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* New questions and spell check (#181)

Added new questions related with KVM, Libvirt and DNF

* Create a new perl folder and move the content of the actual section
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@ -74,7 +74,7 @@
<td align="center"><a href="#puppet"><img src="images/puppet.png" width="75px;" height="75px;" alt="puppet"/><br /><b>Puppet</b></a></td>
<td align="center"><a href="#distributed"><img src="images/distributed.png" width="110px;" height="75px;" alt="Distributed"/><br /><b>Distributed</b></a></td>
<td align="center"><a href="#questions-you-ask"><img src="images/you.png" width="90px;" height="75px;" alt="you"/><br /><b>Questions you can ask</b></a></td>
<td align="center"><a href="#perl"><img src="images/perl.png" width="75px;" height="75px;" alt="perl"/><br /><b>Perl</b></a></td>
<td align="center"><a href="exercises/perl/README.md"><img src="images/perl.png" width="75px;" height="75px;" alt="perl"/><br /><b>Perl</b></a></td>
</tr>
</table>
</center>
@ -5034,518 +5034,6 @@ Output: 3
[Golang container/heap package](https://golang.org/pkg/container/heap/)
</b></details>
## Perl
### Perl Self Assessment
<details>
<summary>What is Perl?</summary><br><b>
From the official [docs](https://perldoc.perl.org/):
"Perl officially stands for Practical Extraction and Report Language, except when it doesn't."
It's a general purpose programming language developed for manipulating texts mainly. It has been used to perform system administration tasks, networking, building websites and more.
</b></details>
<details>
<summary>What data types Perl has? And how can we define it?</summary><br><b>
- Scalar: This is a simple variable that stores single data items. It can be a string, number or reference.
```
my $number = 5;
```
- Arrays: This is a list of scalars.
```
my @numbers = (1, 2, 3, 4, 5);
# or using the `qw` keyword (quote word):
my @numbers = qw/1 2 3 4 5/;
# '/' can be another symbol, e.g qw@1 2 3 4 5@
```
- Hashes (or associative arrays): This is an unordered collection of key-value pairs. We can access to a hash using the keys.
```
my %numbers = (
First => '1',
Second => '2',
Third => '3'
);
```
</b></details>
<details>
<summary>How can you access to a hash value, add and delete a key/value pair and modify a hash?</summary><br><b>
```
my %numbers = (
'First' => '1',
'Second' => '2',
'Third' => '3'
);
```
- Access:
```
print($numbers{'First'});
```
- Add:
```
$numbers{'Fourth'} = 4;
```
- Delete:
```
delete $numbers{'Third'};
```
- Modify:
```
$numbers{'Fifth'} = 6;
$numbers{'Fifth'} = 5;
```
</b></details>
<details>
<summary>How can you iterate an array? And a hash?</summary><br><b>
- Array:
```
my @numbers = qw/1 2 3 4 5/;
# Using `$_` that represents the current iteration in a loop. It starts from index array 0 until the last index.
foreach (@numbers) {
print($_);
}
# Output: 12345
# "$#" returns the max index of an array. That's the reason because we can iterate accesing to the array from the index 0 to the max index.
for my $i (0..$#numbers) {
print($numbers[$i]);
}
# Output: 12345
# Using the `map` keyword:
print map {$_} @numbers;
# Output: 12345
# Using `while`. We should take care with this option. When we use `shift` we're deleting the first element of the array and assigning it to the `element` variable.
# After this `loop` the `numbers` array will not have elements.
while (my $element = shift(@numbers)) {
print($element);
}
# Output: 12345
```
- Hashes:
```
my %capital_cities = (
'Madrid' => 'Spain',
'Rome' => 'Italy',
'Berlin' => 'Germany'
);
# Iterate and get the `keys`:
foreach my $city (keys %capital_cities) {
print($city . "\n");
}
# Iterate and get the `values`:
foreach my $country (values %capital_cities) {
print($country . "\n");
}
# Iterate and get the values and keys (first option):
foreach my $city (keys %capital_cities) {
print("City: $city - Country: $capital_cities{$city}" . "\n");
}
# Iterate and get the values and keys (first option):
while(my ($city, $country) = each %capital_cities) {
print("City: $city - Country: $capital_cities{$city}" . "\n");
}
```
</b></details>
<details>
<summary>What is a Perl subroutine? How to define it?</summary><br><b>
It's the perl model for user defined functions (this is also called function like other programming languages). We can define a subroute with the keyword `sub`.
