xref: /freebsd/bin/ln/symlink.7 (revision 6e5dcc6113da649a79e5bc2c3ea9329bcd1d85d5)
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29.\"	@(#)symlink.7	8.3 (Berkeley) 3/31/94
30.\"
31.Dd February 16, 2015
32.Dt SYMLINK 7
33.Os
34.Sh NAME
35.Nm symlink
36.Nd symbolic link handling
37.Sh SYMBOLIC LINK HANDLING
38Symbolic links are files that act as pointers to other files.
39To understand their behavior, you must first understand how hard links
40work.
41A hard link to a file is indistinguishable from the original file because
42it is a reference to the object underlying the original file name.
43Changes to a file are independent of the name used to reference the
44file.
45Hard links may not refer to directories and may not reference files
46on different file systems.
47A symbolic link contains the name of the file to which it is linked,
48i.e., it is a pointer to another name, and not to an underlying object.
49For this reason, symbolic links may reference directories and may span
50file systems.
51.Pp
52Because a symbolic link and its referenced object coexist in the file system
53name space, confusion can arise in distinguishing between the link itself
54and the referenced object.
55Historically, commands and system calls have adopted their own link
56following conventions in a somewhat ad-hoc fashion.
57Rules for more a uniform approach, as they are implemented in this system,
58are outlined here.
59It is important that local applications conform to these rules, too,
60so that the user interface can be as consistent as possible.
61.Pp
62Symbolic links are handled either by operating on the link itself,
63or by operating on the object referenced by the link.
64In the latter case,
65an application or system call is said to
66.Dq follow
67the link.
68Symbolic links may reference other symbolic links,
69in which case the links are dereferenced until an object that is
70not a symbolic link is found,
71a symbolic link which references a file which does not exist is found,
72or a loop is detected.
73(Loop detection is done by placing an upper limit on the number of
74links that may be followed, and an error results if this limit is
75exceeded.)
76.Pp
77There are three separate areas that need to be discussed.
78They are as follows:
79.Pp
80.Bl -enum -compact -offset indent
81.It
82Symbolic links used as file name arguments for system calls.
83.It
84Symbolic links specified as command line arguments to utilities that
85are not traversing a file tree.
86.It
87Symbolic links encountered by utilities that are traversing a file tree
88(either specified on the command line or encountered as part of the
89file hierarchy walk).
90.El
91.Ss System calls.
92The first area is symbolic links used as file name arguments for
93system calls.
94.Pp
95Except as noted below, all system calls follow symbolic links.
96For example, if there were a symbolic link
97.Dq Li slink
98which pointed to a file named
99.Dq Li afile ,
100the system call
101.Dq Li open("slink" ...\&)
102would return a file descriptor to the file
103.Dq afile .
104.Pp
105There are thirteen system calls that do not follow links, and which operate
106on the symbolic link itself.
107They are:
108.Xr lchflags 2 ,
109.Xr lchmod 2 ,
110.Xr lchown 2 ,
111.Xr lpathconf 2 ,
112.Xr lstat 2 ,
113.Xr lutimes 2 ,
114.Xr readlink 2 ,
115.Xr readlinkat 2 ,
116.Xr rename 2 ,
117.Xr renameat 2 ,
118.Xr rmdir 2 ,
119.Xr unlink 2 ,
120and
121.Xr unlinkat 2 .
122Because
123.Xr remove 3
124is an alias for
125.Xr unlink 2 ,
126it also does not follow symbolic links.
127When
128.Xr rmdir 2
129or
130.Xr unlinkat 2
131with the
132.Dv AT_REMOVEDIR
133flag
134is applied to a symbolic link, it fails with the error
135.Er ENOTDIR .
136.Pp
137The
138.Xr linkat 2
139system call does not follow symbolic links
140unless given the
141.Dv AT_SYMLINK_FOLLOW
142flag.
143.Pp
144The following system calls follow symbolic links
145unless given the
146.Dv AT_SYMLINK_NOFOLLOW
147flag:
148.Xr chflagsat 2 ,
149.Xr fchmodat 2 ,
150.Xr fchownat 2 ,
151.Xr fstatat 2
152and
153.Xr utimensat 2 .
154.Pp
155The owner and group of an existing symbolic link can be changed by
156means of the
157.Xr lchown 2
158system call.
