NAME | DESCRIPTION | SEE ALSO | COLOPHON

SYMLINK(7)                Linux Programmer's Manual               SYMLINK(7)

NAME         top

       symlink - symbolic link handling

DESCRIPTION         top

       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 i-node number, where an i-node number is an
       index into the i-node 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 i-node 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.

   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.

       The 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.)  The upcoming POSIX.1 revision changes 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 the 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         top

       chgrp(1), chmod(1), find(1), ln(1), ls(1), mv(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         top

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Linux                            2014-04-06                       SYMLINK(7)