1.\" $OpenBSD: symlink.7,v 1.21 2019/09/02 21:18:41 deraadt Exp $ 2.\" $NetBSD: symlink.7,v 1.4 1996/04/25 15:44:56 mycroft Exp $ 3.\" 4.\" Copyright (c) 1992, 1993, 1994 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)symlink.7 8.3 (Berkeley) 3/31/94 32.\" 33.Dd $Mdocdate: September 2 2019 $ 34.Dt SYMLINK 7 35.Os 36.Sh NAME 37.Nm symlink 38.Nd symbolic link handling 39.Sh DESCRIPTION 40Symbolic links are files that act as pointers to other files. 41To understand their behavior, it is necessary to understand how hard links 42work. 43A hard link to a file is indistinguishable from the original file because 44it is a reference to the object underlying the original file name. 45Changes to a file are independent of the name used to reference the 46file. 47Hard links may not refer to directories and may not reference files 48on different file systems. 49A symbolic link contains the name of the file to which it is linked; 50i.e., it is a pointer to a name, and not to an underlying object. 51For this reason, symbolic links may reference directories and may span 52file systems. 53.Pp 54Because a symbolic link and its referenced object coexist in the filesystem 55name space, confusion can arise in distinguishing between the link itself 56and the referenced object. 57Historically, commands and system calls have adopted their own 58link-following conventions in a somewhat ad hoc fashion. 59Rules for a more uniform approach, as they are implemented in this system, 60are outlined here. 61It is important that local applications conform to these rules, too, 62so that the user interface can be as consistent as possible. 63.Pp 64Symbolic links are handled either by operating on the link itself, 65or by operating on the object referenced by the link. 66In the latter case, 67an application or system call is said to 68.Dq follow 69the link. 70Symbolic links may reference other symbolic links, 71in which case the links are dereferenced until an object that is 72not a symbolic link is found, 73a symbolic link which references a file which doesn't exist is found, 74or a loop is detected. 75(Loop detection is done by placing an upper limit on the number of 76links that may be followed, with an error resulting if this limit is 77exceeded.) 78.Pp 79There are three separate areas that need to be discussed. 80They are as follows: 81.Pp 82.Bl -enum -compact -offset indent 83.It 84Symbolic links used as file name arguments for system calls. 85.It 86Symbolic links specified as command-line arguments to utilities that 87are not traversing a file tree. 88.It 89Symbolic links encountered by utilities that are traversing a file tree 90(either specified on the command line or encountered as part of the 91file hierarchy walk). 92.El 93.Ss System calls 94The first area is symbolic links used as file name arguments for 95system calls. 96.Pp 97Except as noted below, all system calls follow symbolic links. 98For example, if there were a symbolic link 99.Dq Li slink 100which pointed to a file named 101.Dq Li afile , 102the system call 103.Dq Li open("slink" ...) 104would return a file descriptor to the file 105.Dq afile . 106.Pp 107There are at least five system calls that do not follow links, and which 108operate on the symbolic link itself. 109They are: 110.Xr lchown 2 , 111.Xr lstat 2 , 112.Xr readlink 2 , 113.Xr rename 2 , 114and 115.Xr unlink 2 . 116Because 117.Xr remove 3 118is an alias for 119.Xr unlink 2 , 120it also does not follow symbolic links. 121.Pp 122Additionally, the following system calls accept a 123.Fa flag 124argument to control whether or not to follow symbolic links: 125.Xr chflagsat 2 , 126.Xr fchmodat 2 , 127.Xr fchownat 2 , 128.Xr fstatat 2 , 129.Xr linkat 2 , 130and 131.Xr utimensat 2 . 132.Pp 133The 134.Bx 4.4 135system differs from historical 4BSD systems in that the system call 136.Xr chown 2 137has been changed to follow symbolic links. 138The 139.Xr lchown 2 140system call was added later when the limitations of the new 141.Xr chown 2 142became apparent. 