TweetFollow Us on Twitter

A Brief Look at Perl

Volume Number: 18 (2002)
Issue Number: 8
Column Tag: Section 7

A Brief Look at Perl

a real gem of a scripting language

by Rich Morin

Last month's column closed with some short descriptions of BSD's scripting languages. It offered some opinions, but stopped short of recommending any particular language. This month, I'll get a bit braver, explaining why you might want to use Perl for most of your BSDish scripting needs.

First, however, I should caution that Perl is not always the appropriate choice. If you're modifying a system shell script, don't try to rewrite it in Perl. Just use the language the script is written in (typically the Bourne Shell). If you are just mechanizing a simple list of commands, Perl is probably overkill, but see below. Finally, if the script has to run early in the startup process, the Perl interpreter may not be available.

For new, substantial scripts, however, I would strongly recommend that you use Perl. Here are some reasons:

  • efficiency - The Perl interpreter, unlike the shells, seldom has to start up new processes. This means that substantial Perl scripts will often run much faster than equivalent shell scripts. I found this out several years ago, when I transliterated several large shell scripts to Perl. The run times went down by a factor of five!

  • syntax - Most shells have very weak notions of syntax. One result of this, in Mac OS X, is that white space in file names can be interpreted as splitting the names into multiple tokens. Perl handles strings in a much more sophisticated manner, so it doesn't get confused.

  • integration - Unlike shell scripts, which may stitch together dozens of commands, Perl is an integrated language. This eliminates a great deal of hassle and possible confusion.

  • facilities - Perl has powerful data structures, convenient control-flow operators, and access to almost any imaginable system call. The shells have none of these features. As a result, a Perl program can often do things that would be essentially impossible in any shell.

  • support - Perl has numerous books, a vast library of modules, and a very active user community. Most shells have few to none of these resoueces.

  • portability - Perl scripts can be run on essentially any modern operating system. With a little forethought, they can run unmodified on several different systems. The shells, in contrast, only work on BSD and other Unix-like systems.

Having said all of this, perhaps I should tell you some of the bad news about Perl:

  • complexity - Perl is a very large language, with some really peculiar nooks and crannies. Even if you don't use all of these features, you may well encounter them in a module or some other bit of code you "inherit".

  • informality - Perl's motto ("There's More Than One Way To Do It" gives fair warning that this isn't a nice tidy "orthogonal" language. In fact, Larry Wall (Perl's creator) says that Perl is a "diagonal" language; cutting across the middle often speed things up!

  • mutability - Unlike the shells, Perl is still evolving. Perl 5 has (mostly) stabilized, but Perl 6 development is quite active. So, you might need to relearn some things in a few years.

Show me some code!

This being MacTech, you're probably wondering when you're going to see some actual Perl code. Well, here's a short Perl script that I hacked together to do some backups. It's not a full-featured backup utility, by any means, but it gets the job done (and shows off some Perl language features)...

#!/usr/bin/env perl
#
# macbac - Create backup files, using tar(1).
#
# Written by Rich Morin, CFCL, 2002.06
{
  $date = `date +%y%m%d.%H%M`;
  chomp($date);
  @dir = ('/Users/rdm',
          '/Volumes/Work');
  for $dir (@dir) {
    $bac = cvt($dir);
    $cmd = "nice -10 tar czf $bac $dir";
    printf(">>> %s\n", $cmd);
    system($cmd);
  }
}
sub cvt {  # convert the directory name
  my ($tmp) = @_;
 
  $tmp =~ s|/|.|g;
  $tmp =~ s|\s|_|g;
  return("/Backups/$date$tmp.tgz");
}
  

The first line of any BSD script, as discussed previously, tells the system which program should be invoked as the interpreter for the following lines. Because I may decide to install a later version of the Perl interpreter at some point (e.g., in /usr/local/bin), I don't want to specify a full path name for the Perl interpreter. So, I tell the system to run /usr/bin/env, letting it find and run the appropriate version of Perl.

The remainder of the script, in any case, is read by the Perl interpreter. Perl's syntax and feature set are borrowed from a variety of (mostly Unix) languages and tools, including awk, Basic-Plus, C, sed, sh, and tr. This makes Perl seem familiar to Unix aficionados, but can cause some culture shock to others. Stay calm; it's not really all that bad!

Unlike C, Perl has no "block comments". So, I use a column of sharp signs (#) for my header comments. I also like to wrap the "main" routine in braces. This causes its contents to be indented at the same level as the contents of any sub (routine). It also gives me a visual cue that this is a "block" of code.

