Macintosh File Systems

The filesystem for Mac OS 9 and earlier was the inspiration for the Mac-like properties of the Mac OS X filesystem. It consists of volumes using a filesystem of type HFS or HFS+ . Each volume is known by a name, and the volume names coexist at the top level. The volume names can collide; there is no mechanism forcing floppy disks or disk partitions to have unique names. On Mac OS X, the volume names are made distinct by appending a suffix; if you mount two volumes named sonic , they will mount at /Volumes/sonic and /Volumes/sonic_1 .

Absolute filenames use colon as a separator and do not begin with a colon. For example, atlanta:idd:barium salts would be a file named barium salts (with a space in it) in a directory idd at the top level of a volume named atlanta . The Unix equivalent would roughly be /atlanta/idd/barium salts . (For the command name you'd have to use a trick to avoid misunderstanding the space character.)

Relative filenames come in three kinds. The general kind work like the absolute but start with a colon. Therefore :idd:barium salts will look for a file named barium salts in a directory named idd in the current directory.

Multiple colons at the start go up a level. Therefore :::idd:barium salts will look for a file named barium salts in a directory named idd in a directory two levels above the current directory. Multiple colons elsewhere are prohibited (and usually superfluous).

Special case: names that have no colons are relative to the current directory. Therefore barium salts will look for a file named barium salts in the current directory. Because of this, you cannot refer to a volume in a context that expects a file. Volumes are not considered files in classic Mac OS.

The root 'directory', which contains all of the mount points, cannot really hold any files, because it's unclear on which volume they would actually reside. Instead, the Finder, and the file selection dialogs, depict a phony directory called the desktop that appears on the back surface of your screen, outside of any application, behind all windows. This is a union of the icons for the objects:

• one icon for each volume. It behaves as a directory, specifically the root directory for the volume, but has unique properties.
• one icon for each file in each desktop directory for each volume.

Of course all of these lead to more opportunities for confusion, and for objects of the same name to coexist on the desktop without penalty or shame.

Introduction Macintosh File Systems

Apple's Mac OS X tries to combine the two, by appearing to have a MacOS structure, while being implemented at a lower level by Darwin, a descendant of BSD Unix.

The 'System Disk' on the Mac level is the root volume on the BSD level. This is washington in the picture, in yellow. Traditionally it's the first disk icon on the desktop but these days you can change that. It has the directories System and Library that it boots OS X up from, plus other essentials such as the Volumes directory. If it has MacOS 9 on it also, that is often in a directory named System Folder .

Most other volumes are mounted off of the Volumes directory. Typically a directory of the same name is created there, and all files appear under that. If the volume name collides with a preexisting one, it gets a numbered modification such as atlanta-1 . Sometimes MacOS X mounts them elsewhere such as /private/var/automount . And, you can always use the unix mount command to mount a volume anywhere you want by hand.

The Finder and the file selection dialogs depict the filesystem similar to Mac OS 9, as described above. The system volume, and the volumes under /Volumes , all appear on the desktop, along with files from each volume's Desktop Folder directory. Also, each user's home directory has a Desktop directory that gets merged in if that user is logged in.

As before, because these come from distinct directories and/or name spaces, files with the same name can coexist on the desktop. To add to the confusion, the MacOS X Finder often hides suffixes (something that Tactile Interrogator never does). Therefore, a user named Pat might have these icons all displayed on the desktop, each of which appears in the desktop with the same name 'atlanta':
- a volume named atlanta
- a file /Desktop Folder/atlanta
- a file /Volumes/chicago/Desktop Folder/atlanta
- a file /Volumes/atlanta/Desktop Folder/atlanta
- a file /Users/pat/Desktop/atlanta
- a directory /Users/pat/Desktop/atlanta.app

In Interrogator, you access each of these directories individually. Interrogator has no 'desktop' concept because of the inconvenience when icons on the desktop become buried under windows. The desktop is like a window that always stays behind all other windows. A normal window is more servicable.

There are a number of files, directories and details that are unavailable or difficult to get to in the Mac OS X filesystem using Mac programs like the Finder. The file-open dialogs behave the same way. Much of this is to shield the consumer public from the ugly pipes under the hood, and to shield the system from slightly more ambitious tinkerers. They are nevertheless available to Unix programs such as Tactile Interrogator and to the command line. These include:

.hidden files Unix traditionally treats files whose name starts with a dot to be loosely hidden. ls will ignore them, but you can see them if you use ls -a . The Finder will not show them, although you can coax it to open subdirectories with open on the command line. There are lots of them in your home directory. In Interrogator, you can see them or hide them by choosing hide Hidden files from the View menu, or double clicking on the '(4 hidden files)' icon. Invisible files If a file has the Mac-culture Invisible bit set, the Finder will also refuse to show it. Most of the Unix side ignores this bit (use GetFileInfo if you want to see it). In Interrogator, on the View menu, hide Invisible files can let you hide or show these. You can get the Finder to open subdirectories with the open command. Two good examples are at the root of every volume: files named Desktop DB and Desktop DF . The traditional Unix directories /bin, /sbin, /cores, /usr These come marked Invisible. The /automount and /Desktop Folder and the Mach kernel files that match /mach* These are not marked Invisible but are nontheless. The Trash directory at the top of each disk These are not marked Invisible but are nontheless. Clicking the Trash icon in the Dock will open a window showing the union of all Trash directories, including in your personal directory. The /Network directory This actually shows up as a volume, with its own icon, named Network, that is supposed to list servers. It is not visible as a subdirectory of the root directory, although that's what it is. The traditional Unix directories /etc, /tmp and /var These are not visible although do not have the Invisible bit set. Each is a symlink into the /private directory which holds their actual contents. /private is Invisible. The /dev directory This traditional Unix directory is invisible, without the Invisible bit, and is totally inaccessible from any Mac program. It contains mostly special device files . the insides of packages and applications Try this in the Finder. Make a new directory and name it s.app . As soon as the .app suffix is added, the Finder hides the suffix and pretends like it's an application, regardless of its contents. Double clicking will attempt to run it. There are other kinds of packages in Mac OS X, and the list keeps growing. Most of the others are effectively data files for programs; double clicking them will open them in the program The Finder DOES allow you to open it, though with the contextual menu. To open it in Tactile Interrogator, select it and use slash. file suffixes in general Files have another attribute bit called Hide Suffix. If this is on, the Finder and other Mac programs will hide the filename's suffix from your delicate consumer eyes. We have decided, however, that this is confusing and disruptive to the serious user. For example, you could have the files verdict.txt , verdict.doc and verdict.html , all in the same directory, and appearing to all have the same name. Interrogator never hides a file suffix. It ignores the Hide Suffix attribute bit except to display it in Full Disclosure.