Hard Link - Definition and Overview

There are two types of links on UNIX systems: hard links and symbolic links (or soft links). A hard link is, for all intents and purposes, another name for a file. Hard links between files can be placed in different directories as long as they remain on the same physical partition. The file is deleted only when all hard links to that file are deleted and all hard links to a file are considered equal. Technically, hard links refer to the same file inode, whilst symbolic links do not.

UNIX systems generally only support hard-links to files, not directories, and only on the same volume. To link to directories or across filesystems, on Unix soft links must be used. Windows NT also supports hard links for files. For linking to directories or across volumes, it supports junction points instead.

An advantage of hard links over soft links is that if the target is moved, the link is not invalidated; and invalid hard links cannot exist (unless there has been filesystem corruption), whereas symbolic links can point to non-existent files.

Under Unix, hard links are created with the ln program.