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Programming In UNIX

Perl Programming

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Perl Overview

How to use Perl, quickly: Structure Syntax Data Statements

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But first...

Perl Author: Larry WallPractical Extraction and Report LanguagePurpose: UNIX system administrationTwo major characteristics

Rich: Does a lot of things Simple: Easy on the eyes

Ubiquitous: Freely available, versions available for most major operating systems

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Recommended Reading

Other Good book: Programming Perl by Wall, Christiansen,

and Schwartz, O’Reilly & Associates, ISBN: 1-56592-149-6

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Structure

A compiled - interpreted language Your code is compiled into an internal format,

then interpreted as it runs. You do not need to “compile” it explicitly as you do “c” programs.

A compiler is in the works Currently, you have the source code to run a

Perl program. This is a disadvantage for commercial use where it may violate copyrights to distribute the source.

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Structure

Particulars: Don’t declare variables, just use them No data typing: data interpreted in the

context where it is used Data can expand dynamically: You don’t

size arrays UNIX-centric: If you know UNIX, you

know a lot of Perl

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Structure

Perl is a procedural language, with object capabilities Statements are executed in sequence,

from top to bottom, unless a branching instruction gets in the way

You can declare and use subroutines, and pass parameters to and from The subroutine is still a good thing to

use...

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Syntax

Statements end with a semicolon - “;”White space is an important delimiterYour program ends when the interpreter

reaches the end of the file No fancy “exit” or “end” statements

Use commas to separate operands

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Data

Perl is not strongly typed: Whether the contents of a variable are

used as a number or a string depends on the usage itself

If an operator expects a number like + does, then Perl will see the value as a number.

If an operator expects a string like . does, then Perl will see the value as a string.

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Data

Perl provides the following data structures: Scalar: single-value - $name Array: indexed list - @names is the array,

$names[1] is an individual value of one cell in the array

Hash: associative array - %person, $person{name}

Variables are “declared” when you first use them Values entered by assignment: $name = “mike”;

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Data: Scalars

Some examples$number = 234; # integer$pi = 3.1415; # real$name = “bob”; # string$answer = “My name is $name”; # w/

interpolation$price = ‘$49.95’; #w/o

interpolation$dirlist = `ls`; #command

outputs your directory (*note ` are grave accents)

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Data: Scalars

The simplest data that Perl uses Is either a number or a stringNumbers can be integers (whole numbers)

or floating point numbers (i.e. 1.25 or 7.25e45 or -12e-24

String is a sequence of characters within single or double quotes

Statements are terminated by a semi-colonComments starts with a “#”, comment ends

at the end of the line

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Data: Scalars

Single quotes - are not part of the string, just to ID the beginning and ending of the string

Exceptions to be aware of: to get a single quote into a string

precede it by a backslash to get a backslash into a string precede

it with another backslash \n for a new line doesn’t work here

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Data: Scalars

Double quoted strings. Quotes are still not part of the string but the backslash takes on its full power

String representations (see page 24) \n newline \r return \t tab

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Data: Scalars

Operators for Strings the “.” operator concatenates string values

“hello”.”world” shows up as helloworld, you would need “ “ between your values to get a space between i.e. “hello” . ” “ . ”world” gives you hello world

string repetition uses x to take its left operand (a string) and makes as many concatenated copies as indicated i.e. “fred” x 3 gives the output of fredfredfred

Order of operations - Perl follows the normal math method of resolving mathematical operations

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Data: Scalars

Two classes of operators: one for comparing numbers, one for comparing strings:

Meaning Numeric StringEqual = = eqNot equal ! = neGreater than > gtLess than < ltGreater or equal > = geLess or equal < = le

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Data: Scalars

if I == 5 (by the use of == you know this needs to be a numeric field) Note: you are not giving I the value of 5 you are comparing if I is equal to the value of 5

if name eq ‘jane’ (by the use of eq you know that this is a string

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Data: Scalars

Variable - name for a container that holds one or more values, which is what a scalar is

Assignment - way to give a variable a value $name=“mike”

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Data: Scalars

Autoincrement old way $a +=1 or new way ++$a

Autodecrement $x=12; --$x the value of $x is now

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Data: Scalars

The chop and chomp functions chop takes a single argument and removes

the last character from the sting use caution you may only want to remove the

newline at the end of a string chomp removes only a newline character* Note: If you type in a value when you are

assigning a value to a field, and follow that with pressing the enter key, Perl keeps that carriage return as the last character of the value that you just stored

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Data: Scalars

<STDIN> means standard inputPerl reads the next complete text

line form standard input and uses that string as the value of <STDIN>. If there’s nothing waiting to be read, the Perl program will stop and wait for you to enter some characters followed by a newline (return).

