Lately, I've been getting annoyed at the state of my music and Audiobook collection. Each Audiobook can often be made up of hundreds (one has over a thousand) of small MP3s that allow me to easily skip through the book and also easily remember where I was up to.

But unfortunately, these small MP3s are not tagged and named correctly. Often, they are in correct order on the file system (alphabetically, by their filenames), but not by MP3 ID3 tag. This makes it a pain to play in my media player, especially on my iPod where there are practically no sorting functions.

I looked at various renaming and retagging solutions out on the web and after one of them completely scrambled one of my albums by putting the tag of each song on another song, I decided I needed something that just worked and was really flexible.

I always imagined how good it would be if I could just whip up a quick program to run through those thousand MP3s and name them correctly. So today I decided to create such a solution.

I wrote a small (~90 lines) console application in C++ called ID3CL (ID3 Command-Line) that uses the open source id3lib library to edit the ID3 tags of MP3 files. It takes in command-line arguments and retags a single MP3 file. Its command-line syntax is as follows: 

Usage: id3cl <mp3 filename> -set <fieldname> <value>
       [-set <field name> <value> [..]]

Fields: tracknum, artist, album, title, year
        comment, genre

You basically invoke it like this: id3cl mysong.mp3 -set artist "DJ DC" -set title "Foobar on rocks". That will set the artist and title of the mysong.mp3 file.

Of course, this one-file-retagged-per-program-execution solution doesn't seem like it'd help me with retagging over 1000 MP3s does it? That's where scripting comes in.

I've recently been going nuts over PowerShell, the newish scripting language from Microsoft which is out to get rid of batch files (yay!). Writing PowerShell scripts is kind of a cross between writing C# and writing Bash. Its got some odd things in it (like '"{0:2D}" -f 2' will format 2 to be 02) which can make it almost as incomprehensible as Bash, but most of the time its a pleasure to work with (like C# and unlike Bash).

So, by writing a script in PowerShell which invokes my little C++ app (ID3CL), I can write tiny programs that retag my MP3 files any way I want.

Here's a little PowerShell script that takes MP3 files from the folder that the script is run in (and any files in folders under that one as well (recursively)) and changes their track numbers so the first one is 1 and the second 2, and so on. 

$id3cl = "& 'D:\My Documents\Visual Studio 2005\Projects\ID3CL\release\id3cl.exe' "

$mp3s = Get-ChildItem * -Include *.mp3 -Recurse

$tracknum = 1

foreach ($mp3 in $mp3s)
{
    $cmdline = '"' + $mp3.FullName + '" -set tracknum ' + $tracknum
    Invoke-Expression ($id3cl + $cmdline)
    $tracknum++
    if ($tracknum -eq 256)
    {
        $tracknum = 1
    }
}

This script is useful when I've got a two CD album, and I've got each CD from the album in its own folder. Each CD is treated like its own album with tracks starting from 1 and going on. But the thing is, I don't want to treat the album as two albums, I want one album with in-order track numbers. So that script will take CD1 and set the track numbers from 1 to X and then take CD2 and set the track numbers from X + 1 to Y. All automatically.

So you can see the power of this little system I've created. Unfortunately, only a programmer would be able to make use of this, since you've got to write scripts to do anything useful. But that's what makes it so powerful.

ID3CL is definitely me-ware. It's not user-friendly. It'll do silly things like if you get it to change the tag on a file that doesn't exist, it'll create a music-less MP3 and put your tags on it silently with no error. I can't be bothered fixing such bugs because it works perfectly when you treat it nicely and give it exactly what it expects. This initially made me not want to put it online for you guys to use, but I think I will anyway. Soon™ :). But if it errors because you did something odd with it, you'll have to figure out its unhelpful error messages.

However, I think its worth it for the power it gives you to tag your MP3 collection.