As if one name for a file weren't enough, sometimes you want to have two, three, or a dozen names for the same file. This operation of creating alternate names for a file is called linking . The two major forms of linking are hard links and symbolic links (also called symlinks or soft links). Not all kinds of filesystems support both of these or even either of them. This section describes filesystems under POSIX.
A hard link to a file is indistinguishable from the original name for the file; there's no particular link that is more the "real name" for the file than any other.
The operating system keeps track of how many hard links reference the file at any particular time. When a file is first created, it starts with one link. Each new hard link increases the count. Each removed link reduces the count. When the last link to a file disappears, and the file is closed, the file goes away.
Every hard link to a file must reside on the same mounted filesystem (usually a disk or a part of a disk). Because of this, you cannot make a new hard link to a file that is on a different mounted filesystem.
Under most systems, hard links are also restricted for directories. To keep the directory structure as a tree rather than an arbitrary mish-mash, a directory is allowed only one name from the root, a link from the dot file within itself, and a bunch of dot-dot hard links from each of its subdirectories. If ...