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.

   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 a 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.  (Note that there are
       some "magic" symbolic links in the /proc directory tree—for example,
       the /proc/[pid]/fd/* files—that have different permissions.)

   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).

   System calls
       The first area is symbolic links used as filename arguments for system
       calls.

       Except as noted below, all system calls follow symbolic links.  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, 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.  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—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 5.04 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                             2016-10-08                        SYMLINK(7)