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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.

   Magic links
       There is a special class of symbolic-link-like objects known as "magic links",  which  can
       be  found in certain pseudofilesystems such as proc(5) (examples include /proc/pid/exe and
       /proc/pid/fd/*).  Unlike normal symbolic links,  magic  links  are  not  resolved  through
       pathname-expansion,   but   instead   act   as  direct  references  to  the  kernel's  own
       representation of a file handle.  As such, these magic links allow users to  access  files
       which cannot be referenced with normal paths (such as unlinked files still referenced by a
       running program ).

       Because they can bypass ordinary mount_namespaces(7)-based restrictions, magic links  have
       been used as attack vectors in various exploits.

   Symbolic link ownership, permissions, and timestamps
       The  owner  and  group  of  an existing symbolic link can be changed using lchown(2).  The
       ownership of a symbolic link matters when the link  is  being  removed  or  renamed  in  a
       directory  that  has the sticky bit set (see inode(7)), and when the fs.protected_symlinks
       sysctl is set (see proc(5)).

       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 an ordinary 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.

       However,  magic links do not follow this rule.  They can have a non-0777 mode, though this
       mode is not currently used in any permission checks.

   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:

       •  Symbolic links used as filename arguments for system calls.

       •  Symbolic links specified as command-line arguments to utilities that are not traversing
          a file tree.

       •  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).

       Before describing the treatment of symbolic links by system calls and commands, we require
       some  terminology.  Given a pathname of the form a/b/c, the part preceding the final slash
       (i.e., a/b) is called the dirname component, and the part following the final slash (i.e.,
       c) is called the basename component.

   Treatment of symbolic links in system calls
       The first area is symbolic links used as filename arguments for system calls.

       The treatment of symbolic links within a pathname passed to a system call is as follows:

       (1)  Within  the  dirname  component  of a pathname, symbolic links are always followed in
            nearly every system call.  (This is also true for commands.)  The  one  exception  is
            openat2(2),  which provides flags that can be used to explicitly prevent following of
            symbolic links in the dirname component.

       (2)  Except as noted below, all  system  calls  follow  symbolic  links  in  the  basename
            component  of  a  pathname.   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 in the basename component of a pathname, 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 in the basename component of a
       pathname.     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—then  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)