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Mac OS X Hidden Files & Directories

By Gordon Davisson

Copyright (c) 2005, Westwind Computing inc.

Mac OS X volumes contain quite a few files and directories (aka folders) that are invisible from the Finder (and many other interfaces). For the most part, they're hidden for good reason -- you generally don't need to worry about them, and in fact, probably shouldn't muck with them unless you know what you're doing. But just in case you find yourself needing to deal with them, here's a quick summary of what they're for.

Mac OS X Hidden Files & Directories

._whatever These files are created on volumes that don't natively support full HFS file characteristics (e.g. ufs volumes, Windows fileshares, etc). When a Mac file is copied to such a volume, its data fork is stored under the file's regular name, and the additional HFS information (resource fork, type & creator codes, etc) is stored in a second file (in AppleDouble format), with a name that starts with "._". (These files are, of course, invisible as far as OS-X is concerned, but not to other OS's; this can sometimes be annoying...)
.DS_Store This file in created by the Finder to keep track of folder view options, icon positions, and other visual information about folders. A separate .DS_Store file is created in each directory to store information about that directory, so you'll find them appearing all over the directory tree, in pretty much every folder you've visited with the OS X Finder.
~/.Trash Used to store files & folders from the boot volume that a particular user has thrown in the trash, but that haven't been erased yet.
/.Spotlight-V100 Used to store metadata indexes and indexing rules for Spotlight (version 1.00 apparently). Only created under Mac OS X 10.4.
/Volumes/(whatever)/.Trashes On volumes other than the boot volume, a .Trashes folder is used to hold files & folders that've been put in the trash but not yet deleted. Since each user has their own personal trash can, subfolders are created under .Trashes for different users, named according to their user ID number. For example, if user #501 throws something on a volume named "Data" into the trash, it'd be moved to a directory named /Volumes/Data/.Trashes/501/.
Permissions on this folder are set so that you can only access a trash can if you can guess the users' ID -- that is, you cannot view a list of which users actually have trash cans in existance. If you're trying to free disk space, this can make it rather tricky to find & delete the files in other users' trash cans...
/.hidden This contains a list of files for the Finder to hide -- it's one of three ways a file can be made invisible in OS X. This file is semi-obsolete -- i.e. it does not exist in a standard installation of Mac OS X 10.4, but the Finder will still respect it if it exists...
/.hotfiles.btree Used to track commonly-used small files so their position on disk can be optimized (a process called "adaptive hot file clustering").
/.vol This pseudo-directory is used to access files by their ID number (aka inode number) rather than by name. For example, /.vol/234881034/105486 is file #105486 on volume #234881034.
/automount Used to handle "quasi-static" mounts of network volumes under OS X 10.1. Under most unixes, if a network volume is statically mounted on a client, it's mounted somewhere in the file system, so it looks shows up like a normal directory. Under OS X 10.1, a statically-mounted network volume will actually be mounted in /automount, and a symbolic link pointing to it will be placed where the volume would normally be mounted, thus emulating the normal result.
(Compare this with how "network" mounts are handled via /private/Network.)
/bin This is one of several places where unix-style binaries (that is, programs, or command-line commands) are kept. The programs in /bin include the more common and fundamental things that are used from the unix command line (e.g. ls and rm), as well as several shells (the programs that provide the command line itself).
The other places where similar files are stored are /sbin, /usr/bin, /usr/sbin, and possibly /usr/local/bin, /usr/local/sbin, and maybe even ~/bin/powerpc-apple-macos; collectively, these can be thought of as the command line's equivalent of /Applications.
/cores (This is actually stored in /private/cores; /cores is really just a symbolic link.)
Under some conditions, when a program crashes, it'll "dump core" (essentially, store a copy of the program state at the time it crashed) into this directory. This is really only useful for programmers trying to debug their own programs.
/dev This directory contains what're technically known as device special files. These are not really files in the usual sense, they're more like placeholders that the system uses to keep track of the devices (disks, keyboards, monitors, network connections, etc) attached to it.
/etc (This is actually stored in /private/etc; /etc is really just a symbolic link.)
On a typical unix system the /etc folder will contain all the configuration files for a system, including both documents specifying config information as well as scripts for actually performing various configuration tasks. On OS X, some of the config information stored here is overridden by NetInfo or other directory services, but the /etc files still exist.