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Data: Arrays

A list is ordered scalar dataAn array is a variable that holds a listEach element of the array is a

separate scalar variable with it’s own value

Can have any number of elementsThe smallest has no elements, the

largest can fill all available memory

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Data: Arrays

Some examples: @team = (“bob”, “carol”, “ted”, “alice”);

initializes a four-cell arrayPerl arrays are based at zero: ($team[0] eq

“bob”) is true ($member1, $member2, $member3,

$member4) = @team;reads first four cells into scalars

$team[4] = “bud”;adds bud to the team array

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Data: Arrays

qw short for quote word function - creates a list from the nonwhitespace parts between the parentheses

@team = qw(bob carol ted alice); or

@team = qw ( bob carol ted alice);

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Data: Arrays

The array elements of team would have the following values

bob = $team[0] carol = $team[1] ted = $team[2] alice=$team[3] bud=$team[4]

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Data: Arrays

Other ways to access data in an array [-1] Counts from the back end of an

array so this would give you the value “bud”

# gives the index value of the last element in an array for example print $#team would give the value of 4

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Data: Arrays

push and pop will add or remove elements from the right-hand side of an array your array may be set up like this @mylist = (1,2,3); push (@mylist,4,5,6); mylist now has the value (1,2,3,4,5,6) pop@mylistmylist now has the value (1,2,3,4,5)

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Data: Arrays

shift and unshift functions perform the same functions as the push and pop, however to the left side of a list shift removes unshift adds

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Data: Arrays

The reverse function reverses the order of the elements of it’s argument for example@a=(7,8,9);

@b=reverse(@a); #the value is now #9,8,7 stored in the

array #@b@b=reverse(@b); #gives the reverse of itself

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Data: Arrays

The sort functionTakes it’s arguments, and sorts them as if they were single stringsi.e.;

@temp=(2,4,6,8,10,12,14);@y= sort(@temp); # gives

#10,12,14,2,4,6,8

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Statements

$input = <> ; #gets input from the keyboard “<>” is the line-reading operator

When empty, refers to all files on the command line, or STDIN if there are none

$input is a variable that we came up withDon’t have to declare itIt’s a scalar variable, denoted by the “$” (more

later)

“=“ assigns the value of the right side to the left

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Statements

Try this program: #!/usr/local/bin/perl

print “Enter your name: “;$name = <>;print “Your name is $name\n”;print “…and that’s it.”;

Notice the double-space between the output from the first print and the second in what you see on the screen

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Statements

$name contains the response you typed, including the carriage return character.

Insert this statement before the first print: chomp $name; #removes trailing newline

Or, do it like this: chomp($name = <>);

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Statements: If

if (condition) {statements} # if statement

{…} is a block; don’t need “;” after itExample:

if ($input eq “yes”) { print “Okay, I’ll do it”;}else { print “Heck no, I won’t do it”;}

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Statements: While

while (condition) {statements} #while loop

Don’t forget to insure the condition eventually evaluates to false in the loop

Example: while ($input ne “rumplestiltskin”) {

print “Enter it until you get it RIGHT: “; chomp ($input = < >);}

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Statements: For, Foreach

for ($i = 1; $i < 10; $i++) {print “$i\n”;}foreach $name (@class) {print “$name\

n”}; foreach VAR (LIST): iterates through the

LIST, putting the next element in VARforeach $name ( sort values %team )

{print “$name\n”;} values is a subroutine that returns a

list of the values contained in %team sort is a subroutine that sorts the list of

values

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$_: The Default Input

A global special variable: it’s already there $_ is assumed in a lot of places where you

could put a variable, but choose not to: while (< >) { # reads into $_ print; # prints $_ foreach (%name) { # each value from

#%name goes in $_ Good and Bad:

Good: provides brevity of expression Bad: Not intuitively obvious what is being

manipulated...

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Built-In Function: Split

($name, $grade) = split /:/, $line; split /PATTERN/, EXPR

/PATTERN/ is the delimiter to recognize in splitting

EXPR is the string to be split

Above would take a value of $line like “bob:40” and put “bob” in $name and “40” in $grade. The delimiter is lost in this case.

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Built-In Function: Open

open (MYFILE, “thefile.txt”); Opens the file so named, assigns

it to a file handle for subsequent use

You can write a short-circuit version of an if statement like this: open (STUFF, “stuff”) or die

“Can’t open stuff”; die is a function that causes an

orderly exit from the program, writing a message to stderr

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Lab1 - Hints

How do you invoke a system command from within Perl? There are two ways. system

system “who”; exec

exec “who”;