--- tags: - devops - l1 - flashcard-deck - python-automation --- [Portal](../../../../library/portal/index.md) | **Level:** [L1: Foundations](../../../../library/portal/levels.md) | **Topics:** [Python Automation](../../../../library/portal/topics.md) | **Domain:** DevOps & Tooling id category difficulty tags question answer source_path perl/02f49f6f474a perl easy perl, modules, regex What is a Perl package? And a module? With a Perl package we are defining a namespace.\nA Perl module in one simple word can be defined as a `class`. When we create a `class` in Perl we use the `package` keyword. A module can be used with the `use` keyword. projects/knowledge/interview/perl/027-what-is-a-perl-package-and-a-module.txt perl/113cb5c32ee3 perl easy perl, references, regex What is the purpose of the bless function? The function os the `bless` function is used to turning a plain data structure into an object.\n\nExample: perl -ne 'print if /ERROR/' access.log — prints lines containing ERROR. The -n flag wraps the code in a while(<>){...} loop.\n\nRemember: "perl -ne = line-by-line filter, perl -pe = line-by-line transform." projects/knowledge/interview/perl/019-what-is-the-purpose-of-the-bless-function.txt perl/1f78392b9860 perl medium perl, references, modules, text-processing How can you call a method of an inherited class? "```\n# Class `A` with `printA` method.\npackage A;\n\nsub new { return bless {}, shift; };\nsub printA { print ""A""; };\n\n# Class `B` that extends or use the parent class `A`.\npackage B;\n\nuse parent -norequire, 'A';\n\nsub new { return bless {}, shift; };\n\n# Instance class `B` allows call the inherited method\nmy $b = B->new();\n$b->printA();\n```" projects/knowledge/interview/perl/023-how-can-you-call-a-method-of-an-inherited-class.txt perl/25502d5487a3 perl medium perl, one-liner, cpan, modules How can you install cpanm and a Perl module? There are some different alternatives to install Perl modules. We will use `cpanm`.\n\n- Install `cpanm`:\n\n```\n$ cpan App::cpanminus\n```\n\n- Install the `Test` module with `cpanm`:\n\n```\ncpanm Test\n```\n\nNow we can test the `Test` installed module:\n\n```\n$ perl -M'Test::Simple tests => 1' -e 'ok( 1 + 1 == 2 );'\n1..1\nok 1\n```\n\n```\n$ perl -M'Test::Simple tests => 1' -e 'ok( 1 + 1 == 3 );'\n1..1\nnot ok 1\n# Failed test at -e line 1.\n# Looks like you failed 1 test of 1.\n``` projects/knowledge/interview/perl/031-how-can-you-install-cpanm-and-a-perl-module.txt perl/28838430e1e3 perl medium perl, regex, text-processing How can we evaluate and capture an exception in Perl? "From the official [eval docs](https://perldoc.perl.org/functions/eval):\n\n""`eval` in all its forms is used to execute a little Perl program, trapping any errors encountered so they don't crash the calling program."".\n\ne.g:\n\n```\neval {\n die;\n};\nif ($@) {\n print ""Error. Details: $@"";\n}\n```\n\nIf we execute this we get the next output:\n\n```\nError. Details: Died at eval.pl line 2.\n```\n\nThe `eval` (`try` in another programming languages) is trying to execute a code." projects/knowledge/interview/perl/024-how-can-we-evaluate-and-capture-an-exception-in-pe.txt perl/2a6486512573 perl easy perl, text-processing, regex What is Perl and what are its common use cases in system administration? "From the official [docs](https://perldoc.perl.org/):\n\n""Perl officially stands for Practical Extraction and Report Language, except when it doesn't.""\n\nIt'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." projects/knowledge/interview/perl/001-what-is-perl.txt perl/305fa8158616 perl easy perl, fileio, regex What is Perl Open3? "From the official [IPC::Open3 docs](https://perldoc.perl.org/IPC::Open3):\n\n""IPC::Open3 - open a process for reading, writing, and error handling using open3()"".