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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Cloud 4.1.1 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
OmniFocus 2.7.1 - GTD task manager with...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
CleanApp 5.1.1 - Application deinstaller...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
ForkLift 3.0 Beta 2 - Powerful file mana...
ForkLift is a powerful file manager and ferociously fast FTP client clothed in a clean and versatile UI that offers the combination of absolute simplicity and raw power expected from a well-executed... Read more
Sublime Text 3126 - Sophisticated text e...
Sublime Text is a sophisticated text editor for code, markup, and prose. You'll love the slick user interface, extraordinary features, and amazing performance. Features Goto Anything. Use Goto... Read more
1Password 6.3.3 - Powerful password mana...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
WhatsApp 0.2.1880 - Desktop client for W...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more
NeoFinder 6.9.3 - Catalog your external...
NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs all your data, so you stay in control of your data archive or disk... Read more
Amadeus Pro 2.3.1 - Multitrack sound rec...
Amadeus Pro lets you use your Mac computer for any audio-related task, such as live audio recording, digitizing tapes and records, converting between a variety of sound formats, etc. Thanks to its... Read more
Yasu 4.0.0 β - System maintenance app; p...
Yasu was 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 quickly within a... Read more

Our 5 Favorite iMessage Sticker Packs
At long last, iMessage joins the ranks of messaging apps the likes of LINE and Whatsapp, adding an impressive collection of stickers. They’re a great way to add a little something extra to your daily conversations. [Read more] | Read more »
How to get past Vulture Island's tr...
Vulture Island is a colorful and quirky mish-mash of platforming and puzzles. It’s creative and fresh, but sometimes the game can throw a curveball at you, leaving you stuck as to how you should progress. These tips will help you explore smoothly... | Read more »
The new Clash of Kings is just for Weste...
If you’ve played the original Clash of Kings, you’ll probably recognise the city building, alliance forging and strategic battles in Clash of Kings: The West. What sets this version apart is that it’s tailor made for a Western audience and the... | Read more »
Frost - Survival card game (Games)
Frost - Survival card game 1.12.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.12.1 (iTunes) Description: *Warning: the game will work on iPhone 5C and above and iPad Pro / 4. Other devices are not supported* | Read more »
How to build and care for your team in D...
Before you hit the trail and become a dog sledding legend, there’s actually a fair bit of prep work to be done. In Dog Sled Saga, you’re not only racing, you’re also building and caring for a team of furry friends. There’s a lot to consider—... | Read more »
How to win every race in Dog Sled Saga
If I had to guess, I’d say Dog Sled Saga is the most adorable racing game on the App Store right now. It’s a dog sled racing sim full of adorable, loyal puppies. Just look at those fluffy little tails wagging. Behind that cute, pixelated facade is... | Read more »
Let the war games commence in Gunship Ba...
Buzz Lightyear famously said, “This isn’t flying, this is falling – with style!” In the case of Gunship Battle: Second War, though, this really is flying - with style! The flight simulator app from Joycity puts you in control of 20 faithfully... | Read more »
How to get a high score in Fired Up
Fired Up is Noodlecake Games’ high score chasing, firefighting adventure. You take control of a wayward firefighter who propels himself up the side of a highrise with blasts of water. Sound silly? It is. It’s also pretty difficult. You can’t... | Read more »
NBA 2K17 (Games)
NBA 2K17 1.0 Device: iOS iPhone Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: Following the record-breaking launch of NBA 2K16, the NBA 2K franchise continues to stake its claim as the most authentic sports video... | Read more »
Dog Sled Saga (Games)
Dog Sled Saga 1.0.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.1 (iTunes) Description: A game by Dan + Lisa As a rookie musher, foster a dogsledding team whose skills will grow if they're treated right. Week by... | Read more »

Price Scanner via MacPrices.net

Toughbook Celebrates 20 Years of Ruggedized M...
Panasonic System Communications Company of North America, Division of Panasonic Corporation of North America (Panasonic) today celebrates the 20th anniversary of its industry-leading Toughbook mobile... Read more
13-inch 2.5GHz MacBook Pro (Apple refurbished...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros... Read more
Save 30% on Camera Guard’s Secure Protection...
To celebrate the release of macOS Sierra, Miami-based security solutions company, ProtectStar has announced a special 30% discount on Camera Guard Professional for Mac 2016. This innovative security... Read more
DVDFab Special Deal – Get a 1-Year Free Licen...
Beijing, China based specialist in the field of DVD, Blu-ray and video backup solutions, Fengtao Software has launched its Autumn Special Deals 2016, giving a 1-year free license of a randomly picked... Read more
21-inch iMacs on sale for up to $120 off MSRP
B&H Photo has 21″ iMacs on sale for up to $120 off MSRP including free shipping plus NY sales tax only: - 21″ 3.1GHz iMac 4K: $1379 $120 off MSRP - 21″ 2.8GHz iMac: $1199.99 $100 off MSRP - 21″ 1... Read more
13-inch 2.7GHz/256GB Retina MacBook Pro on sa...
Amazon.com has the 13″ 2.7GHz/256GB Retina Apple MacBook Pro on sale for $151 off MSRP including free shipping: - 13″ 2.7GHz/256GB Retina MacBook Pro (sku MF840LL/A): $1348 $151 off MSRP Read more
Apple TVs on sale for up to $50 off MSRP
Best Buy has 32GB and 64GB Apple TVs on sale for $40-$50 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store... Read more
Apple refurbished 13-inch Retina MacBook Pros...
Apple has Certified Refurbished 13″ Retina MacBook Pros available for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz... Read more
Duplicate Sweeper Free On Mac App Store For O...
To celebrate the launch of Apple’s latest macOS Sierra, Stafford, United Kingdom based Wide Angle Software has announced that its duplicate file finder software, Duplicate Sweeper, is now available... Read more
13-inch Retina MacBook Pros on sale for up to...
B&H Photo has 13″ Retina Apple MacBook Pros on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1174.99 $125 off MSRP - 13... Read more

Jobs Board

Sr. *Apple* Mac Engineer - Net2Source Inc....
…staffing, training and technology. We have following position open with our client. Sr. Apple Mac Engineer6+ Months CTH Start date : 19th Sept Travelling Job If Read more
*Apple* Retail - Multiple Positions-Norfolk,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Lead *Apple* Solutions Consultant - Apple (...
# Lead Apple Solutions Consultant Job Number: 51829230 Detroit, Michigan, United States Posted: Sep. 19, 2016 Weekly Hours: 40.00 **Job Summary** The Lead ASC is an Read more
US- *Apple* Store Leader Program - Apple (Un...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.