Perl% - OS3 · Perl% ESA2012/2013% Adam%Belloum% [email protected]% % % Material%Prepared%by%Eelco%Schatborn%%

Perl

ESA 2012/2013 Adam Belloum

[email protected]

Material Prepared by Eelco Schatborn

ESA: Perl

•  Today: 1. Perl introducDon 2. Basic Perl: types, variables, statements, . . . 3. Regular expression 4. File and I/O 5. Packages 6. Object Oriented Perl 7. DocumentaDon

IntroducDon

•  “PracDcal ExtracDon and Report Language” •  Created as a way to make report processing easier. •  Developed by Larry Wall, as system administrator at NASA, in the late 1980s

•  It's an interpreted language, but can be compiled as well

•  Is now being used for: –  System administra5on automa5on –  Glue between systems, conversion –  CGI backend for websites – Much more. . .

Things to know about perl

•  Perl vocabulary is extremely rich and its grammar is very flexible.

•  Perl programmers usually say "There's more than one way to do it".

•  In fact, you can literally write a Perl script your way, with your own style. Some people even do "poetry" in Perl

•  Because of this, some Perl scripts can be very difficult to read. WriDng them though, is always a pleasure.

Clement Lefebvre: h`p://www.linuxforums.org/arDcles/learn-‐perl-‐in-‐10-‐easy-‐lessons-‐lesson-‐1_120.html

A Perl program

h`p://www.perlmonks.org/?node_id=1590

Poetry in Perl

h`p://www.perlmonks.org/?node_id=1590

Perl documentaDon

•  Every release of Perl comes with documentaDon in a set of files. Most releases have over 1,700 pages of documentaDon included in reference books, user guides, FAQs, and so on.

•  On most operaDng systems, a uDlity called perldoc is installed as part of the Perl system.

•  The perldoc uDlity can search for and format Perl documentaDon for you.

•  To use perldoc to look up the basic syntax for perl, open a terminal or console and issue the command: perldoc perl

More on perldoc •  The Perl documentaDon is divided into parts by purpose:

–  perlfunc (Perl funcDons) –  perlfaq (Perl FAQs) –  perlop (Perl operators)

•  To search for a parDcular keyword, use the –k opDons. For example to look up the print keyword: perldoc –tf print

•  To search the FAQs use –q as an opDon: perldoc –q perl

What are Perl 4, Perl 5, or Perl 6? (contributed by brian d foy) In short, Perl 4 is the past, Perl 5 is the present, and Perl 6 is the future.

Perl interpreter

•  Perl is an interpreted language.

•  This means that the code you will write using the Perl language will need a program called the Perl interpreter in order to be run.

•  For instance, if you write a Perl script in a file called myScript.pl (.pl is commonly used as an extension for Perl scripts), there are two ways to run a perl script. call the interpreter to run it: $ perl myScript.pl

Clement Lefebvre: h`p://www.linuxforums.org/arDcles/learn-‐perl-‐in-‐10-‐easy-‐lessons-‐lesson-‐1_120.html

Perl interpreter: debugger

•  Part of the Perl interpreter is a debugger that you can use to examine the execuDon of your Perl scripts.

•  The debugger is built into every Perl interpreter and is acDvated with the –d opDon when launching the interpreter:

perl –d myprog.txt

Using strict

•  Perl has a keyword called strict.

•  Using strict is a good way to enhance your programming abiliDes. To do this, simply put: use strict;

at the top of your code.

Simple Perl

A simple program to start with: #!/usr/bin/perl

# This is a sample of Perl code # and this how you write comments in Perl

print "Hi there!\n"; print "This is a very";

print "\tsimple program.\n";

ESA: Perl

•  Today: 1. Perl introducDon 2. Basic Perl: types, variables, statements, subrouDnes 3. Regular expression 4. File and I/O 5. Packages 6. Object Oriented Perl 7. DocumentaDon

Variables

•  Three different types of variables: scalars, arrays and hashes

•  Start with the dollar (‘$’), at (‘@’) or (‘%’) depending on the type of variable $a = 2; $message = "Hello World!"; @colors = ("red", "green", "blue"); @primaryNumbers = (1, 2, 3, 5, 7); %phoneNumbers = (Alicia => "090-64-773315", Tom => "085-153-3214", Jimmy => "085-285-4545");

•  Variable names are a punctua5on character, a leBer or underscore, and one or more alphanumeric characters or underscores .