```
sub hello {
print "hello";
}
```
</b></details>
<details>
<summary>Describe the different ways to receive parameters in a subroutine</summary><br><b>
- List assignment: Using the `@_` array. It's a list with the elements that are being passed as parameters.
```
sub power {
my ($b, $e) = @_;
return $b ** $e;
}
&power(2, 3);
```
- Individual assigment: We should access to every element of the `@_` array. It starts from zero.
```
sub power {
my $b = $_[0];
my $e = $_[1];
return $b ** $e;
}
&power(2, 3);
```
- Using `shift` keyword: It's used to remove the first value of an array and it's returned.
```
sub power {
my $b = shift;
my $3 = shift;
return $b ** $e;
}
&power(2, 3);
```
[Source](https://stackoverflow.com/a/21465275/12771230)
We can also read the best way in the same S.O answer.
</b></details>
<details>
<summary>What is lexical and dynamic scoping?</summary><br><b>
</b></details>
<details>
<summary>How to apply referencing and dereferencing?</summary><br><b>
</b></details>
<details>
<summary>Does Perl have conventions?</summary><br><b>
You can check [perlstyle](https://perldoc.perl.org/perlstyle)
</b></details>
<details>
<summary>What is Perl POD? Can you code an example?</summary><br><b>
From the official [docs](https://perldoc.perl.org/perlpod):
"Pod is a simple-to-use markup language used for writing documentation for Perl, Perl programs, and Perl modules."
```
=item
This function returns the factorial of a number.
Input: $n (number you wanna calculate).
Output: number factorial.
=cut
sub factorial {
my ($i, $result, $n) = (1, 1, shift);
$result = $result *= $i && $i++ while $i <= $n;
return $result;
}
```
</b></details>
### Perl Regex
<details>
<summary>Check if the word `electroencefalografista` exists in a string</summary><br><b>
```
my $string = "The longest accepted word by RAE is: electroencefalografista";
if ($string =~ /electroencefalografista/) {
print "Match!";
}
```
</b></details>
<details>
<summary>Check if the word `electroencefalografista` does not exists in a string</summary><br><b>
```
my $string = "The longest not accepted word by RAE is: Ciclopentanoperhidrofenantreno";
if ($string !~ /electroencefalografista/) {
print "Does not match!";
}
```
</b></details>
<details>
<summary>Replace the word `amazing`</summary><br><b>
```
my $string = "Perl is amazing!";
$string =~ s/amazing/incredible/;
print $string;
# Perl is incredible!
```
</b></details>
<details>
<summary>Extract `hh:mm:ss` with capturing group `()` in the following datetime</summary><br><b>
```
my $date = "Fri Nov 19 20:09:37 CET 2021";
my @matches = $date =~ /(.*)(\d{2}:\d{2}:\d{2})(.*)/;
print $matches[1];
# Output: 20:09:37
```
</b></details>
<details>
<summary>Extract all the elements that are numbers in an array</summary><br><b>
```
my @array = ('a', 1, 'b', 2, 'c', 3);
my @numbers = grep (/\d/, @array); # Note: \d involves more digits than 0-9
map {print $_ . "\n" } @numbers;
```
</b></details>
<details>
<summary>Print all the linux system users that starts with d or D</summary><br><b>
- With a Perl one liner :D
```
open(my $fh, '<', '/etc/passwd');
my @user_info = <$fh>;
map { print $& . "\n" if $_ =~ /^d([^:]*)/ } @user_info;
close $fh;
```
- Avoiding one-liners
```
foreach my $user_line (@user_info) {
if ($user_line =~ /^d([^:]*)/) {
print $& . "\n";
}
}
```
</b></details>
### Perl Files Handle
<details>
<summary>Mention the different modes in File Handling</summary><br><b>
- Read only: `<`
- Write mode. It creates the file if doesn't exist: `>`
- Append mode. It creates the file if doesn't exist: `>>`
- Read and write mode: `+<`
- Read, clear and write mode. It creates the file if doesn't exist: `+>`
- Read and append. It creates the file if doesn't exist: `+>>`
</b></details>
<details>
<summary>How to write into a file?</summary><br><b>
```
# We can use:
# '>' Write (it clears a previous content if exists).
# '>>' Append.
open(my $fh, '>>', 'file_name.ext') or die "Error: file can't be opened";
print $fh "writing text...\n";
close($fh);
```
</b></details>
<details>
<summary>How can you read a file and print every line?</summary><br><b>
```
open(my $fh, '<', 'file_to_read.ext') or die "Error: file can't be opened";
my @file = <$fh>;
foreach my $line (@file) {
print $line;
}
```
We can use the file handle without assigning it to an array:
```
open(my $fh, '<', 'file_to_read.ext') or die "Error: file can't be opened";
foreach my $line (<$fh>) {
print $line;
}
```
</b></details>
### Perl OOP
<details>
<summary>Does Perl have support for OOP?</summary><br><b>
From the official [docs](https://perldoc.perl.org/perlootut):
"By default, Perl's built-in OO system is very minimal, leaving you to do most of the work."
