Symbolic link
Encyclopedia
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. 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 system
s 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
.
Symbolic links operate transparently for most operations: programs which 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 link
s. Hard links do not normally point to directories, and they cannot link paths on different volumes
or file systems
. Symbolic links may point to any file or directory irrespective of the volumes
on which the source and destination reside.
Whereas hard links always refer to an existing file, symbolic links may contain arbitrary text that doesn't 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 and 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.
-compliant operating systems, symbolic links are created with the symlink system call. The
The following command creates a symbolic link at the command-line interface
(shell):
source_file is the relative or absolute path to which the symlink should point. Usually the target will exist, although symbolic links may be created to non-existent targets. link_name is the desired name 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.,
The POSIX directory listing application,
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 structure
s used for storing file information on disk (inode
s). 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.
distributions, 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.
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". 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. Vista has a limit of 31 symbolic links in a single path. Only users with the new Create Symbolic Link privilege, which only administrators have by default, can create symbolic links. If this is not the desired behavior, it must be changed in the Local Security Policy management console.
In Windows 7 and Windows Vista
, 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 in at least some POSIX systems, including Linux
.
version of NTFS
introduced reparse points
, which enabled, among other things, the use of Volume Mount Point
s and junction points
. Junction points are for directories only, and moreover, local directories only; junction points to remote shares are unsupported. 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
.
, 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:
Another difference are the capabilities of the mechanism:
They are implemented as ordinary folders (which need to have the read only and/or system attribute) containing a shortcut named target.lnk which refers to the target and a (hidden) desktop.ini with (at least) the following contents:
Folder shortcuts are created and used from the Windows shell in the network neighborhood for example.
A minimal implementation is (the CLSID {00000000-0000-0000-0000-000000000000} is used as a placeholder):
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.
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.
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
The Cygwin User's Guide has more information on this topic.
Amiga
The command creating symbolic links is
shell will follow links automatically.
operating system, symbolic links resemble shadows
in the graphical Workplace Shell
.
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 links may include user and or environment specific information.
Operating system
s that make use of variant symbolic links include NetBSD
, DragonFly BSD
and Domain/OS
.
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.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, a symbolic link (also symlink or soft link) is a special type of file
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
that contains a reference to another file or directory in the form of an absolute or relative path
Path (computing)
A path, the general form of a filename or of a directory name, specifies a unique location in a file system. A path points to a file system location by following the directory tree hierarchy expressed in a string of characters in which path components, separated by a delimiting character, represent...
and that affects pathname resolution. Symbolic links were already present by 1978 in mini-computer operating systems from DEC
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
and Data General's RDOS
RDOS
RDOS was a real-time operating system released in 1972 for the popular Data General Nova and Eclipse minicomputers...
. Today they are supported by the POSIX
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
operating-system standard, most Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s such as FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
, GNU/Linux, and Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
, and also Windows operating systems such as Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
, Windows 7 and to some degree in Windows 2000
Windows 2000
Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...
and Windows XP
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
.
Symbolic links operate transparently for most operations: programs which 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 link
Hard link
In computing, a hard link is a directory entry that associates a name with a file on a file system. . The term is used in file systems which allow multiple hard links to be created for the same file. This has the effect of creating multiple names for the same file, causing an aliasing effect: e.g...
s. Hard links do not normally point to directories, and they cannot link paths on different volumes
Volume (computing)
In the context of computer operating systems, volume is the term used to describe a single accessible storage area with a single file system, typically resident on a single partition of a hard disk. Similarly, it refers to the logical interface used by an operating system to access data stored on...
or file systems
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
. Symbolic links may point to any file or directory irrespective of the volumes
Volume (computing)
In the context of computer operating systems, volume is the term used to describe a single accessible storage area with a single file system, typically resident on a single partition of a hard disk. Similarly, it refers to the logical interface used by an operating system to access data stored on...
on which the source and destination reside.
Whereas hard links always refer to an existing file, symbolic links may contain arbitrary text that doesn't point to anything.
Some Unix as well as Linux distributions use symbolic links extensively in an effort to reorder the file system
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
hierarchy. This is accomplished with several mechanisms, such as variant and 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
In POSIXPOSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
-compliant operating systems, symbolic links are created with the symlink system call. The
lnLn (Unix)ln is a standard Unix command used to create links to files.- Link files :Links allow more than one file name to refer to the same file, elsewhere.There are two types of links, both of which are created by ln:...
shell command normally uses the link 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.The following command creates a symbolic link at the command-line interface
Command-line interface
A command-line interface is a mechanism for interacting with a computer operating system or software by typing commands to perform specific tasks...
(shell):
ln -s source_file link_name
source_file is the relative or absolute path to which the symlink should point. Usually the target will exist, although symbolic links may be created to non-existent targets. link_name is the desired name 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.,
cpCp (Unix)cp is a UNIX command used to copy a file. Files can be copied either to the same directory or to a completely different directory, possibly on a different file system or hard disk drive. If the file is copied to the same directory, the new file must have a different name to the original; in all...
