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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 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 pre‐
       vent the possibility of loops within the filesystem tree,  which	 would
       confuse many programs) and may not refer to files on different filesys‐
       tems (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 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 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, com‐
       mands  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 3.54 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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