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On a number of occasions, I’ve noticed a file named Sentinel in the iTunes base directory. I did a Google search around a year ago and didn’t find anything explaining what it was and, after noticing it again recently, searched again to no avail. I decided to figure out what the file was by seeing what actions updated it (I noticed the last modified date was yesterday.) In this guide, I explain what the file is for those that are curious.
Behavior of the Sentinel File
I was pretty sure the Sentinel file was okay to have and didn’t pose any kind of threat, but I wanted to be sure.
To determine the purpose of the file (and why it was updated yesterday), I started changing things in iTunes to see what happened. Finally, I got the timestamp to change when I deleted a song from my library. This caused the iTunes Library.itl to change as well as the Sentinel file.
I then played some more and noticed it updating again. I ran some commands to see when the file was being accessed and by what processes and found it was being accessed when iTunes started. Then, I deleted the file, reopened iTunes and noticed it was created again.
Finally, I changed the date on the file (I figured I was on to something now and wanted to prove a point) and noticed it was recreated when I opened iTunes.
Conclusion
The sentinel file is a integrity file that is updated when the library is updated. iTunes uses this file to determine the integrity of the iTunes library (that the timestamps match.) If there is a difference in timestamps, iTunes will run an integrity check on the iTunes library. By using this method, the overhead of running this check every time is removed.
About Rich
Rich is the owner and creator of Windows Guides; he spends his time breaking things on his PC so he can write how-to guides to fix them.
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Enlighten us.
that seems logical to me. The name sentinel seems to imply that. I found another post that said it was something to do with “genius.”
http://discussions.apple.com/thread.jspa?messageID=8496105
Sidenote – I think your method to determine what the file is used for was sound.
In short, you’re wrong, sorry.
The sentinel file, contrary to mis-information here – https://discussions.apple.com/thread/1740158?start=15&tstart=0
Is actually a copy of the iTunes library persistent key, which is also stored in the ‘iTunes Library.itl’ and ‘iTunes Music Library.xml’ and is what iTunes & iPhone/iPod use to identify the sync’ed iTunes instance. There is more though.Prior to the arrival of the ‘sentinel’ file it was possible to patch those files and change their persistent library id, so you could sync your iPhone/iPod with different iTunes libraries, this was very handy for those of us who want to sync multiple contacts / calendars and be able to put tunes onto our iOS devices from multiple machines
Anyway, the sentinel file is actually checked against some other file (or perhaps a memory cache) however, my investigations on this have only been about 15 mins, so I don’t have that info at hand. (Google brought me here searching for more info.)
So that’s what it is… if I find out more, I’ll post it here.
Interesting. Please keep us updated.
For windows users, it’s possible to patch the persistent id with iTunes DB Cloner.
I don’t have further info on ‘sentinel’. I’ve read people mention that it’s held in ram and it can be edited there, however patching the file with iTunes and all it’s helper software closed down, and then rebooting, still doesn’t work.
I don’t know what iTunes DB Cloner is doing, so I can’t repeat the process on Mac os x, (I downgraded to iTunes v9.0 to patch it.)
More info if I find it.
Thanks, this was very helpful.
Wrong. Sentinel contains a null terminated hex string which is the same as the persistent library ID. The purpose, appears to be to prevent you from tweaking the ID to allow you to sync with multiple libraries… I see Jason below has also hit on this… I’ve been trying to change it, but it’s been proving to be difficult.