1SYMLINK(7) Linux Programmer's 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
10 understand their behavior, you must first understand how hard links
11 work.
12 A hard link to a file is indistinguishable from the original file
14 because it is a reference to the object underlying the original file‐
15 name. (To be precise: each of the hard links to a file is a reference
16 to the same i-node number, where an i-node number is an index into the
17 i-node table, which contains metadata about all files on a file system.
18 See stat(2).) Changes to a file are independent of the name used to
19 reference the file. Hard links may not refer to directories (to pre‐
20 vent the possibility of loops within the file system tree, which would
21 confuse many programs) and may not refer to files on different file
22 systems (because i-node numbers are not unique across file systems).
23 A symbolic link is a special type of file whose contents are a string
25 that is the pathname another file, the file to which the link refers.
26 In other words, a symbolic link is a pointer to another name, and not
27 to an underlying object. For this reason, symbolic links may refer to
28 directories and may cross file system 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 file
35 system name space, confusion can arise in distinguishing between the
36 link itself and the referenced object. On historical systems, commands
37 and system calls adopted their own link-following conventions in a
38 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 Symbolic link ownership, permissions, and timestamps
44 The owner and group of an existing symbolic link can be changed using
45 lchown(2). The only time that the ownership of a symbolic link matters
46 is when the link is being removed or renamed in a directory that has
47 the sticky bit set (see stat(2)).
48 The last access and last modification timestamps of a symbolic link can
50 be changed using utimensat(2) or lutimes(3).
51 On Linux, the permissions of a symbolic link are not used in any opera‐
53 tions; the permissions are always 0777 (read, write, and execute for
54 all user categories), and can't be changed.
55 Handling of symbolic links by system calls and commands
57 Symbolic links are handled either by operating on the link itself, or
58 by operating on the object referred to by the link. In the latter
59 case, an application or system call is said to follow the link. Sym‐
60 bolic links may refer to other symbolic links, in which case the links
61 are dereferenced until an object that is not a symbolic link is found,
62 a symbolic link that refers to a file which does not exist is found, or
63 a loop is detected. (Loop detection is done by placing an upper limit
64 on the number of links that may be followed, and an error results if
65 this limit is exceeded.)
66 There are three separate areas that need to be discussed. They are as
68 follows:
69 1. Symbolic links used as filename arguments for system calls.
71 2. Symbolic links specified as command-line arguments to utilities that
73 are not traversing a file tree.
74 3. Symbolic links encountered by utilities that are traversing a file
76 tree (either specified on the command line or encountered as part of
77 the file hierarchy walk).
78 System calls
80 The first area is symbolic links used as filename arguments for system
81 calls.
82 Except as noted below, all system calls follow symbolic links. For
84 example, if there were a symbolic link slink which pointed to a file
85 named afile, the system call open("slink" ...) would return a file
86 descriptor referring to the file afile.
87 Various system calls do not follow links, and operate on the symbolic
89 link itself. They are: lchown(2), lgetxattr(2), llistxattr(2), lre‐
90 movexattr(2), lsetxattr(2), lstat(2), readlink(2), rename(2), rmdir(2),
91 and unlink(2). Certain other system calls optionally follow symbolic
92 links. They are: faccessat(2), fchownat(2), fstatat(2), linkat(2),
93 open(2), openat(2), and utimensat(2); see their manual pages for
94 details. Because remove(3) is an alias for unlink(2), that library
95 function also does not follow symbolic links. When rmdir(2) is applied
96 to a symbolic link, it fails with the error ENOTDIR. The link(2) war‐
97 rants special discussion. POSIX.1-2001 specifies that link(2) should
98 dereference oldpath if it is a symbolic link. However, Linux does not
99 do this. (By default Solaris is the same, but the POSIX.1-2001 speci‐
100 fied behavior can be obtained with suitable compiler options.) The
101 upcoming POSIX.1 revision changes the specification to allow either
102 behavior in an implementation.
103 Commands not traversing a file tree
105 The second area is symbolic links, specified as command-line filename
106 arguments, to commands which are not traversing a file tree.
107 Except as noted below, commands follow symbolic links named as command-
109 line arguments. For example, if there were a symbolic link slink which
110 pointed to a file named afile, the command cat slink would display the
111 contents of the file afile.
112 It is important to realize that this rule includes commands which may
114 optionally traverse file trees, e.g., the command chown file is
115 included in this rule, while the command chown -R file, which performs
116 a tree traversal, is not. (The latter is described in the third area,
117 below.)
118 If it is explicitly intended that the command operate on the symbolic
120 link instead of following the symbolic link, e.g., it is desired that
121 chown slink change the ownership of the file that slink is, whether it
122 is a symbolic link or not, the -h option should be used. In the above
123 example, chown root slink would change the ownership of the file
124 referred to by slink, while chown -h root slink would change the owner‐
125 ship of slink itself.
126 There are some exceptions to this rule:
128 * The mv(1) and rm(1) commands do not follow symbolic links named as
130 arguments, but respectively attempt to rename and delete them.
