symlink(7)


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 inode number, where an inode number is  an  index
   into  the  inode  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 inode  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.  (Note that there are
   some "magic" symbolic links in the /proc  directory  tree---for  example,
   the /proc/[pid]/fd/* files---that have different permissions.)

   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.

   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.)   POSIX.1-2008  changed  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  that 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), namei(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 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.





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