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symlink(7)             Miscellaneous Information 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.

   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-expan-
       sion,  but  instead act as direct references to the kernel's own repre-
       sentation of a file handle.  As such, these magic links allow users  to
       access  files which cannot be referenced with normal paths (such as un-
       linked files still referenced by a running program ).

       Because they can  bypass  ordinary  mount_namespaces(7)-based  restric-
       tions,  magic  links  have  been  used as attack vectors in various ex-
       ploits.

   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  exe-
       cute 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  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:

       o  Symbolic links used as filename arguments for system calls.

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

       o  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 al-
            ways 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  re-
            ferring 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 de-
       tails.  Because remove(3) is an alias for unlink(2), that library func-
       tion  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  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--then  the  -h  option  should  be
       used.   In  the above example, chown root slink would change the owner-
       ship 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:

       o  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.)

       o  The  ls(1)  command is also an exception to this rule.  For compati-
          bility with historic systems (when ls(1) is not doing a tree  walk--
          that is, -R option is not specified), the ls(1) command follows sym-
          bolic 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) com-
          mand is the only command where the -H and -L options affect its  be-
          havior even though it is not doing a walk of a file tree.)

       o  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:

       o  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 speci-
          fying 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  derefer-
          enced  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.

       o  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.

       o  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:

       o  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.

       o  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)

Linux man-pages 6.04              2023-04-03                        symlink(7)

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