ln(1p) — Linux manual page

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.1‐2017, 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         top

       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         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.1‐2017, 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         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.1‐2017 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         top

       None.

SEE ALSO         top

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

       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(3p),
       unlink(3p)

COPYRIGHT         top

       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)

Pages that refer to this page: cp(1p)link(1p)mv(1p)