[accesscomp] Dan's Tip of the day
- From: "Bob Acosta" <boacosta@xxxxxxxxxxx>
- To: "tektalk discussion" <tektalkdiscussion@xxxxxxxxxxxxxxxxxxx>
- Date: Tue, 7 Aug 2012 11:27:34 -0700
Symbolic links|symlinks|hard links|soft links|junctions|Windows 7
(i)
File and directory aliases can be very useful. Several types called hard
links, symbolic links (symlinks), and junctions are available in Windows 7.
They are described and their uses explained.
It is very common for operating systems to have a way of creating aliases
for files and directories. For example, Unix and its numerous progeny and
relatives provide for aliases called "hard links" and "soft links". On the
other hand, Windows with FAT16 and FAT32 file systems made no use of this
type of feature. Some provision was made in the NTFS file system in Windows
2000 and beginning with Windows Vista, several types of aliases are
available that provide useful functions - hard links, symbolic (soft)
links, and junctions (introduced in Windows 2000). In some ways, these
aliases resemble the well-known shortcut file but there are fundamental
differences.
In this article I will describe the various kinds of links; a companion
article explains how they are created and how they are used.
http://windows7tips.com/symlinks-creation.html
How hard links and symbolic links differ from conventional shortcut files
First, let's try to clarify what a traditional shortcut is. It is an actual
file with various standard file properties and an extension lnk. (The
extension is usually hidden.) It also contains a reference to an object
like a folder or another file and has the special property that the Windows
shell knows that double-clicking it should open the object that is
referenced in the file. However, applications will generally operate on the
shortcut file itself and not on the object that the shortcut file
references.
For example, if you have a shortcut to a text file and try to open the
shortcut directly with Notepad, it will try to read the contents of the
shortcut file, not the referenced text file.
Programmers often use the term "transparent" to indicate if applications can
act as if the link were actually the referenced file or folder. Standard
Windows shortcut files are not transparent but hard links and symbolic
links are.
An advantage of shortcuts over symbolic links is that shortcuts maintain
their references to their targets even when the target is moved or renamed.
However, neither will work if the target is deleted.
A glossary of terms
The Microsoft documentation on Windows symbolic links, junctions, and
related subjects is inconsistent. Also, I find articles and various
descriptions of this subject on the web to be confusing and often
contradictory. Here are the meanings of various terms as I understand them
after digging through the MSDN documentation (no models of clarity). These
apply to Windows and may be somewhat different for Unix and its relatives.
Symbolic link or symlink
This was introduced beginning with Windows Vista and is sometimes equated to
a "soft link". A Windows Vista/7/8 symbolic link is defined by Microsoft as
"a file-system object that points to another file system object". It can
refer to files or folders (directories) or even to remote shares . However,
the reference is just to a name and not directly to a file object. A
symbolic link contains a text string that is interpreted by the operating
system as a path to another file or directory called the target. If the
target is moved, renamed, or deleted, the symbolic link remains but leads
nowhere. It is then said to be "broken". Unlike shortcut files, symbolic
links are transparent to most applications. However, important exceptions
to transparency are certain backup programs.
Hard link
A hard link is not a file itself but an additional address or alias for a
file. It allows a file to be accessed from several locations. Addresses
must all be on the same volume. In Windows, only files can have hard links.
Only one copy of the file will actually exist but it will appear to be
located in several places. File edits made at any address appear at all
addresses. Deletion of a hard link only removes the address and not the
file as long as at least one address remains.
Soft link
Used in several different senses. Often equated to "symbolic link" but
sometimes equated to "junction" or to both. There seems to be some
confusion in various places because what Microsoft calls a soft link is a
file object and in many Unix-type systems, a soft link is a lower level
entry in the file system.
Junctions
Junction points were introduced in Windows 2000.They use "reparse points"
and are an older way of linking local directories. They are sometimes
erroneously called hard links, even by supposed experts. A junction is, in
fact, a soft link with path information about a target directory. Although
superseded in some respects by Microsoft's version of symbolic links,
junctions are used in Windows Vista/7 to redirect certain system folders
from their old Windows XP names to new names. For example, "My Documents"
gets redirected to "Documents". Junctions are limited to linking local
directories but they can be on separate volumes.
Reparse point
A collection of user-defined data contained in a file or directory. When the
file system opens a file with a reparse point, the file is processed as
directed by the reparse data and an associated file system filter. Used for
folder redirection and for mounting folders.
Dan Thompson
dthompson5@xxxxxxxxx
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