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31 .\" @(#)symlink.7 8.3 (Berkeley) 3/31/94
32 .\" $FreeBSD: src/bin/ln/symlink.7,v 1.30 2005/02/13 22:25:09 ru Exp $
34 .\" 2008-06-11, mtk, Taken from FreeBSD 6.2 and heavily edited for
35 .\" specific Linux details, improved readability, and man-pages style.
37 .TH SYMLINK 7 2014-04-06 "Linux" "Linux Programmer's Manual"
39 symlink \- symbolic link handling
41 Symbolic links are files that act as pointers to other files.
42 To understand their behavior, you must first understand how hard links
45 A hard link to a file is indistinguishable from the original file because
46 it is a reference to the object underlying the original filename.
47 (To be precise: each of the hard links to a file is a reference to
50 where an inode number is an index into the inode table,
51 which contains metadata about all files on a filesystem.
54 Changes to a file are independent of the name used to reference the file.
55 Hard links may not refer to directories
56 (to prevent the possibility of loops within the filesystem tree,
57 which would confuse many programs)
58 and may not refer to files on different filesystems
59 (because inode numbers are not unique across filesystems).
61 A symbolic link is a special type of file whose contents are a string
62 that is the pathname of another file, the file to which the link refers.
63 (The contents of a symbolic link can be read using
65 In other words, a symbolic link is a pointer to another name,
66 and not to an underlying object.
67 For this reason, symbolic links may refer to directories and may cross
68 filesystem boundaries.
70 There is no requirement that the pathname referred to by a symbolic link
72 A symbolic link that refers to a pathname that does not exist is said
76 Because a symbolic link and its referenced object coexist in the filesystem
77 name space, confusion can arise in distinguishing between the link itself
78 and the referenced object.
79 On historical systems,
80 commands and system calls adopted their own link-following
81 conventions in a somewhat ad-hoc fashion.
82 Rules for a more uniform approach,
83 as they are implemented on Linux and other systems,
85 It is important that site-local applications also conform to these rules,
86 so that the user interface can be as consistent as possible.
87 .SS Symbolic link ownership, permissions, and timestamps
88 The owner and group of an existing symbolic link can be changed
91 The only time that the ownership of a symbolic link matters is
92 when the link is being removed or renamed in a directory that
93 has the sticky bit set (see
96 The last access and last modification timestamps
97 of a symbolic link can be changed using
102 On Linux, the permissions of a symbolic link are not used
103 in any operations; the permissions are always
104 0777 (read, write, and execute for all user categories),
105 .\" Linux does not currently implement an lchmod(2).
106 and can't be changed.
110 .\" system differs from historical
112 .\" systems in that the system call
114 .\" has been changed to follow symbolic links.
117 .\" system call was added later when the limitations of the new
120 .SS Obtaining a file descriptor that refers to a symbolic link
121 Using the combination of the
127 yields a file descriptor that can be passed as the
129 argument in system calls such as
136 in order to operate on the symbolic link itself
137 (rather than the file to which it refers).
141 .BR AT_SYMLINK_FOLLOW
142 flag is not specified), if
143 .BR name_to_handle_at (2)
144 is applied to a symbolic link, it yields a handle for the symbolic link
145 (rather than the file to which it refers).
146 One can then obtain a file descriptor for the symbolic link
147 (rather than the file to which it refers)
150 flag in a subsequent call to
151 .BR open_by_handle_at (2).
152 Again, that file descriptor can be used in the
153 aforementioned system calls to operate on the symbolic link itself.
154 .SS Handling of symbolic links by system calls and commands
155 Symbolic links are handled either by operating on the link itself,
156 or by operating on the object referred to by the link.
158 an application or system call is said to
161 Symbolic links may refer to other symbolic links,
162 in which case the links are dereferenced until an object that is
163 not a symbolic link is found,
164 a symbolic link that refers to a file which does not exist is found,
165 or a loop is detected.
166 (Loop detection is done by placing an upper limit on the number of
167 links that may be followed, and an error results if this limit is
170 There are three separate areas that need to be discussed.
173 Symbolic links used as filename arguments for system calls.
175 Symbolic links specified as command-line arguments to utilities that
176 are not traversing a file tree.
178 Symbolic links encountered by utilities that are traversing a file tree
179 (either specified on the command line or encountered as part of the
180 file hierarchy walk).
182 The first area is symbolic links used as filename arguments for
185 Except as noted below, all system calls follow symbolic links.
186 For example, if there were a symbolic link
188 which pointed to a file named
191 .I "open(""slink"" ...\&)"
192 would return a file descriptor referring to the file
195 Various system calls do not follow links, and operate
196 on the symbolic link itself.
201 .BR lremovexattr (2),
210 Certain other system calls optionally follow symbolic links.
213 .\" Maybe one day: .BR fchownat (2)
217 .BR name_to_handle_at (2),
220 .BR open_by_handle_at (2),
223 see their manual pages for details.
228 that library function also does not follow symbolic links.
231 is applied to a symbolic link, it fails with the error
236 warrants special discussion.
237 POSIX.1-2001 specifies that
241 if it is a symbolic link.
242 However, Linux does not do this.
243 (By default Solaris is the same,
244 but the POSIX.1-2001 specified behavior can be obtained with
245 suitable compiler options.)
