1symlink(7) Miscellaneous Information Manual symlink(7)
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6 symlink - symbolic link handling
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9 Symbolic links are files that act as pointers to other files. To un‐
10 derstand their behavior, you must first understand how hard links work.
11 A hard link to a file is indistinguishable from the original file be‐
13 cause it is a reference to the object underlying the original filename.
14 (To be precise: each of the hard links to a file is a reference to the
15 same inode number, where an inode number is an index into the inode ta‐
16 ble, which contains metadata about all files on a filesystem. See
17 stat(2).) Changes to a file are independent of the name used to refer‐
18 ence the file. Hard links may not refer to directories (to prevent the
19 possibility of loops within the filesystem tree, which would confuse
20 many programs) and may not refer to files on different filesystems (be‐
21 cause inode numbers are not unique across filesystems).
22 A symbolic link is a special type of file whose contents are a string
24 that is the pathname of another file, the file to which the link
25 refers. (The contents of a symbolic link can be read using read‐
26 link(2).) In other words, a symbolic link is a pointer to another
27 name, and not to an underlying object. For this reason, symbolic links
28 may refer to directories and may cross filesystem boundaries.
29 There is no requirement that the pathname referred to by a symbolic
31 link should exist. A symbolic link that refers to a pathname that does
32 not exist is said to be a dangling link.
33 Because a symbolic link and its referenced object coexist in the
35 filesystem name space, confusion can arise in distinguishing between
36 the link itself and the referenced object. On historical systems, com‐
37 mands and system calls adopted their own link-following conventions in
38 a somewhat ad-hoc fashion. Rules for a more uniform approach, as they
39 are implemented on Linux and other systems, are outlined here. It is
40 important that site-local applications also conform to these rules, so
41 that the user interface can be as consistent as possible.
42 Magic links
44 There is a special class of symbolic-link-like objects known as "magic
45 links", which can be found in certain pseudofilesystems such as proc(5)
46 (examples include /proc/pid/exe and /proc/pid/fd/*). Unlike normal
47 symbolic links, magic links are not resolved through pathname-expan‐
48 sion, but instead act as direct references to the kernel's own repre‐
49 sentation of a file handle. As such, these magic links allow users to
50 access files which cannot be referenced with normal paths (such as un‐
51 linked files still referenced by a running program ).
52 Because they can bypass ordinary mount_namespaces(7)-based restric‐
54 tions, magic links have been used as attack vectors in various ex‐
55 ploits.
56 Symbolic link ownership, permissions, and timestamps
58 The owner and group of an existing symbolic link can be changed using
59 lchown(2). The ownership of a symbolic link matters when the link is
60 being removed or renamed in a directory that has the sticky bit set
61 (see inode(7)), and when the fs.protected_symlinks sysctl is set (see
62 proc(5)).
63 The last access and last modification timestamps of a symbolic link can
65 be changed using utimensat(2) or lutimes(3).
66 On Linux, the permissions of an ordinary symbolic link are not used in
68 any operations; the permissions are always 0777 (read, write, and exe‐
69 cute for all user categories), and can't be changed.
70 However, magic links do not follow this rule. They can have a non-0777
72 mode, though this mode is not currently used in any permission checks.
73 Obtaining a file descriptor that refers to a symbolic link
75 Using the combination of the O_PATH and O_NOFOLLOW flags to open(2)
76 yields a file descriptor that can be passed as the dirfd argument in
77 system calls such as fstatat(2), fchownat(2), fchmodat(2), linkat(2),
78 and readlinkat(2), in order to operate on the symbolic link itself
79 (rather than the file to which it refers).
80 By default (i.e., if the AT_SYMLINK_FOLLOW flag is not specified), if
82 name_to_handle_at(2) is applied to a symbolic link, it yields a handle
83 for the symbolic link (rather than the file to which it refers). One
84 can then obtain a file descriptor for the symbolic link (rather than
85 the file to which it refers) by specifying the O_PATH flag in a subse‐
86 quent call to open_by_handle_at(2). Again, that file descriptor can be
87 used in the aforementioned system calls to operate on the symbolic link
88 itself.