/lost+found If Disk Utility or fsck discover "orphaned" files (i.e. files that exist, but aren't actually in any directory), they'll be placed here.
/Network This is the "real" location of the Network item that appears at the Computer level in the finder. It provides a place to attach network-wide resources and server volumes. Under OS X 10.1, network resources actually tend to get mounted in /private/Network, and symbolic links to them created in /Network.
In OS 10.3, various network resources (mainly servers) appear dynamically in /Network (thanks to some virtual filesystem magic).
The Mach kernel (which runs at the very core of Mac OS X), along with a couple of shortcuts for getting at it in various ways.
/private In OS X certain root level directories are actually symbolic links (similar to aliases) to directories in /private. Examples are /cores, /etc, and /var which are respectively linked to /private/cores, /private/etc, and /private/var. /private also contains a directory of drivers for certain peripherals.
/private/Network Used to handle "network" (non-static) mounts of network volumes under OS X 10.1. Under previous versions, network mounts were mounted in /Network, but in 10.1 they're actually mounted in /private/network, and a symbolic link is placed in /Network pointing to the actual mount point.
(Compare this with how "quasi-static" mounts are handled via /automount.)
/sbin The /sbin directory is like /bin except it contains binaries that are specifically used for system administration (e.g. mount and fsck).
/tmp (This is actually stored in /private/tmp; /tmp is really just a symbolic link.)
Programs that need temporary space on the hard disk are usally set up to write temporary files to the /tmp directory (although some use /var/tmp instead).
/usr The /usr directory contains many subdirectories that have binaries and files specifically of use to the normal (unix) user.
/usr/bin Another place where unix binaries are kept.
/usr/lib Libraries available for use by progrmming on Mac OS X. Unless you install the Developer Tools, this'll be mostly empty.
Note that this has no relation to Mac OS X's various "Library" directories."
/usr/libexec Holds various daemon programs, system maintenance scripts, and other unix-style programs that usually aren't run directly by humans.
/usr/local As in most unixes, this directory is used to store local customizations and additions to the standard OS installation (e.g. /usr/local/bin would be likely to hold unix binaries added by the system administrator). This directory can be thought of roughly as the unix equivalent of Mac OS X's local library. In the standard install of Mac OS X it is (not surprisingly) completely empty.
Note: As of OS X 10.2, these directories are no longer in the default search path for command-line executables; as a result, anything installed here will not be useable without taking extra steps of one sort or another.
/usr/sbin Yet another place where unix binaries are kept.
/usr/share Contains various data and text files that can, in principle, be shared by multiple architectures (a distinction which makes a lot more sense under other flavors of unix than it does under Mac OS X).
/usr/standalone Contains boot loader programs for (potentially) various computer architectures. In the installs I've looked at, this is simply a duplicate of the BootX loader (also found in /System/Library/CoreServices/BootX); I'm not sure why both copies are needed.
/var (This is actually stored in /private/var; /var is really just a symbolic link.)
Sometimes processes controlled by the operating system need a place to store variable files. Processes like printing and programs that store log files will use subdirectories in the /var directory to store those files.
It also holds a fair bit of configuration information (especially in /var/db).
/var/backups Used to store backups of critical system information (mainly, the nightly dumps of NetInfo databases).
/var/db Holds various databases of system information. The most notable are the NetInfo databases (stored in /var/db/netinfo), shadow password files (in /var/db/shadow/hash), and the system's network configuration database (/var/db/SystemConfiguration/preferences.xml - although it moved to /Library/Preferences/SystemConfiguration/ in 10.3), which together store much of the system and network configuration information that a traditional unix admin would expect to find in /etc, and a Mac OS 9 admin would expect to find in System Folder:Preferences.
/var/log This is where many of the system event logs are kept (others are kept in /Library/Logs).
/var/root The root (superuser) account's home directory. Note that this directory will exist even if you haven't enabled the root account.
/var/run Stores various status information about processes (especially daemons) running on the system.
/var/tmp A place for programs to store temporary data, just like /tmp. Some programs use one, some use the other, so Mac OS X provides both.
/var/vm Used to store the swap files for Mac OS X's virtual memory.
/var/vm/app_profile Holds information about various applications' virtual memory useage.
/Volumes The /Volumes directory is the mount point for all of the drives (other than the boot volume) connected to the system. The Finder hides the Volumes directory itself, but displays its contents at the Computer level.

