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NAME

       symlink - symbolic link handling

DESCRIPTION

       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
       filename.   (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
       filesystem.  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 prevent the possibility of loops within the  filesystem
       tree,  which would confuse many programs) and may not refer to files on
       different filesystems (because i-node numbers  are  not  unique  across
       filesystems).

       A  symbolic  link is a special type of file whose contents are a string
       that is the pathname of another  file,  the  file  to  which  the  link
       refers.    (The   contents  of  a  symbolic  link  can  be  read  using
       readlink(2).)  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 filesystem 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
       filesystem  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
       operations; the permissions are always 0777 (read, write,  and  execute
       for all user categories), and can't be changed.

   Obtaining a file descriptor that refers to a symbolic link
       Using  the  combination  of  the O_PATH and O_NOFOLLOW flags to open(2)
       yields a file descriptor that can be passed as the  dirfd  argument  in
       system  calls  such as fstatat(2), fchownat(2), fchmodat(2), linkat(2),
       and readlinkat(2), in order to operate  on  the  symbolic  link  itself
       (rather than the file to which it refers).

       By  default  (i.e., if the AT_SYMLINK_FOLLOW flag is not specified), if
       name_to_handle_at(2) is applied to a symbolic link, it yields a  handle
       for  the  symbolic link (rather than the file to which it refers).  One
       can then obtain a file descriptor for the symbolic  link  (rather  than
       the  file  to  which  it  refers)  by  specifying  the O_PATH flag in a
       subsequent call to open_by_handle_at(2).  Again, that  file  descriptor
       can  be  used  in  the  aforementioned  system  calls to operate on the
       symbolic link itself.

   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.
       Symbolic  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),
       lremovexattr(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), name_to_handle_at(2),
       open(2),  openat(2),  open_by_handle_at(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)  warrants 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 specified 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; for example, 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—for example, 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
       ownership 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
         working, since the path may no longer be correct.)

       * The  ls(1)  command  is  also  an  exception  to  this   rule.    For
         compatibility  with  historic systems (when ls(1) is not doing a tree
         walk—that is, -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
       performed 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
       "logical"  walk  (where  symbolic  links  the  refer to directories are
       followed).

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

       * A  command  can  be  made  to  follow any symbolic links named on the
         command line, regardless of the  type  of  file  they  reference,  by
         specifying  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
         command  line,  as  well as any symbolic links encountered during the
         traversal,  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  commands 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),
       utimensat(2), lutimes(3), path_resolution(7)

COLOPHON

       This  page  is  part of release 3.65 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/.



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