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

Civilization VI 1.0.1 - Next iteration o...
Sid Meier’s Civilization VI is the next entry in the popular Civilization franchise. Originally created by legendary game designer Sid Meier, Civilization is a strategy game in which you attempt to... Read more
Chromium 55.0.2883.75 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 55.0.2883.75: Security fixes: High CVE-2016... Read more
Google Chrome 55.0.2883.75 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more
VirtualBox 5.1.10 - x86 virtualization s...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
Pixa 1.1.9 - Quickly and easily organize...
Pixa is an image-organizing application. The new app functions well, is easy to use, and helps people organize their images quickly and easily on their computers. For those who prefer not to use the... Read more
VirtualBox 5.1.10 - x86 virtualization s...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
Pixa 1.1.9 - Quickly and easily organize...
Pixa is an image-organizing application. The new app functions well, is easy to use, and helps people organize their images quickly and easily on their computers. For those who prefer not to use the... Read more
Civilization VI 1.0.1 - Next iteration o...
Sid Meier’s Civilization VI is the next entry in the popular Civilization franchise. Originally created by legendary game designer Sid Meier, Civilization is a strategy game in which you attempt to... Read more
Google Chrome 55.0.2883.75 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more
Chromium 55.0.2883.75 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 55.0.2883.75: Security fixes: High CVE-2016... Read more

Latest Forum Discussions

See All

Amateur Surgeon 4 Guide: Become the worl...
It's time to wield your trusty pizza cutter again, as Amateur Surgeon has returned with a whole fresh set of challenges (and some old, familiar ones, too). Starting anew isn't easy, especially when all you have at your disposal is a lighter, the... | Read more »
Le Parker: Sous Chef Extraordinaire (Ga...
Le Parker: Sous Chef Extraordinaire 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Telltale Games really is working on a Gu...
Telltale Games' next episodic adventure is indeed Guardians of the Galaxy. A document tied to the voice actors strike suggested that the project was in the work, but now we have direct confirmation following an announcement at the Game Awards that... | Read more »
Amateur Surgeon returns to iOS and Andro...
Amateur Surgeon and its two sequels disappeared from the App Store some time and it was sad days for all. But now, just in time for the holidays, the Adult Swim favorite makes its joyous return in the shape of Amateur Surgeon 4, a remake with... | Read more »
The best board games on mobile
Sometimes you need to ditch all of the high speed, high action games in favor of something a little more traditional. If you don't feel like parting ways from your mobile device, though, there are still plenty of ways to get that old-school fix.... | Read more »
The best Facebook Messenger Instant Game...
Facebook's new Instant Games is now here, meaning you can play games with your friends directly via Facebook. It's a fun new way to connect with friends, of course, but it's also proving to be a solid gaming experience in its own right, with a... | Read more »
You can now play game's on Facebook...
Facebook launched its new Instant Games platform in an exciting new attempt to engage its user base. As a result, you can now play a number of different games directly through Facebook Messenger. All of these games run with HTML5, meaning you play... | Read more »
Apollo Justice Ace Attorney (Games)
Apollo Justice Ace Attorney 1.00.00 Device: iOS Universal Category: Games Price: $.99, Version: 1.00.00 (iTunes) Description: Court Is Back In Session Star as rookie defense attorney, Apollo Justice, as he visits crime scenes,... | Read more »
KORG iWAVESTATION (Music)
KORG iWAVESTATION 1.0 Device: iOS Universal Category: Music Price: $19.99, Version: 1.0 (iTunes) Description: A revolutionary new world of sound.The Wave Sequence Synthesizer for iPad - KORG iWAVESTATION | Read more »
Don't Grind Guide: Tips for becomin...
Don’t Grind is a surprising, derpy little one touch game with fun hand-drawn graphics. The goal is simple -- get the high score without being chopped to bits. That can be tough when you’re not used to the game, and that’s compounded by the fact... | Read more »

Price Scanner via MacPrices.net

13-inch Silver Touch Bar MacBook Pro in stock...
Amazon has the new 2016 13″ 2.9GHz/256GB Silver Touch Bar MacBook Pro (MLVP2LL/A) in stock today and on sale for $1749 including free shipping. That’s $50 off MSRP, and it’s the lowest price... Read more
Parallels Toolbox 1.3 for Mac Offers 25 Singl...
Parallels has launched Parallels Toolbox 1.3 for Mac, an upgrade that adds five new utilities to the stand-alone application which was released in August and is available exclusively online at http... Read more
OWC Mercury Elite Pro Dual mini Ultra-Portabl...
OWC has introduced the new OWC Mercury Elite Pro Dual mini, a powerful yet ultra-portable dual-drive RAID solution. The new Mercury Elite Pro Dual mini packs phenomenal performance into a small... Read more
Clearance 13-inch Retina MacBook Pros availab...
B&H Photo has clearance 2015 13″ Retina Apple MacBook Pros available for up to $200 off original MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $... Read more
Roundup of 2016 13-inch 2.0GHz MacBook Pro sa...
B&H has the non-Touch Bar 13″ MacBook Pros in stock today for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray (MLL42LL/A): $1449 $... Read more
New 13-inch 2.0GHz Space Gray MacBook Pro in...
Adorama has the new 13″ 2.0GHz Space Gray MacBook Pro (non-Touch Bar, MLL42LL/A) in stock for $1499 including a free 3-year AppleCare Protection Plan. Shipping is free, and Adorama charges sales tax... Read more
Finnair Adopts iOS Enterprise iPad Apps from...
Finnair and IBM have announced a first-of-its-kind agreement to utilize iOS enterprise apps from IBM to support the airline’s overall digital transformation. Finnair is focused on Asia-Europe traffic... Read more
Tech21 Launches Evo Go iPhone 7 Case Availabl...
Tech21 has announced the launch of the Evo Go case for Apple iPhone 7 and iPhone 7 Plus, exclusively at T-Mobile. Available online and at participating T-Mobile stores nationwide, Evo Go cases start... Read more
Apple Turns (RED) with More Ways to Join the...
In recognition of World AIDS Day, Apple is offering more ways than ever for customers to join (RED) in its mission to create an AIDS-free generation. Apple is the worlds largest corporate contributor... Read more
Deals on new 15-inch Touch Bar MacBook Pros,...
B&H Photo has new 2016 Apple 15″ Touch Bar MacBook Pro models in stock today with some available for $50 off MSRP, each including free shipping plus NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more

Jobs Board

*Apple* Brand Ambassador (Macy's) - The...
…(T-ROC), is proud of its unprecedented relationship with our partner and client, APPLE ,in bringing amazing" APPLE ADVOCATES"to "non" Apple store locations. 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
*Apple* Retail - Multiple Positions- White P...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Automotive Detailer - *Apple* Used Autos -...
We are currently conductinginterviews and will be accepting applications for a part-time detailer. Apple Used Autos is a great place to work andstart a career. We Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.