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Operating Systems Solaris Difference between hard link and copy command Post 302378252 by jim mcnamara on Monday 7th of December 2009 10:35:59 AM
Old 12-07-2009
Here is an oversimplified analogy:

Think of a hard link as a pointer in the filesystem. Just like in C, it has to be dereferenced. But instead of what a pointer does: refer to a separate place in memory, with a hardlink we are referred to a place on the physical disk. The link acts as a proxy for the "real" filename.

The C pointer uses very little memory, but can reference a giant object in memory.

A hard link uses very little disks space but can reference a huge file.

How hard links are implemented is not by a universally defined object, but is up to the filesystem and the drivers to resolve. This is like C: a pointer can be a "big-endian" or a "little-endian" object in memory, and the compiler works out how to reference the object the pointer is "aimed" at. The filesystem + driver does the same. We do not worry about how to figure it out at all.

Links have consequences. One of them is that you do not ever want dangling links - links "aimed" at non-existent. It is possible to create a series: link -> link -> physical file. This is where it is easy to get in trouble. You can create circular links in some circumstances.

Copy duplicates a whole file. Link references a whole file.

Last edited by jim mcnamara; 12-07-2009 at 12:10 PM..
 

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ln(1)							      General Commands Manual							     ln(1)

Name
       ln - link to a file

Syntax
       ln [ -f ] [ -i ] [ -s ] name1 [name2]
       ln [ -f ] [ -i ] [ -s ] name ... directory

Description
       A  link is a directory entry referring to a file.  A file, together with its size and all its protection information may have several links
       to it.  There are two kinds of links: hard links and symbolic links.

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

       Given  one or two arguments, creates a link to an existing file name1.  If name2 is given, the link has that name.  The name2 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	is
       made to the last component of name1.

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

Options
       -f   Forces existing destination pathnames to be removed before linking without prompting for confirmation.

       -i   Write a prompt to standard output requesting information for each link that would overwrite an existing file.  If  the  response  from
	    standard input is affirmative, and if permissions allow, the link is done. The -i option has this effect even if the standard input is
	    not a terminal.

       -s   Creates a symbolic link.

	    A symbolic link contains the name of the file to which it is linked.  The referenced file is used when an operation  is  performed	on
	    the  link.	 A  on a symbolic link returns the linked-to file.  An must be done to obtain information about the link.  The call may be
	    used to read the contents of a symbolic link.  Symbolic links may span file systems and may refer to directories.

See Also
       cp(1), mv(1), rm(1), link(2), readlink(2), stat(2), symlink(2)

																	     ln(1)
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