LN(1POSIX)

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 .

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