Making sense of OS X's folder structure

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Reader Mark Walerysiak is frustrated by what he views as redundancy in OS X. He writes:

I’m very happy with all things Apple but one thing I’ve never understood about OS X is the multiple Library folders. Today I was looking for the StartupItems folder because I had to delete something. I went to Hard Drive/System/Library/ StartupItems. And I found nothing in that StartupItems folder. I then went to my Home folder/Library, only to find no StartupItem folder. Finally, I found what I was looking for in Hard Drive/Library/StartupItems folder. What the heck is going on?

This question goes to the heart of the way OS X organizes users and directories. It breaks down this way:

Items in the System folder are for OS X’s use—items in its Library folder are, for the most part, placed there when OS X is installed or when the OS is updated. Unless you’re geeking out, there are few reasons to venture into this folder.

The Library folder at the root level of the hard drive is often populated with third-party items installed by an Administrator. In the case of startup items, they’re placed here because they may be needed by every user who has an account on that Mac. If a user doesn’t have access to a particular application that uses one of these startup items—because they have a controlled account, for example—no worries. It doesn’t hurt to have that item loaded. It would hurt, however, if a startup item was loaded for one account and not another.

This helps explain why there’s no StartupItems folder in the Library folder within your user account. (Underscoring this notion is the fact that startup items load before a user logs in.) Note that StartupItems are not the same thing as the applications you've designated as Login Items for your account. StartupItems work deeper-level background mojo than Login Items.

The Library folder within a user’s folder contains, as you might expect, items that apply to that specific user—preferences, audio files, plug-ins, mailboxes, log files for applications used by that user, and so on. This is the folder you want to dig around in when something in your account is acting up.

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