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Concept
Hard link
Summary
In computing, a hard link is a directory entry (in a directory-based ) that associates a name with a . Thus, each file must have at least one hard link. Creating additional hard links for a file makes the contents of that file accessible via additional paths (i.e., via different names or in different directories). This causes an alias effect: a process can open the file by any one of its paths and change its content. By contrast, a soft link or to a file is not a direct link to the data itself, but rather a reference to a hard link or another soft link.
Every directory is itself a special file, only it contains a list of file names. Hence, multiple hard links to directories are possible, which could create a circular directory structure, rather than a branching structure like a tree. For that reason, some file systems forbid the creation of hard links to directories.
POSIX-compliant operating systems, such as Linux, Android, macOS, and the Windows NT family, support multiple hard links to the same file, depending on the file system. For instance, NTFS and ReFS support hard links, while does not.
Let two hard links, named "LINK A.TXT" and "LINK B.TXT", point to the same physical data. A text editor opens "LINK A.TXT", modifies it and saves it. When the editor (or any other app) opens "LINK B.TXT", it can see those changes made to "LINK A.TXT", since both file names point to the same data. So from a user's point of view this is one file with several filenames. Editing any filename modifies "all" files, however deleting "any" filename except the last one keeps the file around.
However, some editors, such as GNU Emacs, break the hard link concept. When opening a file for editing, e.g., "LINK B.TXT", emacs renames "LINK B.TXT" to "LINK B.TXT~", loads "LINK B.TXT~" into the editor, and saves the modified contents to a newly created "LINK B.TXT". Now, "LINK A.TXT" and "LINK B.TXT" no longer shares the same data. (This behavior can be changed using the emacs variable backup-by-copying.)
Any number of hard links to the physical data may be created.
Official source
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