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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.\" $FreeBSD$ 31.\" 32.Dd February 16, 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 , 152.Xr fstatat 2 153and 154.Xr utimensat 2 . 155.Pp 156The owner and group of an existing symbolic link can be changed by 157means of the 158.Xr lchown 2 159system call. 160The flags, access permissions, owner/group and modification time of 161an existing symbolic link can be changed by means of the 162.Xr lchflags 2 , 163.Xr lchmod 2 , 164.Xr lchown 2 , 165and 166.Xr lutimes 2 167system calls, respectively. 168Of these, only the flags and ownership are used by the system; 169the access permissions are ignored. 170.Pp 171The 172.Bx 4.4 173system differs from historical 174.Bx 4 175systems in that the system call 176.Xr chown 2 177has been changed to follow symbolic links. 178The 179.Xr lchown 2 180system call was added later when the limitations of the new 181.Xr chown 2 182became apparent. 183.Ss Commands not traversing a file tree. 184The second area is symbolic links, specified as command line file 185name arguments, to commands which are not traversing a file tree. 186.Pp 187Except as noted below, commands follow symbolic links named as command 188line arguments. 189For example, if there were a symbolic link 190.Dq Li slink 191which pointed to a file named 192.Dq Li afile , 193the command 194.Dq Li cat slink 195would display the contents of the file 196.Dq Li afile . 197.Pp 198It is important to realize that this rule includes commands which may 199optionally traverse file trees, e.g.\& the command 200.Dq Li "chown file" 201is included in this rule, while the command 202.Dq Li "chown -R file" 203is not. 204(The latter is described in the third area, below.) 205.Pp 206If it is explicitly intended that the command operate on the symbolic 207link instead of following the symbolic link, e.g., it is desired that 208.Dq Li "chown slink" 209change the ownership of the file that 210.Dq Li slink 211is, whether it is a symbolic link or not, the 212.Fl h 213option should be used. 214In the above example, 215.Dq Li "chown root slink" 216would change the ownership of the file referenced by 217.Dq Li slink , 218while 219.Dq Li "chown -h root slink" 220would change the ownership of 221.Dq Li slink 222itself. 223.Pp 224There are five exceptions to this rule. 225The 226.Xr mv 1 227and 228.Xr rm 1 229commands do not follow symbolic links named as arguments, 230but respectively attempt to rename and delete them. 231(Note, if the symbolic link references a file via a relative path, 232moving it to another directory may very well cause it to stop working, 233since the path may no longer be correct.) 234.Pp 235The 236.Xr ls 1 237command is also an exception to this rule. 238For compatibility with historic systems (when 239.Nm ls 240is not doing a tree walk, i.e., the 241.Fl R 242option is not specified), 243the 244.Nm ls 245command follows symbolic links named as arguments if the 246.Fl H 247or 248.Fl L 249option is specified, 250or if the 251.Fl F , 252.Fl d 253or 254.Fl l 255options are not specified. 256(The 257.Nm ls 258command is the only command where the 259.Fl H 260and 261.Fl L 262options affect its behavior even though it is not doing a walk of 263a file tree.) 264.Pp 265The 266.Xr file 1 267and 268.Xr stat 1 269commands are also exceptions to this rule. 270These 271commands do not follow symbolic links named as argument by default, 272but do follow symbolic links named as argument if the 273.Fl L 274option is specified. 275.Pp 276The 277.Bx 4.4 278system differs from historical 279.Bx 4 280systems in that the 281.Nm chown 282and 283.Nm chgrp 284commands follow symbolic links specified on the command line. 285.Ss Commands traversing a file tree. 286The following commands either optionally or always traverse file trees: 287.Xr chflags 1 , 288.Xr chgrp 1 , 289.Xr chmod 1 , 290.Xr cp 1 , 291.Xr du 1 , 292.Xr find 1 , 293.Xr ls 1 , 294.Xr pax 1 , 295.Xr rm 1 , 296.Xr tar 1 297and 298.Xr chown 8 . 299.Pp 300It is important to realize that the following rules apply equally to 301symbolic links encountered during the file tree traversal and symbolic 302links listed as command line arguments. 303.Pp 304The first rule applies to symbolic links that reference files that are 305not of type directory. 306Operations that apply to symbolic links are performed on the links 307themselves, but otherwise the links are ignored. 