Symbolic link

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In computing, a symbolic link (also symlink or soft link) is a special type of file that contains a reference to another file or directory in the form of an absolute or relative path and that affects pathname resolution.[1] Symbolic links were already present by 1978 in mini-computer operating systems from DEC and Data General's RDOS. Today they are supported by the POSIX operating-system standard, most Unix-like operating systems such as FreeBSD, GNU/Linux, and Mac OS X, and also Windows operating systems such as Windows Vista, Windows 7 and to some degree in Windows 2000 and Windows XP in the form of Shortcut files.

Symbolic links operate transparently for most operations: programs that read or write to files named by a symbolic link will behave as if operating directly on the target file. However, programs that need to handle symbolic links specially (e.g., backup utilities) may identify and manipulate them directly.

A symbolic link contains a text string that is automatically interpreted and followed by the operating system as a path to another file or directory. This other file or directory is called the "target". The symbolic link is a second file that exists independently of its target. If a symbolic link is deleted, its target remains unaffected. If a symbolic link points to a target, and sometime later that target is moved, renamed or deleted, the symbolic link is not automatically updated or deleted, but continues to exist and still points to the old target, now a non-existing location or file. Symbolic links pointing to moved or non-existing targets are sometimes called broken, orphaned, dead, or dangling.

Symbolic links are different from hard links. Hard links do not link paths on different volumes or file systems, whereas symbolic links may point to any file or directory irrespective of the volumes on which the link and target reside. Hard links always refer to an existing file, whereas symbolic links may contain an arbitrary path that does not point to anything.

Some Unix as well as Linux distributions use symbolic links extensively in an effort to reorder the file system hierarchy. This is accomplished with several mechanisms, such as variant, context-dependent symbolic links. This offers the opportunity to create a more intuitive or application-specific directory tree and to reorganize the system without having to redesign the core set of system functions and utilities.

POSIX and Unix-like operating systems[edit]

In POSIX-compliant operating systems, symbolic links are created with the symlink[2] system call. The ln shell command normally uses the link[3] system call, which creates a hard link. When the ln -s flag is specified, the symlink() system call is used instead, creating a symbolic link. Symlinks were introduced in 4.2BSD Unix from U.C. Berkeley.

The following command creates a symbolic link at the command-line interface (shell):

 ln -s target_path link_path

target_path is the relative or absolute path to which the symbolic link should point. Usually the target will exist, although symbolic links may be created to non-existent targets. link_path is the path of the symbolic link.

After creating the symbolic link, it may generally be treated as an alias for the target. Any file system management commands (e.g., cp, rm) may be used on the symbolic link. Commands which read or write file contents will access the contents of the target file. The rm (delete file) command, however, removes the link itself, not the target file.

The POSIX directory listing application, ls, denotes symbolic links with an arrow after the name, pointing to the name of the target file (see following example), when the long directory list is requested (-l option). When a directory listing of a symbolic link that points to a directory is requested, only the link itself will be displayed. In order to obtain a listing of the linked directory, the path must include a trailing directory separator character ('/', slash).


Note: In Below Example do not create "three" directory before creation of link in /tmp directory.
 
 $ mkdir -p /tmp/one/two
 $ echo "test_a" >/tmp/one/two/a
 $ echo "test_b" >/tmp/one/two/b
 $ cd /tmp/one/two
 $ ls -l
 -rw-r--r-- 1 user group 7 Jan 01 10:01 a
 -rw-r--r-- 1 user group 7 Jan 01 10:01 b
 
 $ cd /tmp
 $ ln -s /tmp/one/two three
 $ ls -l /tmp/three
 lrwxrwxrwx 1 user group 12 Jul 22 10:02 /tmp/three -> /tmp/one/two
 $ ls -l /tmp/three/
 -rw-r--r-- 1 user group 7 Jan 01 10:01 a
 -rw-r--r-- 1 user group 7 Jan 01 10:01 b
 
 $ cd three
 $ ls -l
 -rw-r--r-- 1 user group 7 Jan 01 10:01 a
 -rw-r--r-- 1 user group 7 Jan 01 10:01 b
 $ cat a
 test_a
 $ cat /tmp/one/two/a
 test_a
 $ echo "test_c" >/tmp/one/two/a
 $ cat /tmp/one/two/a
 test_c
 $ cat /tmp/three/a
 test_c

Storage of symbolic links[edit]

Early implementations of symbolic links stored the symbolic link information as data in regular files. The file contained the textual reference to the link’s target, and an indicator[clarification needed] denoting it as a symbolic link.