I could have asked Perl to grab and format the date information (and should have, if I were trying for cross-OS portability), but the method above shows off Perl's ability to run BSD commands and retrieve their results. The backquotes tell Perl to run the enclosed command, returning the result as a text string. The result gets put into a scalar variable, $date. The chomp() function, by the way, removes the trailing newline from date's output.

The script then tells Perl to create an array variable named @dir and load it with a list containing two text strings. I use single quotes to wrap these strings, indicating that I don't want Perl to do any variable interpolation (see below) or other tricks.

The for loop sets $dir, successively, to each of the values in @dir. Note that the sigil (e.g., $, @) is part of a Perl variable's name, so @dir and $dir are two different variables. This seems a bit weird at first, but ends up being quite handy as you get used to it.

The cvt() routine shows off some of Perl's capabilities and peculiarities. First, it grabs @_ (the array of calling parameters) and copies the contents into a private list of variables. In this case, the list only has one element, but it might well have more.

The next two lines tell Perl to do global substitutions of periods for slashes and underscores for "white space". This gives me "flattened" names (no directory levels) without any annoying spaces, tabs, etc. This is a trivial example of Perl's powerful "regular expression" capability. Regular expressions can be used to perform all sorts of magic on text strings. In fact, there is a substantial book on regular expressions alone!

The last line tells Perl to "interpolate" the variables $bac and $date into a text string. The use of double quotes tells Perl to look for dollar signs and other "magic" characters. Note that, although $tmp is a private variable, $date is shared with the main routine. Finally, the return is not strictly needed (the value of the last expression evaluated in a sub is automatically returned), but I think it adds to the clarity of the code.

Returning to the main routine, we build up a command string, print it (ala C), and hand it off to the operating system to be run. Look up the man pages for nice and tar to see what their roles are in this script.

In a script of this size, there isn't much room to get into Perl's fancier aspects. Next month, I'll give you a more substantial taste of its data structures and control flow, as well as listing some useful Perl resources. If you can't wait to get started, however, just bop over to www.perl.{org,com}...


Rich Morin has been using computers since 1970, Unix since 1983, and Mac-based Unix since 1986 (when he helped Apple create A/UX 1.0). When he isn't writing this column, Rich runs Prime Time Freeware (www.ptf.com), a publisher of books and CD-ROMs for the Free and Open Source software community. Feel free to write to Rich at rdm@ptf.com.

 
AAPL
$99.76
Apple Inc.
+2.09
MSFT
$44.08
Microsoft Corpora
+0.45
GOOG
$520.84
Google Inc.
+9.67

MacTech Search:
Community Search:

Software Updates via MacUpdate

Macgo Blu-ray Player 2.10.9.1750 - Blu-r...
Macgo Mac Blu-ray Player can bring you the most unforgettable Blu-ray experience on your Mac. Overview Macgo Mac Blu-ray Player can satisfy just about every need you could possibly have in a Blu-ray... Read more
Apple iOS 8.1 - The latest version of Ap...
The latest version of iOS can be downloaded through iTunes. Apple iOS 8 comes with big updates to apps you use every day, like Messages and Photos. A whole new way to share content with your family.... Read more
TechTool Pro 7.0.5 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PDFKey Pro 4.0.2 - Edit and print passwo...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Yasu 2.9.1 - System maintenance app; per...
Yasu was originally created with System Administrators who service large groups of workstations in mind, Yasu (Yet Another System Utility) was made to do a specific group of maintenance tasks... Read more
Hazel 3.3 - Create rules for organizing...
Hazel is your personal housekeeper, organizing and cleaning folders based on rules you define. Hazel can also manage your trash and uninstall your applications. Organize your files using a... Read more
Autopano Giga 3.7 - Stitch multiple imag...
Autopano Giga allows you to stitch 2, 20, or 2,000 images. Version 3.0 integrates impressive new features that will definitely make you adopt Autopano Pro or Autopano Giga: Choose between 9... Read more
MenuMeters 1.8 - CPU, memory, disk, and...
MenuMeters is a set of CPU, memory, disk, and network monitoring tools for Mac OS X. Although there are numerous other programs which do the same thing, none had quite the feature set I was looking... Read more
Coda 2.5 - One-window Web development su...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few... Read more
Arq 4.6.1 - Online backup to Google Driv...
Arq is super-easy online backup for the Mac. Back up to your own Google Drive storage (15GB free storage), your own Amazon Glacier ($.01/GB per month storage) or S3, or any SFTP server. Arq backs up... Read more

