TweetFollow Us on Twitter

Getting Started w Perl

Volume Number: 16 (2000)
Issue Number: 9
Column Tag: Tools of the Trade

Getting Started with Perl

By Larry Taylor, Edited by Steve & Patricia Sheets

Open Source power scripting for Macs

Introduction

Perl is a programming/scripting language developed under Unix, which is distributed under the GNU license and now runs on most platforms, including MacOS. It is the language of choice for Unix system administration, CGI scripts and other goodies. More relevantly, it can really expand your ability to accomplish things on the Mac. In this article I describe a frustrating problem I had and a step by step Perl solution. I hope this example will encourage you to learn Perl and use it. Perl scripts are just text files and so are fairly easily portable across platforms making Perl even more useful if you need to solve the same problem on several platforms. Learning Perl is not difficult and it looks great on your resume, so why not give it a try?

Mac + Perl = MacPerl

Perl arouse because many UNIX programmers wanted a quick alternative to C, with many of C's features. The result was a full-featured, easy to use, C-like programming language. Perl has been ported to the Mac where it can be used to create pseudo-applications called droplets. I call them pseudo because they do not have individual types and creators and so they must either be opened by double clicking or by dragging a document onto them. They are interpreted and so need the Perl interpreter in order to run. No Mac interface is needed to get information in or out, so Perl is ideal for projects that involve reading some data, analyzing it, and outputting some conclusions, projects for which the event-loop paradigm is more of an annoyance then a help (although Cmd-period will stop runaway Perl droplets). One can construct compiled applications with a full Mac interface, but the files are large and the advantages over C largely evaporate. I use Perl for tasks as varied as extracting data from files to emailing students in a class their exam scores.

Perl is "open source" software. The interpreter is available to download for free at <http://www.iis.ee.ethz.ch/~neeri/macintosh/perl.html>, or the book "MacPerl, Power and Ease" by Vicki Brown and Chris Nandor (#1-881957-32-2) from Prime Time Freeware <http://www.ptf.com> contains a CD with the interpreter and lots of other useful stuff. The book itself is a nice introduction to programming in general and Perl in particular. Additional Perl stuff can be gleaned from the net. Try starting at <http://www.perl.com>.

The Problem

Got one of those cool digital cameras that saves images to floppies? Then you know the files are labeled automatically, MVC-01L.JPG, MVC-02L.JPG, etc. Copy the images to your computer and you're in business. But suppose you went wild and filled up two disks? Or ten? Files on different floppies often have the same name, so you can't just copy them to the same folder. So you copy one floppy, change all the names of the files, copy the second, etc. - bummer. Even with just a few images, you tend to put them in a folder with a useful name since otherwise you won't remember what the pictures are about, can't search for them with Sherlock, etc. Wouldn't it be nice to have them named, whatever1.jpg, whatever2.jpg, etc.? This is a perfect job for a script.

The script should begin with a folder named whatever and look inside it for all the MVC files and rename them as whatever1.jpg, whatever2.jpg, etc. It should even be a bit smarter. If there are going to be ten or more, the first should be whatever01.jpg: if there are 100 or more, whatever001.jpg, if there are ... but you get the idea. Even more, if there are already some whatever files, it should number the new MVC files to fit into the pattern. Specifically, it should look at the creation time of the first MVC file and the first whatever file. If the MVC time is later, the MVC files should come after the whatever files, but otherwise the whatever files should be renamed and the MVC files should come first. If the user trashed a few of the whatever files so they are no longer in sequence, the whatever files should be renamed so as to be in sequence.

Using this script, you can copy one disk worth of images into a folder, run the script, copy the next disk, run the script, etc. At any time during the process, the images can be viewed and those that are unwanted can be deleted.. At the end, all of the "keepers" are named consecutively in the order in which they were taken, no matter the order in which they are copied or removed.

The Script

Open the MacPerl application and select New from the File menu and you're ready to start. Line 1 should be #!perl. This is a holdover from the Unix world where this line tells the operating system to feed this file to the Perl interpreter. You can also do things with it in MacPerl, but we don't here. Now save the file. Name it what you will. At the bottom of the dialog box is a pop up menu labeled "Type:" (reading "Plain Text"). Set the menu to "Droplet" and save.

