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.\" @(#)symlink.7 8.3 (Berkeley) 3/31/94 30.\" $FreeBSD: head/bin/ln/symlink.7 278848 2015-02-16 12:56:55Z jilles $ 31.\" 32.Dd March 23, 2015 33.Dt SYMLINK 7 34.Os 35.Sh NAME 36.Nm symlink 37.Nd symbolic link handling 38.Sh SYMBOLIC LINK HANDLING 39Symbolic links are files that act as pointers to other files. 40To understand their behavior, you must first understand how hard links 41work. 42A hard link to a file is indistinguishable from the original file because 43it is a reference to the object underlying the original file name. 44Changes to a file are independent of the name used to reference the 45file. 46Hard links may not refer to directories and may not reference files 47on different file systems. 48A symbolic link contains the name of the file to which it is linked, 49i.e., it is a pointer to another name, and not to an underlying object. 50For this reason, symbolic links may reference directories and may span 51file systems. 52.Pp 53Because a symbolic link and its referenced object coexist in the file system 54name space, confusion can arise in distinguishing between the link itself 55and the referenced object. 56Historically, commands and system calls have adopted their own link 57following conventions in a somewhat ad-hoc fashion. 58Rules for more a uniform approach, as they are implemented in this system, 59are outlined here. 60It is important that local applications conform to these rules, too, 61so that the user interface can be as consistent as possible. 62.Pp 63Symbolic links are handled either by operating on the link itself, 64or by operating on the object referenced by the link. 65In the latter case, 66an application or system call is said to 67.Dq follow 68the link. 69Symbolic links may reference other symbolic links, 70in which case the links are dereferenced until an object that is 71not a symbolic link is found, 72a symbolic link which references a file which does not exist is found, 73or a loop is detected. 74(Loop detection is done by placing an upper limit on the number of 75links that may be followed, and an error results if this limit is 76exceeded.) 77.Pp 78There are three separate areas that need to be discussed. 79They are as follows: 80.Pp 81.Bl -enum -compact -offset indent 82.It 83Symbolic links used as file name arguments for system calls. 84.It 85Symbolic links specified as command line arguments to utilities that 86are not traversing a file tree. 87.It 88Symbolic links encountered by utilities that are traversing a file tree 89(either specified on the command line or encountered as part of the 90file hierarchy walk). 91.El 92.Ss System calls. 93The first area is symbolic links used as file name arguments for 94system calls. 95.Pp 96Except as noted below, all system calls follow symbolic links. 97For example, if there were a symbolic link 98.Dq Li slink 99which pointed to a file named 100.Dq Li afile , 101the system call 102.Dq Li open("slink" ...\&) 103would return a file descriptor to the file 104.Dq afile . 105.Pp 106There are thirteen system calls that do not follow links, and which operate 107on the symbolic link itself. 108They are: 109.Xr lchflags 2 , 110.Xr lchmod 2 , 111.Xr lchown 2 , 112.Xr lpathconf 2 , 113.Xr lstat 2 , 114.Xr lutimes 2 , 115.Xr readlink 2 , 116.Xr readlinkat 2 , 117.Xr rename 2 , 118.Xr renameat 2 , 119.Xr rmdir 2 , 120.Xr unlink 2 , 121and 122.Xr unlinkat 2 . 123Because 124.Xr remove 3 125is an alias for 126.Xr unlink 2 , 127it also does not follow symbolic links. 128When 129.Xr rmdir 2 130or 131.Xr unlinkat 2 132with the 133.Dv AT_REMOVEDIR 134flag 135is applied to a symbolic link, it fails with the error 136.Er ENOTDIR . 137.Pp 138The 139.Xr linkat 2 140system call does not follow symbolic links 141unless given the 142.Dv AT_SYMLINK_FOLLOW 143flag. 144.Pp 145The following system calls follow symbolic links 146unless given the 147.Dv AT_SYMLINK_NOFOLLOW 148flag: 149.Xr chflagsat 2 , 150.Xr fchmodat 2 , 151.Xr fchownat 2 , 152and 153.Xr fstatat 2 . 154.\"and 155.\".Xr utimensat 2 . 156.Pp 157The owner and group of an existing symbolic link can be changed by 158means of the 159.Xr lchown 2 160system call. 161The flags, access permissions, owner/group and modification time of 162an existing symbolic link can be changed by means of the 163.Xr lchflags 2 , 164.Xr lchmod 2 , 165.Xr lchown 2 , 166and 167.Xr lutimes 2 168system calls, respectively. 