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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 2016-10-08 "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.
107 (Note that there are some "magic" symbolic links in the
109 directory tree\(emfor example, the
111 files\(emthat have different permissions.)
115 .\" system differs from historical
117 .\" systems in that the system call
119 .\" has been changed to follow symbolic links.
122 .\" system call was added later when the limitations of the new
125 .SS Obtaining a file descriptor that refers to a symbolic link
126 Using the combination of the
132 yields a file descriptor that can be passed as the
134 argument in system calls such as
141 in order to operate on the symbolic link itself
142 (rather than the file to which it refers).
146 .BR AT_SYMLINK_FOLLOW
147 flag is not specified), if
148 .BR name_to_handle_at (2)
149 is applied to a symbolic link, it yields a handle for the symbolic link
150 (rather than the file to which it refers).
151 One can then obtain a file descriptor for the symbolic link
152 (rather than the file to which it refers)
155 flag in a subsequent call to
156 .BR open_by_handle_at (2).
157 Again, that file descriptor can be used in the
158 aforementioned system calls to operate on the symbolic link itself.
159 .SS Handling of symbolic links by system calls and commands
160 Symbolic links are handled either by operating on the link itself,
161 or by operating on the object referred to by the link.
163 an application or system call is said to
166 Symbolic links may refer to other symbolic links,
167 in which case the links are dereferenced until an object that is
168 not a symbolic link is found,
169 a symbolic link that refers to a file which does not exist is found,
170 or a loop is detected.
171 (Loop detection is done by placing an upper limit on the number of
172 links that may be followed, and an error results if this limit is
175 There are three separate areas that need to be discussed.
178 Symbolic links used as filename arguments for system calls.
180 Symbolic links specified as command-line arguments to utilities that
181 are not traversing a file tree.
183 Symbolic links encountered by utilities that are traversing a file tree
184 (either specified on the command line or encountered as part of the
185 file hierarchy walk).
187 The first area is symbolic links used as filename arguments for
190 Except as noted below, all system calls follow symbolic links.
191 For example, if there were a symbolic link
193 which pointed to a file named
196 .I "open(""slink"" ...\&)"
197 would return a file descriptor referring to the file
200 Various system calls do not follow links, and operate
201 on the symbolic link itself.
206 .BR lremovexattr (2),
215 Certain other system calls optionally follow symbolic links.
218 .\" Maybe one day: .BR fchownat (2)
222 .BR name_to_handle_at (2),
225 .BR open_by_handle_at (2),
228 see their manual pages for details.
233 that library function also does not follow symbolic links.
236 is applied to a symbolic link, it fails with the error
240 warrants special discussion.
241 POSIX.1-2001 specifies that
245 if it is a symbolic link.
246 However, Linux does not do this.
247 (By default, Solaris is the same,
248 but the POSIX.1-2001 specified behavior can be obtained with
249 suitable compiler options.)
250 POSIX.1-2008 changed the specification to allow
251 either behavior in an implementation.
252 .SS Commands not traversing a file tree
253 The second area is symbolic links, specified as command-line
254 filename arguments, to commands which are not traversing a file tree.
256 Except as noted below, commands follow symbolic links named as
257 command-line arguments.
258 For example, if there were a symbolic link
260 which pointed to a file named
264 would display the contents of the file
267 It is important to realize that this rule includes commands which may
268 optionally traverse file trees; for example, the command
270 is included in this rule, while the command
271 .IR "chown\ \-R file" ,
272 which performs a tree traversal, is not.
273 (The latter is described in the third area, below.)
275 If it is explicitly intended that the command operate on the symbolic
276 link instead of following the symbolic link\(emfor example, it is desired that
278 change the ownership of the file that
280 is, whether it is a symbolic link or not\(emthe
282 option should be used.
283 In the above example,
284 .I "chown root slink"
285 would change the ownership of the file referred to by
288 .I "chown\ \-h root slink"
289 would change the ownership of
293 There are some exceptions to this rule:
299 commands do not follow symbolic links named as arguments,
300 but respectively attempt to rename and delete them.
