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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 behav-
       ior, you must first understand how hard links work.

       A hard link to a file is indistinguishable from the original file because it is	a  refer-
       ence  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  differ-
       ent 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
       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, commands and system calls adopted their own link-following conven-
       tions in a somewhat ad-hoc fashion.  Rules for a more uniform approach, as they are imple-
       mented  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  permis-
       sions  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 speci-
	  fied 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 derefer-
       ence 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 com-
       mands 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 fol-
       lowing 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 speci-
	 fied, 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 argu-
       ments.

       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 other-
       wise the links are ignored.

       The command rm -r slink directory will remove slink, as well as any symbolic links encoun-
       tered  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 "phys-
       ical" 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 derefer-
	 enced	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 "physi-
	 cal") 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 differ-
	 ently.  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_resolu-
       tion(7)

COLOPHON
       This  page  is  part of release 3.55 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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