143.Ss Commands not traversing a file tree 144The second area is symbolic links, specified as command-line file 145name arguments, to commands which are not traversing a file tree. 146.Pp 147Except as noted below, commands follow symbolic links named as 148command-line arguments. 149For example, if there were a symbolic link 150.Dq Li slink 151which pointed to a file named 152.Dq Li afile , 153the command 154.Dq Li cat slink 155would display the contents of the file 156.Dq Li afile . 157.Pp 158It is important to realize that this rule includes commands which may 159optionally traverse file trees, e.g., the command 160.Dq Li "chown owner file" 161is included in this rule, while the command 162.Dq Li "chown -R owner file" 163is not. 164(The latter is described in the third area, below.) 165.Pp 166If it is explicitly intended that the command operate on the symbolic 167link instead of following the symbolic link \(em e.g., it is desired that 168.Dq Li "chown owner slink" 169change the ownership of 170.Dq Li slink , 171not of what it points to \(em the 172.Fl h 173option should be used. 174In the above example, 175.Dq Li "chown owner slink" 176would change the owner of 177.Dq Li afile 178to 179.Dq Li owner , 180while 181.Dq Li "chown -h owner slink" 182would change the ownership of 183.Dq Li slink . 184.Pp 185There are several exceptions to this rule. 186The 187.Xr mv 1 188and 189.Xr rm 1 190commands do not follow symbolic links named as arguments, 191but respectively attempt to rename and delete them. 192(Note that if the symbolic link references a file via a relative path, 193moving it to another directory may very well cause it to stop working, 194since the path may no longer be correct.) 195.Pp 196The 197.Xr ls 1 198command is also an exception to this rule. 199For compatibility with historic systems (when 200.Nm ls 201is not doing a tree walk, i.e., the 202.Fl R 203option is not specified), 204the 205.Nm ls 206command follows symbolic links named as arguments if the 207.Fl L 208option is specified, 209or if the 210.Fl F , 211.Fl d , 212or 213.Fl l 214options are not specified. 215(If the 216.Fl L 217option is specified, 218.Nm ls 219always follows symbolic links. 220The 221.Fl L 222option affects its behavior even though it is not doing a walk of 223a file tree.) 224.Pp 225The 226.Xr file 1 227command behaves as 228.Xr ls 1 229in that the 230.Fl L 231option makes it follow a symbolic link. 232By default, 233.Dq Li "file slink" 234will report that 235.Dq Li slink 236is a symbolic link. 237This behavior is different from 238.Xr file 1 239on some other systems, where the 240.Fl h 241convention is followed. 242.Pp 243The 244.Bx 4.4 245system differs from historical 4BSD systems in that the 246.Xr chown 8 , 247.Xr chgrp 1 , 248and 249.Xr file 1 250commands follow symbolic links specified on the command line 251(unless the 252.Fl h 253option is used). 254.Ss Commands traversing a file tree 255The following commands either optionally or always traverse file trees: 256.Xr chflags 1 , 257.Xr chgrp 1 , 258.Xr chmod 1 , 259.Xr cp 1 , 260.Xr du 1 , 261.Xr find 1 , 262.Xr ls 1 , 263.Xr pax 1 , 264.Xr rm 1 , 265.Xr tar 1 , 266and 267.Xr chown 8 . 268.Pp 269It is important to realize that the following rules apply equally to 270symbolic links encountered during the file tree traversal and symbolic 271links listed as command-line arguments. 272.Pp 273The first rule applies to symbolic links that reference files that are 274not of type directory. 275Operations that apply to symbolic links are performed on the links 276themselves, but otherwise the links are ignored. 277.Pp 278For example, the command 279.Dq Li "chown -R user slink directory" 280will ignore 281.Dq Li slink , 282because the 283.Fl h 284option was not given. 285Any symbolic links encountered during the tree traversal will also be 286ignored. 287The command 288.Dq Li "rm -r slink directory" 289will remove 290.Dq Li slink , 291as well as any symbolic links encountered in the tree traversal of 292.Dq Li directory , 293because symbolic links may be removed. 294In no case will either 295.Xr chown 8 296or 297.Xr rm 1 298follow the symlink to affect the file which 299.Dq Li slink 300references. 301.Pp 302The second rule applies to symbolic links that reference files of type 303directory. 