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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 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 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 prevent
       the possibility of loops within the file system tree, which would confuse many pro-
       grams) 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 sym-
       bolic 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 bound-
       aries.

       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 refer-
       enced 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  operations;  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 sys-
       tem 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 sym-
       bolic 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),  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 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.)
       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  argu-
       ments.   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 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 ref-
         erences 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 his-
         toric 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 directo-
       ries.  Operations that apply to symbolic links are performed  on  the  links  them-
       selves, 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 "logical" walk (where symbolic
       links the refer to directories are followed).

       Certain conventions are (should be) followed as consistently as  possible  by  com-
       mands 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 pre-
         viously  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  per-
         formed  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
         "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 spec-
       ify  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), utimensat(2),  lutimes(3),
       path_resolution(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-06-18                        SYMLINK(7)

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