169Of these, only the flags and ownership are used by the system; 170the access permissions are ignored. 171.Pp 172The 173.Bx 4.4 174system differs from historical 175.Bx 4 176systems in that the system call 177.Xr chown 2 178has been changed to follow symbolic links. 179The 180.Xr lchown 2 181system call was added later when the limitations of the new 182.Xr chown 2 183became apparent. 184.Ss Commands not traversing a file tree. 185The second area is symbolic links, specified as command line file 186name arguments, to commands which are not traversing a file tree. 187.Pp 188Except as noted below, commands follow symbolic links named as command 189line arguments. 190For example, if there were a symbolic link 191.Dq Li slink 192which pointed to a file named 193.Dq Li afile , 194the command 195.Dq Li cat slink 196would display the contents of the file 197.Dq Li afile . 198.Pp 199It is important to realize that this rule includes commands which may 200optionally traverse file trees, e.g.\& the command 201.Dq Li "chown file" 202is included in this rule, while the command 203.Dq Li "chown -R file" 204is not. 205(The latter is described in the third area, below.) 206.Pp 207If it is explicitly intended that the command operate on the symbolic 208link instead of following the symbolic link, e.g., it is desired that 209.Dq Li "chown slink" 210change the ownership of the file that 211.Dq Li slink 212is, whether it is a symbolic link or not, the 213.Fl h 214option should be used. 215In the above example, 216.Dq Li "chown root slink" 217would change the ownership of the file referenced by 218.Dq Li slink , 219while 220.Dq Li "chown -h root slink" 221would change the ownership of 222.Dq Li slink 223itself. 224.Pp 225There are five exceptions to this rule. 226The 227.Xr mv 1 228and 229.Xr rm 1 230commands do not follow symbolic links named as arguments, 231but respectively attempt to rename and delete them. 232(Note, if the symbolic link references a file via a relative path, 233moving it to another directory may very well cause it to stop working, 234since the path may no longer be correct.) 235.Pp 236The 237.Xr ls 1 238command is also an exception to this rule. 239For compatibility with historic systems (when 240.Nm ls 241is not doing a tree walk, i.e., the 242.Fl R 243option is not specified), 244the 245.Nm ls 246command follows symbolic links named as arguments if the 247.Fl H 248or 249.Fl L 250option is specified, 251or if the 252.Fl F , 253.Fl d 254or 255.Fl l 256options are not specified. 257(The 258.Nm ls 259command is the only command where the 260.Fl H 261and 262.Fl L 263options affect its behavior even though it is not doing a walk of 264a file tree.) 265.Pp 266The 267.Xr file 1 268and 269.Xr stat 1 270commands are also exceptions to this rule. 271These 272commands do not follow symbolic links named as argument by default, 273but do follow symbolic links named as argument if the 274.Fl L 275option is specified. 276.Pp 277The 278.Bx 4.4 279system differs from historical 280.Bx 4 281systems in that the 282.Nm chown 283and 284.Nm chgrp 285commands follow symbolic links specified on the command line. 286.Ss Commands traversing a file tree. 287The following commands either optionally or always traverse file trees: 288.Xr chflags 1 , 289.Xr chgrp 1 , 290.Xr chmod 1 , 291.Xr cp 1 , 292.Xr du 1 , 293.Xr find 1 , 294.Xr ls 1 , 295.Xr pax 1 , 296.Xr rm 1 , 297.Xr tar 1 298and 299.Xr chown 8 . 300.Pp 301It is important to realize that the following rules apply equally to 302symbolic links encountered during the file tree traversal and symbolic 303links listed as command line arguments. 304.Pp 305The first rule applies to symbolic links that reference files that are 306not of type directory. 307Operations that apply to symbolic links are performed on the links 308themselves, but otherwise the links are ignored. 309.Pp 310The command 311.Dq Li "rm -r slink directory" 312will remove 313.Dq Li slink , 314as well as any symbolic links encountered in the tree traversal of 315.Dq Li directory , 316because symbolic links may be removed. 