159The flags, access permissions, owner/group and modification time of
160an existing symbolic link can be changed by means of the
161.Xr lchflags 2 ,
162.Xr lchmod 2 ,
163.Xr lchown 2 ,
164and
165.Xr lutimes 2
166system calls, respectively.
167Of these, only the flags and ownership are used by the system;
168the access permissions are ignored.
169.Pp
170The
171.Bx 4.4
172system differs from historical
173.Bx 4
174systems in that the system call
175.Xr chown 2
176has been changed to follow symbolic links.
177The
178.Xr lchown 2
179system call was added later when the limitations of the new
180.Xr chown 2
181became apparent.
182.Ss Commands not traversing a file tree.
183The second area is symbolic links, specified as command line file
184name arguments, to commands which are not traversing a file tree.
185.Pp
186Except as noted below, commands follow symbolic links named as command
187line arguments.
188For example, if there were a symbolic link
189.Dq Li slink
190which pointed to a file named
191.Dq Li afile ,
192the command
193.Dq Li cat slink
194would display the contents of the file
195.Dq Li afile .
196.Pp
197It is important to realize that this rule includes commands which may
198optionally traverse file trees, e.g.\& the command
199.Dq Li "chown file"
200is included in this rule, while the command
201.Dq Li "chown -R file"
202is not.
203(The latter is described in the third area, below.)
204.Pp
205If it is explicitly intended that the command operate on the symbolic
206link instead of following the symbolic link, e.g., it is desired that
207.Dq Li "chown slink"
208change the ownership of the file that
209.Dq Li slink
210is, whether it is a symbolic link or not, the
211.Fl h
212option should be used.
213In the above example,
214.Dq Li "chown root slink"
215would change the ownership of the file referenced by
216.Dq Li slink ,
217while
218.Dq Li "chown -h root slink"
219would change the ownership of
220.Dq Li slink
221itself.
222.Pp
223There are five exceptions to this rule.
224The
225.Xr mv 1
226and
227.Xr rm 1
228commands do not follow symbolic links named as arguments,
229but respectively attempt to rename and delete them.
230(Note, if the symbolic link references a file via a relative path,
231moving it to another directory may very well cause it to stop working,
232since the path may no longer be correct.)
233.Pp
234The
235.Xr ls 1
236command is also an exception to this rule.
237For compatibility with historic systems (when
238.Nm ls
239is not doing a tree walk, i.e., the
240.Fl R
241option is not specified),
242the
243.Nm ls
244command follows symbolic links named as arguments if the
245.Fl H
246or
247.Fl L
248option is specified,
249or if the
250.Fl F ,
251.Fl d
252or
253.Fl l
254options are not specified.
255(The
256.Nm ls
257command is the only command where the
258.Fl H
259and
260.Fl L
261options affect its behavior even though it is not doing a walk of
262a file tree.)
263.Pp
264The
265.Xr file 1
266and
267.Xr stat 1
268commands are also exceptions to this rule.
269These
270commands do not follow symbolic links named as argument by default,
271but do follow symbolic links named as argument if the
272.Fl L
273option is specified.
274.Pp
275The
276.Bx 4.4
277system differs from historical
278.Bx 4
279systems in that the
280.Nm chown
281and
282.Nm chgrp
283commands follow symbolic links specified on the command line.
284.Ss Commands traversing a file tree.
285The following commands either optionally or always traverse file trees:
286.Xr chflags 1 ,
287.Xr chgrp 1 ,
288.Xr chmod 1 ,
289.Xr cp 1 ,
290.Xr du 1 ,
291.Xr find 1 ,
292.Xr ls 1 ,
293.Xr pax 1 ,
294.Xr rm 1 ,
295.Xr tar 1
296and
297.Xr chown 8 .
298.Pp
299It is important to realize that the following rules apply equally to
300symbolic links encountered during the file tree traversal and symbolic
301links listed as command line arguments.
302.Pp
303The first rule applies to symbolic links that reference files that are
304not of type directory.
305Operations that apply to symbolic links are performed on the links
306themselves, but otherwise the links are ignored.
307.Pp
308The command
309.Dq Li "rm -r slink directory"
310will remove
311.Dq Li slink ,
312as well as any symbolic links encountered in the tree traversal of
313.Dq Li directory ,
314because symbolic links may be removed.
315In no case will
316.Nm rm
317affect the file which
318.Dq Li slink
319references in any way.
320.Pp
321The second rule applies to symbolic links that reference files of type
322directory.
