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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
       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
       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 prevent 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
       operations;  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.
       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), 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)
       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,  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
       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, 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
       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

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