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Perl

• "Practical Extraction and Reporting Language"• written by Larry Wall and first released in 1987• Perl has become a very large system of modules• name came first, then the acronym• designed to be a "glue" language to fill the gap

between compiled programs (output of "gcc", etc.) and scripting languages

• "Perl is a language for easily manipulating text, files and processes": originally aimed at systems administrators and developers

What is Perl?

Perl is a High-level Scripting languageFaster than sh or csh, slower than CNo need for sed, awk, head, wc, tr, …Compiles at run-timeAvailable for Unix, PC, MacBest Regular Expressions on Earth

Executing Perl scripts

• "bang path" convention for scripts:• can invoke Perl at the command line, or• add #!/public/bin/perl at the beginning of the script• exact value of path depends upon your platform (use "which perl"

to find the path)•one execution method:

% perlprint "Hello, World!\n";CTRL-DHello, World!

•preferred method: set bang-path and ensure executable flag is set on the script file

Perl Basics

Comment lines begin with: #File Naming Scheme

• filename.pl (programs)• filename.pm (modules)

Example prog: print “Hello, World!\n”;

Statements must end with semicolon$a = 0;

Should call exit() function when finishedExit value of zero means success

exit (0); # successfulExit value non-zero means failure

exit (2); # failure

Data Types

Integer• 25 750000 1_000_000_000• 8#100 16#FFFF0000

Floating Point• 1.25 50.0 6.02e23 -1.6E-8

String• ‘hi there’ “hi there, $name” qq(tin can)• print “Text Utility, version $ver\n”;

Boolean0 0.0 “” "0" represent Falseall other valuesrepresent True

Variable Types

Scalar• $num = 14;• $fullname = “John H. Smith”;• Variable Names are Case Sensitive• Underlines Allowed: $Program_Version = 1.0;

Scalars

• usage of scalars: print ("pi is equal to: $pi\n");

print "pi is still equal to: ", $pi, "\n"; $c = $a + $b

• important! A scalar variable can be "used" before it is first assigned a value• result depends on context• either a blank string ("") or a zero (0)• this is a source of very subtle bugs• if variable name is mispelled — what should be the result?• do not let yourself get caught by this – use the "-w" flag in

the bang path:#!/public/bin/perl -w

Operators

Math• The usual suspects: + - * / %

$total = $subtotal * (1 + $tax / 100.0);• Exponentiation: **

$cube = $value ** 3; $cuberoot = $value ** (1.0/3);

• Bit-level Operations left-shift: << $val = $bits << 1; right-shift: >> $val = $bits >> 8;

Assignments

As usual: = += -= *= /= **= <<= >>=

$value *= 5;$longword <<= 16;

Increment: ++$counter++ ++$counter

Decrement: --$num_tries-- --$num_tries

Arithmetic

• Perl operators are the same as in C and Java• these are only good for numbers

• but beware:$b = "3" + "5";print $b, "\n"; # prints the

number 8• if a string can be interpreted as a number given arithmetic

operators, it will be• what is the value of $b?:

$b = "3" + "five" + 6?• Perl semantics can be tricky to completely understand

Conditionals

Numeric string

Equal: == eqLess/Greater Than: < > lt gt Less/Greater or equal: <= >= le geZero and empty-string means FalseAll other values equate to True

Comparison: <=>cmp

Results in a value of -1, 0, or 1Logical Not: !

if (! $done) {print “keep

going”;}

Control Structures

“if” statement - second style• statement if condition;

print “\$index is $index” if $DEBUG;• Single statements only• Simple expressions only

• “unless” is a reverse “if”• statement unless condition;

print “millenium is here!” unless $year < 2000;

Subroutines (Functions)

Calling a Subroutine• &subname; # no args, no return value• &subname (args);• retval = &subname (args);• The “&” is optional so long as…

subname is not a reserved word subroutine was defined before being called

Passing ArgumentsPasses the valueLists are expanded

@a = (5,10,15);@b = (20,25);&mysub(@a,@b);

this passes five arguments: 5,10,15,20,25mysub can receive them as 5 scalars, or one array

Command Line Args

$0 = program name@ARGV array of arguments to programzero-based index (default for all arrays)Example

• yourprog -a somefile $0 is “yourprog” $ARGV[0] is “-a” $ARGV[1] is “somefile”

Basic File I/O

Reading a File• open (FILEHANDLE, “$filename”) || die \ “open of $filename failed: $!”;

while (<FILEHANDLE>) {chomp $_; # or just: chomp;print “$_\n”;

}close FILEHANDLE;

Writing a Fileopen (FILEHANDLE, “>$filename”) || die \ “open of $filename failed: $!”;while (@data) {

print FILEHANDLE “$_\n”;

# note, no comma!}close FILEHANDLE;

Debugging in Perl

-w option is great!• #!/bin/perl -w• tells you about…

misused variables using uninitialized data/varables identifiers that are used only once and more

Debug mode: perl -d filename [args]Display Commands

hExtended help

h hAbbreviated help

l (lowercase-L) list lines of codel sub list subroutine subl 5 list line 5l 3-6 list lines 3 through 6 inclusivel

list next window of lines

Thank you