308.Pp 309The command 310.Dq Li "rm -r slink directory" 311will remove 312.Dq Li slink , 313as well as any symbolic links encountered in the tree traversal of 314.Dq Li directory , 315because symbolic links may be removed. 316In no case will 317.Nm rm 318affect the file which 319.Dq Li slink 320references in any way. 321.Pp 322The second rule applies to symbolic links that reference files of type 323directory. 324Symbolic links which reference files of type directory are never 325.Dq followed 326by default. 327This is often referred to as a 328.Dq physical 329walk, as opposed to a 330.Dq logical 331walk (where symbolic links referencing directories are followed). 332.Pp 333As consistently as possible, you can make commands doing a file tree 334walk follow any symbolic links named on the command line, regardless 335of the type of file they reference, by specifying the 336.Fl H 337(for 338.Dq half\-logical ) 339flag. 340This flag is intended to make the command line name space look 341like the logical name space. 342(Note, for commands that do not always do file tree traversals, the 343.Fl H 344flag will be ignored if the 345.Fl R 346flag is not also specified.) 347.Pp 348For example, the command 349.Dq Li "chown -HR user slink" 350will traverse the file hierarchy rooted in the file pointed to by 351.Dq Li slink . 352Note, the 353.Fl H 354is not the same as the previously discussed 355.Fl h 356flag. 357The 358.Fl H 359flag causes symbolic links specified on the command line to be 360dereferenced both for the purposes of the action to be performed 361and the tree walk, and it is as if the user had specified the 362name of the file to which the symbolic link pointed. 363.Pp 364As consistently as possible, you can make commands doing a file tree 365walk follow any symbolic links named on the command line, as well as 366any symbolic links encountered during the traversal, regardless of 367the type of file they reference, by specifying the 368.Fl L 369(for 370.Dq logical ) 371flag. 372This flag is intended to make the entire name space look like 373the logical name space. 374(Note, for commands that do not always do file tree traversals, the 375.Fl L 376flag will be ignored if the 377.Fl R 378flag is not also specified.) 379.Pp 380For example, the command 381.Dq Li "chown -LR user slink" 382will change the owner of the file referenced by 383.Dq Li slink . 384If 385.Dq Li slink 386references a directory, 387.Nm chown 388will traverse the file hierarchy rooted in the directory that it 389references. 390In addition, if any symbolic links are encountered in any file tree that 391.Nm chown 392traverses, they will be treated in the same fashion as 393.Dq Li slink . 394.Pp 395As consistently as possible, you can specify the default behavior by 396specifying the 397.Fl P 398(for 399.Dq physical ) 400flag. 401This flag is intended to make the entire name space look like the 402physical name space. 403.Pp 404For commands that do not by default do file tree traversals, the 405.Fl H , 406.Fl L 407and 408.Fl P 409flags are ignored if the 410.Fl R 411flag is not also specified. 412In addition, you may specify the 413.Fl H , 414.Fl L 415and 416.Fl P 417options more than once; the last one specified determines the 418command's behavior. 419This is intended to permit you to alias commands to behave one way 420or the other, and then override that behavior on the command line. 421.Pp 422The 423.Xr ls 1 424and 425.Xr rm 1 426commands have exceptions to these rules. 427The 428.Nm rm 429command operates on the symbolic link, and not the file it references, 430and therefore never follows a symbolic link. 431The 432.Nm rm 433command does not support the 434.Fl H , 435.Fl L 436or 437.Fl P 438options. 439.Pp 440To maintain compatibility with historic systems, 441the 442.Nm ls 443command acts a little differently. 444If you do not specify the 445.Fl F , 446.Fl d 447or 448.Fl l 449options, 450.Nm ls 451will follow symbolic links specified on the command line. 452If the 453.Fl L 454flag is specified, 455.Nm ls 456follows all symbolic links, 457regardless of their type, 458whether specified on the command line or encountered in the tree walk. 459.Sh SEE ALSO 460.Xr chflags 1 , 461.Xr chgrp 1 , 462.Xr chmod 1 , 463.Xr cp 1 , 464.Xr du 1 , 465.Xr find 1 , 466.Xr ln 1 , 467.Xr ls 1 , 468.Xr mv 1 , 469.Xr pax 1 , 470.Xr rm 1 , 471.Xr tar 1 , 472.Xr lchflags 2 , 473.Xr lchmod 2 , 474.Xr lchown 2 , 475.Xr lstat 2 , 476.Xr lutimes 2 , 477.Xr readlink 2 , 478.Xr rename 2 , 479.Xr symlink 2 , 480.Xr unlink 2 , 481.Xr fts 3 , 482.Xr remove 3 , 483.Xr chown 8 484