301 (Note, if the symbolic link references a file via a relative path,
302 moving it to another directory may very well cause it to stop working,
303 since the path may no longer be correct.)
307 command is also an exception to this rule.
308 For compatibility with historic systems (when
310 is not doing a tree walk\(emthat is,
312 option is not specified),
315 command follows symbolic links named as arguments if the
325 options are not specified.
328 command is the only command where the
332 options affect its behavior even though it is not doing a walk of
337 command is also an exception to this rule.
340 command does not follow symbolic links named as argument by default.
343 command does follow symbolic links named as argument if the
347 .\"The 4.4BSD system differs from historical 4BSD systems in that the
351 .\"commands follow symbolic links specified on the command line.
352 .SS Commands traversing a file tree
353 The following commands either optionally or always traverse file trees:
366 It is important to realize that the following rules apply equally to
367 symbolic links encountered during the file tree traversal and symbolic
368 links listed as command-line arguments.
370 The \fIfirst rule\fP applies to symbolic links that reference files other
372 Operations that apply to symbolic links are performed on the links
373 themselves, but otherwise the links are ignored.
376 .I "rm\ \-r slink directory"
379 as well as any symbolic links encountered in the tree traversal of
381 because symbolic links may be removed.
384 affect the file referred to by
387 The \fIsecond rule\fP applies to symbolic links that refer to directories.
388 Symbolic links that refer to directories are never followed by default.
389 This is often referred to as a "physical" walk, as opposed to a "logical"
390 walk (where symbolic links that refer to directories are followed).
392 Certain conventions are (should be) followed as consistently as
393 possible by commands that perform file tree walks:
395 A command can be made to follow
396 any symbolic links named on the command line,
397 regardless of the type of file they reference, by specifying the
399 (for "half-logical") flag.
400 This flag is intended to make the command-line name space look
401 like the logical name space.
402 (Note, for commands that do not always do file tree traversals, the
404 flag will be ignored if the
406 flag is not also specified.)
408 For example, the command
409 .I "chown\ \-HR user slink"
410 will traverse the file hierarchy rooted in the file pointed to by
414 is not the same as the previously discussed
419 flag causes symbolic links specified on the command line to be
420 dereferenced for the purposes of both the action to be performed
421 and the tree walk, and it is as if the user had specified the
422 name of the file to which the symbolic link pointed.
424 A command can be made to
425 follow any symbolic links named on the command line,
426 as well as any symbolic links encountered during the traversal,
427 regardless of the type of file they reference, by specifying the
429 (for "logical") flag.
430 This flag is intended to make the entire name space look like
431 the logical name space.
432 (Note, for commands that do not always do file tree traversals, the
434 flag will be ignored if the
436 flag is not also specified.)
438 For example, the command
439 .I "chown\ \-LR user slink"
440 will change the owner of the file referred to by
444 refers to a directory,
446 will traverse the file hierarchy rooted in the directory that it
448 In addition, if any symbolic links are encountered in any file tree that
450 traverses, they will be treated in the same fashion as
453 A command can be made to
454 provide the default behavior by specifying the
456 (for "physical") flag.
457 This flag is intended to make the entire name space look like the
460 For commands that do not by default do file tree traversals, the
465 flags are ignored if the
467 flag is not also specified.
468 In addition, you may specify the
473 options more than once;
474 the last one specified determines the command's behavior.
475 This is intended to permit you to alias commands to behave one way
476 or the other, and then override that behavior on the command line.
482 commands have exceptions to these rules:
486 command operates on the symbolic link, and not the file it references,
487 and therefore never follows a symbolic link.
490 command does not support the
497 To maintain compatibility with historic systems,
500 command acts a little differently.
501 If you do not specify the
508 will follow symbolic links specified on the command line.
513 follows all symbolic links,
514 regardless of their type,
515 whether specified on the command line or encountered in the tree walk.
534 .BR path_resolution (7)