\n\nWith `open3` we can have the full control of the STDIN, STDOUT, STDERR. It's usually used to execute commands." projects/knowledge/interview/perl/025-what-is-perl-open3.txt perl/35a35b535419 perl easy perl, modules, regex What is the difference between .pl and .pm extensions? There's no a real difference between a `.pm` and `.pl` extensions. Perl use `.pm` extensions just to difference it as a perl module (a class). `.pl` extensions are usually named for perl scripts without OOP classes. projects/knowledge/interview/perl/028-what-is-the-difference-between-pl-and-pm-extension.txt perl/431a7f1e8a74 perl hard perl, references, modules, fileio Does Perl have polymorphism? What is method overriding? "Yes, it has polymorphism. In fact method overriding is a way to apply it in Perl.\n\nMethod overriding in simple words appears when we have a class with a method that already exist in a parent class.\n\nExample:\n\n```\npackage A;\n\nsub new { return bless {}, shift; };\nsub printMethod { print ""A\n""; };\n\npackage B;\n\nuse parent -norequire, 'A';\n\nsub new { return bless {}, shift; };\nsub printMethod { print ""B\n""; };\n\nmy $a = A->new();\nmy $b = B->new();\n\nA->new()->printMethod();\nB->new()->printMethod();\n\n# Output:\n# A\n# B\n```" projects/knowledge/interview/perl/022-does-perl-have-polymorphism-what-is-method-overrid.txt perl/45c58ce36ce5 perl medium perl, text-processing, regex Extract all the elements that are numbers in an array "```\nmy @array = ('a', 1, 'b', 2, 'c', 3);\nmy @numbers = grep (/\d/, @array); # Note: \d involves more digits than 0-9\nmap {print $_ . ""\n"" } @numbers;\n```"\n\nRemember: In regex, $1 is the first capture group. Example: if ($line =~ /user=(\w+)/) { print $1; } projects/knowledge/interview/perl/013-extract-all-the-elements-that-are-numbers-in-an-ar.txt perl/45fc9f3c734c perl medium perl, regex, text-processing Check if the word `electroencefalografista` exists in a string "```\nmy $string = ""The longest accepted word by RAE is: electroencefalografista"";\nif ($string =~ /electroencefalografista/) { \n print ""Match!"";\n}\n```"\n\nExample: use strict; use warnings; catches undeclared variables and common mistakes. Always start scripts with these two lines.\n\nGotcha: Without strict, Perl silently creates global variables on first use — leading to typo-induced bugs. projects/knowledge/interview/perl/009-check-if-the-word-electroencefalografista-exists-i.txt perl/47028ef94658 perl medium perl, regex, text-processing Check if the word `electroencefalografista` does not exists in a string "```\nmy $string = ""The longest not accepted word by RAE is: Ciclopentanoperhidrofenantreno"";\nif ($string !~ /electroencefalografista/) {\n print ""Does not match!"";\n}\n```"\n\nExample: my $count = 0; declares a lexically scoped variable. my @items = (1,2,3); for arrays, my %map = (a => 1); for hashes.\n\nRemember: "my = mine (local scope), our = shared (package scope)." projects/knowledge/interview/perl/010-check-if-the-word-electroencefalografista-does-not.txt perl/4749a3cf066c perl medium perl, text-processing, regex How can you access to a hash value, add and delete a key/value pair and modify a hash? ```\nmy %numbers = (\n 'First' => '1',\n 'Second' => '2',\n 'Third' => '3'\n);\n```\n\n- Access:\n\n```\nprint($numbers{'First'});\n```\n\n- Add:\n\n```\n$numbers{'Fourth'} = 4;\n```\n\n- Delete:\n\n```\ndelete $numbers{'Third'};\n```\n\n- Modify:\n\n```\n$numbers{'Fifth'} = 6;\n$numbers{'Fifth'} = 5;\n```\n\nExample: @colors = ("red", "green", "blue"); print $colors[0]; # "red". Use push/pop for stack ops, shift/unshift for queue ops.\n\nRemember: "@ = array (think 'at' multiple items), $ = scalar (single)." projects/knowledge/interview/perl/003-how-can-you-access-to-a-hash-value-add-and-delete-.txt perl/477c868e8927 perl easy perl, text-processing, regex What is a Perl subroutine? How to define it? "It's the perl model for user defined functions (this is also called function like other programming languages). We can define a subroutine with the keyword `sub`. \n\n```\nsub hello {\n print ""hello"";\n}\n```" projects/knowledge/interview/perl/005-what-is-a-perl-subroutine-how-to-define-it.txt perl/4ea4451df614 perl medium perl, references, modules, fileio Why a Perl class (or module) should return something at the end of the file? Check the example. "If we want to `use` a Perl module (`import` a class), this module should end in a value different than 0. This is necessary because if we try to import the class and it has a false value, we will not be able to use it.\n\n```\npackage A;\n\nsub new { return bless {}, shift; };\nsub printMethod { print ""A\n""; };\n\n1;\n```" projects/knowledge/interview/perl/029-why-a-perl-class-or-module-should-return-something.txt perl/58825e6ba56d perl medium perl, fileio, text-processing How to write into a file? "```\n# We can use:\n# '>' Write (it clears a previous content if exists).\n# '>>' Append.\nopen(my $fh, '>>', 'file_name.ext') or die ""Error: file can't be opened"";\nprint $fh ""writing text...\n"";\nclose($fh);\n```"\n\nRemember: "chomp eats the newline, chop eats any last char." Always chomp after reading input. projects/knowledge/interview/perl/016-how-to-write-into-a-file.txt perl/691751593d72 perl easy perl, modules, regex What is Perl POD? Can you code an example? "From the official [docs](https://perldoc.perl.org/perlpod):\n\n""Pod is a simple-to-use markup language used for writing documentation for Perl, Perl programs, and Perl modules.""\n\n```\n=item\n This function returns the factorial of a number.\n Input: $n (number you wanna calculate).\n Output: number factorial.\n=cut\nsub factorial {\n my ($i, $result, $n) = (1, 1, shift);\n $result = $result *= $i && $i++ while $i <= $n;\n return $result;\n}\n```" projects/knowledge/interview/perl/008-what-is-perl-pod-can-you-code-an-example.txt perl/6bcac3bb00d1 perl medium perl, references, modules, text-processing How to create a Perl class? How can you call a method? "- Let's create the package: `Example.pm`\n\n```\npackage Example;\n\nsub new {\n my $class = shift;\n my $self = {};\n bless $self, $class;\n return $self;\n}\n\nsub is_working {\n print ""Working!"";\n}\n\n1;\n```\n\n- Now we can instance the `Example` class and call `is_working` method:\n\n```\nmy $e = new Example();\n$e->is_working();\n# Output: Working!\n```" projects/knowledge/interview/perl/020-how-to-create-a-perl-class-how-can-you-call-a-meth.txt perl/71cffdfa3eb5 perl medium perl, modules, fileio, text-processing How can you read a file and print every line? "```\nopen(my $fh, '<', 'file_to_read.ext') or die ""Error: file can't be opened"";\nmy @file = <$fh>;\nforeach my $line (@file) {\n print $line;\n}\n```\n\nWe can use the file handle without assigning it to an array:\n\n```\nopen(my $fh, '<', 'file_to_read.ext') or die ""Error: file can't be opened"";\n\nforeach my $line (<$fh>) {\n print $line;\n}\n```" projects/knowledge/interview/perl/017-how-can-you-read-a-file-and-print-every-line.txt perl/7a54001f3c96 perl hard perl, modules, fileio, text-processing Using Open3: Create a file with the size of 15MB and check it's created successfully "- Code:\n\n```\nuse IPC::Open3;\nuse Data::Dumper;\n\nsub execute_command {\n my @command_to_execute = @_;\n my ($stdin, $stdout, $stderr);\n eval {\n open3($stdin, $stdout, $stderr, @command_to_execute);\n };\n if ($@) {\n print ""Error. Details: $@"";\n }\n close($stdin);\n return $stdout;\n}\n\nmy $file_name = 'perl_open3_test';\n&execute_command('truncate', '-s', '15M', $file_name);\nmy $result = &execute_command('stat', '-c', '%s', $file_name);\nprint Dumper(<$result>);\n```\n\n- Result:\n\n```\n$ -> perl command.pl \n$VAR1 = '15728640\n';\n```" projects/knowledge/interview/perl/026-using-open3-create-a-file-with-the-size-of-15mb-an.txt perl/82edaa9fa7cf perl medium perl, regex, text-processing Extract `hh:mm:ss` with capturing group `()` in the following datetime "```\nmy $date = ""Fri Nov 19 20:09:37 CET 2021"";\nmy @matches = $date =~ /(.*)(\d{2}:\d{2}:\d{2})(.*)/;\nprint $matches[1];\n# Output: 20:09:37\n```"\n\nRemember: "$_ is Perl's 'it' pronoun." Many functions default to $_ when no argument is given. Example: for (@items) { print; } prints each item because print defaults to $_. projects/knowledge/interview/perl/012-extract-hhmmss-with-capturing-group-in-the-followi.txt perl/8bc40e832f68 perl hard perl, modules, text-processing How can you iterate an array? And a hash? "- Array:\n\n```\nmy @numbers = qw/1 2 3 4 5/;\n\n# Using `$_` that represents the current iteration in a loop. It starts from index array 0 until the last index.\nforeach (@numbers) {\n print($_);\n}\n# Output: 12345\n\n\n# ""$#"" returns the max index of an array. That's the reason because we can iterate accessing to the array from the index 0 to the max index.\nfor my $i (0..$#numbers) {\n print($numbers[$i]);\n}\n# Output: 12345\n\n\n# Using the `map` keyword:\nprint map {$_} @numbers;\n# Output: 12345\n\n# Using `while`. We should take care with this option." projects/knowledge/interview/perl/004-how-can-you-iterate-an-array-and-a-hash.txt perl/ad1a9d9710d0 perl medium perl, regex, text-processing Replace the word `amazing` "```\nmy $string = ""Perl is amazing!"";\n$string =~ s/amazing/incredible/;\nprint $string;\n# Perl is incredible!\n```"\n\nRemember: "Perl hash = key-value store." Declared with %. Access with $hash{key}. Use keys/values/each to iterate. projects/knowledge/interview/perl/011-replace-the-word-amazing.txt perl/cdca5c5d4c43 perl medium perl, modules, fileio Does Perl have inheritance? What is the `SUPER` keyword? Yes, Perl supports inheritance. We can read about it in the official [docs](https://perldoc.perl.org/perlobj#Inheritance). \nWe 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.\n\nGotcha: Perl has no boolean type — 0, "", "0", and undef are all false. Everything else is true. projects/knowledge/interview/perl/021-does-perl-have-inheritance-what-is-the-super-keywo.txt perl/d1bd5226d672 perl medium perl, regex, one-liner Does Perl have conventions? You can check [perlstyle](https://perldoc.perl.org/perlstyle)\n\nExample: perl -i.bak -pe 's/foo/bar/g' *.txt — replaces foo with bar in all .txt files, creating .bak backups.\n\nGotcha: -i without extension on some systems destroys the original with no backup. projects/knowledge/interview/perl/007-does-perl-have-conventions.txt perl/d6a194442c79 perl easy perl, cpan, modules What is cpan? And cpanm? "CPAN is the Comprehensive Perl Archive Network. \n\nCPANM From the official [App::cpanminus](https://metacpan.org/pod/App::cpanminus):\n""App::cpanminus - get, unpack, build and install modules from CPAN"".\n\n[Find CPAN modules](https://metacpan.org/)"\n\nExample: my %ages = ("alice" => 30, "bob" => 25); print $ages{"alice"}; # 30. Use keys %hash to iterate.