PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

LN(1P)                    POSIX Programmer's Manual                   LN(1P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       ln — link files

SYNOPSIS         top

       ln [−fs] [−L|−P] source_file target_file

       ln [−fs] [−L|−P] source_file... target_dir

DESCRIPTION         top

       In the first synopsis form, the ln utility shall create a new
       directory entry (link) at the destination path specified by the
       target_file operand. If the −s option is specified, a symbolic link
       shall be created for the file specified by the source_file operand.
       This first synopsis form shall be assumed when the final operand does
       not name an existing directory; if more than two operands are
       specified and the final is not an existing directory, an error shall
       result.

       In the second synopsis form, the ln utility shall create a new
       directory entry (link), or if the −s option is specified a symbolic
       link, for each file specified by a source_file operand, at a
       destination path in the existing directory named by target_dir.

       If the last operand specifies an existing file of a type not
       specified by the System Interfaces volume of POSIX.1‐2008, the
       behavior is implementation-defined.

       The corresponding destination path for each source_file shall be the
       concatenation of the target directory pathname, a <slash> character
       if the target directory pathname did not end in a <slash>, and the
       last pathname component of the source_file.  The second synopsis form
       shall be assumed when the final operand names an existing directory.

       For each source_file:

        1. If the destination path exists and was created by a previous
           step, it is unspecified whether ln shall write a diagnostic
           message to standard error, do nothing more with the current
           source_file, and go on to any remaining source_files; or will
           continue processing the current source_file.  If the destination
           path exists:

            a. If the −f option is not specified, ln shall write a
               diagnostic message to standard error, do nothing more with
               the current source_file, and go on to any remaining
               source_files.

            b. If destination names the same directory entry as the current
               source_file ln shall write a diagnostic message to standard
               error, do nothing more with the current source_file, and go
               on to any remaining source_files.

            c. Actions shall be performed equivalent to the unlink()
               function defined in the System Interfaces volume of
               POSIX.1‐2008, called using destination as the path argument.
               If this fails for any reason, ln shall write a diagnostic
               message to standard error, do nothing more with the current
               source_file, and go on to any remaining source_files.

        2. If the −s option is specified, actions shall be performed
           equivalent to the symlink() function with source_file as the
           path1 argument and the destination path as the path2 argument.
           The ln utility shall do nothing more with source_file and shall
           go on to any remaining files.

        3. If source_file is a symbolic link:

            a. If the −P option is in effect, actions shall be performed
               equivalent to the linkat() function with source_file as the
               path1 argument, the destination path as the path2 argument,
               AT_FDCWD as the fd1 and fd2 arguments, and zero as the flag
               argument.

            b. If the −L option is in effect, actions shall be performed
               equivalent to the linkat() function with source_file as the
               path1 argument, the destination path as the path2 argument,
               AT_FDCWD as the fd1 and fd2 arguments, and AT_SYMLINK_FOLLOW
               as the flag argument.

           The ln utility shall do nothing more with source_file and shall
           go on to any remaining files.

        4. Actions shall be performed equivalent to the link() function
           defined in the System Interfaces volume of POSIX.1‐2008 using
           source_file as the path1 argument, and the destination path as
           the path2 argument.

OPTIONS         top

       The ln utility shall conform to the Base Definitions volume of
       POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines.

       The following options shall be supported:

       −f        Force existing destination pathnames to be removed to allow
                 the link.

       −L        For each source_file operand that names a file of type
                 symbolic link, create a (hard) link to the file referenced
                 by the symbolic link.

       −P        For each source_file operand that names a file of type
                 symbolic link, create a (hard) link to the symbolic link
                 itself.

       −s        Create symbolic links instead of hard links. If the −s
                 option is specified, the −L and −P options shall be
                 silently ignored.

       Specifying more than one of the mutually-exclusive options −L and −P
       shall not be considered an error. The last option specified shall
       determine the behavior of the utility (unless the −s option causes it
       to be ignored).

       If the −s option is not specified and neither a −L nor a −P option is
       specified, it is implementation-defined which of the −L and −P
       options will be used as the default.

OPERANDS         top

       The following operands shall be supported:

       source_file
                 A pathname of a file to be linked. If the −s option is
                 specified, no restrictions on the type of file or on its
                 existence shall be made. If the −s option is not specified,
                 whether a directory can be linked is implementation-
                 defined.

       target_file
                 The pathname of the new directory entry to be created.

       target_dir
                 A pathname of an existing directory in which the new
                 directory entries are created.