</b></details>
<details>
<summary>What is the purpose of the bless function?</summary><br><b>
The function os the `bless` function is used to turning a plain data structure into an object.
</b></details>
<details>
<summary>How to create a Perl class? How can you call a method?</summary><br><b>
- Let's create the package: `Example.pm`
```
package Example;
sub new {
my $class = shift;
my $self = {};
bless $self, $class;
return $self;
}
sub is_working {
print "Working!";
}
1;
```
- Now we can instance the `Example` class and call `is_working` method:
```
my $e = new Example();
$e->is_working();
# Output: Working!
```
</b></details>
<details>
<summary>Does Perl have inheritance? What is the `SUPER` keyword?</summary><br><b>
Yes, Perl supports inheritance. We can read about it in the official [docs](https://perldoc.perl.org/perlobj#Inheritance).
We also can read about `SUPER` keyword that is used to call a method from the parent class. It gives an example about how we can apply inheritance.
</b></details>
<details>
<summary>Does Perl have polymorphism? What is method overriding?</summary><br><b>
Yes, it has polymorphism. In fact method overriding is a way to apply it in Perl.
Method overriding in simple words appears when we have a class with a method that already exist in a parent class.
Example:
```
package A;
sub new { return bless {}, shift; };
sub printMethod { print "A\n"; };
package B;
use parent -norequire, 'A';
sub new { return bless {}, shift; };
sub printMethod { print "B\n"; };
my $a = A->new();
my $b = B->new();
A->new()->printMethod();
B->new()->printMethod();
# Output:
# A
# B
```
</b></details>
<details>
<summary>How can you call a method of an inherited class?</summary><br><b>
```
# Class `A` with `printA` method.
package A;
sub new { return bless {}, shift; };
sub printA { print "A"; };
# Class `B` that extends or use the parent class `A`.
package B;
use parent -norequire, 'A';
sub new { return bless {}, shift; };
# Instance class `B` allows call the inherated method
my $b = B->new();
$b->printA();
```
</b></details>
### Perl OS
<details>
<summary>What is Perl Open3?</summary><br><b>
</b></details>
### Perl Packages & Modules
<details>
<summary>What is a Perl package? And a module?</summary><br><b>
</b></details>
<details>
<summary>What is the difference between .pl and .pm extensions?</summary><br><b>
</b></details>
<details>
<summary>What is cpan? And cpanm?</summary><br><b>
</b></details>
<details>
<summary>How can you install a Perl module?</summary><br><b>
</b></details>
## Mongo
<details>

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@ -0,0 +1,511 @@
## Perl
### Perl Self Assessment
<details>
<summary>What is Perl?</summary><br><b>
From the official [docs](https://perldoc.perl.org/):
"Perl officially stands for Practical Extraction and Report Language, except when it doesn't."
It's a general purpose programming language developed for manipulating texts mainly. It has been used to perform system administration tasks, networking, building websites and more.
</b></details>
<details>
<summary>What data types Perl has? And how can we define it?</summary><br><b>
- Scalar: This is a simple variable that stores single data items. It can be a string, number or reference.
```
my $number = 5;
```
- Arrays: This is a list of scalars.
```
my @numbers = (1, 2, 3, 4, 5);
# or using the `qw` keyword (quote word):
my @numbers = qw/1 2 3 4 5/;
# '/' can be another symbol, e.g qw@1 2 3 4 5@
```
- Hashes (or associative arrays): This is an unordered collection of key-value pairs. We can access to a hash using the keys.
```
my %numbers = (
First => '1',
Second => '2',
Third => '3'
);
```
</b></details>
<details>
<summary>How can you access to a hash value, add and delete a key/value pair and modify a hash?</summary><br><b>
```
my %numbers = (
'First' => '1',
'Second' => '2',
'Third' => '3'
);
```
- Access:
```
print($numbers{'First'});
```
- Add:
```
$numbers{'Fourth'} = 4;
```
- Delete:
```
delete $numbers{'Third'};
```
- Modify:
```
$numbers{'Fifth'} = 6;
$numbers{'Fifth'} = 5;
```
</b></details>
<details>
<summary>How can you iterate an array? And a hash?</summary><br><b>
- Array:
```
my @numbers = qw/1 2 3 4 5/;
# Using `$_` that represents the current iteration in a loop. It starts from index array 0 until the last index.
foreach (@numbers) {
print($_);
}
# Output: 12345
# "$#" returns the max index of an array. That's the reason because we can iterate accesing to the array from the index 0 to the max index.
for my $i (0..$#numbers) {
print($numbers[$i]);
}
# Output: 12345
# Using the `map` keyword:
print map {$_} @numbers;
# Output: 12345
# Using `while`. We should take care with this option. When we use `shift` we're deleting the first element of the array and assigning it to the `element` variable.
