ln (1p) - Linux Man Pages
ln: link files
PROLOGThis 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] source_file target_file
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 IEEE Std 1003.1-2001, 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, 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:
If the destination path exists:
- 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.
- Actions shall be performed equivalent to the unlink() function defined in the System Interfaces volume of IEEE Std 1003.1-2001, 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.
- If the -s option is specified, ln shall create a symbolic link named by the destination path and containing as its pathname source_file. The ln utility shall do nothing more with source_file and shall go on to any remaining files.
- If source_file is a symbolic link, actions shall be performed equivalent to the link() function using the object that source_file references 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.
- Actions shall be performed equivalent to the link() function defined in the System Interfaces volume of IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following option shall be supported:
- Force existing destination pathnames to be removed to allow the link.
Create symbolic links instead of hard links.
The following operands shall be supported:
- 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.
- The pathname of the new directory entry to be created.
A pathname of an existing directory in which the new directory entries
The following environment variables shall affect the execution of ln:
- Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
- If set to a non-empty string value, override the values of all the other internationalization variables.
- 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).
- Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
Determine the location of message catalogs for the processing of LC_MESSAGES
The following exit values shall be returned:
- All the specified files were linked successfully.
- An error occurred.
CONSEQUENCES OF ERRORS
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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 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.
COPYRIGHTPortions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .