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SYMLINK(7)                 Linux Programmer's Manual                SYMLINK(7)



NAME
       symlink - symbolic link handling

SYMBOLIC LINK HANDLING
       Symbolic  links  are  files  that  act  as pointers to other files.  To
       understand their behavior, you must first  understand  how  hard  links
       work.

       A  hard  link  to  a  file  is indistinguishable from the original file
       because it is a reference to the object underlying the  original  file-
       name.   (To be precise: each of the hard links to a file is a reference
       to the same i-node number, where an i-node number is an index into  the
       i-node table, which contains metadata about all files on a file system.
       See stat(2).)  Changes to a file are independent of the  name  used  to
       reference  the  file.  Hard links may not refer to directories (to pre-
       vent the possibility of loops within the file system tree, which  would
       confuse  many  programs)  and  may not refer to files on different file
       systems (because i-node numbers are not unique across file systems).

       A symbolic link is a special type of file whose contents are  a  string
       that  is  the pathname another file, the file to which the link refers.
       In other words, a symbolic link is a pointer to another name,  and  not
       to  an underlying object.  For this reason, symbolic links may refer to
       directories and may cross file system boundaries.

       There is no requirement that the pathname referred  to  by  a  symbolic
       link should exist.  A symbolic link that refers to a pathname that does
       not exist is said to be a dangling link.

       Because a symbolic link and its referenced object coexist in  the  file
       system  name  space,  confusion can arise in distinguishing between the
       link itself and the referenced object.  On historical systems, commands
       and  system  calls  adopted  their  own link-following conventions in a
       somewhat ad-hoc fashion.  Rules for a more uniform  approach,  as  they
       are  implemented  on Linux and other systems, are outlined here.  It is
       important that site-local applications also conform to these rules,  so
       that the user interface can be as consistent as possible.

   Symbolic link ownership, permissions, and timestamps
       The  owner  and group of an existing symbolic link can be changed using
       lchown(2).  The only time that the ownership of a symbolic link matters
       is  when  the  link is being removed or renamed in a directory that has
       the sticky bit set (see stat(2)).

       The last access and last modification timestamps of a symbolic link can
       be changed using utimensat(2) or lutimes(3).

       On Linux, the permissions of a symbolic link are not used in any opera-
       tions; the permissions are always 0777 (read, write,  and  execute  for
       all user categories), and can't be changed.

   Handling of symbolic links by system calls and commands
       Symbolic  links  are handled either by operating on the link itself, or
       by operating on the object referred to by  the  link.   In  the  latter
       case,  an  application or system call is said to follow the link.  Sym-
       bolic links may refer to other symbolic links, in which case the  links
       are  dereferenced until an object that is not a symbolic link is found,
       a symbolic link that refers to a file which does not exist is found, or
       a  loop is detected.  (Loop detection is done by placing an upper limit
       on the number of links that may be followed, and an  error  results  if
       this limit is exceeded.)

       There  are three separate areas that need to be discussed.  They are as
       follows:

       1. Symbolic links used as filename arguments for system calls.

       2. Symbolic links specified as command-line arguments to utilities that
          are not traversing a file tree.

       3. Symbolic  links  encountered by utilities that are traversing a file
          tree (either specified on the command line or encountered as part of
          the file hierarchy walk).

   System calls
       The  first area is symbolic links used as filename arguments for system
       calls.

       Except as noted below, all system calls  follow  symbolic  links.   For
       example,  if  there  were a symbolic link slink which pointed to a file
       named afile, the system call open("slink"  ...)  would  return  a  file
       descriptor referring to the file afile.

       Various  system  calls do not follow links, and operate on the symbolic
       link itself.  They are: lchown(2),  lgetxattr(2),  llistxattr(2),  lre-
       movexattr(2), lsetxattr(2), lstat(2), readlink(2), rename(2), rmdir(2),
       and unlink(2).  Certain other system calls optionally  follow  symbolic
       links.   They  are:  faccessat(2),  fchownat(2), fstatat(2), linkat(2),
       open(2), openat(2),  and  utimensat(2);  see  their  manual  pages  for
       details.   Because  remove(3)  is  an alias for unlink(2), that library
       function also does not follow symbolic links.  When rmdir(2) is applied
       to  a symbolic link, it fails with the error ENOTDIR.  The link(2) war-
       rants special discussion.  POSIX.1-2001 specifies that  link(2)  should
       dereference  oldpath if it is a symbolic link.  However, Linux does not
       do this.  (By default Solaris is the same, but the POSIX.1-2001  speci-
       fied  behavior  can  be  obtained with suitable compiler options.)  The
       upcoming POSIX.1 revision changes the  specification  to  allow  either
       behavior in an implementation.

   Commands not traversing a file tree
       The  second  area is symbolic links, specified as command-line filename
       arguments, to commands which are not traversing a file tree.

       Except as noted below, commands follow symbolic links named as command-
       line arguments.  For example, if there were a symbolic link slink which
       pointed to a file named afile, the command cat slink would display  the
       contents of the file afile.

       It  is  important to realize that this rule includes commands which may
       optionally traverse file trees, e.g. the command chown file is included
       in  this  rule,  while the command chown -R file, which performs a tree
       traversal, is not.  (The latter is described in the third area, below.)