, rmRm (Unix)rm is a basic UNIX command used to remove objects such as files, directories, device nodes, symbolic links, and so on from the filesystem...
) 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,
lsLsIn computing, ls is a command to list files in Unix and Unix-like operating systems. ls is specified by POSIX and the Single UNIX Specification.- History :An ls utility appeared in the original version of AT&T UNIX...
, 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).
$ 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
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 denoting it as a symbolic link.This method was slow and an inefficient use of disk-space
Disk storage
Disk storage or disc storage is a general category of storage mechanisms, in which data are digitally recorded by various electronic, magnetic, optical, or mechanical methods on a surface layer deposited of one or more planar, round and rotating disks...
on small systems. An improvement, called fast symlinks, allowed storage of the target path within the data structure
Data structure
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
s used for storing file information on disk (inode
Inode
In computing, an inode is a data structure on a traditional Unix-style file system such as UFS. An inode stores all the information about a regular file, directory, or other file system object, except its data and name....
s). This space normally stores a list of disk block
Block (data storage)
In computing , a block is a sequence of bytes or bits, having a nominal length . Data thus structured are said to be blocked. The process of putting data into blocks is called blocking. Blocking is used to facilitate the handling of the data-stream by the computer program receiving the data...
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
Retronym
A retronym is a type of neologism that provides a new name for an object or concept to differentiate the original form or version of it from a more recent form or version. The original name is most often augmented with an adjective to account for later developments of the object or concept itself...
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
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
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
File system permissions
Most current file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users to view or make changes to the contents of the filesystem....
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
In Mac OS and some LinuxLinux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
distributions, applications or users can also employ aliases
Alias (Mac OS)
In Mac OS System 7 and later, an alias is a small file that represents another object in a local, remote, or removable file system and provides a dynamic link to it; the target object may be moved, and the alias will still link to it...
, which have the added feature of following the target, even if it is moved to another location on the same volume.
Windows 7 & Vista symbolic link
Windows 7 and Windows VistaWindows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
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
Server Message Block
In computer networking, Server Message Block , also known as Common Internet File System operates as an application-layer network protocol mainly used to provide shared access to files, printers, serial ports, and miscellaneous communications between nodes on a network. It also provides an...
(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
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
operating systems. Microsoft aimed for Vista's symbolic links to "function just like UNIX links". 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. Vista has a limit of 31 symbolic links in a single path. Only users with the new Create Symbolic Link privilege, which only administrators have by default, can create symbolic links. If this is not the desired behavior, it must be changed in the Local Security Policy management console.
In Windows 7 and Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
, 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 in at least some POSIX systems, including Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
.
NTFS Junction points
The Windows 2000Windows 2000
Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...
version of NTFS
NTFS
NTFS is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7....
introduced reparse points
NTFS reparse point
An NTFS reparse point is a type of NTFS file system object. It is available with the NTFS v3.0 found in Windows 2000 or later versions. Reparse points provide a way to extend the NTFS filesystem by adding extra information to the directory entry, so a file system filter can interpret how the...
, which enabled, among other things, the use of Volume Mount Point
Volume Mount Point
Volume Mount Points are specialized NTFS filesystem objects which are used to mount and provide an entry point to other volumes. Mount points can be created in a directory on an NTFS file system, which gives a reference to the root directory of the mounted volume. In fact, any empty directory can...
s and junction points
NTFS junction point
An NTFS junction point is a feature of the NTFS file system that provides the ability to create a symbolic link to a directory which then functions as an alias of that directory...
. Junction points are for directories only, and moreover, local directories only; junction points to remote shares are unsupported. 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
Sysinternals
Windows Sysinternals is a part of the Microsoft TechNet website which offers technical resources and utilities to manage, diagnose, troubleshoot, and monitor a Microsoft Windows environment. Originally, the Sysinternals website was created in 1996 and was operated by the company Winternals...
' Mark Russinovich
Mark Russinovich
Mark E. Russinovich is a Technical Fellow in the Platform and Services Division at Microsoft. He was a cofounder of software producers Winternals before it was acquired by Microsoft in 2006.-Early life and education:...
.
Shortcuts
ShortcutsComputer shortcut
A file shortcut in Microsoft Windows is a small file containing a target URI or GUID to an object, or the name of a target program file that the shortcut represents. The shortcut might additionally specify parameters to be passed to the target program when it is run. Each shortcut can have its own...
, 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 WindowsMicrosoft WindowsMicrosoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
shortcuts normally refer to a destination by an absolute path (starting from the root directoryRoot directoryIn computer file systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the root of a tree — the starting point where all branches originate.-Metaphor:...
), 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 prefixPrefixA prefix is an affix which is placed before the root of a word. Particularly in the study of languages,a prefix is also called a preformative, because it alters the form of the words to which it is affixed.Examples of prefixes:...
, but that prefix is not yet known when the symbolic link is created (e.g., in an archive fileArchive fileAn archive file is a file that is composed of one or more files along with metadata that can include source volume and medium information, file directory structure, error detection and recovery information, file comments, and usually employs some form of lossless compression. Archive files may be...
