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NAME

       symlink - symbolic link handling

SYMBOLIC LINK HANDLING

       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 file system.
       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
       file system tree, which would confuse many  programs)  and  may  not  refer  to  files  on
       different file systems (because i-node numbers are not unique across file systems).

       A symbolic link is a special type of file whose contents are a string that is the pathname
       another file, the file to which the link refers.  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 file system 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 file system 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.

   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),
       open(2), openat(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,  e.g.,  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, e.g., 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, i.e., the -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

       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

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