Declaring variables

•  Unlike many programming languages, variables do not need to be declared prior to use with Perl. –  When the variable is assigned an iniDal value, Perl can figure out the data type.

•  If you try to use an uninitalized variable, Perl will –  use the value zero for a numeric, –  or Null for a string.

•  Avoid uniniDalized variables as much as possible, as results can be unpredictable.

Variables: Scalars

•  Start with the dollar (‘$’) sign. •  Can be a number or a string (or an object reference)

•  Perl automa5cally converts between numbers and strings. $n = 8 + "2” à $n holds "10” $n = 8 . "2" à $n holds "82”

Note: concatenaDon of strings using a period `.’

Variable: Scalars (Strings)

•  Escape sequences represent special characters: \n, \t, . . .

•  Only in double quoted (‘ “ ’) strings •  For verba5m strings use single quotes (‘ ‘ ’) Perl: print "Hi there! \n \t It's me.\n";

print 'And me \n \t as well!';

•  output: Hi there! It's me. And me \n \t as well!

The q and qq operators

•  Perl allows you to use these structures: –  q( ) –  qq( ) Instead of single and double quotes, respecDvely. So, qq(This is a test) is the same as “This is a test”

•  These can be handy when embedding marks that would otherwise need escaping: qq(He said “Help!” then “Now!”)

Variable Scalars (Numbers)

•  simple numbers as per usual: –  decimal: 12, -‐17, 255 . . . –  octal, start with 0: 015, -‐023, 0777 . . . –  hexadecimal, start with 0x: 0xc, -‐0x11, 0XfF, . . . either case allowed for the x or hex digits abcdef.

FloaDng point numbers: –  “one and a quarter": 1.25 –  “7.25 Dmes 10 to the 45th power": 7.25e45. –  “negaDve 12 Dmes 10 to the -‐24th": -‐12e-‐24.

Variables: Array (1)

•  Start with the at (`@') sign. •  Arrays represent ordered lists of scalars. •  DefiniDon using parenthesized, comma delimited lists: @numbers = ( 1, 2, 4, 8, 16 );

@strings = ( "one", "two", "3" );

•  The index of an array starts at 0, not 1 •  Processing single items as a scalar using `[' and `]':

print $strings[1]; # prints "two"

$strings[1] = "2"; # we changed @strings

Variables: Array (2)

•  Arrays can be created using scalar assignment: $names[0] = "Eelco"; à creates @names to hold ("Eelco")

•  The length of an array: $#strings, returns the number of items in the array minus one. –  If equal to -‐1, the array does not exist or is empty. print $#strings; à prints 2 –  Assignment to $#strings changes the size of the array. $#strings = 0; à@strings now holds ( "one" )

Variables: Hash (1)

•  Start with the percentage (`%') sign. •  Made up of keys and values. •  Each key has exactly one corresponding value. •  DefiniDon using parenthesized, comma delimited lists of keys and values: %numbers = ("one" => "one", "two" => 2);

•  Processing a single hash as a scalar using `{' and `}': print $numbers{"two"}; à prints "2" $numbers{"one"} = "1”; à Change %numbers

Variables: Array Hash (2)

•  Like arrays, hashes can be created using scalar assignment:

$strings{"1"} = "one"; à creates %strings to hold ( "1" => "one" )

•  The keys <hash> funcDon will return an array filled with all the keys in the hash.

print keys %numbers; à prints ( "one", "two" )

Variables: $_

•  The $_ variable is used by Perl as a default variable. You can use it in place of variable names in your scripts: $_=“This is a test”; print;

•  Use the default operator carefully as it can easily be confusing.