Latest Forum Discussions

See All

This Week at 148Apps: October 13-17, 201...
Expert App Reviewers   So little time and so very many apps. What’s a poor iPhone/iPad lover to do? Fortunately, 148Apps is here to give you the rundown on the latest and greatest releases. And we even have a tremendous back catalog of reviews; just... | Read more »
Angry Birds Transformers Review
Angry Birds Transformers Review By Jennifer Allen on October 20th, 2014 Our Rating: :: TRANSFORMED BIRDSUniversal App - Designed for iPhone and iPad Transformed in a way you wouldn’t expect, Angry Birds Transformers is a quite... | Read more »
GAMEVIL Announces the Upcoming Launch of...
GAMEVIL Announces the Upcoming Launch of Mark of the Dragon Posted by Jessica Fisher on October 20th, 2014 [ permalink ] Mark of the Dragon, by GAMEVIL, put | Read more »
Interview With the Angry Birds Transform...
Angry Birds Transformers recently transformed and rolled out worldwide. This run-and-gun title is a hit with young Transformers fans, but the ample references to classic Transformers fandom has also earned it a place in the hearts of long-time... | Read more »
Find Free Food on Campus with Ypay
Find Free Food on Campus with Ypay Posted by Jessica Fisher on October 20th, 2014 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
Strung Along Review
Strung Along Review By Jordan Minor on October 20th, 2014 Our Rating: :: GOT NO STRINGSUniversal App - Designed for iPhone and iPad A cool gimmick and a great art style keep Strung Along from completely falling apart.   | Read more »
P2P file transferring app Send Anywhere...
File sharing services like Dropbox have security issues. Email attachments can be problematic when it comes to sharing large files. USB dongles don’t fit into your phone. Send Anywhere, a peer-to-peer file transferring application, solves all of... | Read more »
Zero Age Review
Zero Age Review By Jordan Minor on October 20th, 2014 Our Rating: :: MORE THAN ZEROiPad Only App - Designed for the iPad With its mind-bending puzzles and spellbinding visuals, Zero Age has it all.   | Read more »
Hay Ewe Review
Hay Ewe Review By Campbell Bird on October 20th, 2014 Our Rating: :: SAVE YOUR SHEEPLEUniversal App - Designed for iPhone and iPad Pave the way for your flock in this line drawing puzzle game from the creators of Worms.   | Read more »
My Very Hungry Caterpillar (Education)
My Very Hungry Caterpillar 1.0.0 Device: iOS Universal Category: Education Price: $3.99, Version: 1.0.0 (iTunes) Description: Care for your very own Very Hungry Caterpillar! My Very Hungry Caterpillar will captivate you as he crawls... | Read more »

Price Scanner via MacPrices.net

2013 15-inch 2.0GHz Retina MacBook Pro availa...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
Updated iPad Prices
We’ve updated our iPad Air Price Tracker and our iPad mini Price Tracker with the latest information on prices and availability from Apple and other resellers, including the new iPad Air 2 and the... Read more
Apple Pay Available to Millions of Visa Cardh...
Visa Inc. brings secure, convenient payments to iPad Air 2 and iPad mini 3as well as iPhone 6 and 6 Plus. Starting October 20th, eligible Visa cardholders in the U.S. will be able to use Apple Pay,... Read more
Textkraft Pocket – the missing TextEdit for i...
infovole GmbH has announced the release and immediate availability of Textkraft Pocket 1.0, a professional text editor and note taking app for Apple’s iPhone. In March 2014 rumors were all about... Read more
C Spire to offer iPad Air 2 and iPad mini 3,...
C Spire on Friday announced that it will offer iPad Air 2 and iPad mini 3, both with Wi-Fi + Cellular, on its 4G+ LTE network in the coming weeks. C Spire will offer the new iPads with a range of... Read more
Belkin Announces Full Line of Keyboards and C...
Belkin International has unveiled a new lineup of keyboard cases and accessories for Apple’s newest iPads, featuring three QODE keyboards and a collection of thin, lightweight folios for both the... Read more
Verizon offers new iPad Air 2 preorders for $...
Verizon Wireless is accepting preorders for the new iPad Air 2, cellular models, for $100 off MSRP with a 2-year service agreement: - 16GB iPad Air 2 WiFi + Cellular: $529.99 - 64GB iPad Air 2 WiFi... Read more
Price drops on refurbished Mac minis, now ava...
The Apple Store has dropped prices on Apple Certified Refurbished previous-generation Mac minis, with models now available starting at $419. Apple’s one-year warranty is included with each mini, and... Read more
Apple refurbished 2014 MacBook Airs available...
The Apple Store has Apple Certified Refurbished 2014 MacBook Airs available for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is free.... Read more
Refurbished 2013 MacBook Pros available for u...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available starting at $929. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros (4GB RAM/... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.