89 Handling of symbolic links by system calls and commands
91 Symbolic links are handled either by operating on the link itself, or
92 by operating on the object referred to by the link. In the latter
93 case, an application or system call is said to follow the link. Sym‐
94 bolic links may refer to other symbolic links, in which case the links
95 are dereferenced until an object that is not a symbolic link is found,
96 a symbolic link that refers to a file which does not exist is found, or
97 a loop is detected. (Loop detection is done by placing an upper limit
98 on the number of links that may be followed, and an error results if
99 this limit is exceeded.)
100 There are three separate areas that need to be discussed. They are as
102 follows:
103 • Symbolic links used as filename arguments for system calls.
105 • Symbolic links specified as command-line arguments to utilities that
107 are not traversing a file tree.
108 • Symbolic links encountered by utilities that are traversing a file
110 tree (either specified on the command line or encountered as part of
111 the file hierarchy walk).
112 Before describing the treatment of symbolic links by system calls and
114 commands, we require some terminology. Given a pathname of the form
115 a/b/c, the part preceding the final slash (i.e., a/b) is called the
116 dirname component, and the part following the final slash (i.e., c) is
117 called the basename component.
118 Treatment of symbolic links in system calls
120 The first area is symbolic links used as filename arguments for system
121 calls.
122 The treatment of symbolic links within a pathname passed to a system
124 call is as follows:
125 (1) Within the dirname component of a pathname, symbolic links are al‐
127 ways followed in nearly every system call. (This is also true for
128 commands.) The one exception is openat2(2), which provides flags
129 that can be used to explicitly prevent following of symbolic links
130 in the dirname component.
131 (2) Except as noted below, all system calls follow symbolic links in
133 the basename component of a pathname. For example, if there were
134 a symbolic link slink which pointed to a file named afile, the
135 system call open("slink" ...) would return a file descriptor re‐
136 ferring to the file afile.
137 Various system calls do not follow links in the basename component of a
139 pathname, and operate on the symbolic link itself. They are:
140 lchown(2), lgetxattr(2), llistxattr(2), lremovexattr(2), lsetxattr(2),
141 lstat(2), readlink(2), rename(2), rmdir(2), and unlink(2).
142 Certain other system calls optionally follow symbolic links in the
144 basename component of a pathname. They are: faccessat(2), fchownat(2),
145 fstatat(2), linkat(2), name_to_handle_at(2), open(2), openat(2),
146 open_by_handle_at(2), and utimensat(2); see their manual pages for de‐
147 tails. Because remove(3) is an alias for unlink(2), that library func‐
148 tion also does not follow symbolic links. When rmdir(2) is applied to
149 a symbolic link, it fails with the error ENOTDIR.
150 link(2) warrants special discussion. POSIX.1-2001 specifies that
152 link(2) should dereference oldpath if it is a symbolic link. However,
153 Linux does not do this. (By default, Solaris is the same, but the
154 POSIX.1-2001 specified behavior can be obtained with suitable compiler
155 options.) POSIX.1-2008 changed the specification to allow either be‐
156 havior in an implementation.
157 Commands not traversing a file tree
159 The second area is symbolic links, specified as command-line filename
160 arguments, to commands which are not traversing a file tree.
161 Except as noted below, commands follow symbolic links named as command-
163 line arguments. For example, if there were a symbolic link slink which
164 pointed to a file named afile, the command cat slink would display the
165 contents of the file afile.
166 It is important to realize that this rule includes commands which may
168 optionally traverse file trees; for example, the command chown file is
169 included in this rule, while the command chown -R file, which performs
170 a tree traversal, is not. (The latter is described in the third area,
171 below.)
172 If it is explicitly intended that the command operate on the symbolic
174 link instead of following the symbolic link—for example, it is desired
175 that chown slink change the ownership of the file that slink is,
176 whether it is a symbolic link or not—then the -h option should be used.
177 In the above example, chown root slink would change the ownership of
178 the file referred to by slink, while chown -h root slink would change
179 the ownership of slink itself.
180 There are some exceptions to this rule:
182 • The mv(1) and rm(1) commands do not follow symbolic links named as
184 arguments, but respectively attempt to rename and delete them.
185 (Note, if the symbolic link references a file via a relative path,
186 moving it to another directory may very well cause it to stop work‐
187 ing, since the path may no longer be correct.)
188 • The ls(1) command is also an exception to this rule. For compati‐
190 bility with historic systems (when ls(1) is not doing a tree walk—
191 that is, -R option is not specified), the ls(1) command follows sym‐
192 bolic links named as arguments if the -H or -L option is specified,
193 or if the -F, -d, or -l options are not specified. (The ls(1) com‐
194 mand is the only command where the -H and -L options affect its be‐
195 havior even though it is not doing a walk of a file tree.)