131 (Note, if the symbolic link references a file via a relative path,
132 moving it to another directory may very well cause it to stop work‐
133 ing, since the path may no longer be correct.)
134 * The ls(1) command is also an exception to this rule. For compatibil‐
136 ity with historic systems (when ls(1) is not doing a tree walk, i.e.,
137 the -R option is not specified), the ls(1) command follows symbolic
138 links named as arguments if the -H or -L option is specified, or if
139 the -F, -d, or -l options are not specified. (The ls(1) command is
140 the only command where the -H and -L options affect its behavior even
141 though it is not doing a walk of a file tree.)
142 * The file(1) command is also an exception to this rule. The file(1)
144 command does not follow symbolic links named as argument by default.
145 The file(1) command does follow symbolic links named as argument if
146 the -L option is specified.
147 Commands traversing a file tree
149 The following commands either optionally or always traverse file trees:
150 chgrp(1), chmod(1), chown(1), cp(1), du(1), find(1), ls(1), pax(1),
151 rm(1), and tar(1).
152 It is important to realize that the following rules apply equally to
154 symbolic links encountered during the file tree traversal and symbolic
155 links listed as command-line arguments.
156 The first rule applies to symbolic links that reference files other
158 than directories. Operations that apply to symbolic links are per‐
159 formed on the links themselves, but otherwise the links are ignored.
160 The command rm -r slink directory will remove slink, as well as any
162 symbolic links encountered in the tree traversal of directory, because
163 symbolic links may be removed. In no case will rm(1) affect the file
164 referred to by slink.
165 The second rule applies to symbolic links that refer to directories.
167 Symbolic links that refer to directories are never followed by default.
168 This is often referred to as a "physical" walk, as opposed to a "logi‐
169 cal" walk (where symbolic links the refer to directories are followed).
170 Certain conventions are (should be) followed as consistently as possi‐
172 ble by commands that perform file tree walks:
173 * A command can be made to follow any symbolic links named on the com‐
175 mand line, regardless of the type of file they reference, by specify‐
176 ing the -H (for "half-logical") flag. This flag is intended to make
177 the command-line name space look like the logical name space. (Note,
178 for commands that do not always do file tree traversals, the -H flag
179 will be ignored if the -R flag is not also specified.)
180 For example, the command chown -HR user slink will traverse the file
182 hierarchy rooted in the file pointed to by slink. Note, the -H is
183 not the same as the previously discussed -h flag. The -H flag causes
184 symbolic links specified on the command line to be dereferenced for
185 the purposes of both the action to be performed and the tree walk,
186 and it is as if the user had specified the name of the file to which
187 the symbolic link pointed.
188 * A command can be made to follow any symbolic links named on the com‐
190 mand line, as well as any symbolic links encountered during the tra‐
191 versal, regardless of the type of file they reference, by specifying
192 the -L (for "logical") flag. This flag is intended to make the
193 entire name space look like the logical name space. (Note, for com‐
194 mands that do not always do file tree traversals, the -L flag will be
195 ignored if the -R flag is not also specified.)
196 For example, the command chown -LR user slink will change the owner
198 of the file referred to by slink. If slink refers to a directory,
199 chown will traverse the file hierarchy rooted in the directory that
200 it references. In addition, if any symbolic links are encountered in
201 any file tree that chown traverses, they will be treated in the same
202 fashion as slink.
203 * A command can be made to provide the default behavior by specifying
205 the -P (for "physical") flag. This flag is intended to make the
206 entire name space look like the physical name space.
207 For commands that do not by default do file tree traversals, the -H,
209 -L, and -P flags are ignored if the -R flag is not also specified. In
210 addition, you may specify the -H, -L, and -P options more than once;
211 the last one specified determines the command's behavior. This is
212 intended to permit you to alias commands to behave one way or the
213 other, and then override that behavior on the command line.
214 The ls(1) and rm(1) commands have exceptions to these rules:
216 * The rm(1) command operates on the symbolic link, and not the file it
218 references, and therefore never follows a symbolic link. The rm(1)
219 command does not support the -H, -L, or -P options.
220 * To maintain compatibility with historic systems, the ls(1) command
222 acts a little differently. If you do not specify the -F, -d or -l
223 options, ls(1) will follow symbolic links specified on the command
224 line. If the -L flag is specified, ls(1) follows all symbolic links,
225 regardless of their type, whether specified on the command line or
226 encountered in the tree walk.
227
229 chgrp(1), chmod(1), find(1), ln(1), ls(1), mv(1), rm(1), lchown(2),
230 link(2), lstat(2), readlink(2), rename(2), symlink(2), unlink(2), uti‐
231 mensat(2), lutimes(3), path_resolution(7)
232
234 This page is part of release 3.25 of the Linux man-pages project. A
235 description of the project, and information about reporting bugs, can
236 be found at http://www.kernel.org/doc/man-pages/.
237Linux 2008-06-18 SYMLINK(7)