symlink

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 un-
       derstand their behavior, you must first understand how hard links work.

       A hard link to a file is indistinguishable from the original  file  be-
       cause 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 inode number, where an inode number is an index into the inode ta-
       ble, which contains metadata about all  files  on  a  filesystem.   See
       stat(2).)  Changes to a file are independent of the name used to refer-
       ence 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 different filesystems (be-
       cause 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 ex-
       ample, if there were a symbolic link slink  which  pointed  to  a  file
       named  afile, the system call open("slink" ...) would return a file de-
       scriptor 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 be-
       havior 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 en-
         tire 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  en-
         tire 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  in-
       tended  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),  un-
       link(2), utimensat(2), lutimes(3), path_resolution(7)

COLOPHON
       This  page  is  part of release 5.05 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2016-10-08                        SYMLINK(7)