xref: /freebsd/bin/ln/symlink.7 (revision 6990ffd8a95caaba6858ad44ff1b3157d1efba8f)
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32.\"	@(#)symlink.7	8.3 (Berkeley) 3/31/94
33.\" $FreeBSD$
34.\"
35.Dd March 31, 1994
36.Dt SYMLINK 7
37.Os
38.Sh NAME
39.Nm symlink
40.Nd symbolic link handling
41.Sh SYMBOLIC LINK HANDLING
42Symbolic links are files that act as pointers to other files.
43To understand their behavior, you must first understand how hard links
44work.
45A hard link to a file is indistinguishable from the original file because
46it is a reference to the object underlying the original file name.
47Changes to a file are independent of the name used to reference the
48file.
49Hard links may not refer to directories and may not reference files
50on different file systems.
51A symbolic link contains the name of the file to which it is linked,
52i.e. it is a pointer to another name, and not to an underlying object.
53For this reason, symbolic links may reference directories and may span
54file systems.
55.Pp
56Because a symbolic link and its referenced object coexist in the filesystem
57name space, confusion can arise in distinguishing between the link itself
58and the referenced object.
59Historically, commands and system calls have adopted their own link
60following conventions in a somewhat ad-hoc fashion.
61Rules for more a uniform approach, as they are implemented in this system,
62are outlined here.
63It is important that local applications conform to these rules, too,
64so that the user interface can be as consistent as possible.
65.Pp
66Symbolic links are handled either by operating on the link itself,
67or by operating on the object referenced by the link.
68In the latter case,
69an application or system call is said to
70.Dq follow
71the link.
72Symbolic links may reference other symbolic links,
73in which case the links are dereferenced until an object that is
74not a symbolic link is found,
75a symbolic link which references a file which doesn't exist is found,
76or a loop is detected.
77(Loop detection is done by placing an upper limit on the number of
78links that may be followed, and an error results if this limit is
79exceeded.)
80.Pp
81There are three separate areas that need to be discussed.
82They are as follows:
83.Pp
84.Bl -enum -compact -offset indent
85.It
86Symbolic links used as file name arguments for system calls.
87.It
88Symbolic links specified as command line arguments to utilities that
89are not traversing a file tree.
90.It
91Symbolic links encountered by utilities that are traversing a file tree
92(either specified on the command line or encountered as part of the
93file hierarchy walk).
94.El
95.Ss System calls.
96The first area is symbolic links used as file name arguments for
97system calls.
98.Pp
99Except as noted below, all system calls follow symbolic links.
100For example, if there were a symbolic link
101.Dq Li slink
102which pointed to a file named
103.Dq Li afile ,
104the system call
105.Dq Li open("slink" ...\&)
106would return a file descriptor to the file
107.Dq afile .
108.Pp
109There are six system calls that do not follow links, and which operate
110on the symbolic link itself.
111They are:
112.Xr lchown 2 ,
113.Xr lstat 2 ,
114.Xr readlink 2 ,
115.Xr rename 2 ,
116.Xr rmdir 2 ,
117and
118.Xr unlink 2 .
119Because
120.Xr remove 3
121is an alias for
122.Xr unlink 2 ,
123it also does not follow symbolic links.
124When
125.Xr rmdir 2
126is applied to a symbolic link, it fails with the error
127.Er ENOTDIR .
128.Pp
129The owner and group of an existing symbolic link can be changed by
130means of the
131.Xr lchown 2
132system call.
133The other file attributes, such as the modification time and access
134permissions, are not used by the system and cannot be changed.
135.Pp
136The
137.Bx 4.4
138system differs from historical
139.Bx 4
140systems in that the system call
141.Xr chown 2
142has been changed to follow symbolic links.
143The
144.Xr lchown 2
145system call was added later when the limitations of the new
146.Xr chown 2
147became apparent.
148.Ss Commands not traversing a file tree.
149The second area is symbolic links, specified as command line file
150name arguments, to commands which are not traversing a file tree.
151.Pp
152Except as noted below, commands follow symbolic links named as command
153line arguments.