Standard operators

•  Perl supports the standard mathemaDcal operators +, -‐, *, /, % (modulus) and ** (exponent)

•  Operator order of precedence applies according to standard rules; parentheses group operaDons

•  Operators can be combined in statements: $num1=$num2 * ((3 + 8)*$num3/2);

•  MulDple assignments can be made on one line: $num1=$num2=$num3=7;

Boolean operators

•  Perl supports to Boolean AND and OR operators in the same way as other high-‐level languages: && AND || OR

•  These operators are o�en used for condiDons: if (($num1 < 10) && ($num1 > 5))

•  Perl also allows the use of the words “and” and “or”: if ($num1 < 10 and $num1 > 5)

String operators (relaDonships)

•  There are relaDonal operators for strings (actually, for non-‐numbers): eq equal to ne not equal to gt greater than lt less than ge greater than or equal to le less than or equal to

•  Comparisons are le�-‐to-‐right, using ASCII values

Your second Perl script, the Calculator! #!/usr/bin/perl $nbArguments = $#ARGV + 1; print "number of arguments: $nbArgumentsn"; exit(1) unless $nbArguments == 3; $a = $ARGV[0]; $b = $ARGV[2]; $operation = $ARGV[1]; if ($operation eq "+") { $result = $a + $b;} elsif ($operation eq "-") { $result = $a - $b;} elsif ($operation eq "/") { $result = $a / $b;} elsif ($operation eq "x") { $result = $a * $b;} print "$a $operation $b = $resultn"; Make the script executable: $ chmod a+rx calculator.pl Run the script : $ ./calculator.pl 5 + 6./calculator.pl 11 - 2./calculator.pl 4 x 3./

calculator.pl 33 / 3

The command line arguments are stored in array @ARGV -‐ first element $ARGV[0] -‐ second element $ARGV[1]... etc. -‐ the index of the last element in the array can be obtained by $#ARGV.

Two operators for comparing scalar variables == and eq Unless is used as a control statement

ESA: Perl

•  Today: 1. Perl introducDon 2. Basic Perl: types, variables, control statements, subrouDne

3. Regular expression 4. File and I/O 5. Packages 6. Object Oriented Perl 7. DocumentaDon

Code blocks

•  A block of code contain a list of statements. •  Code blocks are delimited by `{' and `}’ •  Each code block is treated as a unit by Perl, although execuDon is sDll always top to boòm unless moderated by control structures

•  Usually blocks will be associated with other statements, such as if condiDons.

Comparison operators

•  Perl supports the standard comparison operators: > greater than < less than >= greater than or equal to <= less than or equal to == exactly equal to != not equal to

True and false •  In Perl, any condiDon that evaluate to false is assigned a value of zero. Anything that is non-‐zero is true.

•  This applies to condiDons in statements (such as the if ) as well as for numeric evaluaDon: 0 false 3 true (non-‐zero) 5-‐5 false (evaluates to zero) 0.00 false (it’s zero with precision) “” Null string is false “ ” String with a space or anything not null is true

Control Statements: CondiDons (1)

•  CondiDonal statements: if ( test ) { . . . } elsif { . . . }

else { . . . } unless ( test ) { . . . } else { . . . } while ( test ) { . . . } until ( test ) { . . . }

•  Note: comparison of strings using èq'. 0xa == " 10 " à true

0xa eq " 10 " à false

Control Statements: CondiDons (2) if ($a == 5) { print "It's five!\n";

} elsif ($a == 6) { print "It's six!\n";

} else { print "It's something else.\n";

} unless ($pie eq 'apple') { print "Ew, I don't like $pie flavored pie.\n";

} else { print "Apple! My favorite!\n";

}

Control Statements: loops (3)

$a = 0; while ($a != 3) { $a++;

print "Counting up to $a...\n";

}

until ($a == 0) { $a--;

print "Counting down to $a...\n";

}

For loop

•  For loops are for repeDDon •  Using a list and an index variable a block of code will be executed for each value in the list. for $i ( 1, 2, 3, 4 ) { . . . }

•  Using `..' you can also give a range: for $i ( 1 .. 3, 4 ) { . . . }

•  The list can contain any scalars or other lists. @range = ( 1 .. 3 ); for $i ( @range, 4 ) { . . . }

The exit statement

•  The exit statement is used to terminate a Perl script at any Dme.

