I've been using Linux for a long time and I still am completely in the dark about how file permissions really work. With that in mind, does anyone have any books or thorough guides I could read to really understand things completely? I've done my fair share of sysadminning, so I know the easy stuff like making directories readable and writable, making files executable, and changing the owner of a file, but on sharing files across users, I'm lost.

Here's my main problem. I have a number of machines across which I intend to synchronize my music library. I've been using Unison for a while now and it's a great choice as I can easily run it over SSH on my local network which I just set up. Win-win.

Up until this point, I've been synchronizing computers using a 2TB external hard drive. (computer 1 unisons to HD, computer 2 unisons to HD, etc.) This is tedious at best, especially since I encrypted the drive, making it a huge hassle to hook it up to all of my machines and sync it. Anyway, the drive is running ext4 (in TrueCrypt), so it maintains all Unix filesystem info like owners and groups. I just set up a new machine and just Unison'd it to get the music on it, and I realized that now, all of my permissions are fubar. I had to run Unison as root since that was the only way I could get the files to come off of the external drive. Apparently, since I'm using a different user name on this machine than my usual "rfkrocktk" across all machines, this essentially throws a huge wrench in the gears.

Here's my use case. This laptop has two effective users, "leandra" and "rfkrocktk". I want to share music between these two users, so I symlinked /home/rfkrocktk/Music to point to /home/leandra/Music. How do I (a) allow both users access to read/write/delete files in this folder, and (b) keep everything nicely in sync without messing up file ownership?