       If it is explicitly intended that the command operate on  the  symbolic
       link  instead  of following the symbolic link, e.g., it is desired that
       chown slink change the ownership of the file that slink is, whether  it
       is  a symbolic link or not, the -h option should be used.  In the above
       example, chown root slink  would  change  the  ownership  of  the  file
       referred to by slink, while chown -h root slink would change the owner-
       ship of slink itself.

       There are some exceptions to this rule:

       * The mv(1) and rm(1) commands do not follow symbolic  links  named  as
         arguments,  but  respectively  attempt  to  rename  and  delete them.
         (Note, if the symbolic link references a file via  a  relative  path,
         moving  it  to another directory may very well cause it to stop work-
         ing, since the path may no longer be correct.)

       * The ls(1) command is also an exception to this rule.  For compatibil-
         ity with historic systems (when ls(1) is not doing a tree walk, i.e.,
         the -R option is not specified), the ls(1) command  follows  symbolic
         links  named  as arguments if the -H or -L option is specified, or if
         the -F, -d, or -l options are not specified.  (The ls(1)  command  is
         the only command where the -H and -L options affect its behavior even
         though it is not doing a walk of a file tree.)

       * The file(1) command is also an exception to this rule.   The  file(1)
         command  does not follow symbolic links named as argument by default.
         The file(1) command does follow symbolic links named as  argument  if
         the -L option is specified.

   Commands traversing a file tree
       The following commands either optionally or always traverse file trees:
       chgrp(1), chmod(1), chown(1), cp(1),  du(1),  find(1),  ls(1),  pax(1),
       rm(1), and tar(1).

       It  is  important  to realize that the following rules apply equally to
       symbolic links encountered during the file tree traversal and  symbolic
       links listed as command-line arguments.

       The  first  rule  applies  to symbolic links that reference files other
       than directories.  Operations that apply to  symbolic  links  are  per-
       formed on the links themselves, but otherwise the links are ignored.

       The  command  rm -r  slink  directory will remove slink, as well as any
       symbolic links encountered in the tree traversal of directory,  because
       symbolic  links  may be removed.  In no case will rm(1) affect the file
       referred to by slink.

       The second rule applies to symbolic links that  refer  to  directories.
       Symbolic links that refer to directories are never followed by default.
       This is often referred to as a "physical" walk, as opposed to a  "logi-
       cal" walk (where symbolic links the refer to directories are followed).

       Certain  conventions are (should be) followed as consistently as possi-
       ble by commands that perform file tree walks:

       * A command can be made to follow any symbolic links named on the  com-
         mand line, regardless of the type of file they reference, by specify-
         ing the -H (for "half-logical") flag.  This flag is intended to  make
         the command-line name space look like the logical name space.  (Note,
         for commands that do not always do file tree traversals, the -H  flag
         will be ignored if the -R flag is not also specified.)

         For  example, the command chown -HR user slink will traverse the file
         hierarchy rooted in the file pointed to by slink.  Note,  the  -H  is
         not the same as the previously discussed -h flag.  The -H flag causes
         symbolic links specified on the command line to be  dereferenced  for
         the  purposes  of  both the action to be performed and the tree walk,
         and it is as if the user had specified the name of the file to  which
         the symbolic link pointed.

       * A  command can be made to follow any symbolic links named on the com-
         mand line, as well as any symbolic links encountered during the  tra-
         versal,  regardless of the type of file they reference, by specifying
         the -L (for "logical") flag.  This  flag  is  intended  to  make  the
         entire  name space look like the logical name space.  (Note, for com-
         mands that do not always do file tree traversals, the -L flag will be
         ignored if the -R flag is not also specified.)

         For  example,  the command chown -LR user slink will change the owner
         of the file referred to by slink.  If slink refers  to  a  directory,
         chown  will  traverse the file hierarchy rooted in the directory that
         it references.  In addition, if any symbolic links are encountered in
         any  file tree that chown traverses, they will be treated in the same
         fashion as slink.

       * A command can be made to provide the default behavior  by  specifying
         the  -P  (for  "physical")  flag.   This flag is intended to make the
         entire name space look like the physical name space.

       For commands that do not by default do file tree  traversals,  the  -H,
       -L,  and -P flags are ignored if the -R flag is not also specified.  In
       addition, you may specify the -H, -L, and -P options  more  than  once;
       the  last  one  specified  determines  the command's behavior.  This is
       intended to permit you to alias commands  to  behave  one  way  or  the
       other, and then override that behavior on the command line.

       The ls(1) and rm(1) commands have exceptions to these rules:

       * The  rm(1) command operates on the symbolic link, and not the file it
         references, and therefore never follows a symbolic link.   The  rm(1)
         command does not support the -H, -L, or -P options.

       * To  maintain  compatibility  with historic systems, the ls(1) command
         acts a little differently.  If you do not specify the -F,  -d  or  -l
         options,  ls(1)  will  follow symbolic links specified on the command
         line.  If the -L flag is specified, ls(1) follows all symbolic links,
         regardless  of  their  type, whether specified on the command line or
         encountered in the tree walk.

SEE ALSO
       chgrp(1), chmod(1), find(1), ln(1),  ls(1),  mv(1),  rm(1),  lchown(2),
       link(2),  lstat(2), readlink(2), rename(2), symlink(2), unlink(2), uti-
       mensat(2), lutimes(3), path_resolution(7)

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-06-18                        SYMLINK(7)