bitrot: A shell script to detect changes in the audio component of ALAC files
by, 01-22-2014 at 09:05 PM (1528 Views)
One of the best things about FLAC is that it contains an internal checksum, so you can see if the audio portion of the file has been changed (i.e., corrupted in some way, or has suffered "bit rot"). You can change the tags embedded in the file all you want, and it won't alter this checksum. The only thing that will cause it to change is damage to the audio content of the file.
Wouldn't it be nice if ALAC had the same feature? It turns out the Apple OS X command-line utility can do this for lossless ALAC, aiff, and a few otehr types of files. In the case of AIFF, it can actually embed the checksum in the file itself, but it can't do that for ALAC files. However, using another OS X command-line utility, xattr, you can add this (or anything you please) to the resource fork associated with the ALAC file.
The following is a shell script that will do exactly this for you, for ALAC files. In the future, I might expand this to do it for aiff as well, or you could modify it easily enough. Anyway, here is the shell script on google code:
Download it, make it executable ( chmod a+x bitrot ) and stick it in your $PATH, and Bob's your uncle. (For compatibility with the example launchd plist file below, I suggest putting it into /usr/local/bin ). Read the shell script for more details. [I wrote it in zsh because it is better than bash. Try doing recursive globbing ( **/*.m4a ) in bash.]
I recommend running it as a background process manually when you want to create the checksums. Then you can run the actual checking process from a lauchd script or /etc/periodic however often suits you. It is set to log only changes it detects in ~/Library/Logs/bitrot No news (or log file) is good news. If you want a reality check, run bitrot -l . It goes fairly quickly. I haven't tested this on a system in which Spotlight has been deactivated, but it might not work properly on those. Let me know.
Example launchd plist file. You can name this local.bitrot.checker.plist and put it into your user's Library/LaunchAgents folder. Make sure you edit the WorkingDirectory string entry first, providing your own username, and the actual path to where your music files reside. This will check your files every sunday at 1 am. You may wish to adjust this according to your listening habits and degree of paranoia.
Code:<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0"> <dict> <key>Label</key> <string>local.bitrot.checker</string> <key>WorkingDirectory</key> <string>/Users/yourusername/Music/iTunes/iTunes Media/Music</string> <key>ProgramArguments</key> <array> <string>/usr/local/bin/bitrot</string> <string>-c</string> </array> <key>StartCalendarInterval</key> <dict> <key>Hour</key> <integer>1</integer> <key>Minute</key> <integer>3</integer> <key>Weekday</key> <integer>0</integer> </dict> </dict> </plist>