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29 .\" @(#)symlink.7 8.3 (Berkeley) 3/31/94
37 .Nd symbolic link handling
38 .Sh SYMBOLIC LINK HANDLING
39 Symbolic links are files that act as pointers to other files.
40 To understand their behavior, you must first understand how hard links
42 A hard link to a file is indistinguishable from the original file because
43 it is a reference to the object underlying the original file name.
44 Changes to a file are independent of the name used to reference the
46 Hard links may not refer to directories and may not reference files
47 on different file systems.
48 A symbolic link contains the name of the file to which it is linked,
49 i.e., it is a pointer to another name, and not to an underlying object.
50 For this reason, symbolic links may reference directories and may span
53 Because a symbolic link and its referenced object coexist in the file system
54 name space, confusion can arise in distinguishing between the link itself
55 and the referenced object.
56 Historically, commands and system calls have adopted their own link
57 following conventions in a somewhat ad-hoc fashion.
58 Rules for more a uniform approach, as they are implemented in this system,
60 It is important that local applications conform to these rules, too,
61 so that the user interface can be as consistent as possible.
63 Symbolic links are handled either by operating on the link itself,
64 or by operating on the object referenced by the link.
66 an application or system call is said to
69 Symbolic links may reference other symbolic links,
70 in which case the links are dereferenced until an object that is
71 not a symbolic link is found,
72 a symbolic link which references a file which does not exist is found,
73 or a loop is detected.
74 (Loop detection is done by placing an upper limit on the number of
75 links that may be followed, and an error results if this limit is
78 There are three separate areas that need to be discussed.
81 .Bl -enum -compact -offset indent
83 Symbolic links used as file name arguments for system calls.
85 Symbolic links specified as command line arguments to utilities that
86 are not traversing a file tree.
88 Symbolic links encountered by utilities that are traversing a file tree
89 (either specified on the command line or encountered as part of the
93 The first area is symbolic links used as file name arguments for
96 Except as noted below, all system calls follow symbolic links.
97 For example, if there were a symbolic link
99 which pointed to a file named
102 .Dq Li open("slink" ...\&)
103 would return a file descriptor to the file
106 There are thirteen system calls that do not follow links, and which operate
107 on the symbolic link itself.
127 it also does not follow symbolic links.
135 is applied to a symbolic link, it fails with the error
140 system call does not follow symbolic links
142 .Dv AT_SYMLINK_FOLLOW
145 The following system calls follow symbolic links
147 .Dv AT_SYMLINK_NOFOLLOW
155 The owner and group of an existing symbolic link can be changed by
159 The flags, access permissions, owner/group and modification time of
160 an existing symbolic link can be changed by means of the
166 system calls, respectively.
167 Of these, only the flags and ownership are used by the system;
168 the access permissions are ignored.
172 system differs from historical
174 systems in that the system call
176 has been changed to follow symbolic links.
179 system call was added later when the limitations of the new
182 .Ss Commands not traversing a file tree.
183 The second area is symbolic links, specified as command line file
184 name arguments, to commands which are not traversing a file tree.
186 Except as noted below, commands follow symbolic links named as command
188 For example, if there were a symbolic link
190 which pointed to a file named
194 would display the contents of the file
197 It is important to realize that this rule includes commands which may
198 optionally traverse file trees, e.g.\& the command
200 is included in this rule, while the command
201 .Dq Li "chown -R file"
203 (The latter is described in the third area, below.)
205 If it is explicitly intended that the command operate on the symbolic
206 link instead of following the symbolic link, e.g., it is desired that
208 change the ownership of the file that
210 is, whether it is a symbolic link or not, the
212 option should be used.
213 In the above example,
214 .Dq Li "chown root slink"
215 would change the ownership of the file referenced by
218 .Dq Li "chown -h root slink"
219 would change the ownership of
223 There are five exceptions to this rule.
228 commands do not follow symbolic links named as arguments,
229 but respectively attempt to rename and delete them.
230 (Note, if the symbolic link references a file via a relative path,
231 moving it to another directory may very well cause it to stop working,
232 since the path may no longer be correct.)
236 command is also an exception to this rule.
237 For compatibility with historic systems (when
239 is not doing a tree walk, i.e., the
241 option is not specified),
244 command follows symbolic links named as arguments if the
254 options are not specified.
257 command is the only command where the
261 options affect its behavior even though it is not doing a walk of
268 commands are also exceptions to this rule.
270 commands do not follow symbolic links named as argument by default,
271 but do follow symbolic links named as argument if the
277 system differs from historical
283 commands follow symbolic links specified on the command line.
284 .Ss Commands traversing a file tree.
285 The following commands either optionally or always traverse file trees:
299 It is important to realize that the following rules apply equally to
300 symbolic links encountered during the file tree traversal and symbolic
301 links listed as command line arguments.
303 The first rule applies to symbolic links that reference files that are
304 not of type directory.
305 Operations that apply to symbolic links are performed on the links
306 themselves, but otherwise the links are ignored.
309 .Dq Li "rm -r slink directory"
312 as well as any symbolic links encountered in the tree traversal of
314 because symbolic links may be removed.
317 affect the file which
319 references in any way.
321 The second rule applies to symbolic links that reference files of type
323 Symbolic links which reference files of type directory are never
326 This is often referred to as a
328 walk, as opposed to a
330 walk (where symbolic links referencing directories are followed).
332 As consistently as possible, you can make commands doing a file tree
333 walk follow any symbolic links named on the command line, regardless
334 of the type of file they reference, by specifying the
339 This flag is intended to make the command line name space look
340 like the logical name space.
341 (Note, for commands that do not always do file tree traversals, the
343 flag will be ignored if the
345 flag is not also specified.)
347 For example, the command
348 .Dq Li "chown -HR user slink"
349 will traverse the file hierarchy rooted in the file pointed to by
353 is not the same as the previously discussed
358 flag causes symbolic links specified on the command line to be
359 dereferenced both for the purposes of the action to be performed
360 and the tree walk, and it is as if the user had specified the
361 name of the file to which the symbolic link pointed.
363 As consistently as possible, you can make commands doing a file tree
364 walk follow any symbolic links named on the command line, as well as
365 any symbolic links encountered during the traversal, regardless of
366 the type of file they reference, by specifying the
371 This flag is intended to make the entire name space look like
372 the logical name space.
373 (Note, for commands that do not always do file tree traversals, the
375 flag will be ignored if the
377 flag is not also specified.)
379 For example, the command
380 .Dq Li "chown -LR user slink"
381 will change the owner of the file referenced by
385 references a directory,
387 will traverse the file hierarchy rooted in the directory that it
389 In addition, if any symbolic links are encountered in any file tree that
391 traverses, they will be treated in the same fashion as
394 As consistently as possible, you can specify the default behavior by
400 This flag is intended to make the entire name space look like the
403 For commands that do not by default do file tree traversals, the
408 flags are ignored if the
410 flag is not also specified.
411 In addition, you may specify the
416 options more than once; the last one specified determines the
418 This is intended to permit you to alias commands to behave one way
419 or the other, and then override that behavior on the command line.
425 commands have exceptions to these rules.
428 command operates on the symbolic link, and not the file it references,
429 and therefore never follows a symbolic link.
432 command does not support the
439 To maintain compatibility with historic systems,
442 command acts a little differently.
443 If you do not specify the
450 will follow symbolic links specified on the command line.
455 follows all symbolic links,
456 regardless of their type,
457 whether specified on the command line or encountered in the tree walk.