hard link

file system One of several directory entries which refer to

the same Unix file. A hard link is created with the "ln"

(link) command:

ln old name new name

where old name and new name are pathnames within the

same file system. Hard links to the same file are

indistinguishable from each other except that they have

different pathnames. They all refer to the same inode and

the inode contains all the information about a file.

The standard ln command does not usually allow you to create a

hard link to a directory, chiefly because the standard rm

and rmdir commands do not allow you to delete such a link.

Some systems provide link and unlink commands which give

direct access to the system calls of the same name, for

which no such restrictions apply.

Normally all hard links to a file must be in the same filesystem because a directory entry just relates a pathname to

an inode within the same file system. The only exception is a

mount point.

The restrictions on hard links to directories and between

file systems are very common but are not mandated by POSIX.

Symbolic links are often used instead of hard links because

they do not suffer from these restrictions.

The space associated with a file is not freed until all the

hard links to the file are deleted. This explains why the

system call to delete a file is called "unlink".

(1997-10-22)