Takeaway: CPAN comes with an MP3::Tag module, which provides ready-made tools to read and extract metadata from MP3 files, making it a simple matter to identify the title, artist, and genre of a particular MP3 track. Here is some example code showing how you can use the module to create a categorizing application.
This article is also available as a TechRepublic download, including the code samples in a more manageable text file format.
If you're like most people, you probably have a bunch of MP3s scattered around your hard disk. And if you're like most people, you probably also have on your to-do list a plan to inspect and catalog them so that you know exactly what you're listening to. It's just that you haven't gotten around to it yet.
Well, with a little help from Perl, it's possible for you to finally cross that item off your list. CPANMP3::Tag module, which provides ready-made tools to read and extract metadata from MP3 files, making it a simple matter to identify the title, artist, and genre of a particular MP3 track. This can then be used with Perl's file functions to efficiently (and automatically) build an index of all your MP3 content. And if you'd like to, you can even edit the file metadata with built-in module functions. comes with an
This article will discuss both functions, showing you how to use the MP3::Tag module to read and write MP3 file metadata. It assumes that you have a working Perl installation with the MP3::Tag module installed; if you don't have this module, you can download it from CPAN.
Reading MP3 tags
Let's start with the basics: reading ID3 tags embedded in MP3 files. Listing A is a simple example, which demonstrates how this may be done with MP3::Tag:
Listing A
#!/usr/bin/perl
use MP3::Tag;
$mp3 = MP3::Tag->new('track1.mp3'); # create object
$mp3->get_tags(); # read tags
if (exists $mp3->{ID3v1}) { # print track information
print "Filename: $filename\n";
print "Artist: " . $mp3->{ID3v1}->artist . "\n";
print "Title: " . $mp3->{ID3v1}->title . "\n";
print "Album: " . $mp3->{ID3v1}->album . "\n";
print "Year: " . $mp3->{ID3v1}->year . "\n";
print "Genre: " . $mp3->{ID3v1}->genre . "\n";
}
$mp3->close(); # destroy object Nothing too complicated here. First, a new MP3::Tag object is created, with the filename and location of the MP3 file passed to the object constructor as an argument. Next, the object's get_tags() method is used to read the metadata embedded in the file headers and represent it as object properties. These properties can then be accessed and printed in the normal way. Here's a sample of the output:
Filename: track1.mp3
Artist: The Bungers
Title: Techno #1
Album: Bungabom
Year: 2005
Genre: Rancid Trance This capability makes it particularly easy to do what I promised I'd show you at the beginning of this article--create a printable catalog of all your music files. All you need to do is place the code above in a loop, run it on all your *.mp3 files, and format the output for easy readability. Listing B shows you how.
Listing B
#!/usr/bin/perl
use MP3::Tag; # import module
@files = <*.mp3>; # find MP3 files in current directory
# loop over file list
# print tag information
foreach (@files) {
$mp3 = MP3::Tag->new($_);
$mp3->get_tags();
if (exists $mp3->{ID3v1}) {
print $_, "\t", $mp3->{ID3v1}->artist, "\t", $mp3->{ID3v1}->title, "\n";
}
$mp3->close();
} In this case, the list of MP3 files in the current directory is stored in the @files array. A foreachget_tags() method to retrieve and print detailed metadata for each file. loop then iterates over this array, using the
And there you have it--an automatically generated MP3 catalog! As you add new music files to your collection, simply rerun the script above, and they will automatically show up in the catalog listing. Isn't that neat?
Writing MP3 tags
Of course, it doesn't just stop there--you can just as easily use MP3::Tag to write new metadata to an MP3 file. Listing C is an example of how you might do this.
Listing C
#!/usr/bin/perl
use MP3::Tag;
$mp3 = MP3::Tag->new('track1.mp3'); # create object
$mp3->get_tags(); # read tags
if (exists $mp3->{ID3v1}) { # save track information
$mp3->{ID3v1}->title("Techno #3");
$mp3->{ID3v1}->artist("The Bungers");
$mp3->{ID3v1}->album("Bing Bong");
$mp3->{ID3v1}->year("2006");
$mp3->{ID3v1}->write_tag();
}
$mp3->close(); # destroy object If you look closely, you'll see some similarities between this and the previous scripts. As before, the first step is to initialize an object of the MP3::Tag class by passing the object constructor the name of the MP3 file you wish to alter. The get_tags() method is then used to retrieve the current file metadata. Altering this metadata becomes as simple as assigning new values to the appropriate object properties and then saving the new values to the file with a call to the write_tag() method.
Of course, in the real world, it's highly likely that you won't be hard-wiring metadata into your script. Instead, you're more likely to need an interactive application, one that prompts the user for artist, track, and title information and then writes this data to the MP3 file. Luckily, this application is easy to build, knowing what you know now about MP3::Tag. Take a look at Listing D.
Listing D
#!/usr/bin/perl
use MP3::Tag;
$filename = shift; # get filename from command line
$mp3 = MP3::Tag->new($filename);
$mp3->get_tags(); # read tags
print "Enter track title: "; # prompt for title
chomp ($title = <>);
print "Enter artist: "; # prompt for artist
chomp ($artist = <>);
print "Enter album: "; # prompt for album name
chomp ($album = <>);
print "Enter year: "; # prompt for year
chomp ($year = <>);
if (exists $mp3->{ID3v1}) { # save track information
$mp3->{ID3v1}->title($title);
$mp3->{ID3v1}->artist($artist);
$mp3->{ID3v1}->album($album);
$mp3->{ID3v1}->year($year);
$mp3->{ID3v1}->write_tag();
}
$mp3->close(); # destroy object When invoked on the command line, this script expects to be passed the name of the MP3 file to be edited. It then instantiates a new MP3::Tag object for this file and retrieves available tag information. Next, a series of prompts is generated, for the user to interactively enter title, artist, album, and year information. This data is then written back to the MP3 file via the write_tag() method discussed previously.
And that's about it. The scripts above should have given you some idea of what you can do with MP3::Tag and perhaps even helped you organize your music collection. Until next time...happy coding!