This method was slow and an inefficient use of disk-space on small systems. An improvement, called fast symlinks, allowed storage of the target path within the data structures used for storing file information on disk (inodes). This space normally stores a list of disk block addresses allocated to a file. Thus, symlinks with short target paths are accessed quickly. Systems with fast symlinks often fall back to using the original method if the target path exceeds the available inode space. The original style is retroactively termed a slow symlink. It is also used for disk compatibility with other or older versions of operating systems.

Although storing the link value inside the inode saves a disk block and a disk read, the operating system still needs to parse the path name in the link, which always requires reading additional inodes and generally requires reading other, and potentially many, directories, processing both the list of files and the inodes of each of them until it finds a match with the link's path components. Only when a link points to a file in the same directory do "fast symlinks" provide significantly better performance than other symlinks.

The vast majority of POSIX-compliant implementations use fast symlinks. However, the POSIX standard does not require the entire set of file status information common to regular files to be implemented for symlinks. This allows implementations to use other solutions, such as storing symlink data in directory entries.

The file system permissions of a symbolic link usually have relevance only to rename or removal operations of the link itself, not to the access modes of the target file which are controlled by the target file's own permissions.

The reported size of a symlink is the number of characters in the path it points to.

Mac OS aliases[edit]

In Mac OS and some Linux distributions[clarification needed], applications or users can also employ aliases, which have the added feature of following the target, even if it is moved to another location on the same volume.

Microsoft Windows[edit]

Windows symbolic link[edit]

Windows Vista, Windows 7 & Windows 8 support symbolic links for both files and directories with the command line utility mklink. Unlike junction points, a symbolic link can also point to a file or remote Server Message Block (SMB) network path. Additionally, the NTFS symbolic link implementation provides full support for cross-filesystem links. However, the functionality enabling cross-host symbolic links requires that the remote system also support them, which effectively limits their support to Windows Vista and later Windows operating systems.

Symbolic links are designed to aid in migration and application compatibility with POSIX operating systems. Microsoft aimed for Vista's symbolic links to "function just like UNIX links".[4] However, the implementation varies from Unix symbolic links in several ways. For example, Vista users must manually indicate when creating a symbolic link whether it is a file or a directory.[5] Vista has a limit of 31 symbolic links in a single path.[6] Only users with the new Create Symbolic Link privilege, which only administrators have by default, can create symbolic links.[7] If this is not the desired behavior, it must be changed in the Local Security Policy management console.

In Vista, Windows 7 & Windows 8, when the working directory path ends with a symbolic link, the current parent path reference, .., will refer to the parent directory of the symbolic link rather than that of its target. This behaviour is also found at the shell level in at least some POSIX systems, including Linux, but never in accessing files and directories through operating system calls. For instance, bash builtin commands 'pwd' and 'cd' operate on the current logical directory. '/bin/pwd' is often used in scripts to determine the actual current working directory. When any path is used with a system call, any use of '..' will use the actual filesystem parent of the directory containing the '..' pseudo-directory entry. So, "cd ..; cat something" and "cat ../something" may return completely different results.

A third-party driver is available to enable symbolic links on Windows XP.[8]

NTFS Junction points[edit]

The Windows 2000 version of NTFS introduced reparse points, which enabled, among other things, the use of Volume Mount Points and junction points. Junction points are for directories only, and moreover, local directories only; junction points to remote shares are unsupported.[9] The Windows 2000 and XP Resource Kits include a program called linkd to create junction points; a more powerful one named Junction was distributed by Sysinternals' Mark Russinovich.

It is important to point out that not all standard applications support reparse points. Most noticeably, Backup suffers from this problem and will issue an error message 0x80070003[10] when the folders to be backed up contain a reparse point.

Shortcuts[edit]

Shortcuts, which are supported by the graphical file browsers of some operating systems, may resemble symbolic links but differ in a number of important ways. One difference is what type of software is able to follow them:

  • Symbolic links are automatically resolved by the file system. Any software program, upon accessing a symbolic link, will see the target instead, whether the program is aware of symbolic links or not.
  • Shortcuts are treated like ordinary files by the file system and by software programs that are not aware of them. Only software programs that understand shortcuts (such as the Windows shell and file browsers) treat them as references to other files.