The advantage of a droplet is that you can just drop items onto its icon and the information is passed on to the script. In this script we include no other way to input folder/file information, although Perl can do so, even through standard file. The folder/file information is passed to the script as $ARGV[0]for the first folder/file, $ARGV[1] for the second, etc. Droplets allow us to use the Mac GUI to mimic the command line paradigm. Dropping a collection of files on a droplet has the same effect as the command line, droplet_name file1 file2 ...

Before discussing the code, here is an outline for solving the problem.

  • Step 1: Get the folder name. If a folder is dropped, use it; if a file is dropped, use the enclosing folder. If several items are dropped, process them all.
  • Step 2: Collect the names of the MVC files and the whatever files.
  • Step 3: Get the two creation times and figure out the starting numbers for the two sets of files.
  • Step 4: Rename the files.

We do a certain amount of error checking and quit at the first sign of trouble - these may be your only photos of Aunt Rose. Perl borrows much from C, including the tendency to write short functions (subroutines in Perl). One immediate difference is the lack of variable typing (the same variable can be a number or a string, depending on context). Another is the ability to work with arrays whose size is unknown before execution, As a language, Perl is particularly adept at manipulating arrays and strings and it does file management rather well.

Now for the code. We write a sequence of subroutines most of which just do one of the steps outlined above and pass the relevant data on to the next. We try to introduce some interesting features of Perl in discussing each subroutine. More information can be gleaned from the code and its comments. Here is the first routine. The for loop works its way through the dropped items, passing each one in turn to the subroutine do_a_folder which returns false if anything goes wrong. Ordinary Perl variable names start with $; arrays start with @; $#foo is the last index of the array @foo. As with C, the first array element is $foo[0]. If this were C the braces would be optional, but in Perl they are required.

for($ii=0;$ii<=$#ARGV; $ii++) {   # This is a Perl comment.
   if(!do_a_folder($ARGV[$ii])){exit;}
   }

Perl handles file system objects via path names and the $ARGV variables are path names. The first line of the subroutine illustrates the way Perl passes variables to subroutines: the values are in a list/stack named @_ and we can shift them off in order. The rest of the routine is straightforward. Perl has a simple syntax for checking if strings are folders or files, using two simple "if" tests. One wrinkle here is that if you drop two MVC files on the droplet, by the time the second one is ready to be processed, it no longer exists since it was renamed on the first pass. The routine does nothing in this case except return true, which is what we want. In short, this subroutine handles Step 1 for each dropped object and passes the results to the next subroutine.

sub do_a_folder{
$object=shift(@_);      
if( -f $object) {   # -f checks if $object is a file, 
                           # if it is, get enclosing folder.
   $x=rindex($object,':');   # find LAST occurrence of :
   $object=substr($object,0,$x);   # remove last part of
                                             # path name
   }
# $object now path name to folder
$x=rindex($object,':');   # find LAST occurrence of :
$fold_name=substr($object,$x+1);   # get name of folder
if( -d $object) {   # it's a folder
   unlink("$object:MAVICA.HTM");   
      # This deletes a junk file which often gets copied.
   return process_folder($object,$fold_name);
   }
   # else quietly do nothing.
return 1;
}


Extract the relevant files into two arrays. There is no need to specify the size of these arrays in advance since Perl handles these details. The undef's make sure that these arrays are empty at the start. Explicitly initializing variables is usually a good idea. One outstanding feature of Perl is Unix regular expression matching and substitution. Look how easy it is to find the files we want:

if( $files[$i]=~m/^$fold_name\d*\.jpg$/)

This is true if the string on the left contains the expression between the /'s. That expression says the string must begin (^) with $fold_name, have any number of digits (\d*) and then end ($) with a .jpg. The dot is \. because . means match any character. When we find a file of the desired type, the push puts it at the end of the appropriate array. Note that the elseif of C becomes elsif. Finally, the construction \@mvc_files is a way to pass a reference to the entire array to the next subroutine.

