symlink


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 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 pre-
       vent the possibility of loops within the file system tree, which  would
       confuse  many  programs)  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 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 file system 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  file
       system  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 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 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.

         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
         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 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2008-06-18                        SYMLINK(7)
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