PERL
PERL Variable interpolation from string
To interpolate a variable which is part of string (string can be user input or read from file) use the " String-Interpolate-0.32 " library. This is a safer option than doing a search and replace like 's/\$var/$var/'
https://metacpan.org/release/NEILB/String-Interpolate-0.32/view/lib/String/Interpolate.pm
PERL Execute a file as a Script
To execute a file as script use the following code segment.
The code assumes all script component lines starting with "//::". Rest all lines are just printed as is.
use File::Slurp;
use strict;
use warnings;
use utf8;
my $fr_file_name = $ARGV[0]; # input file name
open my $fr, '<', $fr_file_name
or die "$0 : failed to open input file '$fr_file_name' : $!\n";
my $fw_file_name = $fr_file_name.'_execute'; # output file name
open my $fw, '>', $fw_file_name
or die "$0 : failed to open output file '$fw_file_name' : $!\n";
foreach my $line (<$fr>){
chomp $line;
if ($line =~ m|^(\Q//::\E)|) {
$line =~ s/^\/\/:://;
print {$fw} "$line\n";
} else {
$line = quotemeta($line);
$line = "print \"$line\";";
print {$fw} "$line\n";
print {$fw} "print \"\\n\"\;\n";
}
}
close $fw
or warn "$0 : failed to close output file '$fw_file_name' : $!\n";
close $fr
or warn "$0 : failed to close input file '$fr_file_name' : $!\n";
my $VAR = "bar";
print "\n\n";
print "Evaluating command \n";
my $command=read_file('input_file_execute');
eval $command;
print "\n\n";
Installing PERL libraries locally
Use the following code to install PERL libraries locally (downloaded from CPAN)
Directory structure:
Library source code -> ~/perl_libs/perl_lib_source
Library install path -> ~/perl_libs/perl_lib_install
Procedure to install:
Download CPAN library to ~/perl_libs/perl_lib_source (For e.g. File_slurp.tar.gz)
cd ~/perl_libs/perl_lib_source
Add below lines to a file and run as script (in e.g. $1 will be File_slurp.tar.gz)
********************************** START OF SCRIPT**********************************
tar -zxvf $1
set libName=`echo $1 | sed 's/\.tar\.gz//'`
mkdir -p ~/perl_libs/perl_lib_install/$libName
cd $libName
perl Makefile.PL INSTALL_BASE=~/perl_libs/perl_lib_install/$libName
make
make test
make install
cd -
rm -rf $libName
if ( $?PERL5LIB ) then
setenv PERL5LIB $PERL5LIB\:~/perl_libs/perl_lib_install/$libName/lib/perl5
else
setenv PERL5LIB ~/perl_libs/perl_lib_install/$libName/lib/perl5
endif
********************************** END OF SCRIPT ***********************************
Setting Environment variables
foreach list (`ls -d ~/perl_libs/perl_lib_install`)
setenv PERL5LIB $PERL5LIB\:~/perl_libs/perl_libs_install/$list/lib/perl5
end
In-Line editing of files
PERL offers in-line editing feature through command line.
Syntax:
perl -p -i -e <regular_expression> <file/s>
The same feature can be used inside the script (though it comes with certain limitations)
Syntax:
our $^I = '';
our @ARGV = <filelist>; # Either glob or pass an array of filelist
while (<ARGV>) {
#paring through each file
print;
}
Notes:
This method only works with ARGV as filehandler
Setting the PERL variable $^I to certain value will create a backup of the file
The back up file name is appended with the content of the $^I
The print statement handles the write back of the file i.e. internally it uses the select construct to write back.
Unset the $^I variable to return to normal operation
PERL PACKAGES
<PLACE HOLDER>
DETECT LAST ITERATION
If a file is being read, detecting the last iteration can be performed this way (will be useful to add footers)
while(<>){
.....
print "Last iteration" if (eof);
}
Subroutine caller information
The code segment below can be used inside a subroutine to get the information of the caller.
( my $package, my $filename, my $line ) = caller;
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
SEEK and TELL
Two useful file-handle operation in PERL are "seek" and "tell".
SEEK:
This is used to set the FILEHANDLE's position
Syntax:
seek <FILEHANDLE>, POSITION, WHENCE
Where,
POSITION can be obtained from "tell"
WHENCE is encoded as follows,
0 - Set the new position of file handle at POSITION argument
1 - Set the new position of file handle at (Current position + POSITION argument)
2 - Set the new position of file handle at END of file + POSITION argument (POSITION should be negative)
TELL:
This returns the current position of the FILEHANDLE in bytes
PERL array comparison benchmark
use strict;
use warnings;
use Benchmark qw'cmpthese';
my %sections = (
strign_s1 => 1, strign_s2 => 1, strign_s3 => 1, strign_s4 => 1,
string_o1 => 1, string_o2 => 1, string_o3 => 1, string_o4 => 1,
string_o5 => 1, string_o6 => 1, string_o7 => 1, string_o8 => 1,
string_a1 => 1, string_a2 => 1, string_a3 => 1, string_a4 => 1,
string_a5 => 1, string_a6 => 1, string_a7 => 1, string_a8 => 1,
string_b1 => 1, string_b2 => 1, string_b3 => 1, string_b4 => 1,
string_b5 => 1, string_b6 => 1, string_b7 => 1, string_b8 => 1,
string_c1 => 1, string_c2 => 1, string_c3 => 1, string_c4 => 1,
string_c5 => 1, string_c6 => 1, string_c7 => 1, string_c8 => 1,
);
my @somelist = qw(
strign_s1, strign_s2, strign_s3, strign_s4,
string_o1, string_o2, string_o3, string_o4,
string_o5, string_o6, string_o7, string_o8,
string_a1, string_a2, string_a3, string_a4,
string_a5, string_a6, string_a7, string_a8,
string_b1, string_b2, string_b3, string_b4,
string_b5, string_b6, string_b7, string_b8,
string_c1, string_c2, string_c3, string_c4,
string_c5, string_c6, string_c7, string_c8,
);
my @elements = map { 'strign_s' . int(1 + rand(10)) } 1 .. 100;
my $namespace;
cmpthese(10000, {
hash_check => sub {
foreach my $element (@elements) {
$namespace = $element if exists $sections{$element};
}
},
nested_loop => sub {
foreach my $list (@somelist) {
foreach my $element (@elements) {
if ($list eq $element) {
$namespace = $element;
last;
}
}
}
},
});
Result:
Rate nested_loop hash_check
nested_loop 3745/s -- -95%
hash_check 76923/s 1954% --