sub process_folder{
$fold=shift(@_);
$fold_name=shift(@_);
# Make sure names can't be too long for the Finder.
$fold_name=substr($fold_name,0,23);
undef(@fold_name_files);   # Clear old values
undef(@fold_name_files);   # Clear old values
if( opendir(DIR,$fold)) {   # if we can read the directory
   chdir($fold); # change the working directory
   @files=readdir(DIR);   # read all objects into an array
   closedir(DIR);   # close the directory for reading
   for($i=0;$i<=$#files;$i++) {
      if( $files[$i]=~m/^$fold_name\d*\.jpg$/){
            # remember the folder_name files
         push(@fold_name_files,$files[$i]);
         }
      elsif( $files[$i]=~m/^MVC-\d*L\.JPG$/) {   
            # remember the MVC files
         push(@mvc_files,$files[$i]);
         }
      }   
   if($#mvc_files<0 && $#fold_name_files<0) {
      return 1; # Nothing to do.
      }
   else {   # Go rename the files.
   return ( 
      setup_rename(\@mvc_files,\@fold_name_files,$fold_name));
      }
   }
else { print"Failed to open $fold\n"; return 0;}   
}

In the first few lines of the next subroutine, we retrieve the reference to the arrays. The syntax is straightforward: in the previous subroutine @mvc_files was an array: in this subroutine the same array is @$mvc_files. There is no need to use the same name.

Now look at the phrase:

length($#$fold_name_files+$#$mvc_files+1+$startNumber) 

This is an example of how variable type changes: $#$fold_name_files is one less than the number of files in the array @$fold_name_files so the sum is the biggest number in a file name. The function length treats the number as a string and returns its length. If we have more than 9,999 files, we quit since then the file names might be longer than the Finder limit of 31 characters.

Perl has built-in functions to easily extract file information. We have no trouble getting creation times: the function stat returns an array of data and the eleventh element in the array is the creation time. Remember, the first is [0]. We then use this information to determine the starting number for the two sets of file names. This completes Step 3 and we pass the needed information on to the next subroutine.

sub setup_rename{
$mvc_files=shift(@_);
$fold_name_files=shift(@_);
$fold_name=shift(@_);
$startNumber=1;   # The first file is numbered 1.
#
$new_digit_size=length(
         $#$fold_name_files+$#$mvc_files+1+$startNumber);
if($new_digit_size>4){
   print"More than 9,999 files? No way!\n";
   return 1;   # Will process other folders 
   }
#
# Get MVC creation time (if possible).
if( ($#$fold_name_files>=0) ) {
   $time_MVC=(stat($$mvc_files[0]))[10];
   }   
# Get folder_name creation time (if possible).
if($#$fold_name_files>=0) {
   $time_FN=(stat($$fold_name_files[0]))[10];
   }
# Calculate starting numbers.
if($#$mvc_files<0) { $fold_name_startNumber=$startNumber;}
elsif($#$fold_name_files<0) {$mvc_startNumber=$startNumber;}
elsif($time_MVC<$time_FN) {
   $mvc_startNumber=$startNumber;
   $fold_name_startNumber=$#$mvc_files+1+$startNumber;
   }
else {
   $mvc_startNumber=$#$fold_name_files+1+$startNumber;
   $fold_name_startNumber=$startNumber;
   }
return rename_files($mvc_files,$mvc_startNumber,
      $fold_name_files,$fold_name_startNumber,
      $fold_name,$new_digit_size);
}

The rename routine (Step 4) is a bit more complicated. The Perl rename routine is a Unix style routine, so if there already is a file with the new name, the old file is destroyed without warning. The Mac solution is better, but annoying - put up a dialog box and let the user recover. But you don't want dialog boxes, you just want the files renamed. The solution we use is to create a temporary folder, move the files into this folder as we rename them, move them back when we are done, and finally, delete the temporary folder. We put this temporary folder in our enclosing folder so that in the event of an error it should be easy to find all your files.

Here we introduce another way to collect the information passed as the arguments: make a list on the left and set it equal to @_. The mkdir, rmdir functions betray their Unix heritage. Subroutines move the files into the temporary folder and out of it again.

sub rename_files{
# Make temporary folder - the name will be a number
$dir=0;
while( -d $dir || -f $dir ) {$dir++;}
   # Possible infinite loop - but need thousands of 
   # folders/files with numbers as names.Don't worry.
if(!mkdir($dir,0777)) {
   print"Failed to make temporary folder.\n";
   return 0;
   }
($filesA,$startA,$filesB,$startB,$prefix,$digit_size)=@_;
$dir_prefix=":$dir:$prefix";
# Move the first batch of files, then the second.
# Bail if error. 
if(!mv_tmp($startA,$filesA,$dir_prefix,$digit_size)){
   return 0;
   }
if(!mv_tmp($startB,$filesB,$dir_prefix,$digit_size)){
   return 0;
   }
# move the files back. Bail if error.
if(!mv_back($dir)){return 0;}
# Delete the temporary directory
return rmdir($dir);
}