STDIN         top

       Not used.

INPUT FILES         top

       None.

ENVIRONMENT VARIABLES         top

       The following environment variables shall affect the execution of ln:

       LANG      Provide a default value for the internationalization
                 variables that are unset or null. (See the Base Definitions
                 volume of POSIX.1‐2008, Section 8.2, Internationalization
                 Variables for the precedence of internationalization
                 variables used to determine the values of locale
                 categories.)

       LC_ALL    If set to a non-empty string value, override the values of
                 all the other internationalization variables.

       LC_CTYPE  Determine the locale for the interpretation of sequences of
                 bytes of text data as characters (for example, single-byte
                 as opposed to multi-byte characters in arguments).

       LC_MESSAGES
                 Determine the locale that should be used to affect the
                 format and contents of diagnostic messages written to
                 standard error.

       NLSPATH   Determine the location of message catalogs for the
                 processing of LC_MESSAGES.

ASYNCHRONOUS EVENTS         top

       Default.

STDOUT         top

       Not used.

STDERR         top

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES         top

       None.

EXTENDED DESCRIPTION         top

       None.

EXIT STATUS         top

       The following exit values shall be returned:

        0    All the specified files were linked successfully.

       >0    An error occurred.

CONSEQUENCES OF ERRORS         top

       Default.

       The following sections are informative.

APPLICATION USAGE         top

       None.

EXAMPLES         top

       None.

RATIONALE         top

       The CONSEQUENCES OF ERRORS section does not require ln −f a b to
       remove b if a subsequent link operation would fail.

       Some historic versions of ln (including the one specified by the
       SVID) unlink the destination file, if it exists, by default. If the
       mode does not permit writing, these versions prompt for confirmation
       before attempting the unlink. In these versions the −f option causes
       ln not to attempt to prompt for confirmation.

       This allows ln to succeed in creating links when the target file
       already exists, even if the file itself is not writable (although the
       directory must be).  Early proposals specified this functionality.

       This volume of POSIX.1‐2008 does not allow the ln utility to unlink
       existing destination paths by default for the following reasons:

        *  The ln utility has historically been used to provide locking for
           shell applications, a usage that is incompatible with ln
           unlinking the destination path by default. There was no
           corresponding technical advantage to adding this functionality.

        *  This functionality gave ln the ability to destroy the link
           structure of files, which changes the historical behavior of ln.

        *  This functionality is easily replicated with a combination of rm
           and ln.

        *  It is not historical practice in many systems; BSD and BSD-
           derived systems do not support this behavior. Unfortunately,
           whichever behavior is selected can cause scripts written
           expecting the other behavior to fail.

        *  It is preferable that ln perform in the same manner as the link()
           function, which does not permit the target to exist already.

       This volume of POSIX.1‐2008 retains the −f option to provide support
       for shell scripts depending on the SVID semantics. It seems likely
       that shell scripts would not be written to handle prompting by ln and
       would therefore have specified the −f option.

       The −f option is an undocumented feature of many historical versions
       of the ln utility, allowing linking to directories. These versions
       require modification.

       Early proposals of this volume of POSIX.1‐2008 also required a −i
       option, which behaved like the −i options in cp and mv, prompting for
       confirmation before unlinking existing files. This was not historical
       practice for the ln utility and has been omitted.

       The −L and −P options allow for implementing both common behaviors of
       the ln utility. Earlier versions of this standard did not specify
       these options and required the behavior now described for the −L
       option. Many systems by default or as an alternative provided a non-
       conforming ln utility with the behavior now described for the −P
       option. Since applications could not rely on ln following links in
       practice, the −L and −P options were added to specify the desired
       behavior for the application.

       The −L and −P options are ignored when −s is specified in order to
       allow an alias to be created to alter the default behavior when
       creating hard links (for example, alias ln='ln −L').  They serve no
       purpose when −s is specified, since source_file is then just a string
       to be used as the contents of the created symbolic link and need not
       exist as a file.

       The specification ensures that ln a a with or without the −f option
       will not unlink the file a.  Earlier versions of this standard were
       unclear in this case.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       chmod(1p), find(1p), pax(1p), rm(1p)

       The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines

       The System Interfaces volume of POSIX.1‐2008, link(3p), unlink(3p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) In the event of any discrepancy between this version and
       the original IEEE and The Open Group Standard, the original IEEE and
       The Open Group Standard is the referee document. The original
       Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                              LN(1P)