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