Another difference are the capabilities of the mechanism:

  • Microsoft Windows shortcuts normally refer to a destination by an absolute path (starting from the root directory), whereas POSIX symbolic links can refer to destinations via either an absolute or a relative path. The latter is useful if both the location and destination of the symbolic link share a common path prefix, but that prefix is not yet known when the symbolic link is created (e.g., in an archive file that can be unpacked anywhere).
  • Microsoft Windows application shortcuts contain additional metadata that can be associated with the destination, whereas POSIX symbolic links are just strings that will be interpreted as absolute or relative pathnames.
  • Unlike symbolic links, Windows shortcuts maintain their references to their targets even when the target is moved or renamed. Windows domain clients may subscribe to a Windows service called Distributed Link Tracking to track the changes in files and folders to which they are interested. The service maintains the integrity of shortcuts, even when files and folders are moved across the network.[11] Additionally, in Windows 9x and later, Windows shell tries to find the target of a broken shortcut before proposing to delete it.

Folder Shortcuts[12][edit]

Almost like shortcuts, but transparent to the Windows shell. They are implemented as ordinary folders (which need to have the read only and/or system attribute[13]) containing a shortcut named target.lnk which refers to the target and a (hidden) desktop.ini with (at least) the following contents:

 [.ShellClassInfo]
 CLSID2={0AFACED1-E828-11D1-9187-B532F1E9575D}

Folder shortcuts are created and used from the Windows shell in the network neighborhood for example.

Shell Objects[edit]

The shell objects[14] or shell folders are defined in the Windows registry and can be used to implement a sort of symbolic link too. Like folder shortcuts, they are transparent to the Windows shell.

A minimal implementation is (the CLSID {00000000-0000-0000-0000-000000000000} is used as a placeholder):

 [HKEY_CLASSES_ROOT\CLSID\{00000000-0000-0000-0000-000000000000}]
 @="display name"
 [HKEY_CLASSES_ROOT\CLSID\{00000000-0000-0000-0000-000000000000}\DefaultIcon]
 @="..." ; path to icon
 [HKEY_CLASSES_ROOT\CLSID\{00000000-0000-0000-0000-000000000000}\InProcServer32]
 @="%SystemRoot%\\System32\\ShDocVw.Dll"
 "ThreadingModel"="Apartment"
 [HKEY_CLASSES_ROOT\CLSID\{00000000-0000-0000-0000-000000000000}\Instance]
 "CLSID"="{0AFACED1-E828-11D1-9187-B532F1E9575D}"
 [HKEY_CLASSES_ROOT\CLSID\{00000000-0000-0000-0000-000000000000}\Instance\InitPropertyBag]
 "Attributes"=hex:15,00,00,00
 "Target"="..." ; absolute (WITHOUT "TargetKnownFolder" or "TargetSpecialFolder" only)
                ; or relative path to target
 "TargetKnownFolder"="{guidguid-guid-guid-guid-guidguidguid}" ; GUID of target folder, Windows Vista and later
 "TargetSpecialFolder"="0x00xy" ; CSIDL of target
 [HKEY_CLASSES_ROOT\CLSID\{00000000-0000-0000-0000-000000000000}\ShellFolder]
 "Attributes"=hex:00,00,00,00

The My Documents folder on the Desktop as well as the Fonts and the Administrative Tools folders in the Control Panel are examples of shell objects redirected to file-system folders.

Cygwin symbolic links[edit]

Cygwin simulates POSIX-compliant symbolic links in the Microsoft Windows file system. It uses identical programming and user utility interfaces as Unix (see above), but creates Windows shortcuts (.lnk files) with additional information used by Cygwin at the time of symlink resolution. Cygwin symlinks are compliant with both Windows and the POSIX standard.[clarification needed]

Some differences exist, however. Cygwin has no way to specify shortcut-related information – such as working directory or icon – as there is no place for such parameters in ln -s command. To create standard Microsoft .lnk files Cygwin provides the mkshortcut and readshortcut utilities[15]

The Cygwin User's Guide[16] has more information on this topic.

