Ubuntu Unleashed, Second Edition

Because each operator acts on the thing immediately to the right of it, it helps to read

this from right to left (or bottom to top, the way it is written here).

The first thing that acts on the list is the

map operator. It transforms the list into a hash in

which the keys are the list elements and the values are the lengths of each element. This

is where you put in your code that does the transformation by which you want to sort.

The next operator is the

sort function, which sorts the list by the values.

Finally, the hash is transformed back into an array by extracting its keys. The array is now

in the desired order.

Command-Line Processing

Perl is great at parsing the output of various programs. This is a task for which many

people use tools such as

awk and sed. Perl gives you a larger vocabulary for performing

these tasks. The following example is very simple, but illustrates how you might use Perl

to chop up some output and do something with it. In the example, Perl is used to list

only those files that are larger than 10KB:

$ ls -la | perl -nae ‘print “$F[8] is $F[4]\n” if $F[4] > 10000;’

The -n switch indicates that I want the Perl code run for each line of the output. The -a

switch automatically splits the output into the @F array. The -e switch indicates that the

Perl code is going to follow on the command line.

Related Ubuntu and Linux Commands

You will use these commands and tools when using Perl with Linux:

a2p—A filter used to translate awk scripts into Perl

find2perl—A utility used to create Perl code from command lines using the find

command

pcregrep—A utility used to search data using Perl-compatible regular expressions

perlcc—A compiler for Perl programs

perldoc—A Perl utility used to read Perl documentation

s2p—A filter used to translate sed scripts into Perl

vi—The vi (actually vim) text editor

Reference

The first place to look is in the Perl documentation and Linux man pages.

Perl, all of its documentation, and millions of lines of Perl programs are all available free

on the Internet. A number of Usenet newsgroups are also devoted to Perl, as are shelves of

books and a quarterly journal.

CHAPTER 27 Using Perl

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Books

Although your local bookstore might have dozens of titles on Perl, the following are some

of the more highly recommended of these. You might also look at the Camel Critiques

(Tom Christiansen; http://language.perl.com/critiques/index.html) for reviews of other

available Perl books.

. Advanced Perl Programming, by Sriram Srinivasan, O’Reilly & Associates

. Sams Teach Yourself Perl in 21 Days, Second Edition, by Laura Lemay, Sams Publishing

. Sams Teach Yourself Perl in 24 Hours, Second Edition, by Clinton Pierce, Sams

Publishing

. Learning Perl, Third Edition, by Randal L. Schwartz, Tom Phoenix, O’Reilly &

Associates

. Programming Perl, Third Edition, by Larry Wall, Tom Christiansen, and Jon Orwant,

O’Reilly & Associates

. Effective Perl Programming: Writing Better Programs with Perl, by Joseph Hall, Addison-

Wesley Publishing Company

. CGI Programming with Perl, Second Edition, by Gunther Birznieks, Scott Guelich,

Shishir Gundavaram, O’Reilly & Associates

. Mastering Regular Expressions, by Jeffrey Friedl, O’Reilly & Associates

Usenet

Check out the following on Usenet:

. comp.lang.perl.misc—Discusses various aspects of the Perl programming language.

Make sure that your questions are Perl specific, not generic CGI programming ques-

tions. The regulars tend to flame folks who don’t know the difference.

. comp.infosystems.www.authoring.cgi—Discusses authoring of CGI programs, so

much of the discussion is Perl specific. Make sure that your questions are related to

CGI programming, not just Perl. The regulars are very particular about staying on

topic.

WWW

Check these sites on the World Wide Web:

. http://www.perl.com—Tom Christiansen maintains the Perl language home page.

This is the place to find all sorts of information about Perl, from its history and

culture to helpful tips. This is also the place to download the Perl interpreter for

your system.

Reference

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