246 The upcoming POSIX.1 revision changes the specification to allow
247 either behavior in an implementation.
248 .SS Commands not traversing a file tree
249 The second area is symbolic links, specified as command-line
250 filename arguments, to commands which are not traversing a file tree.
252 Except as noted below, commands follow symbolic links named as
253 command-line arguments.
254 For example, if there were a symbolic link
256 which pointed to a file named
260 would display the contents of the file
263 It is important to realize that this rule includes commands which may
264 optionally traverse file trees; for example, the command
266 is included in this rule, while the command
267 .IR "chown\ \-R file" ,
268 which performs a tree traversal, is not.
269 (The latter is described in the third area, below.)
271 If it is explicitly intended that the command operate on the symbolic
272 link instead of following the symbolic link\(emfor example, it is desired that
274 change the ownership of the file that
276 is, whether it is a symbolic link or not\(emthe
278 option should be used.
279 In the above example,
280 .I "chown root slink"
281 would change the ownership of the file referred to by
284 .I "chown\ \-h root slink"
285 would change the ownership of
289 There are some exceptions to this rule:
295 commands do not follow symbolic links named as arguments,
296 but respectively attempt to rename and delete them.
297 (Note, if the symbolic link references a file via a relative path,
298 moving it to another directory may very well cause it to stop working,
299 since the path may no longer be correct.)
303 command is also an exception to this rule.
304 For compatibility with historic systems (when
306 is not doing a tree walk\(emthat is,
308 option is not specified),
311 command follows symbolic links named as arguments if the
321 options are not specified.
324 command is the only command where the
328 options affect its behavior even though it is not doing a walk of
333 command is also an exception to this rule.
336 command does not follow symbolic links named as argument by default.
339 command does follow symbolic links named as argument if the
343 .\"The 4.4BSD system differs from historical 4BSD systems in that the
347 .\"commands follow symbolic links specified on the command line.
348 .SS Commands traversing a file tree
349 The following commands either optionally or always traverse file trees:
362 It is important to realize that the following rules apply equally to
363 symbolic links encountered during the file tree traversal and symbolic
364 links listed as command-line arguments.
366 The \fIfirst rule\fP applies to symbolic links that reference files other
368 Operations that apply to symbolic links are performed on the links
369 themselves, but otherwise the links are ignored.
372 .I "rm\ \-r slink directory"
375 as well as any symbolic links encountered in the tree traversal of
377 because symbolic links may be removed.
380 affect the file referred to by
383 The \fIsecond rule\fP applies to symbolic links that refer to directories.
384 Symbolic links that refer to directories are never followed by default.
385 This is often referred to as a "physical" walk, as opposed to a "logical"
386 walk (where symbolic links the refer to directories are followed).
388 Certain conventions are (should be) followed as consistently as
389 possible by commands that perform file tree walks:
391 A command can be made to follow
392 any symbolic links named on the command line,
393 regardless of the type of file they reference, by specifying the
395 (for "half-logical") flag.
396 This flag is intended to make the command-line name space look
397 like the logical name space.
398 (Note, for commands that do not always do file tree traversals, the
400 flag will be ignored if the
402 flag is not also specified.)
404 For example, the command
405 .I "chown\ \-HR user slink"
406 will traverse the file hierarchy rooted in the file pointed to by
410 is not the same as the previously discussed
415 flag causes symbolic links specified on the command line to be
416 dereferenced for the purposes of both the action to be performed
417 and the tree walk, and it is as if the user had specified the
418 name of the file to which the symbolic link pointed.
420 A command can be made to
421 follow any symbolic links named on the command line,
422 as well as any symbolic links encountered during the traversal,
423 regardless of the type of file they reference, by specifying the
425 (for "logical") flag.
426 This flag is intended to make the entire name space look like
427 the logical name space.
428 (Note, for commands that do not always do file tree traversals, the
430 flag will be ignored if the
432 flag is not also specified.)
434 For example, the command
435 .I "chown\ \-LR user slink"
436 will change the owner of the file referred to by
440 refers to a directory,
442 will traverse the file hierarchy rooted in the directory that it
444 In addition, if any symbolic links are encountered in any file tree that
446 traverses, they will be treated in the same fashion as
449 A command can be made to
450 provide the default behavior by specifying the
452 (for "physical") flag.
453 This flag is intended to make the entire name space look like the
456 For commands that do not by default do file tree traversals, the
461 flags are ignored if the
463 flag is not also specified.
464 In addition, you may specify the
469 options more than once;
470 the last one specified determines the command's behavior.
471 This is intended to permit you to alias commands to behave one way
472 or the other, and then override that behavior on the command line.
478 commands have exceptions to these rules:
482 command operates on the symbolic link, and not the file it references,
483 and therefore never follows a symbolic link.
486 command does not support the
493 To maintain compatibility with historic systems,
496 command acts a little differently.
497 If you do not specify the
504 will follow symbolic links specified on the command line.
509 follows all symbolic links,
510 regardless of their type,
511 whether specified on the command line or encountered in the tree walk.
529 .BR path_resolution (7)
531 This page is part of release 3.79 of the Linux
534 A description of the project,
535 information about reporting bugs,
536 and the latest version of this page,
538 \%http://www.kernel.org/doc/man\-pages/.