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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 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 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  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 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 readlink(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, 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.

   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 descrip‐
       tor that can be passed as the dirfd argument in system calls such  as  fstatat(2),  fchow‐
       nat(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 subsequent
       call to open_by_handle_at(2).  Again, that file descriptor can be used  in  the  aforemen‐
       tioned 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.  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), name_to_handle_at(2), open(2), openat(2), open_by_han‐
       dle_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 ENOTDIR.

       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.)  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 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;  for example, the command chown file is included in this rule, while the com‐
       mand 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—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 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—that is, -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 that 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 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2015-08-08                                 SYMLINK(7)

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