304Symbolic links which reference files of type directory are never 305.Dq followed 306by default. 307This is often referred to as a 308.Dq physical 309walk, as opposed to a 310.Dq logical 311walk (where symbolic links referencing directories are followed). 312.Pp 313As consistently as possible, it is possible to make commands doing a file tree 314walk follow any symbolic links named on the command line, regardless 315of the type of file they reference, by specifying the 316.Fl H 317(for 318.Dq half-logical ) 319flag. 320This flag is intended to make the command-line name space look 321like the logical name space. 322(Note: 323for commands that do not always do file tree traversals, the 324.Fl H 325flag will be ignored if the 326.Fl R 327flag is not also specified.) 328.Pp 329For example, the command 330.Dq Li "chown -HR user slink" 331will traverse the file hierarchy rooted in the file pointed to by 332.Dq Li slink . 333The 334.Fl H 335is not the same as the previously discussed 336.Fl h 337flag. 338The 339.Fl H 340flag causes symbolic links specified on the command line to be 341dereferenced both for the purposes of the action to be performed 342and the tree walk, and it is as if the user had specified the 343name of the file to which the symbolic link pointed. 344.Pp 345As consistently as possible, it is possible to make commands doing a file tree 346walk follow any symbolic links named on the command line, as well as 347any symbolic links encountered during the traversal, regardless of 348the type of file they reference, by specifying the 349.Fl L 350(for 351.Dq logical ) 352flag. 353This flag is intended to make the entire name space look like 354the logical name space. 355(Note: 356for commands that do not always do file tree traversals, the 357.Fl L 358flag will be ignored if the 359.Fl R 360flag is not also specified.) 361.Pp 362For example, the command 363.Dq Li "chown -LR user slink" 364will change the owner of the file referenced by 365.Dq Li slink . 366If 367.Dq Li slink 368references a directory, 369.Nm chown 370will traverse the file hierarchy rooted in the directory that it 371references. 372In addition, if any symbolic links are encountered in any file tree that 373.Nm chown 374traverses, they will be treated in the same fashion as 375.Dq Li slink . 376.Pp 377As consistently as possible, it is possible to specify the default behavior by 378specifying the 379.Fl P 380(for 381.Dq physical ) 382flag. 383This flag is intended to make the entire name space look like the 384physical name space. 385.Pp 386For commands that do not by default do file tree traversals, the 387.Fl H , 388.Fl L , 389and 390.Fl P 391flags are ignored if the 392.Fl R 393flag is not also specified. 394In addition, the 395.Fl H , 396.Fl L , 397and 398.Fl P 399options may be specified more than once; 400the last one specified determines the command's behavior. 401This is intended to permit aliasing commands to behave one way 402or the other, and then override that behavior on the command line. 403.Pp 404The 405.Xr ls 1 406and 407.Xr rm 1 408commands have exceptions to these rules. 409The 410.Nm rm 411command operates on the symbolic link, and not the file it references, 412and therefore never follows a symbolic link. 413The 414.Nm rm 415command does not support the 416.Fl H , 417.Fl L , 418or 419.Fl P 420options. 421.Pp 422To maintain compatibility with historic systems, 423the 424.Nm ls 425command never follows symbolic links unless the 426.Fl L 427flag is specified. 428If the 429.Fl L 430flag is specified, 431.Nm ls 432follows all symbolic links, 433regardless of their type, 434whether specified on the command line or encountered in the tree walk. 435The 436.Nm ls 437command does not support the 438.Fl H 439or 440.Fl P 441options. 442.Sh SEE ALSO 443.Xr chflags 1 , 444.Xr chgrp 1 , 445.Xr chmod 1 , 446.Xr cp 1 , 447.Xr du 1 , 448.Xr find 1 , 449.Xr ln 1 , 450.Xr ls 1 , 451.Xr mv 1 , 452.Xr pax 1 , 453.Xr rm 1 , 454.Xr tar 1 , 455.Xr lchown 2 , 456.Xr lstat 2 , 457.Xr readlink 2 , 458.Xr rename 2 , 459.Xr symlink 2 , 460.Xr unlink 2 , 461.Xr fts_open 3 , 462.Xr remove 3 , 463.Xr chown 8 464