\n\nRemember: "% = hash (think percent = pairs of things)." projects/knowledge/interview/perl/030-what-is-cpan-and-cpanm.txt perl/e427948a2aab perl hard perl, regex, one-liner, fileio Print all the linux system users that starts with d or D "- With a Perl one liner :D\n```\nopen(my $fh, '<', '/etc/passwd');\nmy @user_info = <$fh>;\nmap { print $& . ""\n"" if $_ =~ /^d([^:]*)/ } @user_info;\nclose $fh;\n```\n\n- Avoiding one-liners\n\n```\nforeach my $user_line (@user_info) {\n if ($user_line =~ /^d([^:]*)/) {\n print $& . ""\n"";\n }\n}\n```" projects/knowledge/interview/perl/014-print-all-the-linux-system-users-that-starts-with-.txt perl/e7107d933647 perl hard perl, fileio, regex Mention the different modes in File Handling - Read only: `<`\n- Write mode. It creates the file if doesn't exist: `>`\n- Append mode. It creates the file if doesn't exist: `>>`\n- Read and write mode: `+<`\n- Read, clear and write mode. It creates the file if doesn't exist: `+>`\n- Read and append. It creates the file if doesn't exist: `+>>` projects/knowledge/interview/perl/015-mention-the-different-modes-in-file-handling.txt perl/f4faf379c7f5 perl hard perl, references, text-processing What data types Perl has? And how can we define it? - Scalar: This is a simple variable that stores single data items. It can be a string, number or reference.\n\n```\nmy $number = 5;\n```\n\n- Arrays: This is a list of scalars. \n\n```\nmy @numbers = (1, 2, 3, 4, 5);\n# or using the `qw` keyword (quote word):\nmy @numbers = qw/1 2 3 4 5/; \n# '/' can be another symbol, e.g qw@1 2 3 4 5@\n```\n\n- Hashes (or associative arrays): This is an unordered collection of key-value pairs. We can access to a hash using the keys.\n\n```\nmy %numbers = (\n First => '1',\n Second => '2',\n Third => '3'\n);\n``` projects/knowledge/interview/perl/002-what-data-types-perl-has-and-how-can-we-define-it.txt perl/f5397db5c324 perl hard perl, modules, regex Does Perl have support for OOP? "From the official [docs](https://perldoc.perl.org/perlootut):\n\n""By default, Perl's built-in OO system is very minimal, leaving you to do most of the work."""\n\nFun fact: Perl's motto is "There's More Than One Way To Do It" (TMTOWTDI, pronounced "tim-toady"). projects/knowledge/interview/perl/018-does-perl-have-support-for-oop.txt perl/fcedabe6d5ac perl hard perl, regex, one-liner Describe the different ways to receive parameters in a subroutine - List assignment: Using the `@_` array. It's a list with the elements that are being passed as parameters.\n\n```\nsub power {\n my ($b, $e) = @_;\n return $b ** $e; \n}\n\n&power(2, 3);\n```\n\n- Individual assignment: We should access to every element of the `@_` array. It starts from zero.\n\n```\nsub power {\n my $b = $_[0];\n my $e = $_[1];\n return $b ** $e; \n}\n\n&power(2, 3);\n```\n\n- Using `shift` keyword: It's used to remove the first value of an array and it's returned.\n\n```\nsub power {\n my $b = shift;\n my $3 = shift;\n return $b ** $e; \n}\n\ projects/knowledge/interview/perl/006-describe-the-different-ways-to-receive-parameters-.txt --- ## Wiki Navigation ### Related Content - Python Algorithms Flashcards *(CLI)* (flashcard_deck, L2) — Python Automation - [Python Async & Concurrency](../../../../library/topics/python-async-concurrency/index.md) (Topic Pack, L2) — Python Automation - Python Concepts Flashcards *(CLI)* (flashcard_deck, L0) — Python Automation - [Python Debugging](../../../../library/topics/python-debugging/index.md) (Topic Pack, L1) — Python Automation - [Python Drills](../../../../library/drills/python_drills.md) (Drill, L0) — Python Automation - Python Exercises (Quest Ladder) *(CLI)* (Exercise Set, L0) — Python Automation - Python Flashcards *(CLI)* (flashcard_deck, L1) — Python Automation - Python OOP Flashcards *(CLI)* (flashcard_deck, L2) — Python Automation - [Python Packaging](../../../../library/topics/python-packaging/index.md) (Topic Pack, L2) — Python Automation - [Python for Infrastructure](../../../../library/topics/python-infra/index.md) (Topic Pack, L1) — Python Automation