Incomputing, ahard link is adirectory entry (in adirectory-basedfile system) that associates a name with afile. 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 additionalpaths (i.e., via different names or in different directories).[1] This causes analias effect: a process can open the file by any one of its paths and change its content. By contrast, asoft 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 atree. For that reason, some file systems forbid the creation of additional hard links to directories.
POSIX-compliantoperating systems, such asLinux,Android,macOS, and the non-POSIX-compliantWindows NT family,[2] support multiple hard links to the same file, depending on the file system. For instance,NTFS andReFS support hard links,[3] whileFAT does not.
Let two hard links, named "LINK A.TXT" and "LINK B.TXT", point to the same physical data. Atext 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 asGNU 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. (This behavior can be changed using the emacs variablebackup-by-copying.)
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.
Mostfile systems that support hard links usereference counting. The file system stores aninteger value with each logicaldata section that represents the total number of hard links that have been created to point to the data. When a new link is created, this value is increased by one. When a link is removed, the value is decreased by one. When the counter becomes zero, the operating system frees the logical data section. (The OS may not to do so immediately, e.g., when there are outstanding file handles open, for performance reasons, or to enable theundelete command.)
This is a simple method for the file system to track the use of a given area of storage, as zero values indicate free space and nonzero values indicate used space. The maintenance of this value guarantees that there will be no dangling hard links pointing nowhere. The data section and the associatedinode are preserved as long as a single hard link (directory reference) points to it or any process keeps the associated file open.
OnPOSIX-compliant operating systems, the reference count for a file or directory is returned by thestat() or fstat() system calls in thest_nlink field ofstruct stat.
To prevent loops in the filesystem, and to keep the interpretation of the ".." file (parent directory) consistent, operating systems do not generally allow hard links to directories.UNIX System V allowed them, but only thesuperuser had permission to make such links.[4]Mac OS X v10.5 (Leopard) and newer use hard links on directories for theTime Machine backup mechanism only.[5]
Hard links can be created to files only on the same volume, i.e., within the same file system. (This is because a hard link is a "name → inode" association where the inode is identified by itsnumber, and inode numbers are only unique within their partition. Also, different volumes may have different file systems. There is no guarantee that the target volume's file system is compatible with hard linking.)
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&TUnix System 6, released in 1975, the number of hard links allowed was 127.[6][7] On Unix-like systems the in-memory counter is 4,294,967,295 (on 32-bit machines) or 18,446,744,073,709,551,615 (on 64-bit machines). In some file systems, the number of hard links is limited more strictly by their on-disk format. For example, as ofLinux 3.11, theext4 file system limits the number of hard links on a file to 65,000.[8]Windows enforces a limit of 1024 hard links to a file onNTFS volumes.[9]
OnLinux 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 ahierarchy. Brown notes thatPlan 9 from Bell Labs, the intended successor to Unix, does not include the concept of a hard link.[10]
Windows NT 3.1 and later support hard links on theNTFS file system.[11] Windows 2000 introduces aCreateHardLink() function to create hard links, but only for files, not directories.[12] TheDeleteFile() function can remove them.
To create a hard link on Windows, end-users can use:
fsutil utility (introduced inWindows 2000)[13]mklink internal command ofWindows Command Prompt (introduced inWindows Vista andWindows Server 2008)[14]New-Item cmdlet ofPowerShell[15]To interrogate a file for its hard links, end-users can use:
fsutil utility[13]Get-Item andGet-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 "HardLink" string if the associated file has multiple hard links.[16]TheWindows Component Store uses hard links to keep track of different versions of components stored on the hard disk drive.
OnUnix-like systems, thelink()system call can create additional hard links to existing files. To create hard links, end-users can use:
To interrogate a file for its hard links, end-users can use:
Unix-like emulation or compatibility software running on Microsoft Windows, such asCygwin andSubsystem for UNIX-based Applications, allow the use of POSIX interfaces.
OpenVMS supports hard links on theODS-5 file system.[17] Unlike Unix, VMS can create hard links to directories.
freedup command frees-up disk space by replacing duplicate data stores with automatically generated hard links--link-dest=DIR so that a new backup folder will use hardlinks of the previous backup if not changed[18] as a space and time saving feature (incremental backup)[19][20][21]Establishes a hard link between an existing file and a new file. This function is only supported on the NTFS file system, and only for files, not directories.
If your location is in a FileSystem drive, the following values are allowed: If your location is in a FileSystem drive, the following values are allowed: File[,] Directory[,] Junction[,] HardLink
This option behaves like --copy-dest, but unchanged files are hard linked from DIR to the destination directory. The files must be identical in all preserved attributes (e.g. permissions, possibly ownership) in order for the files to be linked together
The --link-dest argument allows you to hard-link files from a different destination if they haven't changed, saving both time on transfer, as well as disk space
If the file in /backup/server1Old is the same as the file on the remote server then instead of copying it over rsync will create a hard link from the file in /backup/server1Old into /backup/server1New
copying changes into the new directory, and making hard links where possible for unchanged files