154For example, if there were a symbolic link
155.Dq Li slink
156which pointed to a file named
157.Dq Li afile ,
158the command
159.Dq Li cat slink
160would display the contents of the file
161.Dq Li afile .
162.Pp
163It is important to realize that this rule includes commands which may
164optionally traverse file trees, e.g. the command
165.Dq Li "chown file"
166is included in this rule, while the command
167.Dq Li "chown -R file"
168is not.
169(The latter is described in the third area, below.)
170.Pp
171If it is explicitly intended that the command operate on the symbolic
172link instead of following the symbolic link, e.g., it is desired that
173.Dq Li "chown slink"
174change the ownership of the file that
175.Dq Li slink
176is, whether it is a symbolic link or not, the
177.Fl h
178option should be used.
179In the above example,
180.Dq Li "chown root slink"
181would change the ownership of the file referenced by
182.Dq Li slink ,
183while
184.Dq Li "chown -h root slink"
185would change the ownership of
186.Dq Li slink
187itself.
188.Pp
189There are four exceptions to this rule.
190The
191.Xr mv 1
192and
193.Xr rm 1
194commands do not follow symbolic links named as arguments,
195but respectively attempt to rename and delete them.
196(Note, if the symbolic link references a file via a relative path,
197moving it to another directory may very well cause it to stop working,
198since the path may no longer be correct.)
199.Pp
200The
201.Xr ls 1
202command is also an exception to this rule.
203For compatibility with historic systems (when
204.Nm ls
205is not doing a tree walk, i.e. the
206.Fl R
207option is not specified),
208the
209.Nm ls
210command follows symbolic links named as arguments if the
211.Fl H
212or
213.Fl L
214option is specified,
215or if the
216.Fl F ,
217.Fl d
218or
219.Fl l
220options are not specified.  (The
221.Nm ls
222command is the only command where the
223.Fl H
224and
225.Fl L
226options affect its behavior even though it is not doing a walk of
227a file tree.)
228.Pp
229The
230.Xr file 1
231command is also an exception to this rule.
232The
233.Xr file 1
234command does not follow symbolic links named as argument by default.
235The
236.Xr file 1
237command does follow symbolic links named as argument if
238.Fl L
239option is specified.
240.Pp
241The
242.Bx 4.4
243system differs from historical
244.Bx 4
245systems in that the
246.Nm chown
247and
248.Nm chgrp
249commands follow symbolic links specified on the command line.
250.Ss Commands traversing a file tree.
251The following commands either optionally or always traverse file trees:
252.Xr chflags 1 ,
253.Xr chgrp 1 ,
254.Xr chmod 1 ,
255.Xr cp 1 ,
256.Xr du 1 ,
257.Xr find 1 ,
258.Xr ls 1 ,
259.Xr pax 1 ,
260.Xr rm 1 ,
261.Xr tar 1
262and
263.Xr chown 8 .
264.Pp
265It is important to realize that the following rules apply equally to
266symbolic links encountered during the file tree traversal and symbolic
267links listed as command line arguments.
268.Pp
269The first rule applies to symbolic links that reference files that are
270not of type directory.
271Operations that apply to symbolic links are performed on the links
272themselves, but otherwise the links are ignored.
273.Pp
274For example, the command
275.Dq Li "chown -R user slink directory"
276will ignore
277.Dq Li slink ,
278because symbolic links in this system do not have owners.
279Any symbolic links encountered during the tree traversal will also be
280ignored.
281The command
282.Dq Li "rm -r slink directory"
283will remove
284.Dq Li slink ,
285as well as any symbolic links encountered in the tree traversal of
286.Dq Li directory ,
287because symbolic links may be removed.
288In no case will either
289.Nm chown
290or
291.Nm rm
292affect the file which
293.Dq Li slink
294references in any way.
295.Pp
296The second rule applies to symbolic links that reference files of type
297directory.
298Symbolic links which reference files of type directory are never
299.Dq followed
300by default.
301This is often referred to as a
302.Dq physical
303walk, as opposed to a
304.Dq logical
305walk (where symbolic links referencing directories are followed).
