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.
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.