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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 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
       different 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
       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.  (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  subsequent
       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.  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
          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),    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 ENOTDIR.

       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 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
         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  performed  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 "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
         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 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 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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