306.Pp
307As consistently as possible, you can make commands doing a file tree
308walk follow any symbolic links named on the command line, regardless
309of the type of file they reference, by specifying the
310.Fl H
311(for
312.Dq half\-logical )
313flag.
314This flag is intended to make the command line name space look
315like the logical name space.
316(Note, for commands that do not always do file tree traversals, the
317.Fl H
318flag will be ignored if the
319.Fl R
320flag is not also specified.)
321.Pp
322For example, the command
323.Dq Li "chown -HR user slink"
324will traverse the file hierarchy rooted in the file pointed to by
325.Dq Li slink .
326Note, the
327.Fl H
328is not the same as the previously discussed
329.Fl h
330flag.
331The
332.Fl H
333flag causes symbolic links specified on the command line to be
334dereferenced both for the purposes of the action to be performed
335and the tree walk, and it is as if the user had specified the
336name of the file to which the symbolic link pointed.
337.Pp
338As consistently as possible, you can make commands doing a file tree
339walk follow any symbolic links named on the command line, as well as
340any symbolic links encountered during the traversal, regardless of
341the type of file they reference, by specifying the
342.Fl L
343(for
344.Dq logical )
345flag.
346This flag is intended to make the entire name space look like
347the logical name space.
348(Note, for commands that do not always do file tree traversals, the
349.Fl L
350flag will be ignored if the
351.Fl R
352flag is not also specified.)
353.Pp
354For example, the command
355.Dq Li "chown -LR user slink"
356will change the owner of the file referenced by
357.Dq Li slink .
358If
359.Dq Li slink
360references a directory,
361.Nm chown
362will traverse the file hierarchy rooted in the directory that it
363references.
364In addition, if any symbolic links are encountered in any file tree that
365.Nm chown
366traverses, they will be treated in the same fashion as
367.Dq Li slink .
368.Pp
369As consistently as possible, you can specify the default behavior by
370specifying the
371.Fl P
372(for
373.Dq physical )
374flag.
375This flag is intended to make the entire name space look like the
376physical name space.
377.Pp
378For commands that do not by default do file tree traversals, the
379.Fl H ,
380.Fl L
381and
382.Fl P
383flags are ignored if the
384.Fl R
385flag is not also specified.
386In addition, you may specify the
387.Fl H ,
388.Fl L
389and
390.Fl P
391options more than once; the last one specified determines the
392command's behavior.
393This is intended to permit you to alias commands to behave one way
394or the other, and then override that behavior on the command line.
395.Pp
396The
397.Xr ls 1
398and
399.Xr rm 1
400commands have exceptions to these rules.
401The
402.Nm rm
403command operates on the symbolic link, and not the file it references,
404and therefore never follows a symbolic link.
405The
406.Nm rm
407command does not support the
408.Fl H ,
409.Fl L
410or
411.Fl P
412options.
413.Pp
414To maintain compatibility with historic systems,
415the
416.Nm ls
417command acts a little differently.  If you do not specify the
418.Fl F ,
419.Fl d
420or
421.Fl l
422options,
423.Nm ls
424will follow symbolic links specified on the command line.  If the
425.Fl L
426flag is specified,
427.Nm ls
428follows all symbolic links,
429regardless of their type,
430whether specified on the command line or encountered in the tree walk.
431.Sh SEE ALSO
432.Xr chflags 1 ,
433.Xr chgrp 1 ,
434.Xr chmod 1 ,
435.Xr cp 1 ,
436.Xr du 1 ,
437.Xr find 1 ,
438.Xr ln 1 ,
439.Xr ls 1 ,
440.Xr mv 1 ,
441.Xr pax 1 ,
442.Xr rm 1 ,
443.Xr tar 1 ,
444.Xr lchown 2 ,
445.Xr lstat 2 ,
446.Xr readlink 2 ,
447.Xr rename 2 ,
448.Xr symlink 2 ,
449.Xr unlink 2 ,
450.Xr fts 3 ,
451.Xr remove 3 ,
452.Xr chown 8
453