•  You can send a return status code back to the calling program with exit, if you want, by appending the return code a�er the exit statement: if ($val1 == 0) { exit 0;}

–  This will exit the program with a return code of 0 is $val1 is zero.

Control Statements: next & last

•  The next statement causes execuDon of a loop to restart. –  It is similar to the conDnue statement in some languages. Any statements below the next statement are not executed.

•  The last statement can be used to terminate a loop, regardless of the condiDon’s value. –  The last statement is similar to break in other programming languages

Labels

•  Blocks of code and the for and while statements can all be labeled, and idenDfied by that label. –  To use a label, place it before the statement block followed by a colon: BLOCK1: {statements…}

•  The name of the label can be any valid ASCII characters. –  The convenDon is to use uppercase for labels so there is no conflict with exisDng keywords.

ESA: Perl

•  Today: 1. Perl introducDon 2. Basic Perl: types, variables, control statements, subrouDne 3. Regular expression 4. File and I/O 5. Packages 6. Object Oriented Perl 7. DocumentaDon

SubrouDnes

•  Subs are funcDons, they can have any arguments •  The arguments are handed down using the special array @_

•  A value can be returned using return Perl: sub multiply {

my (@ops) = @_; return $ops[0] * $ops[1];

}

multiply(2, 3); output: 6

Perl funcDons and parentheses

•  Perl funcDons can be wrièn with or without parentheses in most cases, so the statements: index string, search_string;

and

index(string, search_string);

are idenDcal. It is up to you whether you use parentheses.

ESA: Perl

•  Today: 1. Perl introducDon 2. Basic Perl: types, variables, statements, subrouDne 3. Regular expression 4. File and I/O 5. Packages 6. Object Oriented Perl 7. DocumentaDon

Perl Regular expressions (1)

•  Very mature, can be highly complex •  Can be used for matching (tesDng) or transliteraDon.

•  The matching operator `=~ m/… /' is used for tesDng

Perl: if ("This is perl!" =~ m/is/) {

print "Match!";

} output: Match!


•  `^' and `$' match the beginning and end of a string respecDvely.

•  You can use character classes (`[' . . . `]'). •  You can quanDfy matches using `*', `+' or `?' a�er a character or character class.

•  You can quanDfy matches using `{ from , to }’. •  `.' matches any character. •  Use a backslash (`\') to escape characters like:

`.', `^', `$', `/', `\', `{', `}', `[', `]', . . . .


Metacharacters in regular expressions: \d Matches a single digit character \w Matches a single word character (a leèr, digit or underscore) \s Matches a whitespace character

. . . . . .

Flags that can follow a regular expression: i Match case insensiDvely g Remember the current posiDon for the next matching


•  Matching subexpressions: –  You can match subexpressions by encapsulaDng them with `(' and `)’

–  Each subexpression is stored in a variable $1 . . . $n, which can be used later on.

Perl: "This is it!" =~ m/\s(..)/;

print $1;

•  QuesDon: What is printed by the above lines?

Perl Regular expressions (4) Search and replace: •  Using s/ regexp / replacement /

–  Perl: $a = "This is it, or is it?" $a =~ s/\s.(s)/ wa$1/; print $a; –  output: This was it, or is it?

•  Using the g flag the replacement will be made for all matches, instead of just the first one. –  Perl: $a = "This is it, or is it?” $a =~ s/\s.(s)/ wa$1/g; print $a; –  output: This was it, or was it?

ESA: Perl


File Handling •  Opening and closing files •  “open” instrucDon takes two arguments: the name of a filehandle and

the name of the file itself. –  The filehandle is like a variable which represents the handling of the

file within the script. open (CLIENTS, "clients.txt");

•  By default, the file is open in read-‐mode. To open a file in write-‐mode or append-‐mode simply add a “>” or “>>” in front of the file name: open (CLIENTS, ">clients.txt"); open (CLIENTS, ">>clients.txt");

File Handling: open •  “open” instrucDon returns

–  true if it managed to open the file, –  false otherwise.

