H

Hard Link
An alternate name for a file that actually shares the file with the original name in a symmetrical manner.

At a low level, Unix volumes are a collection of files, each of which is uniquely identified within the volume by an integer, called the i-node number. Often the root directory is number 2, and the others are numbered up from there. For a given i-node number, there can be only one file on that volume.

Most people are unaware of their file's i-node numbers; they instead pay attention to the file name and directory.

two filenames point to one file For instance, the file /mmonroe/NormaJean.jpeg has an entry, "NormaJean.jpeg", in the directory file /mmonroe. This entry might say that the file has an i-node number, perhaps 77611. Elsewhere on the volume is a list of i-nodes, and entry number 77611 points to the actual data for the file NormaJean.jpeg.

A hardlink is a situation where one i-node entry has more than one directory entry. In our example, the directory /mmonroe might also have an entry Marilyn.jpeg with i-node 77611, or perhaps another directory like /denise might have an entry NormaJean.jpeg with i-node 77611. All of these represent the same chunk of data.

The new entries are symmetric with the original; if you remove the old or new filename, the file will still exist under the other name, just like a joint bank account. The actual data for the file will only be deleted after the last directory entry has been removed.

Hard links always are on the same volume as their target file; they cannot cross volumes as with symlinks. Hard links are typically not found on non-Unix operating systems, and are usually prohibited on those volume types, such as Windows FAT volumes. For many reasons, Symlinks have become more popular.

For a special case of a symlink, every directory has an entry named . (a single period) that refers to the directory itself, and an entry .. (two periods) that refers to its parent directory. Therefore, if the directory /home has subdirectories /home/veronica, /home/traci and /home/monique, the directory /home actually has five entries pointing to it, each from a different directory:
/home
/home/.
/home/traci/..
/home/veronica/..
/home/monique/..

Similarly, traci, veronica and monique each have at least two entries pointing to them; one in the directory /home and another for the dot entry in the directory itself. Typically a directory will have N+2 hardlinks, where N is the number of subdirectories.

Other than these . and .. entries, hard links to directories are frowned upon and are often prohibited or restricted. Strange ambiguities arise, such as, who's parent does .. point to?

Besides directories, most files are not hardlinked; in other words, they have just one directory entry. You can add more by pasting filenames with altH shift^altV on X11, or ^H ^cmdY on MacOS, although hardlinks are uncommon on MacOS. You can also do it with the command-line command ln.

Hard links can often break through permission and security barriers that symlinks cannot. For instance, if Marilyn.jpeg is hardlinked to NormaJean.jpeg, you can access the file either way. Although they share their mode bits, their directories might prevent access. If Marilyn.jpeg is a symlink, however, you must be able to access both the Marilyn.jpeg symlink as well as the target file NormaJean.jpeg Hard links can even extend access outside of chroot jails (see man chroot).

See also Symlink, man ln.

Header Time
The header time (or header date or ctime or attribute change time or chmod time) on a file is the time at which the file's header attributes were most recently changed. Along with the modify time and the access time, it is one of the main timestamps on a Unix file.

A Unix volume can be thought of as two cooperating data structures: a set of files, each identified by a unique i-node number, and a tree of directories. Each directory is itself a file, and contains a list of filenames and the inodes that the names refer to.

Each file has, in addition to its data bytes, a standard set of attributes that tell permissions, ownership, type, sizes, dates, etc. When those attributes for a file change, the header time is set. Therefore, changing the ownership of a file will set the header time, but changing its name, or moving it to a new directory, won't. Directory header information is changed, however, when the files inside change, or change names.

That's how it's supposed to work. In reality, the details change from one brand of Unix to another, and from one kind of filesystem to another. Sometimes the header time is changed upon a rename. Some people or filesystems interpret the header time, called ctime internally, as "creation time" rather than "header change time".

You should do some experimentation on your volume before you trust the times, especially access times, that you see.

HFS and HFS+
The Hierarchical File System or HFS organizes files for MacOS volumes two gigabytes and smaller. For many years, it was the workhorse filesystem of MacOS. It contains slots for the 4-byte Mac Type and Creator for each file, along with several flag words, dates, coordinates for the Finder window position, Finder color, and other information. An HFS volume can be a System volume for Mac OS 9 and before, but not for Mac OS X.

The HFS+ filesystem is an expansion of HFS to volumes larger than 2GB and also adds Unicode filenames and Unix attributes and features such as the mode, header time, hardlinks, and so on. An HFS volume can be a System volume for Mac OS 9 and for Mac OS X, or both at the same time.

On hard disks, HFS and HFS+ volumes are usually partitions of the disk. Floppy and Zip disks tend to have just one volume that takes all the space. Network served volumes over AFP behave similarly to HFS, but there's subtle differences.

CDs have one or more sessions, each of which may be a different file system type. So a CD or DVD that mounts on MacOS might not mount on Linux or Windows; if so you could say that it only has an HFS+ session. Or if it has Mac-specific features, the disk might have two sessions, an HFS+ session that talks to MacOS, and an ISO session that is seen on other platforms. If it looks the same on Mac and other platforms, probably it's just a regular ISO disk, designed to be cross platform.

Host Window a typical host window
The Host Window shows all of the volumes that are mounted on your host computer.

There is only one Host window in Interrogator; it is either showing or closed. You can open the Host Window by using the keystrokes alt H alt O or on Mac ^H ^O. Opening it a second time simply brings the existing window to the front.

You can select one volume by clicking or typing. You can open a Volume Window onto it by typing slash, or you can open a Directory Window onto its root directory by double clicking or pressing Enter.

Interrogator tries to group the volumes into categories. Hard Disk partitions are grouped by the physical disk they are partitions of. Network volumes are grouped according to the protocol; NFS volumes go under the NFS category and AFP volumes group under the AFP category.

If Interrogator can tell if it's a removable volume like a CD or floppy disk, it goes under a Removable category. The System and Other categories list other volumes that didn't fit into the other categories. Typically they are used internally by the operating system and are unsuitable for storing files, although sometimes Interrogator gets confused and a volume ends up there by accident.


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