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34 .Nd symbolic link handling
35 .Sh SYMBOLIC LINK HANDLING
36 Symbolic links are files that act as pointers to other files.
37 To understand their behavior, you must first understand how hard links
39 A hard link to a file is indistinguishable from the original file because
40 it is a reference to the object underlying the original file name.
41 Changes to a file are independent of the name used to reference the
43 Hard links may not refer to directories and may not reference files
44 on different file systems.
45 A symbolic link contains the name of the file to which it is linked,
46 i.e., it is a pointer to another name, and not to an underlying object.
47 For this reason, symbolic links may reference directories and may span
50 Because a symbolic link and its referenced object coexist in the file system
51 name space, confusion can arise in distinguishing between the link itself
52 and the referenced object.
53 Historically, commands and system calls have adopted their own link
54 following conventions in a somewhat ad-hoc fashion.
55 Rules for more a uniform approach, as they are implemented in this system,
57 It is important that local applications conform to these rules, too,
58 so that the user interface can be as consistent as possible.
60 Symbolic links are handled either by operating on the link itself,
61 or by operating on the object referenced by the link.
63 an application or system call is said to
66 Symbolic links may reference other symbolic links,
67 in which case the links are dereferenced until an object that is
68 not a symbolic link is found,
69 a symbolic link which references a file which does not exist is found,
70 or a loop is detected.
71 (Loop detection is done by placing an upper limit on the number of
72 links that may be followed, and an error results if this limit is
75 There are three separate areas that need to be discussed.
78 .Bl -enum -compact -offset indent
80 Symbolic links used as file name arguments for system calls.
82 Symbolic links specified as command line arguments to utilities that
83 are not traversing a file tree.
85 Symbolic links encountered by utilities that are traversing a file tree
86 (either specified on the command line or encountered as part of the
90 The first area is symbolic links used as file name arguments for
93 Except as noted below, all system calls follow symbolic links.
94 For example, if there were a symbolic link
96 which pointed to a file named
99 .Dq Li open("slink" ...\&)
100 would return a file descriptor to the file
103 There are thirteen system calls that do not follow links, and which operate
104 on the symbolic link itself.
124 it also does not follow symbolic links.
132 is applied to a symbolic link, it fails with the error
137 system call does not follow symbolic links
139 .Dv AT_SYMLINK_FOLLOW
142 The following system calls follow symbolic links
144 .Dv AT_SYMLINK_NOFOLLOW
153 The owner and group of an existing symbolic link can be changed by
157 The flags, access permissions, owner/group and modification time of
158 an existing symbolic link can be changed by means of the
164 system calls, respectively.
165 Of these, only the flags and ownership are used by the system;
166 the access permissions are ignored.
170 system differs from historical
172 systems in that the system call
174 has been changed to follow symbolic links.
177 system call was added later when the limitations of the new
180 .Ss Commands not traversing a file tree.
181 The second area is symbolic links, specified as command line file
182 name arguments, to commands which are not traversing a file tree.
184 Except as noted below, commands follow symbolic links named as command
186 For example, if there were a symbolic link
188 which pointed to a file named
192 would display the contents of the file
195 It is important to realize that this rule includes commands which may
196 optionally traverse file trees, e.g.\& the command
198 is included in this rule, while the command
199 .Dq Li "chown -R file"
201 (The latter is described in the third area, below.)
203 If it is explicitly intended that the command operate on the symbolic
204 link instead of following the symbolic link, e.g., it is desired that
206 change the ownership of the file that
208 is, whether it is a symbolic link or not, the
210 option should be used.
211 In the above example,
212 .Dq Li "chown root slink"
213 would change the ownership of the file referenced by
216 .Dq Li "chown -h root slink"
217 would change the ownership of
221 There are five exceptions to this rule.
226 commands do not follow symbolic links named as arguments,
227 but respectively attempt to rename and delete them.
228 (Note, if the symbolic link references a file via a relative path,
229 moving it to another directory may very well cause it to stop working,
230 since the path may no longer be correct.)
234 command is also an exception to this rule.
235 For compatibility with historic systems (when
237 is not doing a tree walk, i.e., the
239 option is not specified),
242 command follows symbolic links named as arguments if the
252 options are not specified.
255 command is the only command where the
259 options affect its behavior even though it is not doing a walk of
266 commands are also exceptions to this rule.
268 commands do not follow symbolic links named as argument by default,
269 but do follow symbolic links named as argument if the
275 system differs from historical
281 commands follow symbolic links specified on the command line.
282 .Ss Commands traversing a file tree.
283 The following commands either optionally or always traverse file trees:
297 It is important to realize that the following rules apply equally to
298 symbolic links encountered during the file tree traversal and symbolic
299 links listed as command line arguments.
301 The first rule applies to symbolic links that reference files that are
302 not of type directory.
303 Operations that apply to symbolic links are performed on the links
304 themselves, but otherwise the links are ignored.
307 .Dq Li "rm -r slink directory"
310 as well as any symbolic links encountered in the tree traversal of
312 because symbolic links may be removed.
315 affect the file which
317 references in any way.
319 The second rule applies to symbolic links that reference files of type
321 Symbolic links which reference files of type directory are never
324 This is often referred to as a
326 walk, as opposed to a
328 walk (where symbolic links referencing directories are followed).
330 As consistently as possible, you can make commands doing a file tree
331 walk follow any symbolic links named on the command line, regardless
332 of the type of file they reference, by specifying the
337 This flag is intended to make the command line name space look
338 like the logical name space.
339 (Note, for commands that do not always do file tree traversals, the
341 flag will be ignored if the
343 flag is not also specified.)
345 For example, the command
346 .Dq Li "chown -HR user slink"
347 will traverse the file hierarchy rooted in the file pointed to by
351 is not the same as the previously discussed
356 flag causes symbolic links specified on the command line to be
357 dereferenced both for the purposes of the action to be performed
358 and the tree walk, and it is as if the user had specified the
359 name of the file to which the symbolic link pointed.
361 As consistently as possible, you can make commands doing a file tree
362 walk follow any symbolic links named on the command line, as well as
363 any symbolic links encountered during the traversal, regardless of
364 the type of file they reference, by specifying the
369 This flag is intended to make the entire name space look like
370 the logical name space.
371 (Note, for commands that do not always do file tree traversals, the
373 flag will be ignored if the
375 flag is not also specified.)
377 For example, the command
378 .Dq Li "chown -LR user slink"
379 will change the owner of the file referenced by
383 references a directory,
385 will traverse the file hierarchy rooted in the directory that it
387 In addition, if any symbolic links are encountered in any file tree that
389 traverses, they will be treated in the same fashion as
392 As consistently as possible, you can specify the default behavior by
398 This flag is intended to make the entire name space look like the
401 For commands that do not by default do file tree traversals, the
406 flags are ignored if the
408 flag is not also specified.
409 In addition, you may specify the
414 options more than once; the last one specified determines the
416 This is intended to permit you to alias commands to behave one way
417 or the other, and then override that behavior on the command line.
423 commands have exceptions to these rules.
426 command operates on the symbolic link, and not the file it references,
427 and therefore never follows a symbolic link.
430 command does not support the
437 To maintain compatibility with historic systems,
440 command acts a little differently.
441 If you do not specify the
448 will follow symbolic links specified on the command line.
453 follows all symbolic links,
454 regardless of their type,
455 whether specified on the command line or encountered in the tree walk.