317In no case will 318.Nm rm 319affect the file which 320.Dq Li slink 321references in any way. 322.Pp 323The second rule applies to symbolic links that reference files of type 324directory. 325Symbolic links which reference files of type directory are never 326.Dq followed 327by default. 328This is often referred to as a 329.Dq physical 330walk, as opposed to a 331.Dq logical 332walk (where symbolic links referencing directories are followed). 333.Pp 334As consistently as possible, you can make commands doing a file tree 335walk follow any symbolic links named on the command line, regardless 336of the type of file they reference, by specifying the 337.Fl H 338(for 339.Dq half\-logical ) 340flag. 341This flag is intended to make the command line name space look 342like the logical name space. 343(Note, for commands that do not always do file tree traversals, the 344.Fl H 345flag will be ignored if the 346.Fl R 347flag is not also specified.) 348.Pp 349For example, the command 350.Dq Li "chown -HR user slink" 351will traverse the file hierarchy rooted in the file pointed to by 352.Dq Li slink . 353Note, the 354.Fl H 355is not the same as the previously discussed 356.Fl h 357flag. 358The 359.Fl H 360flag causes symbolic links specified on the command line to be 361dereferenced both for the purposes of the action to be performed 362and the tree walk, and it is as if the user had specified the 363name of the file to which the symbolic link pointed. 364.Pp 365As consistently as possible, you can make commands doing a file tree 366walk follow any symbolic links named on the command line, as well as 367any symbolic links encountered during the traversal, regardless of 368the type of file they reference, by specifying the 369.Fl L 370(for 371.Dq logical ) 372flag. 373This flag is intended to make the entire name space look like 374the logical name space. 375(Note, for commands that do not always do file tree traversals, the 376.Fl L 377flag will be ignored if the 378.Fl R 379flag is not also specified.) 380.Pp 381For example, the command 382.Dq Li "chown -LR user slink" 383will change the owner of the file referenced by 384.Dq Li slink . 385If 386.Dq Li slink 387references a directory, 388.Nm chown 389will traverse the file hierarchy rooted in the directory that it 390references. 391In addition, if any symbolic links are encountered in any file tree that 392.Nm chown 393traverses, they will be treated in the same fashion as 394.Dq Li slink . 395.Pp 396As consistently as possible, you can specify the default behavior by 397specifying the 398.Fl P 399(for 400.Dq physical ) 401flag. 402This flag is intended to make the entire name space look like the 403physical name space. 404.Pp 405For commands that do not by default do file tree traversals, the 406.Fl H , 407.Fl L 408and 409.Fl P 410flags are ignored if the 411.Fl R 412flag is not also specified. 413In addition, you may specify the 414.Fl H , 415.Fl L 416and 417.Fl P 418options more than once; the last one specified determines the 419command's behavior. 420This is intended to permit you to alias commands to behave one way 421or the other, and then override that behavior on the command line. 422.Pp 423The 424.Xr ls 1 425and 426.Xr rm 1 427commands have exceptions to these rules. 428The 429.Nm rm 430command operates on the symbolic link, and not the file it references, 431and therefore never follows a symbolic link. 432The 433.Nm rm 434command does not support the 435.Fl H , 436.Fl L 437or 438.Fl P 439options. 440.Pp 441To maintain compatibility with historic systems, 442the 443.Nm ls 444command acts a little differently. 445If you do not specify the 446.Fl F , 447.Fl d 448or 449.Fl l 450options, 451.Nm ls 452will follow symbolic links specified on the command line. 453If the 454.Fl L 455flag is specified, 456.Nm ls 457follows all symbolic links, 458regardless of their type, 459whether specified on the command line or encountered in the tree walk. 460.Sh SEE ALSO 461.Xr chflags 1 , 462.Xr chgrp 1 , 463.Xr chmod 1 , 464.Xr cp 1 , 465.Xr du 1 , 466.Xr find 1 , 467.Xr ln 1 , 468.Xr ls 1 , 469.Xr mv 1 , 470.Xr pax 1 , 471.Xr rm 1 , 472.Xr tar 1 , 473.Xr lchflags 2 , 474.Xr lchmod 2 , 475.Xr lchown 2 , 476.Xr lstat 2 , 477.Xr lutimes 2 , 478.Xr readlink 2 , 479.Xr rename 2 , 480.Xr renameat 2 , 481.Xr symlink 2 , 482.Xr unlink 2 , 483.Xr fts 3 , 484.Xr remove 3 , 485.Xr chown 8 486