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System AdministrationIntroduction to Scripting, PerlSession 3 – Sat 10 Nov 2007

References: chapter 1,

The Unix Programming Environment, Kernighan & Pike, ISBN 0-13-937681-X;

Perl man pages: perlintro

Albert Lingelbach, Jr.alingelb@yahoo.com

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Review of Session 2 Concepts

Unix history, multiuser/timesharing, kernel, shell, man pages, file permissions

Flow control ctrl-C, ctrl-S, ctrl-Q, ctrl-D

Commands: echo, bash, date, who, pwd, ls, cd, touch, rm, cp,

mv, cat, more, gedit, mkdir, rmdir, grep, sort, head, tail, wc, diff, chmod, bc

File paths & wildcards *, ?

I/O management >, >>, <, |, 2>

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Scripts

It is possible to write a series of commands in a file, called a script.

The script can then be run as a program. Scripts can be written in different languages:

bourne shell korn shell c chell perl

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Scripts, continued

Most are similar, and in fact are derived from each other

First line of the script file indicates the language, e.g.#!/usr/bin/env perl#!/bin/sh

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A note on editing

From the command line, if you enter gedit file.txtthe editor will open, BUT the command window will hang until you exit gedit

Also, depending on what you do in gedit, some error messages might print in the command window (they can be ignored).

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Note on editing, continued

To avoid this, you can use the command line:gedit file.txt 2>/dev/null & 2>/dev/null

ignores the errors by sending them to /dev/null &

makes the command run in the "background", which allows the command prompt to come back immediately

you will see a message in the command window when you close gedit, informing you that it has closed; it can be ignored.

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Shell script example

create a file whoison.sh with the following contents:#!/bin/shecho "these users are on the system"who | cut -d\ -f1 | sort | uniq

make the file executable:chmod 755 whoison.sh(this command will have no output)

run the command./whoison.sh

Note there are two spaces here

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Perl

Perl is widely available (standard on Solaris and most Unix systems)

Free (http://www.perl.org) Widely used,

lots of documentation & other resources Flexible & powerful Not the best first language

not strongly typed very flexible syntax, lots of shortcuts

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Perl Documentation

man perl man -M /usr/perl5/man perlintro man -M /usr/perl5/man subject

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Example Perl Script

create a file myscript.pl with the contents:#!/usr/bin/env perlprint "hello world.\n";

make the file executablechmod 755 myscript.pl

run the script./myscript.pl or -perl myscript.pl

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Perl Syntax Every command ends with ; Perl comments begin with # Any text from # to the end of the line is

ignored Whitespace is ignored

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Scalar Variables

a variable is a named storage location a “scalar” variable can hold one thing:

string number

integer floating-point

try the following:#!/usr/bin/env perlmy $string = "hello world.\n";my $number = 17.42;print $string;print "$number\n";

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Scalar variables continued

variable declaration begins with my scalar variable names begin with $ variables are “interpolated” within double

quotes

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Special Operators

= assignmentmy $x = 17;print "$x\n";

. concatenationmy $string = "this" . "that";print "$string\n";

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Math Operators

numbers can be manipulated with standard operators:+ addition- subtraction*multiplication/ division many others; see

man -M /usr/perl5/man perlop try

$myresult = 3 * 8 – 4 – 2;print “$myresult\n”;

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Math Operators continued

note that operators have associativity (left, right) precedence

see man -M /usr/perl5/man perlop for details

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Numerical Comparisons

numbers can be compared with comparison operators:== equals< less than> greater than<= less than or equals>= greater than or equals!= not equal many others; see

man -M /usr/perl5/man perlop

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String Comparisons

strings can be compared with string comparison operators:eq equalityne inequalitylt less thangt greater thanle less than or equalge greater than or equal

inequality is measured by dictionary order, i.e. "apple" < "banana""dog" < "doggy"

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Control Flow - if

if-then-else controls program to do one thing if condition is

true, otherwise do something else else is not required

syntax:if (condition) { ...}else { ...}

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Control Flow – if – continued

example:if ($x == 3) { print "x equals 3\n";}else { print "x is not equal to 3\n";}

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Control Flow – while

while loop allows the repeated execution of a set of

statements, while condition is true syntax:

while (condition) { ...}

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Control Flow – while - continued

examplemy $x = 5;while ($x > 0) { print "x = $x\n"; $x = $x - 1;}

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Control Flow – for

for loop allows the repeated execution of a set of

statements, while counting through an index syntax:

for (initialization; condition; step operation) { ...}

examplefor (my $i = 0; $i < 10; $i++) { print "i = $i\n";}

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Boolean Logic

boolean values are true or false comparisons return boolean values boolean values can be combined using

boolean operators:&& and|| or! not

examples$x < 17 || $y > 10$s eq "this" && $x > 10

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Subroutines

Subroutines break your code into pieces that are easier to

understand and manage Syntax:

sub name { ...}

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Subroutines - continued

Example:sub print1to5 { for (my $i = 1; $i <= 5; $i++) { print "$i "; } print "\n";}

print1to5;

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Subroutine arguments

Subroutines can be given "arguments" (or "parameters");additional information from the caller

these are accessed by the shift statement

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Subroutine arguments – continued

examplesub printXtoY { my $start = shift; my $end = shift; for (my $i = $start; $i <= $end; $i++) {

print "$i ": } print "\n";}

printXtoY (3, 7);

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Functions

Subroutines can return a value, using the return statement

Subroutines that return a value are often called "functions"

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Functions - continued

Example:sub square { my $x = shift; my $y = $x * $x; return $y;}

my $sq = square (3);print "sq = $sq\n";

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Command Line Arguments

The shift command can also be used to get arguments passed to the script from the command line.

Example - create the file mult.pl:#!/usr/bin/env perlmy $x = shift;my $y = shift;print "product = " . $x * $y . "\n";

(Don't forget to make mult.pl executable) Run it from the command line:

./mult.pl 3 4

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Review Scripts; perl; syntax: #comment, statement; Scalar $variables Special operators: = . Math operators: + - * /; precedence,

associativity Numerical comparisons: == < > <= >= != String comparisons: eq ne lt gt le ge Control flow: if, while, for Boolean logic: && || ! Subroutines, arguments/parameters,

functions

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Next up:

Scope of Variables Arrays Hashes Regular Expressions