In computing, a hard link is a directory entry that associates a name with a file on a file system. (A directory is itself a special kind of file that contains a list of such entries.) The term is used in file systems which allow multiple hard links to be created for the same file. This has the effect of creating multiple names for the same file, causing an aliasing effect: e.g. if the file is opened by 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 on such file systems is not a link to a file itself, but to a file name. This also creates aliasing, but in a different way.
Directories are files, so multiple hard links to directories are possible; however, their unrestricted creation is usually avoided, because of the cyclic structures this may create.
Hard links—that is, multiple directory entries to the same file—are supported by POSIX-compliant and partially POSIX-compliant operating systems, such as GNU/Linux, Android, Apple's Mac OS X, Windows NT4 and later Windows NT operating systems.
Support also depends on the type of file system being used. For instance, the NTFS file system supports hard links, while FAT does not.
Read more about Hard Link: Usage, Link Counter, Example, Limitations of Hard Links
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