Nothing much new in the next subroutine except the foreach loop. This works through the array setting $h to the values of the array in order - no need for an index variable. This is not earthshaking, but elegant. The s routine completes the script.

sub mv_tmp{
($first,$list,$dir_prefix,$digitSize)=@_;
foreach $h (@$list) {
   $numStr=substr("00000",0,$digitSize-length($first)).$first;
   if(!rename($h,"$dir_prefix$numStr.jpg") ){
      print"Failed to move $h into $dir\n";
      return 0;
      }
      $first++;
   }
return 1;
}

sub mv_back{
$dir=shift(@_);
if(opendir(DIR,$dir) ){
   @files=readdir(DIR);   # read all objects into an array
   closedir(DIR);   # close the directory for reading
   chdir($dir);
   foreach $h (@files) {
      if(!rename($h,"::$h") ){
         print"Failed to move $h out of $dir\n";
         return 0;
         }
      }
   chdir("::");
   return 1;
   }
else {return 0;}
}



Final Comments

The constructions, syntax and built-in functions discussed in this short article have barely scratched the surface of what is available. And more is coming every day. See <http://www.perl.com> and related links. I hope this example will spark your interest in using Perl for your own projects. Happy scripting.


Larry Taylor is a research mathematician and professor who spends too much time fooling around with this sort of thing. More stuff at http://www.nd.edu/~taylor.

 
AAPL
$116.57
Apple Inc.
+0.26
MSFT
$48.70
Microsoft Corpora
+0.00
GOOG
$538.89
Google Inc.
+4.06

MacTech Search:
Community Search:

Software Updates via MacUpdate

Cyberduck 4.6 - FTP and SFTP browser. (F...
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
Evernote 6.0.1 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
calibre 2.11 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Herald 5.0.1 - Notification plugin for M...
Note: Versions 2.1.3 (for OS X 10.7), 3.0.6 (for OS X 10.8), and 4.0.8 (for OS X 10.9) are no longer supported by the developer. Herald is a notification plugin for Mail.app, Apple's Mac OS X email... Read more
Firetask 3.7 - Innovative task managemen...
Firetask uniquely combines the advantages of classical priority-and-due-date-based task management with GTD. Stay focused and on top of your commitments - Firetask's "Today" view shows all relevant... Read more
TechTool Pro 7.0.6 - 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
PhotoDesk 3.0.1 - Instagram client for p...
PhotoDesk lets you view, like, comment, and download Instagram pictures/videos! (NO Uploads! / Image Posting! Instagram forbids that! AND you *need* an *existing* Instagram account). But you can do... Read more
SuperDuper! 2.7.3 - Advanced disk clonin...
SuperDuper! is an advanced, yet easy to use disk copying program. It can, of course, make a straight copy, or "clone" -- useful when you want to move all your data from one machine to another, or do... Read more
MacJournal 6.1.5 - Create, maintain, and...
MacJournal is the world's most popular journaling software for the Mac. MacJournal 6 adds a calendar mode that show entries from any journal, geolocation, word count, and progress tracking, as well... Read more

