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



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

NAME
       ln -- link files

SYNOPSIS
       ln [[-fs]] [[-L|-P]] source__file target__file

       ln [[-fs]] [[-L|-P]] source__file... target__dir

DESCRIPTION
       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.1-2017, 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 the destination path 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.1-2017, called
               using the destination path 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.1-2017 using
           source__file as the path1 argument, and the destination path as the
           path2 argument.

OPTIONS
       The ln utility shall conform to the Base Definitions volume of
       POSIX.1-2017, 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
       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
       Not used.

INPUT FILES
       None.

ENVIRONMENT VARIABLES
       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.1-2017, 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
       Default.

STDOUT
       Not used.

STDERR
       The standard error shall be used only for diagnostic messages.

OUTPUT FILES
       None.

EXTENDED DESCRIPTION
       None.

EXIT STATUS
       The following exit values shall be returned:

        0    All the specified files were linked successfully.

       >0    An error occurred.

CONSEQUENCES OF ERRORS
       Default.

       The following sections are informative.

APPLICATION USAGE
       None.

EXAMPLES
       None.

RATIONALE
       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.1-2017 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.1-2017 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.1-2017 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 aa aa with or without the -f option
       will not unlink the file aa.  Earlier versions of this standard were
       unclear in this case.

FUTURE DIRECTIONS
       None.

SEE ALSO
       chmod, find, pax, rm

       The Base Definitions volume of POSIX.1-2017, Chapter _8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines

       The System Interfaces volume of POSIX.1-2017, link(), unlink()

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  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.opengroup.org/unix/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                  2017                                LN(1)
