Perl Cheatsheet

Install Perl with Perlbrew

curl -L https://install.perlbrew.pl | bash

source ~/perl5/perlbrew/etc/bashrc

perlbrew available

# install the most recent Perl version, 5.38.2 
perlbrew install perl-5.38.2

# To install the bleeding-edge version of Perl
perlbrew install perl-blead

perlbrew list

Getting started example

#!/usr/bin/env perl
print "Command is: $0\n";
for my $arg ( @ARGV ) {
    print $arg, "\n";
}
exit 0;

• my is a function for declaring variables. We often need a temporary variable to store the variable value obtained in each loop in the for loop. We usually sandwich a my declaration in the for statement, which has the advantage of limiting the scope of $arg to this for loop structure.
• Perl statements end with a semicolon.

Variable Classification

Scalar

Scalars start with a $ followed by a letter or underscore.

my $num1 = 10;         # 10
my $num2 = $num1 * 10; # 100
my ($num3, $num4, $num5) = (1, $num2 + 1, 3);
$num1 = $num2 = $num3 = 0;

## my can assign either one scalar or multiple scalars at a time, and can also assign continuously.

my $str1 = 'ABC';       # ABC
my $str2 = '$str1 xyz'; # $str1 xyz
my $str3 = "$str1 xyz"; # ABC xyz
## If a variable is contained in double quotes, its value is interpolated, i.e., the variable name is replaced by its content. Variables in single quotes are not interpolated.

my $str5 = 'here is \' and \';  # here is ' and \
## In a single-quoted string, all characters except for two special characters (' (single quote) and \ (backslash)) are kept as they are.

my $str6 = $str1 . "_" . "xyz"; # ABC_xyz
## String concatenation using .

Array

Array names must start with @ followed by a letter or underscore.

my @nums = ( 1, 4, 9, 16 );
my @vars1 = ( "AAA", "BBB", "CCC" );
my @vars2 = ( "aaa", "bbb", "ccc" );

$nums[0] = 0;           # now @nums is: (0, 4, 9, 16)
$nums[4] = 17;          # now @nums is: (0, 4, 9, 16, 17)
$vars1[1] = "DDDD";

## $# followed by the array name represents the index of the last element in the array.
print 'last index of @vars1 is: ',  $#vars1, "\n";
# last index of @vars1 is: 2

## scalar function returns the size of the array, i.e., the number of elements it contains.
print '@vars2 has ', scalar @vars2, " element\n";
# @vars2 has 3 element

Hash

Hash names must start with %.

%pcc_of = (
  'ZheJiang'    => 'HangZhou',
  'JiangXi'     => 'NanChang',
  'XiZang'      => 'LaSa',
);
print $pcc_of{'JiangXi'}, "\n";

## Use the delete function to delete an element.
$one_hash{'akey'} = "something";
print $one_hash{'akey'}, "\n";
delete $one_hash{'akey'};
print $one_hash{'akey'}, "\n";

Variable Combination

%array_of = (
  "keyA" =>  ["a1", "a2", "a3"],
  "keyB" =>  ["b1", "b2"],
);

%hash_of = (
  "keyA" => { "k1" => "valueA1",
              "k2" => "valueA2",
            },
  "keyB" => { "k1" => "valueB1",
              "k2" => "valueB2",
            },
);

Loops

For

for my $var ( @someArray ) {
  if ( condition1 ) {
    last;
  }
  ## Other sentences ;
}

• If there are nested loops, last; generally only exits the innermost loop.
• If you need to exit an outer loop, you can use a label.
• To skip the remaining statements in a loop body and go to the next iteration, use the next; command.

#!/usr/bin/env perl

my ($opt, %value_of_opt);
for my $arg ( @ARGV ) {
    if ( $arg =~ /^-/ ) {
        $opt = $arg;
    }
    else {
        $value_of_opt{$opt} = $arg;
    }
}

for my $opt ( keys %value_of_opt ) {
    print "$opt => $value_of_opt{$opt}\n";
}
exit 0;

Conditional Statements

If

if ( condition1 ) {
    # sentences1
} elsif {
    # sentences2
} else {
    # sentences3
}

True & False

• If it has not been assigned, it is false.
• 0, ‘0’, and ‘’ are false.
• Everything else is true.

my $t1 = 0;   # false
my $t2 = '0'; # false
my $t3 = '' ; # false
my $t4 = ' '; # true

Logical Operators

!
&&
||
not
and
or

Reference

Equivalent to pointers in C, but more convenient and safer to use.

• To create a reference, use the backslash “”.
• To parse a reference, use the corresponding symbol according to the data type pointed to by the reference.

## Create.
$sref = \$str;
$aref = \@ARGV;
$href = \%ENV;

## Parse.
$$sref # ($str)  or ${$sref}
@$aref # (@ARGV) or @{$aref}
%$href # (%ENV)  or %{$href}

Subroutines

• Subroutines are like functions in other languages.
• Defined by the keyword sub.
• Enclosed in braces.

my $num = 2;
sub times_three {
  $_[0] = $_[0] * 3;
  print "value: $_[0]\n";
}
times_three($num);
print "num is: $num\n";

Modules

• A module is generally a file.
• The filename suffix is often .pm for “Perl Module”.

perl -e "use perl_module"

#!/usr/bin/env perl
use lib "../perl_module";
use My_perl_module_v1;

System Integration

System Command

## Execute an OS command.
system "ls", "/tmp";
system("ls /tmp");
$re = system("ls /opt");

## Get the output of a system command.
my $user = `whoami`;

## Get and set environment variables.
print "\$USER: $ENV{'USER'}\n";
print "\$PATH: $ENV{'PATH'}\n";
$ENV{'some_env'} = some_value;

Reading Files

#!/usr/bin/env perl
open my $fh_input, '<', "./open_file.pl" or die "read file failed: $!";
while ( my $line = <$fh_input> ) {
    print $line;
}
close $fh_input or die "close file failed: $!";

exit 0

Writing Files

#!/usr/bin/env perl
open my $fh_output, '>', "write_file.txt";
print $fh_output "This is an example\n";
close $fh_output;

exit 0;

Reading Directories

#!/usr/bin/env perl
opendir my $dh, "." or die "Error: read directory failed.";
my @filedirs = readdir $dh;
closedir $dh or die "Error: close directory failed.";
for my $f ( @filedirs ) {
    print $f, "\n";
}

exit 0

Creating Directories

mkdir dir_name[, mask]

Clear terminal screen in Perl

Using ANSI Escape Sequences

print "\033[2J";    #clear the screen
print "\033[0;0H"; #jump to 0,0

use system

## for *nix
system("clear");
## for windows
system("cls");

Using a Module

use Term::ANSIScreen qw(cls);
cls();

Further reading

• Perl one-liners cookbook
• Learning Perl
• https://learnxinyminutes.com/docs/perl
• https://qntm.org/perl_en

Perl Project

• Webmin (https://github.com/webmin/webmin)

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