Perl Scripting Basics Overview - Jack Zheng

9/16/2012

Perl Scripting Basics IT 4423 Unix/Linux Administration

J.G. Zheng Fall 2012

Overview Perl scripting language basics  

Script files Language elements  Variable  Operators  Control flow  Function

Scripting apps  

String processing File processing 2

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Perl Practical extraction and report language (unofficial) Perl is a general purpose programming language 

Used widely for system administration, network programming, web CGI, etc.

Perl is installed on most Linux distros 

Check perl's version and installed directory

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Perl Script File Script file   

File extension .pl Add “#!/usr/bin/perl” as a directive in the first line Case sensitive

Statement delimiter 

Use ; to separate (or end) statements (required)

Use # for comments (same as Bash) #!/usr/bin/perl $var1="morning"; #this is a variable print "Good $var1\n";

#!/bin/bash var1="morning" #this is a variable echo "Good $var1" 4

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Script Execution Similar to bash scripts 



Use the perl process to execute scripts perl [script file name] Or allow the execution permission

Source error 

The program will not execute if there are syntax errors (compilation error). – different from Bash Initially, the file does not have execution permission. Use "perl" to execute it or add execution permission

!! For security reasons, the current directory is not in the search path. Add “./” to force search the current directory 5

Input and Output Output 

Use "print" command the control characters like \n, \t, etc.



Advanced output

 http://www.tizag.com/perlT/perlstrings.php

 http://en.wikibooks.org/wiki/Perl_Programming/Advanced_Output

User input   

Use or just http://www.tizag.com/perlT/perluserinput.php chomp function: http://www.tizag.com/perlT/perlchomp.php

#!/usr/bin/perl

#!/bin/bash

$var1="morning"; #this is a variable print "Enter your name: "; $var2=; different print "good $var1, $var2\n"

var1="morning"; #this is a variable read -p "Enter your name: " var2 echo "good $var1, $var2" 6

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Variables Variable naming  

Variable names are case sensitive Dynamically typed: string, number, etc. – different from bash

Assignment 

Variables are always indicated by $, both in assignment and reference – different from bash No space around the

#!/usr/bin/perl

different

$var1 = "morning"; #this is a variable print "Enter your name: "; $var2=; print "good $var1, $var2\n"

#!/bin/bash

= symbol in bash; no such restriction in perl

var1="morning"; #this is a variable read -p "Enter your name: " var2 echo "good $var1, $var2" 7

Script Arguments Arguments are stored in an array @ARGV and can be accessed through $ARGV[0], $ARGV[1], etc. $#ARGV is the index number of the last array element 

So, $#ARGV+1 is the number of arguments

Script arguments can be supplied at the command line separated by space #!/usr/bin/perl

Use parentheses to force a calculation

print "Number of arguments: ".($#ARGV+1)."\n"; print $ARGV[0]."\n"; print $ARGV[1]."\n"; . is the string print $ARGV[2]."\n";

concatenation operator

#!/bin/bash echo "Number of arguments: $#" echo "$0" echo "$1" echo "$2"

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Arithmetic Operations Basic operators and operator precedence are similar to C style languages  

+, -, *, /, %, ** ++, --, +=, etc.

#!/usr/bin/perl $var1=5; $var2=10; $var3=$var1+$var2; print "$var1+$var2=$var3\n"; $var4=$var1/$var2; print "$var1/$var2=$var4\n";

#!/bin/bash !!Different: • In Bash, arithmetic operations need to be specifically instructed (using "let" or other methods). • In perl, there is no need to.

var1=5 var2=10 var3=var1+var2 echo "$var1+$var2=$var3" let var3=var1+var2 echo "$var1+$var2=$var3" var3=$[var1+var2] echo "$var1+$var2=$var3" 9

Relational and Logical Operations Get more 

http://www.tizag.com/perlT/perloperators.php !!Different: • In Perl, operators are the same for strings or numbers. • In bash, they are different

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Flow Control – If #!/usr/bin/perl

$var1=6; $var2=5;

In Perl, the if structure is much more like C style languages and much simpler

Note the keyword for else if is "elsif"

var1=5 var2=5

Bash syntax for if is much more strict

if [ $var1 = $var2 ]; then echo "Same" else echo "Different" fi

if ($var1==$var2) { print "Same"; } elsif ($var1>$var2) { print "$var1 is greater"; } else { print $var1." is smaller"; } print "\n";

#!/bin/bash

{ } are required

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Compound Conditions Logical operator  

&&: logical AND ||: logical OR

#!/usr/bin/perl

In Perl, the logical comparison is much more like C style languages and much simpler

#!/bin/bash

Bash syntax for if is much more strict

$var1=3;

var1=5

if ($var14) { print "$var1 is Within range"; } else { print "$var1 is out of range"; } print "\n";

if [[ $var1 -lt 10 && $var1 -gt 4 ]]; then echo "Within range" else echo "Out of range" fi

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Flow Control – For Loop

#!/usr/bin/perl

In Perl, the loop structure is much more like C style languages and much simpler

#!/bin/bash for (( i=0 ; i < 3 ; i++ )); do echo "$i" done

for ($i=0 ; $i < 3 ; $i++ ) { print "$i\n"; }

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Flow Control – While Loop More 

http://www.tizag.com/perlT/perlwhile.php

#!/usr/bin/perl

In Perl, the loop structure is much more like C style languages and much simpler

$counter=0; while ($counter