1.\" $OpenBSD: symlink.7,v 1.17 2007/05/31 19:19:14 jmc 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: May 31 2007 $ 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 122The 123.Bx 4.4 124system differs from historical 4BSD systems in that the system call 125.Xr chown 2 126has been changed to follow symbolic links. 127The 128.Xr lchown 2 129system call was added later when the limitations of the new 130.Xr chown 2 131became apparent. 132.Ss Commands not traversing a file tree 133The second area is symbolic links, specified as command-line file 134name arguments, to commands which are not traversing a file tree. 135.Pp 136Except as noted below, commands follow symbolic links named as 137command-line arguments. 138For example, if there were a symbolic link 139.Dq Li slink 140which pointed to a file named 141.Dq Li afile , 142the command 143.Dq Li cat slink 144would display the contents of the file 145.Dq Li afile . 146.Pp 147It is important to realize that this rule includes commands which may 148optionally traverse file trees, e.g., the command 149.Dq Li "chown owner file" 150is included in this rule, while the command 151.Dq Li "chown -R owner file" 152is not. 153(The latter is described in the third area, below.) 154.Pp 155If it is explicitly intended that the command operate on the symbolic 156link instead of following the symbolic link -- e.g., it is desired that 157.Dq Li "chown owner slink" 158change the ownership of 159.Dq Li slink , 160not of what it points to -- the 161.Fl h 162option should be used. 163In the above example, 164.Dq Li "chown owner slink" 165would change the owner of 166.Dq Li afile 167to 168.Dq Li owner , 169while 170.Dq Li "chown -h owner slink" 171would change the ownership of 172.Dq Li slink . 173.Pp 174There are several exceptions to this rule. 175The 176.Xr mv 1 177and 178.Xr rm 1 179commands do not follow symbolic links named as arguments, 180but respectively attempt to rename and delete them. 181(Note that if the symbolic link references a file via a relative path, 182moving it to another directory may very well cause it to stop working, 183since the path may no longer be correct.) 184.Pp 185The 186.Xr ls 1 187command is also an exception to this rule. 188For compatibility with historic systems (when 189.Nm ls 190is not doing a tree walk, i.e., the 191.Fl R 192option is not specified), 193the 194.Nm ls 195command follows symbolic links named as arguments if the 196.Fl L 197option is specified, 198or if the 199.Fl F , 200.Fl d , 201or 202.Fl l 203options are not specified. 204(If the 205.Fl L 206option is specified, 207.Nm ls 208always follows symbolic links. 209The 210.Fl L 211option affects its behavior even though it is not doing a walk of 212a file tree.) 213.Pp 214The 215.Xr file 1 216command behaves as 217.Xr ls 1 218in that the 219.Fl L 220option makes it follow a symbolic link. 221By default, 222.Dq Li "file slink" 223will report that 224.Dq Li slink 225is a symbolic link. 226This behavior is different from 227.Xr file 1 228on some other systems, where the 229.Fl h 230convention is followed. 231.Pp 232The 233.Bx 4.4 234system differs from historical 4BSD systems in that the 235.Xr chown 8 , 236.Xr chgrp 1 , 237and 238.Xr file 1 239commands follow symbolic links specified on the command line 240(unless the 241.Fl h 242option is used). 243.Ss Commands traversing a file tree 244The following commands either optionally or always traverse file trees: 245.Xr chflags 1 , 246.Xr chgrp 1 , 247.Xr chmod 1 , 248.Xr cp 1 , 249.Xr du 1 , 250.Xr find 1 , 251.Xr ls 1 , 252.Xr pax 1 , 253.Xr rm 1 , 254.Xr tar 1 , 255and 256.Xr chown 8 . 257.Pp 258It is important to realize that the following rules apply equally to 259symbolic links encountered during the file tree traversal and symbolic 260links listed as command-line arguments. 261.Pp 262The first rule applies to symbolic links that reference files that are 263not of type directory. 264Operations that apply to symbolic links are performed on the links 265themselves, but otherwise the links are ignored. 266.Pp 267For example, the command 268.Dq Li "chown -R user slink directory" 269will ignore 270.Dq Li slink , 271because the 272.Fl h 273option was not given. 274Any symbolic links encountered during the tree traversal will also be 275ignored. 276The command 277.Dq Li "rm -r slink directory" 278will remove 279.Dq Li slink , 280as well as any symbolic links encountered in the tree traversal of 281.Dq Li directory , 282because symbolic links may be removed. 