CSE271 Lab 10: Perl

Announcements

Exam 2 is being graded.
Homework #6 is due tomorrow.
Homework #7 is defined at the end of this lab.

Introduction

Perl

is an interpreted programming language
is known for its power and flexibility
combines the familiar syntax of C, C++, sed, awk, grep, sh, and csh into one powerful tool
was invented by Larry Wall
is an acronym for "Practical Extraction and Report Language"
is optimized for string manipulation, I/O, and system tasks
has built-in functions for almost everything that's in section 2 of the UNIX manual pages

Perl Basics

Perl scripts can be run in the same way as shell scripts

by perl scriptname
by making the script executable and including an appropriate !# line

Perl ignores extra whitespace -- indent or not, it's up to you. Comments start with a # and go to the end of the line. All Perl statements end in a ;

Perl Variables

Perl variables don't need to be declared before use, and their type is inferred from their use. Variable identifiers are composed of letters, numbers, and the underscore and are case sensitive (as is all of Perl). A scalar variable is a single value, either numeric or a character string. Scalars are prefixed with a $, e.g., $username. Arrays (lists) are prefixed with @, e.g., @array, but individual members subscripted with the correct type, as in $array[4], and allocation is automatic.

$this_is_a_variable = "some string";
$var2 = 44;
@list = ("one", "two");

foreach $i (@list) {
   print "Element: $i\n";
}

Perl Strings

There are several kinds of quotes, which generate different results.

The single quote 'like this' is used to surround literal text -- no variable expansion
Double quotes "like this" are used when variable expansion is permitted

The backtick `like this` surrounds system calls, just as in shell programming

$this_is_a_variable = "some string";
$var2 = 44;
$varstring = 'no expansion of $var2';
$date = `date`;

Perl Functions

Perl functions (built in or user defined) are identified by their unique names. Parameters are comma separated, but parentheses are often optional.

print("length: " ,length("hello world")); 
# prints the same thing as 
print "length: ",1, length "a";

sub functionname {
  statements
}

functionname(list of parameters);

Associative Arrays (Hashes)

Perl provides the built-in ability to handle associative arrays. These are arrays that are ordered not by an integer index, but by an arbitrary string. Usually think of them (the string index values) as keys with associated values.

$principal{"clarinet"} = "Susan Bartlett"; 
$principal{"basson"} = "Andrew Vandesteeg"; 
$principal{"flute"} = "Heidi Lawson"; 
$principal{"oboe"} = "Jeanine Hassel"; 
@woodwinds = keys(%principal); 
@woodwindPrincipals = values(%principal);

Digging Deeper into Perl

Perl is huge; there is always more than one way to do something. It has loops, pointers, object-oriented support, full regular expressions, many supporting libraries, etc. While we have started to scratch the surface here, I recommend that you get more comfortable by skimming through a Perl tutorial. If you are uncertain of your programming skills, I suggest: Picking Up Perl. If you are more confident, try Perl in 20 pages.

There is lots (perhaps too much!) of Perl documentation online and there are books on Perl (such as Learning Perl and Programming Perl) in the Lehigh Safari e-book library.

A first exercise

Now write a very simple Perl script that prints out the parameters given to it, like the echo(1) command. It should work like this:

% ./echo.pl hello world hello world

Note that we haven't reviewed how to examine and interpret script (or function) parameters. You'll have to find that out from a tutorial as mentioned above.

A more significant exercise

When you have worked through a Perl tutorial, you can begin homework 7, which re-implements the apache log file parser in Perl, is due next Tuesday (8 days from how).

Last revised: 2 April 2007, Brian D. Davison.