PERL Commands

🐪 Perl Commands – File Operations

Topic	Question	Answer
Perl	Run script	perl script.pl
Perl	Run with warnings	perl -w script.pl
Perl	Check syntax	perl -c script.pl
Perl	Run one-liner	perl -e 'print "Hello\n";'
Perl	Run with modules	perl -M<Module> script.pl
Perl	Run in place edit	perl -pi -e 's/old/new/g' file.txt
Perl	Print line numbers	perl -ne 'print "$. $_"' file.txt
Perl	File is empty	if (-z "file.txt")
Perl	File exists	if (-e "file.txt")
Perl	File readable	if (-r "file.txt")
Perl	File writable	if (-w "file.txt")
Perl	File executable	if (-x "file.txt")
Perl	Is directory	if (-d "path")
Perl	Is file	if (-f "file.txt")
Perl	Is symlink	if (-l "link")
Perl	Get file size	my $size = -s "file.txt";
Perl	Get file age (days)	my $age = -M "file.txt";
Perl	Get modification time	my $mtime = (stat "file.txt")[9];
Perl	Open for reading	open(my $fh, '<', 'file.txt');
Perl	Open for writing	open(my $fh, '>', 'file.txt');
Perl	Open for appending	open(my $fh, '>>', 'file.txt');
Perl	Open with error check	open(my $fh, '<', 'file') or die "Cannot open: $!";
Perl	Close file	close($fh);
Perl	Read line	my $line = <$fh>;
Perl	Read all lines	my @lines = <$fh>;
Perl	Read file line by line	while (my $line = <$fh>) { }
Perl	Read into array	my @lines = <$fh>;
Perl	Write to file	print $fh "text\n";
Perl	Print to STDOUT	print "text\n";
Perl	Print to STDERR	print STDERR "error\n";
Perl	Printf formatting	printf("Value: %d\n", $number);
Perl	Die with error	die "Error: $!\n";
Perl	Warn with message	warn "Warning: something\n";
Perl	Slurp file	my $content = do { local $/; <$fh> };
Perl	Remove file	unlink('file.txt');
Perl	Rename file	rename('old.txt', 'new.txt');
Perl	Copy file	use File::Copy; copy('src', 'dst');
Perl	Move file	use File::Copy; move('src', 'dst');

🔤 Perl Commands – String & Regex

Topic	Question	Answer
Perl	Search and replace	$line =~ s/old/new/g
Perl	Replace case insensitive	$line =~ s/old/new/gi
Perl	Replace first occurrence	$line =~ s/old/new/
Perl	Match pattern	if ($line =~ /pattern/)
Perl	Negate match	if ($line !~ /pattern/)
Perl	Match case insensitive	if ($line =~ /pattern/i)
Perl	Match with capture	if ($line =~ /(\d+)/) { my $num = $1; }
Perl	Global match	my @matches = $line =~ /pattern/g
Perl	Execute with backticks	my $date = \date`;`
Perl	Execute with system	system("ls -l");
Perl	Execute with qx	my @array = qx(ls);
Perl	Get exit code	my $exit = $? >> 8;
Perl	Split by spaces	my @words = split(/ /, $str);
Perl	Split by commas	my @fruits = split(/,/, $text);
Perl	Split by whitespace	my @words = split(/\s+/, $str);
Perl	Split with limit	my @parts = split(/,/, $str, 3);
Perl	Join array	my $string = join(",", @array);
Perl	String concatenation	my $result = $str1 . $str2;
Perl	String repeat	my $repeated = $str x 3;
Perl	String length	my $len = length($string);
Perl	Substring	my $sub = substr($string, $start, $len);
Perl	Index of substring	my $pos = index($string, $substring);
Perl	Last index	my $pos = rindex($string, $substring);
Perl	Uppercase	my $upper = uc($string);
Perl	Lowercase	my $lower = lc($string);
Perl	Uppercase first	my $ucfirst = ucfirst($string);
Perl	Lowercase first	my $lcfirst = lcfirst($string);
Perl	Trim whitespace	$string =~ s/^\s+|\s+$//g;
Perl	Trim left	$string =~ s/^\s+//;
Perl	Trim right	$string =~ s/\s+$//;
Perl	Check if contains	if (index($string, $substring) != -1)
Perl	String compare	if ($str1 eq $str2)
Perl	String not equal	if ($str1 ne $str2)
Perl	String less than	if ($str1 lt $str2)
Perl	String greater than	if ($str1 gt $str2)
Perl	Numeric compare	if ($num1 == $num2)
Perl	Not equal numeric	if ($num1 != $num2)
Perl	Less than	if ($num1 < $num2)
Perl	Greater than	if ($num1 > $num2)
Perl	Chomp newline	chomp($line);
Perl	Chop last character	chop($string);
Perl	Reverse string	my $reversed = reverse($string);

