ln

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



PROLOG
       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
       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 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
       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(1P)