•  use this value in order to test the success of the operaDon •  The following code opens the file in write-‐mode and prints

“Insufficient privileges” if the script doesn't manage to do so. open (CLIENTS, ">clients.txt") or print "Insufficient

privileges\n";

•  Remember to always close the files once you're finished with them. If you don't your changes might be lost

close (CLIENTS);

File Handling: write

•  Wri5ng into files •  Once the file is open in write mode you can write into it simply by wriDng into its filehandle.

•  The “print” instrucDon writes things on the screen by default, but you can specify a filehandle to write into.

open (CLIENTS, ">>clients.txt") or die "Insufficient privileges: $! \n"; print CLIENTS "Mr John Doen"; close (CLIENTS);

File Handling: read •  Copying the content of the file into an array

–  Each line will then correspond to an element of the array: open(CLIENTS, "clients.txt"); @lines = <CLIENTS>; close(CLIENTS); print @lines;

•  Looping through the filehandle –  you can loop through the filehandle in a while loop by wriDng while ($line = )

•  (think of it as “while” there are lines in the clients file, I'm assigning the current line to $line):

•  open (CLIENTS, "clients.txt"); while ($line = <CLIENTS> ) { print $line; } close (CLIENTS);

Valid tests

•  These tests are all UNIX tests available to Perl: -‐B true if a binary file -‐d true if directory -‐e true if file exists -‐f true if regular file -‐M returns age in days since last modificaDon -‐r true if readable -‐s returns size of file in bytes -‐T true if text file -‐w true if writable -‐z true if file exists but is empty

Your 3rd Perl script, list of employees ! #open the employeesfile open (EMPLOYEES,"employees.txt");

#for each line while ($line =) { #remove the carriage return chomp $line; #split the line between tabs #and get the different elements ($name, $department, $salary) = split /t/, $line; #go to the next line unless the name starts with "Mr “ next unless $name =~ /^Mr /; #go to the next line unless the salary is more than 25000. next unless $salary > 25000; #go to the next line unless the department is R&D. next unless $department eq "R&D"; #since all employees here are male, #remove the parDcle in front of their name $name =~ s/Mr//; print"$namen”; } close (EMPLOYEES);

What this script does to the list of employees ?

What can you learn from the example (new things we did not discuss)

ESA: Perl


Packages (1)

•  Packages are a way to split up perl code into several pieces

•  A package can be included into a piece of code by using use

•  Each package represents a new namespace •  Create a package –  by creaDng a new file, generally ending in .pm –  The code for the class you are creaDng goes in there. –  The package name is the same as the filename, this is easier to use, but not obligatory.

Packages (2) •  Create a new file, for instance MyPackage.pm •  The first line should provide the name of the class in the

package: package MyPackage;

Next comes the code for the package . . . •  The file should end with 1; to indicate to the loading

module that the enDre package was read successfully. •  Example:

package: package MyPackage; print "WOW, it worked!\n"; 1;

Packages (3)

•  To use our package run the following perl code:

#!/usr/bin/perl use MyPackage;

•  The output of the program: WOW, it worked!;

Packages (4) •  You can refer to variables and filehandles in other packages by

prefixing the iden5fier with the package name and a double colon: $Package::Variable

•  You can create subs in a package in the normal way

•  They can be called from the program using the package: $Package::the_sub("well?")

•  Special code blocks can be used for iniDalizaDon and destrucDon: BEGIN {}, END {}, INIT {}, CHECK {}

•  Use perldoc perlmod to get more informaDon

ESA: Perl


Objects (1)

“An Object is simply a reference that happens to know which class it belongs to.”

