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29 .\" @(#)symlink.7 8.3 (Berkeley) 3/31/94
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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 nine system calls that do not follow links, and which operate
107 on the symbolic link itself.
123 it also does not follow symbolic links.
126 is applied to a symbolic link, it fails with the error
129 The owner and group of an existing symbolic link can be changed by
133 The flags, access permissions, owner/group and modification time of
134 an existing symbolic link can be changed by means of the
140 system calls, respectively.
141 Of these, only the flags are used by the system;
142 the access permissions and ownership are ignored.
146 system differs from historical
148 systems in that the system call
150 has been changed to follow symbolic links.
153 system call was added later when the limitations of the new
156 .Ss Commands not traversing a file tree.
157 The second area is symbolic links, specified as command line file
158 name arguments, to commands which are not traversing a file tree.
160 Except as noted below, commands follow symbolic links named as command
162 For example, if there were a symbolic link
164 which pointed to a file named
168 would display the contents of the file
171 It is important to realize that this rule includes commands which may
172 optionally traverse file trees, e.g.\& the command
174 is included in this rule, while the command
175 .Dq Li "chown -R file"
177 (The latter is described in the third area, below.)
179 If it is explicitly intended that the command operate on the symbolic
180 link instead of following the symbolic link, e.g., it is desired that
182 change the ownership of the file that
184 is, whether it is a symbolic link or not, the
186 option should be used.
187 In the above example,
188 .Dq Li "chown root slink"
189 would change the ownership of the file referenced by
192 .Dq Li "chown -h root slink"
193 would change the ownership of
197 There are four exceptions to this rule.
202 commands do not follow symbolic links named as arguments,
203 but respectively attempt to rename and delete them.
204 (Note, if the symbolic link references a file via a relative path,
205 moving it to another directory may very well cause it to stop working,
206 since the path may no longer be correct.)
210 command is also an exception to this rule.
211 For compatibility with historic systems (when
213 is not doing a tree walk, i.e., the
215 option is not specified),
218 command follows symbolic links named as arguments if the
228 options are not specified.
231 command is the only command where the
235 options affect its behavior even though it is not doing a walk of
240 command is also an exception to this rule.
243 command does not follow symbolic links named as argument by default.
246 command does follow symbolic links named as argument if
252 system differs from historical
258 commands follow symbolic links specified on the command line.
259 .Ss Commands traversing a file tree.
260 The following commands either optionally or always traverse file trees:
274 It is important to realize that the following rules apply equally to
275 symbolic links encountered during the file tree traversal and symbolic
276 links listed as command line arguments.
278 The first rule applies to symbolic links that reference files that are
279 not of type directory.
280 Operations that apply to symbolic links are performed on the links
281 themselves, but otherwise the links are ignored.
284 .Dq Li "rm -r slink directory"
287 as well as any symbolic links encountered in the tree traversal of
289 because symbolic links may be removed.
292 affect the file which
294 references in any way.
296 The second rule applies to symbolic links that reference files of type
298 Symbolic links which reference files of type directory are never
301 This is often referred to as a
303 walk, as opposed to a
305 walk (where symbolic links referencing directories are followed).
307 As consistently as possible, you can make commands doing a file tree
308 walk follow any symbolic links named on the command line, regardless
309 of the type of file they reference, by specifying the
314 This flag is intended to make the command line name space look
315 like the logical name space.
316 (Note, for commands that do not always do file tree traversals, the
318 flag will be ignored if the
320 flag is not also specified.)
322 For example, the command
323 .Dq Li "chown -HR user slink"
324 will traverse the file hierarchy rooted in the file pointed to by
328 is not the same as the previously discussed
333 flag causes symbolic links specified on the command line to be
334 dereferenced both for the purposes of the action to be performed
335 and the tree walk, and it is as if the user had specified the
336 name of the file to which the symbolic link pointed.
338 As consistently as possible, you can make commands doing a file tree
339 walk follow any symbolic links named on the command line, as well as
340 any symbolic links encountered during the traversal, regardless of
341 the type of file they reference, by specifying the
346 This flag is intended to make the entire name space look like
347 the logical name space.
348 (Note, for commands that do not always do file tree traversals, the
350 flag will be ignored if the
352 flag is not also specified.)
354 For example, the command
355 .Dq Li "chown -LR user slink"
356 will change the owner of the file referenced by
360 references a directory,
362 will traverse the file hierarchy rooted in the directory that it
364 In addition, if any symbolic links are encountered in any file tree that
366 traverses, they will be treated in the same fashion as
369 As consistently as possible, you can specify the default behavior by
375 This flag is intended to make the entire name space look like the
378 For commands that do not by default do file tree traversals, the
383 flags are ignored if the
385 flag is not also specified.
386 In addition, you may specify the
391 options more than once; the last one specified determines the
393 This is intended to permit you to alias commands to behave one way
394 or the other, and then override that behavior on the command line.
400 commands have exceptions to these rules.
403 command operates on the symbolic link, and not the file it references,
404 and therefore never follows a symbolic link.
407 command does not support the
414 To maintain compatibility with historic systems,
417 command acts a little differently.
418 If you do not specify the
425 will follow symbolic links specified on the command line.
430 follows all symbolic links,
431 regardless of their type,
432 whether specified on the command line or encountered in the tree walk.