# symlink

## 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.
    


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