Ln (Unix)

ln is a shell command for creating a link file to an existing file or directory.^[1] By default, the command creates a hard link, but with the -s command line option, it creates a symbolic link.^[2] Most systems disallow a hard link to a directory since such links could disrupt the structure of a file system and interfere with the operation of other utilities.^[3] The command can create a symbolic link to non-existent file.^[4]

ln

Example usage of ln utility
Original author(s)	AT&T Bell Laboratories
Developer(s)	Various open-source and commercial developers
Initial release	November 3, 1971; 53 years ago (1971-11-03)
Operating system	Unix, Unix-like, IBM i
Platform	Cross-platform
Type	Command
License	coreutils: GPLv3+

The command appeared in Issue 2 of the X/Open Portability Guidelines.^[5] The version in GNU Core Utilities was written by Mike Parker and David MacKenzie.^[6] The command is available in Windows via UnxUtils^[7] and has been ported to IBM i.^[8]

Links

A link allows more than one path to refer to the same file.

A hard link is a directory entry that refers to a file's inode (an internal reference). A file can have multiple hard links – each referring to the same inode.^[9] Creating a hard link does not copy the contents of the file; but merely causes another name to be associated with the same content. When a hard link is created for a file, a counter in its inode is incremented, and removing a hard link results in decrementing the counter. A file is not deleted (to allow reusing its storage space) until its reference count reaches zero.^[1][10] A hard link can only refer to an object on the same file system.^[11]

A symbolic link refers to another file by its path (a symbolic reference).^[4] When encountered during pathname resolution, modify the pathname resolution to be taken to the location which the symbolic link contains. The content of a symbolic link is the target path, which can also be examined via the readlink command.^[12] A symbolic link may specify a path that does not refer to an existing file. Resolving such a symbolic link will fail until a file is created at its specified path. Also, a symbolic link to an existing file will fail to resolve if the file is moved.^[9]

Use

Per the Single Unix Specification, the ln command is specified in the Shell and Utilities (XCU) document.^[13] POSIX includes a similar specification.^[5]

If neither target file nor target directory are specified, links are created in the current working directory.

Options

The specification indicates command line options that must be supported:

• -f (force) Clobbers an existing file if the output path specifies an existing file
• -L If a source operand specifies a symbolic link, create a hard link to the link's target file
• -P If a source operand specifies a symbolic link, create a hard link to the symbolic link file
• -s Create symbolic links instead of hard links; causes -L and -P to be silently ignored

If more than one of the mutually-exclusive options -L and -P is specified, the last option specified determines the resulting behavior.

If the -s option is not specified and neither a -L nor a -P option is specified, the implementation defines which of the -L and -P options is used by default.

Single file invocation

The command creates a new link to the path indicated by source; stored at the path indicated by target. The syntax is as follows:

ln [-fs] [-L|-P] source target

Multiple file invocation

The command creates a new link to each file specified by the source_# operands; stored in an existing directory indicated by target_dir.

ln [-fs] [-L|-P] source_1 source_2 ... target_dir

Examples

The following creates a hard link to file foo called bar.

ln foo bar

The following creates a symbolic link to file foo called bar.

ln -s foo bar

See also

• List of POSIX commands
• NTFS junction point