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. 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 disk, 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 could 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. 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, and also 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 and ReFS do not. On POSIX-compliant and partially POSIX-compliant operating systems, such as all Unix-like systems, additional hard links to existing files are created with the link() system call, or the ln and link command-line utilities. The stat command can reveal how many hard links point to a given file. The link count is also included in the output of ls -l. On Microsoft Windows, hard links can be created using the mklink /H command on Windows NT 6.0 and later systems (such as Windows Vista), and in earlier systems (Windows XP, Windows Server 2003) using fsutil.exe hardlink create.The Windows API from Windows 2000 onwards includes a CreateHardLink() call to create hard links, DeleteFile() is used to remove them, and GetFileInformationByHandle() can be used to determine the number of hard links associated with a file. Hard links require an NTFS partition. Starting with Windows Vista, hard links are used by Windows Component Store (WinSxS) to keep track of different versions of DLLs stored on the hard disk drive. Unix-like emulation or compatibility software running on Windows, such as Cygwin and Subsystem for UNIX-based Applications, allow the use of POSIX interfaces under Windows. The process of unlinking dissociates a name from the data on the volume without destroying the associated data. The data is still accessible, as long as at least one link that points to it still exists. When the last link is removed, the space is considered free. A process called undeleting allows the recreation of links to data that are no longer associated with a name. However, this process is not available on all systems and is often not reliable. When a file is deleted, it is added to a free space map for re-use. If a portion of the deleted file space is claimed by new data, undeletion will be unsuccessful, because some or all of the previous data will have been overwritten, and may result in cross-linking with the new data and leading to filesystem corruption. Additionally, deleted files on solid state drives may be erased at any time by the storage device for reclamation as free space.