1.\"- 2.\" Copyright (c) 1992, 1993, 1994 3.\" The Regents of the University of California. All rights reserved. 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 3. Neither the name of the University nor the names of its contributors 14.\" may be used to endorse or promote products derived from this software 15.\" without specific prior written permission. 16.\" 17.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 18.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 19.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 20.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 21.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 22.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 23.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 24.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 25.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 26.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 27.\" SUCH DAMAGE. 28.\" 29.Dd February 16, 2015 30.Dt SYMLINK 7 31.Os 32.Sh NAME 33.Nm symlink 34.Nd symbolic link handling 35.Sh SYMBOLIC LINK HANDLING 36Symbolic links are files that act as pointers to other files. 37To understand their behavior, you must first understand how hard links 38work. 39A hard link to a file is indistinguishable from the original file because 40it is a reference to the object underlying the original file name. 41Changes to a file are independent of the name used to reference the 42file. 43Hard links may not refer to directories and may not reference files 44on different file systems. 45A symbolic link contains the name of the file to which it is linked, 46i.e., it is a pointer to another name, and not to an underlying object. 47For this reason, symbolic links may reference directories and may span 48file systems. 49.Pp 50Because a symbolic link and its referenced object coexist in the file system 51name space, confusion can arise in distinguishing between the link itself 52and the referenced object. 53Historically, commands and system calls have adopted their own link 54following conventions in a somewhat ad-hoc fashion. 55Rules for more a uniform approach, as they are implemented in this system, 56are outlined here. 57It is important that local applications conform to these rules, too, 58so that the user interface can be as consistent as possible. 59.Pp 60Symbolic links are handled either by operating on the link itself, 61or by operating on the object referenced by the link. 62In the latter case, 63an application or system call is said to 64.Dq follow 65the link. 66Symbolic links may reference other symbolic links, 67in which case the links are dereferenced until an object that is 68not a symbolic link is found, 69a symbolic link which references a file which does not exist is found, 70or a loop is detected. 71(Loop detection is done by placing an upper limit on the number of 72links that may be followed, and an error results if this limit is 73exceeded.) 74.Pp 75There are three separate areas that need to be discussed. 76They are as follows: 77.Pp 78.Bl -enum -compact -offset indent 79.It 80Symbolic links used as file name arguments for system calls. 81.It 82Symbolic links specified as command line arguments to utilities that 83are not traversing a file tree. 84.It 85Symbolic links encountered by utilities that are traversing a file tree 86(either specified on the command line or encountered as part of the 87file hierarchy walk). 88.El 89.Ss System calls. 90The first area is symbolic links used as file name arguments for 91system calls. 92.Pp 93Except as noted below, all system calls follow symbolic links. 94For example, if there were a symbolic link 95.Dq Li slink 96which pointed to a file named 97.Dq Li afile , 98the system call 99.Dq Li open("slink" ...\&) 100would return a file descriptor to the file 101.Dq afile . 102.Pp 103There are thirteen system calls that do not follow links, and which operate 104on the symbolic link itself. 105They are: 106.Xr lchflags 2 , 107.Xr lchmod 2 , 108.Xr lchown 2 , 109.Xr lpathconf 2 , 110.Xr lstat 2 , 111.Xr lutimes 2 , 112.Xr readlink 2 , 113.Xr readlinkat 2 , 114.Xr rename 2 , 115.Xr renameat 2 , 116.Xr rmdir 2 , 117.Xr unlink 2 , 118and 119.Xr unlinkat 2 . 120Because 121.Xr remove 3 122is an alias for 123.Xr unlink 2 , 124it also does not follow symbolic links. 125When 126.Xr rmdir 2 127or 128.Xr unlinkat 2 129with the 130.Dv AT_REMOVEDIR 131flag 132is applied to a symbolic link, it fails with the error 133.Er ENOTDIR . 