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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)symlink.7 8.3 (Berkeley) 3/31/94 29.\" $FreeBSD: src/bin/ln/symlink.7,v 1.13.2.7 2003/03/03 19:04:46 trhodes Exp $ 30.\" 31.Dd July 31, 2010 32.Dt SYMLINK 7 33.Os 34.Sh NAME 35.Nm symlink 36.Nd symbolic link handling 37.Sh SYMBOLIC LINK HANDLING 38Symbolic links are files that act as pointers to other files. 39To understand their behavior, you must first understand how hard links 40work. 41A hard link to a file is indistinguishable from the original file because 42it is a reference to the object underlying the original file name. 43Changes to a file are independent of the name used to reference the 44file. 45Hard links may not refer to directories and may not reference files 46on different file systems. 47A symbolic link contains the name of the file to which it is linked, 48i.e. it is a pointer to another name, and not to an underlying object. 49For this reason, symbolic links may reference directories and may span 50file systems. 51.Pp 52Because a symbolic link and its referenced object coexist in the file system 53name space, confusion can arise in distinguishing between the link itself 54and the referenced object. 55Historically, commands and system calls have adopted their own link 56following conventions in a somewhat ad-hoc fashion. 57Rules for more a uniform approach, as they are implemented in this system, 58are outlined here. 59It is important that local applications conform to these rules, too, 60so that the user interface can be as consistent as possible. 61.Pp 62Symbolic links are handled either by operating on the link itself, 63or by operating on the object referenced by the link. 64In the latter case, 65an application or system call is said to 66.Dq follow 67the link. 68Symbolic links may reference other symbolic links, 69in which case the links are dereferenced until an object that is 70not a symbolic link is found, 71a symbolic link which references a file which doesn't exist is found, 72or a loop is detected. 73(Loop detection is done by placing an upper limit on the number of 74links that may be followed, and an error results if this limit is 75exceeded.) 76.Pp 77There are three separate areas that need to be discussed. 78They are as follows: 79.Pp 80.Bl -enum -compact -offset indent 81.It 82Symbolic links used as file name arguments for system calls. 83.It 84Symbolic links specified as command line arguments to utilities that 85are not traversing a file tree. 86.It 87Symbolic links encountered by utilities that are traversing a file tree 88(either specified on the command line or encountered as part of the 89file hierarchy walk). 90.El 91.Ss System calls. 92The first area is symbolic links used as file name arguments for 93system calls. 94.Pp 95Except as noted below, all system calls follow symbolic links. 96For example, if there were a symbolic link 97.Dq Li slink 98which pointed to a file named 99.Dq Li afile , 100the system call 101.Dq Li open("slink" ...\&) 102would return a file descriptor to the file 103.Dq afile . 104.Pp 105There are six system calls that do not follow links, and which operate 106on the symbolic link itself. 107They are: 108.Xr lchown 2 , 109.Xr lstat 2 , 110.Xr readlink 2 , 111.Xr rename 2 , 112.Xr rmdir 2 , 113and 114.Xr unlink 2 . 115Because 116.Xr remove 3 117is an alias for 118.Xr unlink 2 , 119it also does not follow symbolic links. 120When 121.Xr rmdir 2 122is applied to a symbolic link, it fails with the error 123.Er ENOTDIR . 124.Pp 125The owner and group of an existing symbolic link can be changed by 126means of the 127.Xr lchown 2 128system call. 129The other file attributes, such as the modification time and access 130permissions, are not used by the system and cannot be changed. 131.Pp 132The 133.Bx 4.4 134system differs from historical 135.Bx 4 136systems in that the system call 137.Xr chown 2 138has been changed to follow symbolic links. 139The 140.Xr lchown 2 141system call was added later when the limitations of the new 142.Xr chown 2 143became apparent. 144.Ss Commands not traversing a file tree. 145The second area is symbolic links, specified as command line file 146name arguments, to commands which are not traversing a file tree. 147.Pp 148Except as noted below, commands follow symbolic links named as command 149line arguments. 150For example, if there were a symbolic link 151.Dq Li slink 152which pointed to a file named 153.Dq Li afile , 154the command 155.Dq Li cat slink 156would display the contents of the file 157.Dq Li afile . 158.Pp 159It is important to realize that this rule includes commands which may 160optionally traverse file trees, e.g. the command 161.Dq Li "chown file" 162is included in this rule, while the command 163.Dq Li "chown -R file" 164is not. 165(The latter is described in the third area, below.) 166.Pp 167If it is explicitly intended that the command operate on the symbolic 168link instead of following the symbolic link, e.g., it is desired that 169.Dq Li "chown slink" 170change the ownership of the file that 171.Dq Li slink 172is, whether it is a symbolic link or not, the 173.Fl h 174option should be used. 175In the above example, 176.Dq Li "chown root slink" 177would change the ownership of the file referenced by 178.Dq Li slink , 179while 180.Dq Li "chown -h root slink" 181would change the ownership of 182.Dq Li slink 183itself. 184.Pp 185There are four 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, 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 H 208or 209.Fl L 210option is specified, 211or if the 212.Fl F , 213.Fl d 214or 215.Fl l 216options are not specified. (The 217.Nm ls 218command is the only command where the 219.Fl H 220and 221.Fl L 222options affect its behavior even though it is not doing a walk of 223a file tree.) 224.Pp 225The 226.Xr file 1 227command is also an exception to this rule. 228The 229.Xr file 1 230command does not follow symbolic links named as argument by default. 231The 232.Xr file 1 233command does follow symbolic links named as argument if 234.Fl L 235option is specified. 236.Pp 237The 238.Bx 4.4 239system differs from historical 240.Bx 4 241systems 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 renameat 2 , 445.Xr symlink 2 , 446.Xr unlink 2 , 447.Xr fts 3 , 448.Xr remove 3 , 449.Xr chown 8 450