ln, link

Make hard links and symbolic links.
`ln' creates a new directory entry (linked file) with the same modes as the original file. It is useful for maintaining multiple copies of a file in many places at once without using up storage for the copies; instead, a link 'points' to the original copy. How a link 'points' to a file is one of the differences between a hard and symbolic link.

Syntax
      ln [-fhinsv] OriginalSourceFile [NewLinkFile]

      ln [-fhinsv] OriginalSourceFile... NewLinkFile

      link  OriginalSourceFile  NewLinkFile

Options

     -f	   If the target file already exists, then unlink it so that the link
	   may occur.  (The -f option overrides any previous -i options.)

     -h	   If the NewLinkFile (or directory) is a symbolic link, do not follow
	   it.	This is most useful with the -f option, to replace a symlink
	   which may point to a directory.

     -i	   Cause ln to write a prompt to standard error if NewLinkFile
	   exists.  If the response from the standard input begins with the
	   character 'y' or 'Y', then unlink the NewLinkFile file so that the link
	   may occur.  Otherwise, do not attempt the link.  (The -i option
	   overrides any previous -f options.)

     -n	   Same as -h, for compatibility with other ln implementations.

     -s	   Create a symbolic link.

     -v	   Cause ln to be verbose, showing files as they are processed.

By default, ln makes hard links. A hard link to a file is indistinguishable from the original directory entry; any changes to a file are effectively independent of the name used to reference the file. Hard links may not normally refer to directories and may not span file systems.

A symbolic link contains the name of the file to which it is linked. The referenced file is used when an open(2) operation is performed on the link. A stat(2) on a symbolic link will return the linked-to file; an lstat(2) must be done to obtain information about the link. The readlink(2) call may be used to read the contents of a symbolic link. Symbolic links may span file systems and may refer to directories.

Given one or two arguments, ln creates a link to an existing file OriginalSourceFile. If NewLinkFile is given, the link has that name.

NewLinkFile may also be a directory in which to place the link; otherwise it is placed in the current directory. If only the directory is specified, the link will be made to the last component of OriginalSourceFile.

Given more than two arguments, ln makes links in NewLinkFile to all the named source files. The links made will have the same name as the files being linked to.

Examples

N.b. these assume you have the OSX Auto-Save feature turned off.

1) Hard Link to a file while it is open:

Create a new file called cake.jpg
Open the file cake.jpg in Preview.app
In Terminal, make a hard-link: ln cake.jpg link.jpg
In Preview, rotate the image and then Save and Close - the changes will now be visible in both cake.jpg and link.jpg
Delete cake.jpg : rm cake.jpg
The data will be automatically copied into link.jpg and will still be readable.

2) Delete a file, while hard links to it are still open:

Create a new file called flower.jpg
In Terminal, make a hard-link: ln flower.jpg link.jpg
Open the file link.jpg in Preview.app
In Terminal, delete the original file: rm flower.jpg
In Preview, rotate the image and then Save - the changes should be saved to link.jpg
Note that many applications will in fact fail to save the data in the above scenario, because they opened the original file which has since been deleted.

3) Create a Symbolic Link to a file while it is open:

Create a new file called cake.jpg
Open the file cake.jpg in Preview.app
In Terminal, make a symbolic-link: ln -s cake.jpg sybm.jpg
In Preview, rotate the image and then Save and Close - the changes will now be visible in both cake.jpg and symb.jpg
Delete cake.jpg : rm cake.jpg
The symbolic link symb.jpg now points to a nonexistent file, attempting to open it will give an error.

4) Delete a file, while Symbolic links to it are still open:

Create a new file called flower.jpg
In Terminal, make a symbolic-link: ln -s flower.jpg symb.jpg
Open the file symb.jpg in Preview.app
In Terminal, delete the original file: rm flower.jpg
In Preview, rotate the image and then Save - you will be prompted to save under a new name

"God shows his contempt for wealth by the kind of person he selects to receive it" ~ Austin O'Malley

Related:

ln man page - Apple.com
pathchk - Check file name portability
symlink - Make a new name for a file


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