SYMLINK(7)		   Linux Programmer's Manual		    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	 file-
       name.   (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 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 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	 read-
       link(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, 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.  (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	subse-
       quent 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.  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), 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 ENOT-
       DIR.

       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	 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--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 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 that  refer  to	directories  are  fol-
       lowed).

       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),  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
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Linux				  2016-10-08			    SYMLINK(7)