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SYMLINK(7)		   Linux Programmers 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 cant 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
	 traversal, regardless of the type of file they reference, by specify
	 ing 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 commands 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)




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