Hard link

In computing, a hard link is a directory entry that associates a name with a file on a file system. All directory-based file systems must have (at least) one hard link giving the original name for each file. The term “hard link” is usually only used in file systems that allow more than one hard link for the same file.

Creating a hard link has the effect of giving one file multiple names (e.g. different names in different directories) all of which independently connect to the same data on the disc, none of which depends on any of the others. This causes an alias effect: e.g. if the file is opened by any one of its names, and changes are made to its content, then these changes will also be visible when the file is opened by an alternative name. By contrast, a soft link or “shortcut” to a file is not a direct link to the data itself, but rather is a short file that contains the text of a file name, or a location that gives direct access to yet another file name within some directory. The name contained in or referred to by the soft link may either be a hard link or another soft link. This also creates aliasing, but in a different way.

Every directory is itself a file, only special because it contains a list of file names maintained by the file system. Since directories themselves are files, multiple hard links to directories are possible which would create loops within the structure of the directories, rather than a branching structure like a tree. For that reason, the creation of hard links to directories is sometimes forbidden, even if possible.

Hard links – that is, multiple directory entries to the same file – are supported by POSIX-compliant and partially POSIX-compliant operating systems, such as Linux, Android, macOS, Windows NT4 and later Windows NT operating systems.

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