SYMLINK(7)                (2020-06-09)                 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 con-
          tains 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 direc-
          tories 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 distin-
          guishing 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 pseud-
          ofilesystems such as proc(5) (examples include
          /proc/[pid]/exe and /proc/[pid]/fd/*).  Unlike normal

     Page 1                        Linux             (printed 5/21/22)

     SYMLINK(7)                (2020-06-09)                 SYMLINK(7)

          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 sym-
          bolic 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
          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 spec-
          ified), 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
          Symbolic links are handled either by operating on the link
          itself, or by operating on the object referred to by the

     Page 2                        Linux             (printed 5/21/22)

     SYMLINK(7)                (2020-06-09)                 SYMLINK(7)

          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 sym-
          bolic 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 travers-
             ing 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
          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 exam-
             ple, 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

     Page 3                        Linux             (printed 5/21/22)

     SYMLINK(7)                (2020-06-09)                 SYMLINK(7)

          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 sym-
          bolic 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 com-
          mand 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 exam-
          ple, 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

     Page 4                        Linux             (printed 5/21/22)

     SYMLINK(7)                (2020-06-09)                 SYMLINK(7)

            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 argu-
            ments 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 spec-
            ified.

        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 argu-
          ments.

          The first rule applies to symbolic links that reference
          files other than directories.  Operations that apply to sym-
          bolic links are performed on the links themselves, but oth-
          erwise 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 sym-
          bolic 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

     Page 5                        Linux             (printed 5/21/22)

     SYMLINK(7)                (2020-06-09)                 SYMLINK(7)

            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 tra-
            verse the file hierarchy rooted in the file pointed to by
            slink. Note, the -H is not the same as the previously dis-
            cussed -h flag.  The -H flag causes symbolic links speci-
            fied on the command line to be dereferenced for the pur-
            poses 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 encoun-
            tered 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 hier-
            archy 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 phys-
            ical 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 deter-
          mines 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 sym-
            bolic link.  The rm(1) command does not support the -H,
            -L, or -P options.

     Page 6                        Linux             (printed 5/21/22)

     SYMLINK(7)                (2020-06-09)                 SYMLINK(7)

          * 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 speci-
            fied, 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 5.10 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/.

     Page 7                        Linux             (printed 5/21/22)