# After this `loop` the `numbers` array will not have elements.
while (my $element = shift(@numbers)) {
print($element);
}
# Output: 12345
```
- Hashes:
```
my %capital_cities = (
'Madrid' => 'Spain',
'Rome' => 'Italy',
'Berlin' => 'Germany'
);
# Iterate and get the `keys`:
foreach my $city (keys %capital_cities) {
print($city . "\n");
}
# Iterate and get the `values`:
foreach my $country (values %capital_cities) {
print($country . "\n");
}
# Iterate and get the values and keys (first option):
foreach my $city (keys %capital_cities) {
print("City: $city - Country: $capital_cities{$city}" . "\n");
}
# Iterate and get the values and keys (first option):
while(my ($city, $country) = each %capital_cities) {
print("City: $city - Country: $capital_cities{$city}" . "\n");
}
```
</b></details>
<details>
<summary>What is a Perl subroutine? How to define it?</summary><br><b>
It's the perl model for user defined functions (this is also called function like other programming languages). We can define a subroute with the keyword `sub`.
```
sub hello {
print "hello";
}
```
</b></details>
<details>
<summary>Describe the different ways to receive parameters in a subroutine</summary><br><b>
- List assignment: Using the `@_` array. It's a list with the elements that are being passed as parameters.
```
sub power {
my ($b, $e) = @_;
return $b ** $e;
}
&power(2, 3);
```
- Individual assigment: We should access to every element of the `@_` array. It starts from zero.
```
sub power {
my $b = $_[0];
my $e = $_[1];
return $b ** $e;
}
&power(2, 3);
```
- Using `shift` keyword: It's used to remove the first value of an array and it's returned.
```
sub power {
my $b = shift;
my $3 = shift;
return $b ** $e;
}
&power(2, 3);
```
[Source](https://stackoverflow.com/a/21465275/12771230)
We can also read the best way in the same S.O answer.
</b></details>
<details>
<summary>What is lexical and dynamic scoping?</summary><br><b>
</b></details>
<details>
<summary>How to apply referencing and dereferencing?</summary><br><b>
</b></details>
<details>
<summary>Does Perl have conventions?</summary><br><b>
You can check [perlstyle](https://perldoc.perl.org/perlstyle)
</b></details>
<details>
<summary>What is Perl POD? Can you code an example?</summary><br><b>
From the official [docs](https://perldoc.perl.org/perlpod):
"Pod is a simple-to-use markup language used for writing documentation for Perl, Perl programs, and Perl modules."
```
=item
This function returns the factorial of a number.
Input: $n (number you wanna calculate).
Output: number factorial.
=cut
sub factorial {
my ($i, $result, $n) = (1, 1, shift);
$result = $result *= $i && $i++ while $i <= $n;
return $result;
}
```
</b></details>
### Perl Regex
<details>
<summary>Check if the word `electroencefalografista` exists in a string</summary><br><b>
```
my $string = "The longest accepted word by RAE is: electroencefalografista";
if ($string =~ /electroencefalografista/) {
print "Match!";
}
```
</b></details>
<details>
<summary>Check if the word `electroencefalografista` does not exists in a string</summary><br><b>
```
my $string = "The longest not accepted word by RAE is: Ciclopentanoperhidrofenantreno";
if ($string !~ /electroencefalografista/) {
print "Does not match!";
}
```
</b></details>
<details>
<summary>Replace the word `amazing`</summary><br><b>
```
my $string = "Perl is amazing!";
$string =~ s/amazing/incredible/;
print $string;
