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In terminal, the open -n ~/Applications/iTunes doesn't create a new instance of iTunes, just switches to the one already open. Why?

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iTunes uses a database file to maintain a track of all the meta data on the contents of the iTunes library. Opening multiple instances of iTunes would potentially result in the 2nd instance being unable to gain the correct access to these files as they would be locked by the first instance.

OK, you can have multiple iTunes libraries (hold Option when starting to choose or create a new one), but this is a relatively edge case scenario, and even for users of this functionality, I cannot see many instances where it would be useful to have more than one open simultaneously. Playing 2 files at once would sound awful, and if you want to copy from one to another, you can still gain access to the files directly through the finder.

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and how does the OS restrict this? –  Marty Feb 6 '12 at 21:35
    
Not sure I understand your comment –  stuffe Feb 6 '12 at 21:41
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It doesn't have to be the OS, could also be iTunes itself which determines it's already running when started a second time. –  patrix Feb 6 '12 at 22:10
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I was going to suggest that it was the presence of the LSMultipleInstancesProhibited key set to true in the Info.plist inside the application bundle, but iTunes doesn't appear to have that set. Launch Services may hard-wired to to look for the com.apple.iTunes CFBundleIdentifier and not allow a second instance. You could try changing that manually in a second copy of the app bundle to see if it allowed it to launch. But I'd make backup copies of your library database beforehand. :-) –  NSGod Feb 7 '12 at 6:38
    
@stuffe I meant how does the OS restrict the user from opening a second instance. –  Marty Feb 8 '12 at 0:44

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