🐪 Perl Commands – Arrays & Hashes

Topic	Question	Answer
Perl	Push to array	push(@array, $element);
Perl	Push multiple	push(@array, $el1, $el2);
Perl	Pop from array	my $last = pop(@array);
Perl	Shift from array	my $first = shift(@array);
Perl	Unshift to array	unshift(@array, $element);
Perl	Get array length	my $length = scalar(@array);
Perl	Array size	my $size = $#array + 1;
Perl	Last index	my $last_idx = $#array;
Perl	Reverse array	my @reversed = reverse(@array);
Perl	Sort array	my @sorted = sort(@array);
Perl	Sort numerically	my @sorted = sort {$a <=> $b} @array;
Perl	Sort descending	my @sorted = sort {$b <=> $a} @array;
Perl	Sort by length	my @sorted = sort {length($a) <=> length($b)} @array;
Perl	Check if in array	if (grep {$_ eq $item} @array)
Perl	Filter array	my @filtered = grep {$_ > 10} @array;
Perl	Map array	my @doubled = map {$_ * 2} @array;
Perl	Array slice	my @slice = @array[0..4];
Perl	Join array	my $string = join(",", @array);
Perl	Create hash	my %hash = (key1 => 'value1', key2 => 'value2');
Perl	Access hash value	my $value = $hash{key};
Perl	Set hash value	$hash{key} = 'value';
Perl	Check key exists	if (exists $hash{key})
Perl	Check defined value	if (defined $hash{key})
Perl	Delete hash key	delete $hash{key};
Perl	Get all keys	my @keys = keys(%hash);
Perl	Get all values	my @values = values(%hash);
Perl	Get key-value pairs	my %copy = %hash;
Perl	Iterate hash	while (my ($key, $value) = each %hash) { }
Perl	Iterate keys	foreach my $key (keys %hash) { }
Perl	Hash size	my $size = scalar(keys %hash);
Perl	Clear hash	%hash = ();
Perl	Merge hashes	my %merged = (%hash1, %hash2);

🐪 Perl Commands – Control Structures

Topic	Question	Answer
Perl	For loop	for (my $i = 0; $i < 10; $i++) { }
Perl	For each loop	foreach my $item (@array) { }
Perl	For each with index	for my $i (0..$#array) { }
Perl	While loop	while (condition) { }
Perl	Until loop	until (condition) { }
Perl	Do-while	do { } while (condition);
Perl	Loop with next	next; (continue)
Perl	Loop with last	last; (break)
Perl	Loop with redo	redo;
Perl	If statement	if (condition) { }
Perl	If-else	if (condition) { } else { }
Perl	If-elsif-else	if (cond1) { } elsif (cond2) { } else { }
Perl	Unless statement	unless (condition) { }
Perl	Postfix if	print "yes" if $condition;
Perl	Postfix unless	print "no" unless $condition;
Perl	Ternary operator	my $result = $condition ? $true : $false;
Perl	Logical AND	if ($cond1 && $cond2)
Perl	Logical OR	if ($cond1 || $cond2)
Perl	Logical NOT	if (!$condition)
Perl	Logical and (low precedence)	open my $fh, '<', $file and print "opened";
Perl	Logical or (low precedence)	open my $fh, '<', $file or die "Failed: $!";
Perl	Defined-or operator	my $val = $var // "default";

🐪 Perl Commands – Subroutines & Modules

Topic	Question	Answer
Perl	Define subroutine	sub name { my ($arg1, $arg2) = @_; }
Perl	Call subroutine	name($arg1, $arg2);
Perl	Return from sub	return $value;
Perl	Return multiple	return ($val1, $val2);
Perl	Subroutine prototype	sub name ($$$) { }
Perl	Anonymous subroutine	my $sub = sub { };
Perl	Reference to sub	my $ref = \&name;
Perl	Call sub reference	$ref->($arg);
Perl	Use strict	use strict;
Perl	Use warnings	use warnings;
Perl	Use module	use Module::Name;
Perl	Require module	require Module::Name;
Perl	Import functions	use Module qw(func1 func2);
Perl	Import all	use Module ':all';
Perl	No import	use Module ();
Perl	Package declaration	package MyPackage;
Perl	Get command line args	my $arg = $ARGV[0];
Perl	All CLI arguments	my @args = @ARGV;
Perl	Number of arguments	my $count = @ARGV;
Perl	Environment variable	my $value = $ENV{VAR_NAME};
Perl	Set environment	$ENV{VAR_NAME} = 'value';
Perl	Process ID	my $pid = $$;
Perl	Program name	my $name = $0;

🐪 Perl Commands – Advanced Features

Topic	Question	Answer
Perl	Current directory	use Cwd; my $dir = getcwd();
Perl	Change directory	chdir('/path/to/dir');
Perl	List directory	opendir(my $dh, '.'); my @files = readdir($dh);
Perl	Close directory	closedir($dh);
Perl	Make directory	mkdir('dirname', 0755);
Perl	Remove directory	rmdir('dirname');
Perl	Get current time	my $time = time();
Perl	Format time	use POSIX; strftime("%Y-%m-%d", localtime());
Perl	Sleep seconds	sleep(5);
Perl	Sleep microseconds	use Time::HiRes; usleep(1000);
Perl	Generate random	my $rand = int(rand(100));
Perl	Seed random	srand(42);
Perl	Round number	use POSIX; my $rounded = floor($num);
Perl	Ceiling	use POSIX; my $ceil = ceil($num);
Perl	Absolute value	my $abs = abs($num);
Perl	Square root	my $sqrt = sqrt($num);
Perl	Power	my $pow = $base ** $exp;
Perl	Create reference	my $ref = \$scalar;
Perl	Array reference	my $aref = \@array;
Perl	Hash reference	my $href = \%hash;
Perl	Dereference scalar	my $val = $$ref;
Perl	Dereference array	my @arr = @$aref;
Perl	Dereference hash	my %h = %$href;
Perl	Anonymous array	my $aref = [1, 2, 3];
Perl	Anonymous hash	my $href = {key => 'value'};
Perl	Eval for exceptions	eval { risky_code(); }; if ($@) { }
Perl	Die with exception	die "Error: $!";