Perl documentation

Loops

1. while loop

    my $i = 0;
    while($i < scalar @array) {
    	print $i, ": ", $array[$i];
    	$i++;
    	}

2. until loop

    my $i = 0;
    until($i >= scalar @array) {
    	print $i, ": ", $array[$i];
    	$i++;
    	}

3. C-style loop

    for(my $i = 0; $i < scalar @array; $i++) {
    	print $i, ": ", $array[$i];
    	}

4. foreach loop with arrays

    foreach my $i ( 0 .. $#array ) {
    	print $i, ": ", $array[$i];
    }

5. foreach loop with hash

    foreach my $key (sort keys %scientists) {
    	print $key, ": ", $scientists{$key};
    }

Array functions

• pop: Extracts and returns the final element of the array.
• push: appends extra elements to the end of the array.
• shift: extracts and returns the first element of the array.
• unshift: inserts new elements at the beginning of the array.

File handles

1. open file and read from it

    my $f = "text.txt";
    my $result = open(my $fh, "<", $f) || die "Couldn't open '".$f."' for reading because: ".$!;
    while(my $line = <$fh>) {
    	# process $line...
    	}

2. open file and write to it

    my $f = "text.txt";
    open(my $fh, ">", $f) || die "Couldn't open '".$f."' for writing because: ".$!;
    print $fh2 "The eagles have left the nest";

Regular expressions

• Match operations using =~ m// in scalar context. =~ m//g indicates “group match”

    my $string = "Hello world";
    if($string =~ m/(\w+)\s+(\w+)/) {
    	print "success";
    }

• Match operations using =~ m// in list context. =~ m//g indicates “group match”

    my $string = "colourless green ideas sleep furiously";
    my @matches = $string =~ m/(\w+)\s+((\w+)\s+(\w+))\s+(\w+)\s+(\w+)/;

    print join ", ", map { "'".$_."'" } @matches;
    #prints "'colourless', 'green ideas', 'green', 'ideas', 'sleep', 'furiously'"

• Substitution operations are performed using =~ s///. =~ s///g performs a global search

    my $string = "Good morning world";
    $string =~ s/world/Vietnam/;
    print $string; # "Good morning Vietnam"

References

1. Scalar reference

    use warnings;
    use strict;
    my $x = 10;
    my $xr = \$x;
     
    # change $x via $xr
    $$xr = $$xr * 2;
     
    print("$x\n"); # 20
    print("$$xr \n");  # 20
    print("$xr\n"); # SCALAR(0x1d2e6e4)

2. Array reference

    use warnings;
    use strict;
    my @a = (1..5);
    my $ar = \@a;
     
    my $i = 0;
    for(@$ar){
        print("$ar->[$i++] \n");
    }

3. Hash reference

    #!/usr/bin/perl
    use warnings;
    use strict;
     
    my %months= ( Jan => 1,
           Feb => 2,
           Mar => 3,
           Apr => 4,
           May => 5,  
           Jun => 6,
           Jul => 7,
           Aug => 8,
           Sep => 9,
           Oct => 10,
           Nov => 11,
           Dec => 12);
     
    my $monthr = \%months;    
     
    for(keys %$monthr){
        print("$_  = $monthr->{$_}\n");
    } 

• Command line read file and process
  • -p makes Perl loop over the incoming data and print it out once you’ve finished fiddling with it

    cat test | perl -p -e 's/(.*):(.*)/$2:$1/'

• system commands in perl

    system("net use ${drive} \\\\${server}\\${remote_drv_ltr}\$ ${passwd} /USER:${domain}\\${login}");

• change backup date and rsync
• perl one liners
  • Find .aep files of a nugget eg. N2118

perl -nwl -e '(/N(\s|_)2118\D/i or /(N)2118\D/i) and (/.aep\z/i) and print "$ARGV => $_" ;' 3dlist/*.txt firesafe_hdd/*.txt svnlist/*.list