Emulating Posix links under Windows[edit]

The different forms of symbolic links in Windows cover most user and administrator requirements. Unfortunately they do not fulfill important programmer requirements. They cannot be used to write functions with posix behavior. All symbolic links of Windows have shortcomings and not even a combination of them can be used to write POSIX functions. The crucial shortcomings are:

  • Vista symbolic links need administrator privileges to create them.
  • NTFS Junction points are for directories only.
  • Shortcuts work only for programs that are written to take them into account.
  • Cygwin symbolic links work only inside Cygwin. Outside they have the same shortcomings as shortcuts

Summary[edit]

Property/Action Symbolic link Junction Hard link
When symbolic link/junction/hard link is deleted... target remains unchanged target is deleted (except when using special tools) reference counter is decremented; when it reaches 0, the target is deleted
When target is moved symbolic link becomes invalid junction becomes invalid hard link remains valid
Relative path allowed not allowed (on save becomes an absolute path) not applicable
Drive requirement any drive allowed any drive allowed only same drive
Read of target allowed allowed only same drive (link stored in descriptor)
Windows for files Windows Vista/2008 and later
(administrator rights required)
no yes
for folders yes no
Unix for files yes not applicable yes
for directories yes not applicable no

Amiga[edit]

The command creating symbolic links is makelink, which is also used for hard links. Internally the dos.library returns an error code indicating that a target is a soft link if you try to perform actions on it that are only legal for a file, and applications that wish to follow the symbolic link then needs to explicitly make a call to follow the link and retry the operation. The AmigaDOS shell will follow links automatically.

OS/2[edit]

In the OS/2 operating system, symbolic links somewhat resemble shadows in the graphical Workplace Shell. However, shadows, due to the fully object-oriented System Object Model, are considerably more powerful and robust than a simple link. For example, shadows do not lose their capabilities when renamed or when either the object or subject of the link is relocated.[citation needed]

Variable symbolic links[edit]

Symbolic links may be implemented in a context-dependent or variable fashion, such that the link points to varying targets depending on a configuration parameter, run-time parameter, or other instantaneous condition.

A variable or variant symbolic link is a symbolic link that has a variable name embedded in it. This allows some flexibility in filesystem order that is not possible with a standard symbolic link. Variables embedded in a symbolic link may include user and environment specific information.

Operating systems that make use of variant symbolic links include NetBSD, DragonFly BSD, Domain/OS, and FreeBSD 10.[17]

HP/Tru64 uses a context dependent symbolic link where the context is the cluster member number.

Pyramid Technology's OSx Operating System implemented conditional symbolic links which pointed to different locations depending on which universe a program was running in. The universes supported were AT&T's SysV.3 and the Berkeley Software Distribution (BSD 4.3). For example: if the ps command was run in the att universe, then the symbolic link for the directory /bin would point to /.attbin and the program /.attbin/ps would be executed. Whereas if the ps command was run in the ucb universe, then /bin would point to /.ucbbin and /.ucbbin/ps would be executed. Similar Conditional Symbolic Links were also created for other directories such as /lib, /usr/lib, /usr/include.

See also[edit]

  • Symlink race — a security-vulnerability caused by symbolic links
  • freedup — generates links between identical data automatically

References[edit]

  1. ^ Pathname resolution, POSIX.
  2. ^ symlink, symlinkat. IEEE Std 1003.1, 2013 Edition.
  3. ^ link, linkat. IEEE Std 1003.1, 2013 Edition.
  4. ^ Symbolic Links, MSDN Library, Win32 and COM Development, 2008-01-18
  5. ^ CreateSymbolicLink Function, MSDN Library, Win32 and COM Development
  6. ^ Symbolic Link Programming Considerations, MSDN
  7. ^ Mark Russinovich: Inside the Windows Vista Kernel: Part 1 – File-based symbolic links, Microsoft Technet, February 2007.
  8. ^ [1], Link Shell Extension website
  9. ^ Sysinternals Junction documentation
  10. ^ http://support.microsoft.com/kb/973455
  11. ^ "Distributed Link Tracking and Object Identifiers". Microsoft Developers Network. Microsoft Corporation. 20 March 2011. Retrieved 30 June 2011. 
  12. ^ [2] Specifying a Namespace Extension's Location
  13. ^ [3] Unable to remove Read-Only attribute from folder
  14. ^ Creating Shell Extensions with Shell Instance Objects. msdn.microsoft.com
  15. ^ [4] Microsoft .lnk files in Cygwin
  16. ^ [5] Cygwin User's Guide, Cygwin.
  17. ^ [6] What's new for FreeBSD 10

External links[edit]

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.