134.Pp 135The 136.Xr linkat 2 137system call does not follow symbolic links 138unless given the 139.Dv AT_SYMLINK_FOLLOW 140flag. 141.Pp 142The following system calls follow symbolic links 143unless given the 144.Dv AT_SYMLINK_NOFOLLOW 145flag: 146.Xr chflagsat 2 , 147.Xr fchmodat 2 , 148.Xr fchownat 2 , 149.Xr fstatat 2 150and 151.Xr utimensat 2 . 152.Pp 153The owner and group of an existing symbolic link can be changed by 154means of the 155.Xr lchown 2 156system call. 157The flags, access permissions, owner/group and modification time of 158an existing symbolic link can be changed by means of the 159.Xr lchflags 2 , 160.Xr lchmod 2 , 161.Xr lchown 2 , 162and 163.Xr lutimes 2 164system calls, respectively. 165Of these, only the flags and ownership are used by the system; 166the access permissions are ignored. 167.Pp 168The 169.Bx 4.4 170system differs from historical 171.Bx 4 172systems in that the system call 173.Xr chown 2 174has been changed to follow symbolic links. 175The 176.Xr lchown 2 177system call was added later when the limitations of the new 178.Xr chown 2 179became apparent. 180.Ss Commands not traversing a file tree. 181The second area is symbolic links, specified as command line file 182name arguments, to commands which are not traversing a file tree. 183.Pp 184Except as noted below, commands follow symbolic links named as command 185line arguments. 186For example, if there were a symbolic link 187.Dq Li slink 188which pointed to a file named 189.Dq Li afile , 190the command 191.Dq Li cat slink 192would display the contents of the file 193.Dq Li afile . 194.Pp 195It is important to realize that this rule includes commands which may 196optionally traverse file trees, e.g.\& the command 197.Dq Li "chown file" 198is included in this rule, while the command 199.Dq Li "chown -R file" 200is not. 201(The latter is described in the third area, below.) 202.Pp 203If it is explicitly intended that the command operate on the symbolic 204link instead of following the symbolic link, e.g., it is desired that 205.Dq Li "chown slink" 206change the ownership of the file that 207.Dq Li slink 208is, whether it is a symbolic link or not, the 209.Fl h 210option should be used. 211In the above example, 212.Dq Li "chown root slink" 213would change the ownership of the file referenced by 214.Dq Li slink , 215while 216.Dq Li "chown -h root slink" 217would change the ownership of 218.Dq Li slink 219itself. 220.Pp 221There are five exceptions to this rule. 222The 223.Xr mv 1 224and 225.Xr rm 1 226commands do not follow symbolic links named as arguments, 227but respectively attempt to rename and delete them. 228(Note, if the symbolic link references a file via a relative path, 229moving it to another directory may very well cause it to stop working, 230since the path may no longer be correct.) 231.Pp 232The 233.Xr ls 1 234command is also an exception to this rule. 235For compatibility with historic systems (when 236.Nm ls 237is not doing a tree walk, i.e., the 238.Fl R 239option is not specified), 240the 241.Nm ls 242command follows symbolic links named as arguments if the 243.Fl H 244or 245.Fl L 246option is specified, 247or if the 248.Fl F , 249.Fl d 250or 251.Fl l 252options are not specified. 253(The 254.Nm ls 255command is the only command where the 256.Fl H 257and 258.Fl L 259options affect its behavior even though it is not doing a walk of 260a file tree.) 261.Pp 262The 263.Xr file 1 264and 265.Xr stat 1 266commands are also exceptions to this rule. 267These 268commands do not follow symbolic links named as argument by default, 269but do follow symbolic links named as argument if the 270.Fl L 271option is specified. 272.Pp 273The 274.Bx 4.4 275system differs from historical 276.Bx 4 277systems in that the 278.Nm chown 279and 280.Nm chgrp 281commands follow symbolic links specified on the command line. 282.Ss Commands traversing a file tree. 283The following commands either optionally or always traverse file trees: 284.Xr chflags 1 , 285.Xr chgrp 1 , 286.Xr chmod 1 , 287.Xr cp 1 , 288.Xr du 1 , 289.Xr find 1 , 290.Xr ls 1 , 291.Xr pax 1 , 292.Xr rm 1 , 293.Xr tar 1 294and 295.Xr chown 8 . 296.Pp 297It is important to realize that the following rules apply equally to 298symbolic links encountered during the file tree traversal and symbolic 299links listed as command line arguments. 300.Pp 301The first rule applies to symbolic links that reference files that are 302not of type directory. 303Operations that apply to symbolic links are performed on the links 304themselves, but otherwise the links are ignored. 