323Symbolic links which reference files of type directory are never
324.Dq followed
325by default.
326This is often referred to as a
327.Dq physical
328walk, as opposed to a
329.Dq logical
330walk (where symbolic links referencing directories are followed).
331.Pp
332As consistently as possible, you can make commands doing a file tree
333walk follow any symbolic links named on the command line, regardless
334of the type of file they reference, by specifying the
335.Fl H
336(for
337.Dq half\-logical )
338flag.
339This flag is intended to make the command line name space look
340like the logical name space.
341(Note, for commands that do not always do file tree traversals, the
342.Fl H
343flag will be ignored if the
344.Fl R
345flag is not also specified.)
346.Pp
347For example, the command
348.Dq Li "chown -HR user slink"
349will traverse the file hierarchy rooted in the file pointed to by
350.Dq Li slink .
351Note, the
352.Fl H
353is not the same as the previously discussed
354.Fl h
355flag.
356The
357.Fl H
358flag causes symbolic links specified on the command line to be
359dereferenced both for the purposes of the action to be performed
360and the tree walk, and it is as if the user had specified the
361name of the file to which the symbolic link pointed.
362.Pp
363As consistently as possible, you can make commands doing a file tree
364walk follow any symbolic links named on the command line, as well as
365any symbolic links encountered during the traversal, regardless of
366the type of file they reference, by specifying the
367.Fl L
368(for
369.Dq logical )
370flag.
371This flag is intended to make the entire name space look like
372the logical name space.
373(Note, for commands that do not always do file tree traversals, the
374.Fl L
375flag will be ignored if the
376.Fl R
377flag is not also specified.)
378.Pp
379For example, the command
380.Dq Li "chown -LR user slink"
381will change the owner of the file referenced by
382.Dq Li slink .
383If
384.Dq Li slink
385references a directory,
386.Nm chown
387will traverse the file hierarchy rooted in the directory that it
388references.
389In addition, if any symbolic links are encountered in any file tree that
390.Nm chown
391traverses, they will be treated in the same fashion as
392.Dq Li slink .
393.Pp
394As consistently as possible, you can specify the default behavior by
395specifying the
396.Fl P
397(for
398.Dq physical )
399flag.
400This flag is intended to make the entire name space look like the
401physical name space.
402.Pp
403For commands that do not by default do file tree traversals, the
404.Fl H ,
405.Fl L
406and
407.Fl P
408flags are ignored if the
409.Fl R
410flag is not also specified.
411In addition, you may specify the
412.Fl H ,
413.Fl L
414and
415.Fl P
416options more than once; the last one specified determines the
417command's behavior.
418This is intended to permit you to alias commands to behave one way
419or the other, and then override that behavior on the command line.
420.Pp
421The
422.Xr ls 1
423and
424.Xr rm 1
425commands have exceptions to these rules.
426The
427.Nm rm
428command operates on the symbolic link, and not the file it references,
429and therefore never follows a symbolic link.
430The
431.Nm rm
432command does not support the
433.Fl H ,
434.Fl L
435or
436.Fl P
437options.
438.Pp
439To maintain compatibility with historic systems,
440the
441.Nm ls
442command acts a little differently.
443If you do not specify the
444.Fl F ,
445.Fl d
446or
447.Fl l
448options,
449.Nm ls
450will follow symbolic links specified on the command line.
451If the
452.Fl L
453flag is specified,
454.Nm ls
455follows all symbolic links,
456regardless of their type,
457whether specified on the command line or encountered in the tree walk.
458.Sh SEE ALSO
459.Xr chflags 1 ,
460.Xr chgrp 1 ,
461.Xr chmod 1 ,
462.Xr cp 1 ,
463.Xr du 1 ,
464.Xr find 1 ,
465.Xr ln 1 ,
466.Xr ls 1 ,
467.Xr mv 1 ,
468.Xr pax 1 ,
469.Xr rm 1 ,
470.Xr tar 1 ,
471.Xr lchflags 2 ,
472.Xr lchmod 2 ,
473.Xr lchown 2 ,
474.Xr lstat 2 ,
475.Xr lutimes 2 ,
476.Xr readlink 2 ,
477.Xr rename 2 ,
478.Xr symlink 2 ,
479.Xr unlink 2 ,
480.Xr fts 3 ,
481.Xr remove 3 ,
482.Xr chown 8
483