Hard link


In computing, a hard link is a directory entry that associates a name with a file. 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. 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 “shortcut” 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 on many systems, containing a list of file names instead of other data. 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 additional hard links to directories.
POSIX-compliant operating systems, such as Linux, Android, macOS, and the non-POSIX-compliant 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 FAT does not.

Operation

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 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 share the same data.
Any number of hard links to the physical data may be created. To access the data, a user only needs to specify the name of any existing link; the operating system will resolve the location of the actual data. Even if the user deletes one of the hard links, the data is still accessible through any other link that remains. Once the user deletes all of the links, if no process has the file open, the operating system frees the disk space that the file once occupied.

Limitations

To prevent loops in the filesystem, and to keep the interpretation of the "" file consistent, operating systems do not generally allow hard links to directories. UNIX System V allowed them, but only the superuser had permission to make such links. Mac OS X v10.5 and newer use hard links on directories for the Time Machine backup mechanism only.
Hard links can be created to files only on the same volume, i.e., within the same file system.
The maximum number of hard links to a single file on a particular type of file system is limited by the size of the file system's reference counter and the size of the copy of the reference counter in the operating system's in-memory per-file data structure; it may also be limited by a policy choice in the operating system code. Exceeding the permitted number of links results in an error. In AT&T Unix System 6, released in 1975, the number of hard links allowed was 127. On Unix-like systems the in-memory counter is 4,294,967,295 or 18,446,744,073,709,551,615. In some file systems, the number of hard links is limited more strictly by their on-disk format. For example, as of Linux 3.11, the ext4 file system limits the number of hard links on a file to 65,000. Windows enforces a limit of 1024 hard links to a file on NTFS volumes.
On Linux Weekly News, Neil Brown criticized hard links as high-maintenance, since they complicate the design of programs that handle directory trees, including archivers and disk usage tools. These apps must take care to de-duplicate files that are linked multiple times in a hierarchy. Brown notes that Plan 9 from Bell Labs, the intended successor to Unix, does not include the concept of a hard link.

Platform support

and later support hard links on the NTFS file system. Windows 2000 introduces a CreateHardLink function to create hard links, but only for files, not directories. The DeleteFile function can remove them.
To create a hard link on Windows, end-users can use:
To interrogate a file for its hard links, end-users can use:
  • The fsutil utility
  • The Get-Item and Get-ChildItem cmdlets of PowerShell. These cmdlets represent each file with an object; PowerShell adds a read-only LinkType property to each of them. This property contains the string if the associated file has multiple hard links.
The Windows Component Store uses hard links to keep track of different versions of components stored on the hard disk drive.
On Unix-like systems, the system call can create additional hard links to existing files. To create hard links, end-users can use:
  • The ln utility
  • The link utility
  • The New-Item cmdlet of PowerShell
To interrogate a file for its hard links, end-users can use:
  • The stat command
  • The ls -l command
  • The Get-Item and Get-ChildItem cmdlets of PowerShell
Unix-like emulation or compatibility software running on Microsoft Windows, such as Cygwin and Subsystem for UNIX-based Applications, allow the use of POSIX interfaces.
OpenVMS supports hard links on the ODS-5 file system. Unlike Unix, VMS can create hard links to directories.