symlink

SYMLINK(7)                  Linux Programmer's 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 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 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 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:

       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).

       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)

COLOPHON
       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2021-03-22                         SYMLINK(7)