•  No special syntax for constructors, you have to make your own

•  An object is created using the bless() –  The bless() funcDon creates a reference to an object Anything can become blessed

Objects (2) •  using a known reference:

package Critter; sub newSub { my $self = {}; bless $self; return $self; }

Note: •  {} allocates an anonymous, empty hash and returns a

reference to it •  bless takes that reference and tells the object it references

that it's now a Crièr, and returns the reference •  Using anonymous objects the class will not know itself

h`p://perldoc.perl.org/perlobj.html

Classes (1) “A class is simply a package that happens to provide methods to

deal with object references.” •  A class provides the methods for dealing with its reference •  Methods have a constructor funcDon, generally newSub()

package Critter; sub newSub { my $self = {}; bless $self; return $self; }


Classes (2) •  A method is simply a subrou5ne that expects

–  an object reference (or a package name, for class methods) as the first argument.

•  A constructor can call methods using the reference package Critter; sub newSub { my $self = {}; bless $self; $self->initialize(); return $self; }


Classes (3)

•  Classes can be inherited •  For that use the two-‐argument version of bless •  Bless an object into a class

package Critter; Sub newSub { my $class = shift; my $self = {}; bless $self, $class; $self->initialize(); return $self; }


documentaDon

•  BOOKS!, they are in the back of the classroom. . . •  Use the web, there are a lot of websites on perl •  Check www.perl.com for help. •  Find out about perldoc

Material for these slides was taken from h`p://www.perl.com/pub/a/2000/10/begperl1.html

Assignment Create a simple program that can keep track of your mp3's.

1. first program the following using only packages, no OO 1.1. store the names and locaDons of your mp3's in a large array or hash, and allow for addiDonal parameters to be stored as well, like preference (1-‐5 stars), number of Dmes played, etc. 1.2. create a simple interface for searching/adding/deleDng/ediDng your database. Make sure to use subs for these funcDons. Make use of RE... 2. (OTIONAL) recreate, using the previous code, the assignment using OO. 2.1 create a main program that can run mulDple databases at the same. Each database must be a separate object instance. Provide an interface that allows the user to choose and switch between the databases at will. 3. (BONUS) add funcDons for (re)storing your databases

Perl Script Exercise: Netswitch •  In this exercise, we want to be able to switch between

networks. We defined network configuraDon files in a directory called "networks".

•  For instance, here is the content of ./networks/home: interface=eth2 type=dhcp proxy=none And here is the content of ./networks/work: interface=eth1 type=dhcp proxy=www-‐proxy.work.com proxy_port=8080

SoluDon (1) #!/usr/bin/perl #default values

$interface="none";

$type="none";

$address="none";

$gateway="none";

$dns="none";

$proxy="none";

$proxy_port = "none";

#gather information from the network file $network = $1;

$networkFile = "./networks/$network";

open (NETWORK, "$networkFile") or die "$networkFile not found or not readable \n";

SoluDon (2) while ($line = ) { chomp $line; ($variable, $value) = split /=/, $line; if ($variable eq "interface") $interface = $value; elsif ($variable eq "type") $type = $value; elsif ($variable eq "address") $address = $value; elsif ($variable eq "gateway") $gateway = $value; elsif ($variable eq "dns") $dns = $value; elsif ($variable eq "proxy") $proxy = $value; elsif ($variable eq "proxy_port") $proxy_port = $value; } #make sure the type and interface are defined if ($interface eq "none") die "Interface name not defined \n"; if ($type eq "none") die "Network type (dhcp, static) not define \n"; if ($type eq "dhcp") { print "Network type: dhcpn"; #just get an IP address and settings from the DHCP Server system("dhclient"); } elsif ($type eq "static") { print "Network type: staticn”;

SoluDon (3) #bring the interface up if ($address eq "none") die ("IP address not definedn"); system("ifconfig $interface $address up"); if ($gateway ne "none") { print "Gateway: $gatewayn"; system("route add default gw $gateway"); } if ($dns ne "none") { print "DNS Server: $dnsn"; $strNameServer = "cat "."'"."nameserver $dns"."' > /etc/

resolv.conf"; system($strNameServer); } } else die "Bad network type : $type. Use dhcp or static.n";

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Perl% - OS3 · Perl% ESA2012/2013% Adam%Belloum% [email protected]% % % Material%Prepared%by%Eelco%Schatborn%%