# Perl is incredible!
```
</b></details>
<details>
<summary>Extract `hh:mm:ss` with capturing group `()` in the following datetime</summary><br><b>
```
my $date = "Fri Nov 19 20:09:37 CET 2021";
my @matches = $date =~ /(.*)(\d{2}:\d{2}:\d{2})(.*)/;
print $matches[1];
# Output: 20:09:37
```
</b></details>
<details>
<summary>Extract all the elements that are numbers in an array</summary><br><b>
```
my @array = ('a', 1, 'b', 2, 'c', 3);
my @numbers = grep (/\d/, @array); # Note: \d involves more digits than 0-9
map {print $_ . "\n" } @numbers;
```
</b></details>
<details>
<summary>Print all the linux system users that starts with d or D</summary><br><b>
- With a Perl one liner :D
```
open(my $fh, '<', '/etc/passwd');
my @user_info = <$fh>;
map { print $& . "\n" if $_ =~ /^d([^:]*)/ } @user_info;
close $fh;
```
- Avoiding one-liners
```
foreach my $user_line (@user_info) {
if ($user_line =~ /^d([^:]*)/) {
print $& . "\n";
}
}
```
</b></details>
### Perl Files Handle
<details>
<summary>Mention the different modes in File Handling</summary><br><b>
- Read only: `<`
- Write mode. It creates the file if doesn't exist: `>`
- Append mode. It creates the file if doesn't exist: `>>`
- Read and write mode: `+<`
- Read, clear and write mode. It creates the file if doesn't exist: `+>`
- Read and append. It creates the file if doesn't exist: `+>>`
</b></details>
<details>
<summary>How to write into a file?</summary><br><b>
```
# We can use:
# '>' Write (it clears a previous content if exists).
# '>>' Append.
open(my $fh, '>>', 'file_name.ext') or die "Error: file can't be opened";
print $fh "writing text...\n";
close($fh);
```
</b></details>
<details>
<summary>How can you read a file and print every line?</summary><br><b>
```
open(my $fh, '<', 'file_to_read.ext') or die "Error: file can't be opened";
my @file = <$fh>;
foreach my $line (@file) {
print $line;
}
```
We can use the file handle without assigning it to an array:
```
open(my $fh, '<', 'file_to_read.ext') or die "Error: file can't be opened";
foreach my $line (<$fh>) {
print $line;
}
```
</b></details>
### Perl OOP
<details>
<summary>Does Perl have support for OOP?</summary><br><b>
From the official [docs](https://perldoc.perl.org/perlootut):
"By default, Perl's built-in OO system is very minimal, leaving you to do most of the work."
</b></details>
<details>
<summary>What is the purpose of the bless function?</summary><br><b>
The function os the `bless` function is used to turning a plain data structure into an object.
</b></details>
<details>
<summary>How to create a Perl class? How can you call a method?</summary><br><b>
- Let's create the package: `Example.pm`
```
package Example;
sub new {
my $class = shift;
my $self = {};
bless $self, $class;
return $self;
}
sub is_working {
print "Working!";
}
1;
```
- Now we can instance the `Example` class and call `is_working` method:
```
my $e = new Example();
$e->is_working();
# Output: Working!
```
</b></details>
<details>
<summary>Does Perl have inheritance? What is the `SUPER` keyword?</summary><br><b>
Yes, Perl supports inheritance. We can read about it in the official [docs](https://perldoc.perl.org/perlobj#Inheritance).
We also can read about `SUPER` keyword that is used to call a method from the parent class. It gives an example about how we can apply inheritance.
</b></details>
<details>
<summary>Does Perl have polymorphism? What is method overriding?</summary><br><b>
Yes, it has polymorphism. In fact method overriding is a way to apply it in Perl.
Method overriding in simple words appears when we have a class with a method that already exist in a parent class.
Example:
```
package A;
sub new { return bless {}, shift; };
sub printMethod { print "A\n"; };
package B;
use parent -norequire, 'A';
sub new { return bless {}, shift; };
sub printMethod { print "B\n"; };
my $a = A->new();
my $b = B->new();
A->new()->printMethod();
B->new()->printMethod();
# Output:
# A
# B
```
</b></details>
<details>
<summary>How can you call a method of an inherited class?</summary><br><b>
```
# Class `A` with `printA` method.
package A;
sub new { return bless {}, shift; };
sub printA { print "A"; };
# Class `B` that extends or use the parent class `A`.
package B;
use parent -norequire, 'A';
sub new { return bless {}, shift; };
# Instance class `B` allows call the inherated method
my $b = B->new();
$b->printA();
```
</b></details>
### Perl OS
<details>
<summary>What is Perl Open3?</summary><br><b>
</b></details>
### Perl Packages & Modules
<details>
<summary>What is a Perl package? And a module?</summary><br><b>
</b></details>
<details>
<summary>What is the difference between .pl and .pm extensions?</summary><br><b>
</b></details>
<details>
<summary>What is cpan? And cpanm?</summary><br><b>
</b></details>
<details>
<summary>How can you install a Perl module?</summary><br><b>
</b></details>