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

MacFamilyTree 7.3.3 - Create and explore...
MacFamilyTree gives genealogy a facelift: it's modern, interactive, incredibly fast, and easy to use. We're convinced that generations of chroniclers would have loved to trade in their genealogy... Read more
Skype 7.5.0.738 - Voice-over-internet ph...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
PushPal 3.0 - Mirror Android notificatio...
PushPal is a client for Pushbullet, which automatically shows you all of your phone's notifications right on your computer. This means you can see who's calling or read text messages even if your... Read more
Logic Pro X 10.1.1 - Music creation and...
Apple Logic Pro X is the most advanced version of Logic ever. Sophisticated new tools for professional songwriting, editing, and mixing are built around a modern interface that's designed to get... Read more
VLC Media Player 2.2.0 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Sound Studio 4.7.8 - Robust audio record...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
LibreOffice 4.4.1.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
VueScan 9.5.03 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Freeway Pro 7.0.3 - Drag-and-drop Web de...
Freeway Pro lets you build websites with speed and precision... without writing a line of code! With its user-oriented drag-and-drop interface, Freeway Pro helps you piece together the website of... Read more
Cloud 3.3.0 - 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

This Week at 148Apps: February 23-27, 20...
Final February Fun at 148Apps   How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you’re looking for. The ones we love become Editor’s Choice, standing out... | Read more »
GDC 2015 – Does Not Commute is Definitel...
GDC 2015 – Does Not Commute is Definitely a Game You Should Keep an Eye on Posted by Rob Rich on March 2nd, 2015 [ permalink ] We were teased about Mediocre Games’ (Smash Hit, | Read more »
F84 Games & POW! Announce Stan Lee V...
F84 Games has announced that it is working with legendary comic creator Stan Lee and POW! Entertainment to produce Stan Lee’s Hero Command. The game will be a action adventure of heroic proportions. | Read more »
Setlyst Keeps Your Set Straight So You C...
Setlyst Keeps Your Set Straight So You Can Focus On Rocking Out. Posted by Jessica Fisher on March 2nd, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Space is Vast, So Space Agency Has a Vas...
Space is Vast, So Space Agency Has a Vast New Update! Posted by Jessica Fisher on March 2nd, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Size DOES Matter Review
Size DOES Matter Review By Campbell Bird on March 2nd, 2015 Our Rating: :: HARD TO BEATUniversal App - Designed for iPhone and iPad This rhythm game has a unique control scheme and performance system that make it feel like a true... | Read more »
The first ever action 3D card battler Al...
On the other hand, you probably haven’t played an action 3D card battler – until now. Step forward, All Star Legion. All Star Legion is a 3D QTE-based action RPG card battler, but fear not – the game itself isn’t as convoluted as its description.... | Read more »
Travel Back to the 1980s With the Making...
Headup Games has released a hilarious making of video for its upcoming title, Pixel Heroes: Byte & Magic. The game is a RPG/Roguelike where you control three heroes set to save the township of Pixton from an evil cult called The Sons of Dawn.... | Read more »
Heavenstrike Rivals Review
Heavenstrike Rivals Review By Campbell Bird on March 2nd, 2015 Our Rating: :: HEAVENLY STRATEGICUniversal App - Designed for iPhone and iPad Despite a few flaws, this free-to-play strategy game is a fun mix of new and old strategy... | Read more »
Get The Whole Story – Lone Wolf Complete...
Get The Whole Story – Lone Wolf Complete is Now Available and On Sale Posted by Jessica Fisher on February 27th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »

Price Scanner via MacPrices.net

Sale! 15-inch 2.2GHz Retina MacBook Pro for $...
 Best Buy has the 15″ 2.2GHz Retina MacBook Pro on sale for $1774.99 $1799.99, or $225 off MSRP. Choose free home shipping or free local store pickup (if available). Price valid for online orders... Read more
27-inch 3.5GHz 5K iMac in stock today and on...
 B&H Photo has the 27″ 3.5GHz 5K iMac in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price available for... Read more
Apple Launches Free Web-Based Pages and Other...
Apple’s new Web-only access to iWork productivity apps is a free level of iCloud service available to anyone, including people who don’t own or use Apple devices. The service includes access to Apple... Read more
Survey Reveals Solid State Disk (SSD) Technol...
In a recent SSD technology use survey, Kroll Ontrack, a firm specializing in data recovery, found that while nearly 90 percent of respondents leverage the performance and reliability benefits of SSD... Read more
Save up to $600 with Apple refurbished Mac Pr...
The Apple Store is offering Apple Certified Refurbished Mac Pros for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
Apple CEO Tim Cook to Deliver 2015 George Was...
Apple CEO Tim Cook will deliver the George Washington University’s Commencement address to GWU grads on May 17, at which time he will also be awarded an honorary doctorate of public service from the... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
Save up to $50 on iPad Air 2s, NY tax only, f...
 B&H Photo has iPad Air 2s on sale for $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469.99 $30 off - 64GB iPad Air 2 WiFi: $549 $50 off - 128GB iPad Air 2... Read more
16GB iPad Air 2 on sale for $447, save $52
Walmart has the 16GB iPad Air 2 WiFi on sale for $446.99 on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale price for online orders only,... Read more

Jobs Board

*Apple* Pay Automation Engineer - iOS System...
**Job Summary** At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring passion and dedication to your job Read more
Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
Event Director, *Apple* Retail Marketing -...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global engagement strategy and team. Delivering an overarching brand Read more
*Apple* Pay - Site Reliability Engineer - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.