fh);
}

#
# BUT BEWARE, ACTUALLY OpenTimeOut returns a reference to the same FILEHANDLE in current glob !
# The code below illustrate this:
#
#
	my ($to,$from) = @_;
	$fhto = openTimeOut(\*TO,">>$to");
	$fhfrom = openTimeOut(\*FROM,"<$from");    # This returns same FILEHANDLE reference as $fhto
	while (<$fhfrom>) {print $fhto $_}         # Failed, because now $fhto = $fhfrom, which only open for output
	close($fhfrom);
	close($fhto);

#
# The solution, pass by parameters:
#
sub openTimeOut2(*;$)
{
	my $fh = shift;
	my $filename = shift;
	my $timeout=15;
	while( !open($fh,$filename) ) { sleep 1; --$timeout or die "Time out trying to open file $filename"; }
}

sub printToFile($@)
{
	my $filename = shift;
	openTimeOut2(\*LOG,">$filename");
	print LOG @_;
	close(LOG);
}

#-----------------------------------------------------------------------------------------------------------------------------
#uppercase / lowercase
#
my $lowercase = lc "My StRiNg";                 #mystring
my $uppercase = uc "My StRiNg";	                #MYSTRING
my $firstcharlowercase = lcfirst "My StRiNg";   #my StRiNg
my $firstcharuppercase = ucfirst "My StRiNg";   #My StRiNg

Using Anonymous Hash References

#!/usr/bin/perl

use strict;

my @myarray;

foreach my $iter ( 1..10 )
{
	my $value1 = "value1_".$iter;
	my $value2 = "value2_".$iter;
	my $value3 = "value3_".$iter;

   	print "Creating our \$hashref... ";
   	# Construct { key1 => value1, key2 => value2.... } creates a REFERENCE to an anonymous hash.
   	# Since reference are SCALAR, we assign it to a scalar variable
   	my $hashref = { index1 => $value1, index2 => $value2, index3 => $value3 };
	
	print "Done.\n",
	      "  \$hashref: ",$hashref,"\n";
	print "  content: ",$$hashref{'index1'},",",$$hashref{'index2'},",",$$hashref{'index2'},"\n";

	print "Adding \$hashref to our array... ";
	push( @myarray, $hashref );

	print "Done. There are currently ", scalar(@myarray), " elements in \@myarray.\n";
	print "Accessing last element of our array...";
	print " content: @myarray[$#myarray], ${@myarray[$#myarray]}{'index1'} our better yet @myarray[$#myarray]->{'index2'}\n";
}

print "\n\nNow we will traverse our array again...\n";
foreach ( @myarray )
{
	print 	"$_ containing ",
	 		"index1 => $$_{'index1'},",
			"index2 => $$_{'index2'},",
			"index3 => $$_{'index3'}\n";
	print 	"... or using -> operator: ",
	 		"index1 => $_->{'index1'},",
			"index2 => $_->{'index2'},",
			"index3 => $_->{'index3'}\n";
}

Parsing Command Line Parameters

Command line parameters are parsed through variable ARGV.

print scalar @ARGV;                   #number of parameters
print $#ARGV;                         #... idem
print "1st param: $ARGV[0]";          #positional parameters
print "2nd param: $ARGV[1]";
print "Executable name: $0";          #Name of current executable
usage() unless defined($ARGV[0])      # defined($ARGV[0]) is true if there is a parameter

Simple version

#!/usr/bin/perl

use strict;
use warnings;

my $verbose=0;
my $projectdir;

# Parse command options (-option).
while ($#ARGV>=0 && $ARGV[0] =~ m/^\-/ ) {
    $verbose=1 if $ARGV[0] =~ m/^\-v/i;
    shift @ARGV;
}

# Parse mandatory parameter
usage() unless defined($ARGV[0]);
$projectdir=$ARGV[0];

# Show parsing result
print "verbose=$verbose\n";
print "projectdir=$projectdir\n";

exit 0;

sub usage {
    print "Usage: $0 [options] directory\n";
    print "\n";
    print "  Options:\n";
    print "    -v   verbose mode\n";
    exit;
}

Using GetOpt

use strict;
use Getopt::Long qw(:config no_ignore_case);

my $debug=0;
my $quiet=0;
my username;

# Parse options
GetOptions ("d|debug+"  => \$debug,
            "q|quiet"   => \$quiet,
            "u|user=s"  => \$username ) || usage();

# Parse remaining parameters
my $url = $ARGV[0];

Pitfalls

# Frequent Mistakes in Perl
die "can't run this";

#-----------------------------------------------------------------------------------------------------------------------------------
#Forget to chop the trailing "\n"
#-----------------------------------------------------------------------------------------------------------------------------------
my $path = qx(pwd);                     #NOK! trailing \n will corrupt path construction
chop( my $path = qx(pwd) );             #OK!

#-----------------------------------------------------------------------------------------------------------------------------------
#Mix case in name of package
#-----------------------------------------------------------------------------------------------------------------------------------
# Imagine a module file named Vobs.pm

use Vobs;
use VOBs;                               # NOK --> Will complain about double definition (but will not flag the mix case problem)

#-----------------------------------------------------------------------------------------------------------------------------------
# Beware of operator precedence and strange behaviour
chomp my @emptylist = qx("dir");        #NOK ! @emptylist will be empty
chomp ( my @list = qx("dir") );	        #OK !

CPAN - Perl Packages

First time launch:

$ cpan                                       # ... OR ...
$ perl -MCPAN -e shell                       #  --> yes auto config

To adapt config related to proxy:

cpan> o config init /proxy/                  # (to enter an empty string, simply enter 1 space char as a value)
cpan> o conf commit

To install a Perl package (eg. here package GetOpt::Long):

$ cpan
cpan>install GetOpt::Long

Editing the configruation:

cpan> o conf init                            # Reset the configuration
cpan> o conf http_proxy http://proxy:8080/   # Edit a given variable (eg. here changing proxy settings):
cpan> o conf commit                          # To commit changes in the configuration
cpan> o                                      # to get o options
cpan> o conf                                 # To get o conf option

To edit CPAN database url:

cpan> o conf /urllist/
cpan> o conf init /urllist/
cpan> o conf urllist shift
cpan> o conf urllist unshift ftp://my.new.site/
cpan> o conf commit

To update CPAN itself:

cpan> install Bundle::CPAN
cpan> reload cpan