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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)
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.
ln — link files
ln [−fs] [−L|−P] source_file target_file
ln [−fs] [−L|−P] source_file... target_dir
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.
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.
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.
Not used.
None.
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.
Default.
Not used.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
0 All the specified files were linked successfully.
>0 An error occurred.
Default.
The following sections are informative.
None.
None.
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.
None.
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)
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)
Pages that refer to this page: cp(1p), link(1p), mv(1p)