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 serviceWindows ServiceOn Microsoft Windows operating systems, a Windows service is a long-running executable that performs specific functions and which is designed not to require user intervention. Windows services can be configured to start when the operating system is booted and run in the background as long as...
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.Additionally, in Windows 9x and later, Windows shellWindows ShellThe Windows shell is the main graphical user interface in Microsoft Windows, and since Windows 95 hosted by Windows Explorer. The Windows shell includes well-known Windows components such as the Taskbar and the Start menu...
tries to find the target of a broken shortcut before proposing to delete it.
Folder Shortcuts
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) 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
shell objects or shell folders are defined in the Windows registry and can be used to implement sort of symbolic links 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
CygwinCygwin
Cygwin is a Unix-like environment and command-line interface for Microsoft Windows. Cygwin provides native integration of Windows-based applications, data, and other system resources with applications, software tools, and data of the Unix-like environment...
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.
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 utilitiesThe Cygwin User's Guide has more information on this topic.
AmigaAmigaThe Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...
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 AmigaDOSAmigaDOS
AmigaDOS is the disk operating system of the AmigaOS, which includes file systems, file and directory manipulation, the command-line interface, and file redirection....
shell will follow links automatically.
OS/2
In the OS/2OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
operating system, symbolic links resemble shadows
Shadow (OS/2)
In the graphical Workplace Shell of the OS/2 operating system, a shadow is an object that represents another object.A shadow as a stand-in for any other object on the desktop, such as a document, an application, a folder, a hard disk, a network share or removable medium, or a printer. A target...
in the graphical Workplace Shell
Workplace Shell
The Workplace Shell is a object-oriented desktop shell produced by IBM's Boca Raton development lab for OS/2 2.0. It is based on Common User Access and made a radical shift away from the Program Manager type interface that earlier versions of OS/2 shared with Windows 3.x or the...
.
Variable symbolic links
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 links may include user and or environment specific information.
Operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s that make use of variant symbolic links include NetBSD
NetBSD
NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...
, DragonFly BSD
DragonFly BSD
DragonFly BSD is a free Unix-like operating system created as a fork of FreeBSD 4.8. Matthew Dillon, an Amiga developer in the late 1980s and early 1990s and a FreeBSD developer between 1994 and 2003, began work on DragonFly BSD in June 2003 and announced it on the FreeBSD mailing lists on July...
and Domain/OS
Domain/OS
Domain/OS is the operating system used by the Apollo/Domain line of workstations manufactured by Apollo Computer, Inc. during the late 1980s, as the successor to the one previously used, AEGIS. It was one of the early distributed operating systems...
.
HP/Tru64 uses a context dependent symbolic link where the context is the cluster member number.
Pyramid Technology
Pyramid Technology
Pyramid Technology Corporation was a computer company that produced a number of RISC-based minicomputers at the upper end of the performance range. They also became the second company to ship a multiprocessor Unix system , in 1985, which formed the basis of their product line into the early 1990s...
's OSx Operating System
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
implemented conditional symbolic links which pointed to different locations depending on which universe
Universe (Unix)
In some versions of the Unix operating system, the term universe was used to denote some variant of the working environment. During the late 1980s, most commercial Unix variants were derived from either System V or BSD. Most versions provided both BSD and System V universes and allowed the user to...
a program was running in. The universes supported were AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
's SysV.3
UNIX System V
Unix System V, commonly abbreviated SysV , is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4...
and the Berkeley Software Distribution
Berkeley Software Distribution
Berkeley Software Distribution is a Unix operating system derivative developed and distributed by the Computer Systems Research Group of the University of California, Berkeley, from 1977 to 1995...
(BSD 4.3). For example: if the ps
Ps (Unix)
In most Unix-like operating systems, the ps program displays the currently-running processes. A related Unix utility named top provides a real-time view of the running processes....
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
- Symlink raceSymlink raceA symlink race is a kind of software security vulnerability that results from a program creating files in an insecure manner. A malicious user can create a symbolic link to a file not otherwise accessible to him or her...
, a security-vulnerability caused by symbolic links - freedupFreedupfreedup is a program to scan directories or file lists for duplicate files. The file lists may be provided to an input pipe or internally generated using find with provided options. There are more options to specify the search conditions more detailed. Other options influence the performed actions,...
— generates links between identical data automatically - Hard linkHard linkIn computing, a hard link is a directory entry that associates a name with a file on a file system. . The term is used in file systems which allow multiple hard links to be created for the same file. This has the effect of creating multiple names for the same file, causing an aliasing effect: e.g...
— related but distinct concept
External links
- Q & A: The difference between hard and soft links as applied to Linux
- Junction: maintain NTFS junction points (for Windows 2000 and above)
- FSUtil Hardlink: Microsoft Technet page on using the command-line tool FSUtil to create hardlinks (for Windows 2000 and above)
- Link Shell Extension: Windows Explorer context menu and property sheet handler to maintain hardlinks, NTFS junction points, symbolic links etc.