283In no case will either 284.Xr chown 8 285or 286.Xr rm 1 287follow the symlink to affect the file which 288.Dq Li slink 289references. 290.Pp 291The second rule applies to symbolic links that reference files of type 292directory. 293Symbolic links which reference files of type directory are never 294.Dq followed 295by default. 296This is often referred to as a 297.Dq physical 298walk, as opposed to a 299.Dq logical 300walk (where symbolic links referencing directories are followed). 301.Pp 302As consistently as possible, it is possible to make commands doing a file tree 303walk follow any symbolic links named on the command line, regardless 304of the type of file they reference, by specifying the 305.Fl H 306(for 307.Dq half\-logical ) 308flag. 309This flag is intended to make the command-line name space look 310like the logical name space. 311(Note: 312for commands that do not always do file tree traversals, the 313.Fl H 314flag will be ignored if the 315.Fl R 316flag is not also specified.) 317.Pp 318For example, the command 319.Dq Li "chown -HR user slink" 320will traverse the file hierarchy rooted in the file pointed to by 321.Dq Li slink . 322The 323.Fl H 324is not the same as the previously discussed 325.Fl h 326flag. 327The 328.Fl H 329flag causes symbolic links specified on the command line to be 330dereferenced both for the purposes of the action to be performed 331and the tree walk, and it is as if the user had specified the 332name of the file to which the symbolic link pointed. 333.Pp 334As consistently as possible, it is possible to make commands doing a file tree 335walk follow any symbolic links named on the command line, as well as 336any symbolic links encountered during the traversal, regardless of 337the type of file they reference, by specifying the 338.Fl L 339(for 340.Dq logical ) 341flag. 342This flag is intended to make the entire name space look like 343the logical name space. 344(Note: 345for commands that do not always do file tree traversals, the 346.Fl L 347flag will be ignored if the 348.Fl R 349flag is not also specified.) 350.Pp 351For example, the command 352.Dq Li "chown -LR user slink" 353will change the owner of the file referenced by 354.Dq Li slink . 355If 356.Dq Li slink 357references a directory, 358.Nm chown 359will traverse the file hierarchy rooted in the directory that it 360references. 361In addition, if any symbolic links are encountered in any file tree that 362.Nm chown 363traverses, they will be treated in the same fashion as 364.Dq Li slink . 365.Pp 366As consistently as possible, it is possible to specify the default behavior by 367specifying the 368.Fl P 369(for 370.Dq physical ) 371flag. 372This flag is intended to make the entire name space look like the 373physical name space. 374.Pp 375For commands that do not by default do file tree traversals, the 376.Fl H , 377.Fl L , 378and 379.Fl P 380flags are ignored if the 381.Fl R 382flag is not also specified. 383In addition, the 384.Fl H , 385.Fl L , 386and 387.Fl P 388options may be specified more than once; 389the last one specified determines the command's behavior. 390This is intended to permit aliasing commands to behave one way 391or the other, and then override that behavior on the command line. 392.Pp 393The 394.Xr ls 1 395and 396.Xr rm 1 397commands have exceptions to these rules. 398The 399.Nm rm 400command operates on the symbolic link, and not the file it references, 401and therefore never follows a symbolic link. 402The 403.Nm rm 404command does not support the 405.Fl H , 406.Fl L , 407or 408.Fl P 409options. 410.Pp 411To maintain compatibility with historic systems, 412the 413.Nm ls 414command never follows symbolic links unless the 415.Fl L 416flag is specified. 417If the 418.Fl L 419flag is specified, 420.Nm ls 421follows all symbolic links, 422regardless of their type, 423whether specified on the command line or encountered in the tree walk. 424The 425.Nm ls 426command does not support the 427.Fl H 428or 429.Fl P 430options. 431.Sh SEE ALSO 432.Xr chflags 1 , 433.Xr chgrp 1 , 434.Xr chmod 1 , 435.Xr cp 1 , 436.Xr du 1 , 437.Xr find 1 , 438.Xr ln 1 , 439.Xr ls 1 , 440.Xr mv 1 , 441.Xr pax 1 , 442.Xr rm 1 , 443.Xr tar 1 , 444.Xr lchown 2 , 445.Xr lstat 2 , 446.Xr readlink 2 , 447.Xr rename 2 , 448.Xr symlink 2 , 449.Xr unlink 2 , 450.Xr fts 3 , 451.Xr remove 3 , 452.Xr chown 8 453