196 • The file(1) command is also an exception to this rule. The file(1)
198 command does not follow symbolic links named as argument by default.
199 The file(1) command does follow symbolic links named as argument if
200 the -L option is specified.
201 Commands traversing a file tree
203 The following commands either optionally or always traverse file trees:
204 chgrp(1), chmod(1), chown(1), cp(1), du(1), find(1), ls(1), pax(1),
205 rm(1), and tar(1).
206 It is important to realize that the following rules apply equally to
208 symbolic links encountered during the file tree traversal and symbolic
209 links listed as command-line arguments.
210 The first rule applies to symbolic links that reference files other
212 than directories. Operations that apply to symbolic links are per‐
213 formed on the links themselves, but otherwise the links are ignored.
214 The command rm -r slink directory will remove slink, as well as any
216 symbolic links encountered in the tree traversal of directory, because
217 symbolic links may be removed. In no case will rm(1) affect the file
218 referred to by slink.
219 The second rule applies to symbolic links that refer to directories.
221 Symbolic links that refer to directories are never followed by default.
222 This is often referred to as a "physical" walk, as opposed to a "logi‐
223 cal" walk (where symbolic links that refer to directories are fol‐
224 lowed).
225 Certain conventions are (should be) followed as consistently as possi‐
227 ble by commands that perform file tree walks:
228 • A command can be made to follow any symbolic links named on the com‐
230 mand line, regardless of the type of file they reference, by speci‐
231 fying the -H (for "half-logical") flag. This flag is intended to
232 make the command-line name space look like the logical name space.
233 (Note, for commands that do not always do file tree traversals, the
234 -H flag will be ignored if the -R flag is not also specified.)
235 For example, the command chown -HR user slink will traverse the file
237 hierarchy rooted in the file pointed to by slink. Note, the -H is
238 not the same as the previously discussed -h flag. The -H flag
239 causes symbolic links specified on the command line to be derefer‐
240 enced for the purposes of both the action to be performed and the
241 tree walk, and it is as if the user had specified the name of the
242 file to which the symbolic link pointed.
243 • A command can be made to follow any symbolic links named on the com‐
245 mand line, as well as any symbolic links encountered during the tra‐
246 versal, regardless of the type of file they reference, by specifying
247 the -L (for "logical") flag. This flag is intended to make the en‐
248 tire name space look like the logical name space. (Note, for com‐
249 mands that do not always do file tree traversals, the -L flag will
250 be ignored if the -R flag is not also specified.)
251 For example, the command chown -LR user slink will change the owner
253 of the file referred to by slink. If slink refers to a directory,
254 chown will traverse the file hierarchy rooted in the directory that
255 it references. In addition, if any symbolic links are encountered
256 in any file tree that chown traverses, they will be treated in the
257 same fashion as slink.
258 • A command can be made to provide the default behavior by specifying
260 the -P (for "physical") flag. This flag is intended to make the en‐
261 tire name space look like the physical name space.
262 For commands that do not by default do file tree traversals, the -H,
264 -L, and -P flags are ignored if the -R flag is not also specified. In
265 addition, you may specify the -H, -L, and -P options more than once;
266 the last one specified determines the command's behavior. This is in‐
267 tended to permit you to alias commands to behave one way or the other,
268 and then override that behavior on the command line.
269 The ls(1) and rm(1) commands have exceptions to these rules:
271 • The rm(1) command operates on the symbolic link, and not the file it
273 references, and therefore never follows a symbolic link. The rm(1)
274 command does not support the -H, -L, or -P options.
275 • To maintain compatibility with historic systems, the ls(1) command
277 acts a little differently. If you do not specify the -F, -d, or -l
278 options, ls(1) will follow symbolic links specified on the command
279 line. If the -L flag is specified, ls(1) follows all symbolic
280 links, regardless of their type, whether specified on the command
281 line or encountered in the tree walk.
282
284 chgrp(1), chmod(1), find(1), ln(1), ls(1), mv(1), namei(1), rm(1),
285 lchown(2), link(2), lstat(2), readlink(2), rename(2), symlink(2), un‐
286 link(2), utimensat(2), lutimes(3), path_resolution(7)
287Linux man-pages 6.04 2023-04-03 symlink(7)