Mac OS 9 Invisible Files & Folders

If you dual-boot your computer into Mac OS 9 as well as OS X, you'll find that OS 9 creates additional invisible files and folders mixed in with OS X's. Here are some of the more common examples:
AppleShare PDS Used by OS 9's personal file sharing to keep track of which folders are shared, and what their protections are (note that this is completely unrelated to OS X file sharing settings and permissions).
Cleanup At Startup Contains files created by Mac OS 9 applications and installers that need to be deleted the next time OS 9 starts up.
Desktop DB
Desktop DF
Used in Mac OS 7 through 9 to keep track of known document types, icons, and which Applications can open what document types.
Desktop Folder The Mac OS 9 desktop folder; Mac OS X allows you to see its contents using an alias on the Mac OS X desktop, but if you look for it directly, you won't see it.
Note: only the Desktop Folder on the Mac OS X boot volume is invisible; any other volume's Desktop Folders will be fully visible in the finder.
Network Trash Folder Used by OS 9 filesharing clients to temporarily store trashed files. This folder will be created in the shared folder (aka share point), and although the OS 9 client will attempt to empty its contents before disconnecting, the folder itself will remain.
Shutdown Check Used by Mac OS 9 to keep track of whether the computer shut down cleanly or crashed.
Temporary Items Used by Mac OS 9 applications for temporary storage.
TheFindByContentFolder Stores indexes for Sherlock's Find By Content capability.
TheVolumeSettingsFolder A place for the Finder to hide various info (e.g. the desktop printer database).
Trash Anything that is put in the OS 9 trash is found in this directory after booting into OS X.
VM Storage This is where OS 9 keeps its virtual memory backing storage, and it gets left lying around after you boot into Mac OS X. If you want to recover the disk space, go ahead and delete it.

Why are there 7 binaries directories?

When you enter a command on the Mac OS X command line, there are up to 7 different directories where it looks for the corresponding program. Why so many, you might ask? Well, they are organized by several different characteristics, and since there are many possible combinations of those characteristics... it winds up being a lot of different categories of programs. There are 6 directories that fit a fairly consistent pattern, and one that doesn't quite fit; let me discuss the 6 first.

First distinction: "bin" vs. "sbin". The 6 consistent directories come in matching pairs, named "bin" and "sbin". The "bin" version contains general-use programs, while the "sbin" version contains programs that're generally only used for system administration. The distinction is somewhat arbitrary (for example, the IP ping utility is in an sbin directory, but the AppleTalk version is in a bin directory), and programs in sbin are accessible to all users (although they often will refuse to perform unless run by an administrator, or sometimes root).

Second distinction: / vs. /usr/ vs. /usr/local/. Each of these directories can contain bin and sbin subdirectories. Most programs are kept in the /usr/ version. Some of the more critical, core programs (especially those that're required during the boot process) are kept in the / directories instead. The reason for this is that it's possible to keep /usr on a file server rather than needing a separate copy on every computer on the network. But if this is done, the client computers need to be able to get to the point where they can connect to a server without having access to anything in /usr; programs that absolutely need to be available have to go in /bin or /sbin. Finally, programs that're installed "locally" (i.e. by a system administrator, rather than by Apple as part of the standard system install) go in /usr/local/. If you look there on a standard OS X system, you'll find /usr/local totally empty (/usr/local/bin and /usr/local/sbin don't even exist).
Note: in 10.2, /usr/local/bin and /usr/local/sbin were removed from the default search path, which renders them fairly useless...

Finally, there's an idiosyncratic directory that doesn't fit the pattern of bin/sbin pairs: it's the user's personal bin directory, named ~/bin/powerpc-apple-macos (which doesn't exist unless you take the trouble to create it). Why such an unweildy name? Well, the idea is that your home directory might be shared across several different computer architectures. It might, for example, live on a server, and be accessed whenever you log into any Mac OS X, Linux, Solaris, ... etc computer on the network. Generally, a binary program will not work on more than one of these architectures, so you want different binaries available to you depending on precisely which computer you're using at the moment. This was removed from the default search path in 10.1, so is now only of historical interest.

Reasons for invisibility

In Mac OS X, there are three different ways a file or directory can be made invisible in the finder: it can have the "invisible" attribute set (as in older Mac OS systems), its name can start with "." (as in other unix systems), or its name can be listed in the /.hidden file. Many of the files and directories listed above are actually invisible for multiple reasons (e.g. /bin is listed in /.hidden, as well as having its invisible attribute set).

Note that OS X only respects the .hidden file on its boot volume, so if you boot from another disk, several normally-hidden files will suddenly be visible. Also, since Mac OS 9 (and older versions) only recognize the invisible flag, even more of these files (mainly /.vol, /mach, /mach.sym, and sometimes .DS_Store) will be visible when you boot into Mac OS 9.

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