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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 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  in‐
       ode  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, con‐
       fusion 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 path‐
       name-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 refer‐
       enced 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 owner‐
       ship 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 uti‐
       mensat(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 ob‐
       ject 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  re‐
       sults 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), lremovex‐
       attr(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 function also does not fol‐
       low  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  op‐
       tions.)   POSIX.1-2008  changed  the specification to allow either behavior in an implementa‐
       tion.

   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 follow‐
       ing 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 respec‐
          tively 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 compatibility with historic sys‐
          tems (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  fol‐
          low 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  encoun‐
       tered 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.  Opera‐
       tions  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  encoun‐
       tered  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 re‐
       fer 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 fol‐
       lowed).

       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 ig‐
          nored 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 al‐
          ways do file tree traversals, the -L flag will be ignored if the -R flag is not also spec‐
          ified.)

          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  en‐
          countered  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  "physi‐
          cal")  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 ig‐
       nored 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 differ‐
          ently.  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_reso‐
       lution(7)

Linux man-pages 6.7                          2023-10-31                                   symlink(7)