Latest Forum Discussions

See All

Quest for Revenge (Games)
Quest for Revenge 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: The great Kingdom of the west has fallen. The gods ignore the prayers of the desperate. A dark warlord has extinguished... | Read more »
Threadz is a New Writing Adventure for Y...
Threadz is a New Writing Adventure for You and Your Friends Posted by Jessica Fisher on November 21st, 2014 [ permalink ] In the tradition of round-robin storytelling, | Read more »
SteelSeries Stratus XL Hardware Review
Made by: SteelSeries Price: $59.99 Hardware/iOS Integration Rating: 4 out of 5 stars Usability Rating: 4.5 out of 5 stars Reuse Value Rating: 4.25 out of 5 stars Build Quality Rating: 4.5 out of 5 stars Overall Rating: 4.31 out of 5 stars | Read more »
ACDSee (Photography)
ACDSee 1.0.0 Device: iOS iPhone Category: Photography Price: $1.99, Version: 1.0.0 (iTunes) Description: Capture, perfect, and share your photos with ACDSee. The ACDSee iPhone app combines an innovative camera, a powerful photo... | Read more »
ProTube for YouTube (Entertainment)
ProTube for YouTube 2.0.2 Device: iOS Universal Category: Entertainment Price: $1.99, Version: 2.0.2 (iTunes) Description: ProTube is the ultimate, fully featured YouTube app. With it's highly polished design, ProTube offers ad-free... | Read more »
Weather or Not - Reports and Forecasts...
Weather or Not - Reports and Forecasts for your Calendar 1.0.0 Device: iOS iPhone Category: Weather Price: $2.99, Version: 1.0.0 (iTunes) Description: Weather or Not is a beautiful and intuitive way to check the weather and... | Read more »
Sago Mini Road Trip (Education)
Sago Mini Road Trip 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: Go for a fun-filled drive with Jinja the cat. Pick a destination, select a vehicle and hit the road. What will Jinja... | Read more »
New Tower Defense Game, Kingdom Rush: Or...
New Tower Defense Game, Kingdom Rush: Origins, is Available Today Posted by Jessica Fisher on November 20th, 2014 [ permalink ] iPad Only App - Designed for the iPad | Read more »
Sunburn! (Games)
Sunburn! 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: Your ship is gone. Your crew is scattered. One option remains. Gather your crew... and jump into the sun. Reunite your... | Read more »
Tapventures Review
Tapventures Review By Jennifer Allen on November 20th, 2014 Our Rating: :: ODDLY COMPELLINGUniversal App - Designed for iPhone and iPad Tapventures is an increasingly hands-off one-tap RPG, but expect it to hook you despite your... | Read more »

Price Scanner via MacPrices.net

64GB iPod touch on sale for $249, save $50
Best Buy has the 64GB iPod touch on sale for $249 on their online store for a limited time. Their price is $50 off MSRP. Choose free shipping or free local store pickup (if available). Sale price for... Read more
15″ 2.2GHz Retina MacBook Pro on sale for $17...
 B&H Photo has the 2014 15″ 2.2GHz Retina MacBook Pro on sale for $1799.99 for a limited time. Shipping is free, and B&H charges NY sales tax only. B&H will also include free copies of... Read more
New Logitech AnyAngle Case/Stand Brings Flexi...
Logitec has announced the newest addition to its suite of tablet products — the Logitech AnyAngle. A protective case with an any-angle stand for iPad Air 2 and all iPad mini models, AnyAngle is the... Read more
2013 15-inch 2.0GHz Retina MacBook Pro availa...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros available for $1499 including free shipping plus NY sales tax only. Their price is $500 off original MSRP. B&H will... Read more
16GB Retina iPad mini on sale today for $199,...
 Staples has 2nd generation 16GB Retina iPad minis on sale for $199 on their online store for a limited time. Their price is $100 off MSRP. Choose free shipping or free local store pickup (if... Read more
Developers Start Designing Apps for Apple Wat...
Apple has announced the availability of WatchKit, software that gives developers a set of tools to easily create experiences designed specifically for Apple Watch. Apple’s developer community can now... Read more
C Spire Launches iPad Air 2 and iPad Mini 3 o...
C Spire has announced that iPad Air 2 with Wi-Fi + Cellular and iPad mini 3 with Wi-Fi + Cellular are now available on its 4G LTE network. C Spire offers both new iPads with a range of data plans... Read more
Are You On Your Last PC? – The ‘Book Mystique
Will your current PC be your last? Quite possibly so if you define “personal computer” as a traditional desktop or laptop form factor machine according to some commentators. So then, the upshot that... Read more
Save up to $180 on MacBook Airs with Apple re...
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
16GB iPad mini available for $219, save $30
Walmart has 16GB iPad minis (1st generation) available for $219 on their online store. Their price is $30 off MSRP. Choose free shipping or free store pickup (if available). Price for online orders... Read more

Jobs Board

*Apple* Solutions Consultant (ASC)- Retail S...
**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
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Store Leader Program - College Gradu...
Job Description: Job Summary As an Apple Store Leader Program agent, you can continue your education as you major in the art of leadership at the Apple Store. You'll Read more
*Apple* Retail - Multiple Positions (US) - A...
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
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.