305.Pp 306The command 307.Dq Li "rm -r slink directory" 308will remove 309.Dq Li slink , 310as well as any symbolic links encountered in the tree traversal of 311.Dq Li directory , 312because symbolic links may be removed. 313In no case will 314.Nm rm 315affect the file which 316.Dq Li slink 317references in any way. 318.Pp 319The second rule applies to symbolic links that reference files of type 320directory. 321Symbolic links which reference files of type directory are never 322.Dq followed 323by default. 324This is often referred to as a 325.Dq physical 326walk, as opposed to a 327.Dq logical 328walk (where symbolic links referencing directories are followed). 329.Pp 330As consistently as possible, you can make commands doing a file tree 331walk follow any symbolic links named on the command line, regardless 332of the type of file they reference, by specifying the 333.Fl H 334(for 335.Dq half\-logical ) 336flag. 337This flag is intended to make the command line name space look 338like the logical name space. 339(Note, for commands that do not always do file tree traversals, the 340.Fl H 341flag will be ignored if the 342.Fl R 343flag is not also specified.) 344.Pp 345For example, the command 346.Dq Li "chown -HR user slink" 347will traverse the file hierarchy rooted in the file pointed to by 348.Dq Li slink . 349Note, the 350.Fl H 351is not the same as the previously discussed 352.Fl h 353flag. 354The 355.Fl H 356flag causes symbolic links specified on the command line to be 357dereferenced both for the purposes of the action to be performed 358and the tree walk, and it is as if the user had specified the 359name of the file to which the symbolic link pointed. 360.Pp 361As consistently as possible, you can make commands doing a file tree 362walk follow any symbolic links named on the command line, as well as 363any symbolic links encountered during the traversal, regardless of 364the type of file they reference, by specifying the 365.Fl L 366(for 367.Dq logical ) 368flag. 369This flag is intended to make the entire name space look like 370the logical name space. 371(Note, for commands that do not always do file tree traversals, the 372.Fl L 373flag will be ignored if the 374.Fl R 375flag is not also specified.) 376.Pp 377For example, the command 378.Dq Li "chown -LR user slink" 379will change the owner of the file referenced by 380.Dq Li slink . 381If 382.Dq Li slink 383references a directory, 384.Nm chown 385will traverse the file hierarchy rooted in the directory that it 386references. 387In addition, if any symbolic links are encountered in any file tree that 388.Nm chown 389traverses, they will be treated in the same fashion as 390.Dq Li slink . 391.Pp 392As consistently as possible, you can specify the default behavior by 393specifying the 394.Fl P 395(for 396.Dq physical ) 397flag. 398This flag is intended to make the entire name space look like the 399physical name space. 400.Pp 401For commands that do not by default do file tree traversals, the 402.Fl H , 403.Fl L 404and 405.Fl P 406flags are ignored if the 407.Fl R 408flag is not also specified. 409In addition, you may specify the 410.Fl H , 411.Fl L 412and 413.Fl P 414options more than once; the last one specified determines the 415command's behavior. 416This is intended to permit you to alias commands to behave one way 417or the other, and then override that behavior on the command line. 418.Pp 419The 420.Xr ls 1 421and 422.Xr rm 1 423commands have exceptions to these rules. 424The 425.Nm rm 426command operates on the symbolic link, and not the file it references, 427and therefore never follows a symbolic link. 428The 429.Nm rm 430command does not support the 431.Fl H , 432.Fl L 433or 434.Fl P 435options. 436.Pp 437To maintain compatibility with historic systems, 438the 439.Nm ls 440command acts a little differently. 441If you do not specify the 442.Fl F , 443.Fl d 444or 445.Fl l 446options, 447.Nm ls 448will follow symbolic links specified on the command line. 449If the 450.Fl L 451flag is specified, 452.Nm ls 453follows all symbolic links, 454regardless of their type, 455whether specified on the command line or encountered in the tree walk. 456.Sh SEE ALSO 457.Xr chflags 1 , 458.Xr chgrp 1 , 459.Xr chmod 1 , 460.Xr cp 1 , 461.Xr du 1 , 462.Xr find 1 , 463.Xr ln 1 , 464.Xr ls 1 , 465.Xr mv 1 , 466.Xr pax 1 , 467.Xr rm 1 , 468.Xr tar 1 , 469.Xr lchflags 2 , 470.Xr lchmod 2 , 471.Xr lchown 2 , 472.Xr lstat 2 , 473.Xr lutimes 2 , 474.Xr readlink 2 , 475.Xr rename 2 , 476.Xr symlink 2 , 477.Xr unlink 2 , 478.Xr fts 3 , 479.Xr remove 3 , 480.Xr chown 8 481