IIRC, time machine on os x uses hard links--it copies the file to your
backup drive once, and then uses hard links to show unchanged files/
folders in subsequent backups.
Sent while on the go!
On Mar 4, 2010, at 5:15 AM, Daniel Eggleston <[email protected]> wrote:
I don't tend to use hard links, but they are useful in that you
won't run into the 'broken link' issue. i.e. you can move either
file, but the link won't break. You can also remove either file
without breaking a link. Soft links are less confusing, though,
because it's immediately apparent that one is a link. With hard
links, you need to look at the output of ls -i to see what inode the
file is stored at.
On Wed, Mar 3, 2010 at 10:36 PM, Blues Renegade <[email protected]
> wrote:
Thanks for the clarification, Daniel! Why use hard links at all?
They must exist for a reason, but I have yet to encounter a need
that ONLY a hard link could fill. In commercial UNIX apps I've
installed I've only ever seen soft links used by the software
vendor, never hard links. What am I missing here?
Thanks again, Daniel! Despite my error on hard links, I do hope DOS-
MAN got something out of all I wrote on soft links, or I wasted my
time.
John
Daniel Eggleston wrote:
An important point that should be made: a link is always a link, and
never a copy! A hard link is not a copy. A hard link is a duplicate
inode pointer, so both are "real" files. i.e.:
$ echo "foo" > fileA
$ ln fileA fileB
$ ln -s fileA fileC
$ cat file B
foo
$ echo "bar" > fileA
$ cat fileB
bar
$ cat fileC
bar
$ rm fileA
$ cat fileB
bar
$ cat fileC
*error*
The difference between a hard link and a soft link is, the soft link
points to a file by name. A hard link points by inode, so after you
remove the original file, it still exists with the second filename.
But they're not separate copies, they're the same file!
On Wed, Mar 3, 2010 at 7:32 AM, Blues Renegade <[email protected]
<mailto:[email protected]>> wrote:
I think you're making it much harder for yourself by mentally
cross-referencing everything in Linux back to your DOS/Windows
knowledge. You'll have a much easier time if you "forget" about
DOS/Windows and approach Linux like a kid learning about computers
for the first time.
When I first got into Linux, I decided to go back to where Linux
came from and picked up some old UNIX books where everything was
done on the command line, they didn't even have an X-server yet,
and to this day, most UNIX server admins run a console-system
only, no GUI! Most would never dream of using a GUI, seeing it as
a headache; more programs and config files to break and wreak
havoc on the server's stability.
As for links, they are fairly straightforward. There are 2 kinds:
hard links and soft links.
A hard link is actually just a copy of a file. Personally, I have
never used hard links. I use cp -a (-a archives a file with its
original date/time and preserves ownership and permissions) if I
want to duplicate files.
A soft link is a shortcut to a file; under the covers it's nothing
more than a pointer that contains the path to where the file
exists.
IMPORTANT! The path it stores is the text you enter when you
create the link. The ln command does NOT try and validate your
path. The UNIX/Linux philosophy is that commands are small
programs with a very specific purpose. If you want/need more
functionality, then you are expected to combine the commands using
piping and/or redirection to get the job done. Once you learn
enough commands and know how to combine them, the light bulb will
go on and you'll realize the true power of UNIX (in our case,
Linux).
CREATING LINKS:
For example, you're in your home directory and you want a quick
way to 'cd' to a /home/dos-man/programming/c/linux/utilities.
Here's a FLAWED WAY of creating a soft link... I'm showing you a
pitfall first:
ln -s ./programming/c/linux/utilities/ c-utils
ln = link command
-s = soft link (shortcut, pointer, stored path, however you want
to remember it)
./programming/c/linux/utilities = a relative path (relative
reference) to the directory utilities
The period at the beginning points to the current directory. (Not
very exact is it?!)
c-utils = the filename of your link (shortcut) i.e. this is what
you'll use with cd to save typing that long path
You're working away in /home/dos-man/programming/c/dos/ and you
decide to go work on a linux utility, so you run:
cd c-utils
and surprise, it fails!
What it does is try to cd from your current directory to your
link's path. The cd command replaces the period in the link with
your current directory and tries to change to the new "fangled"
(mangled is more like it) path:
It's as though you entered:
cd /home/dos-man/programming/c/dos/programming/c/linux/utilities/
No such path exists on your system (and if by chance it does, then
you're not where you expect to be!!).
**** TIP: When creating softlinks, include the complete path (AKA
absolute reference).
To fix the problem above, rm c-utils to remove the link you
created.
Re-create it with the complete path this time (an absolute
reference).
ln -s /home/dos-man/programming/c/linux/utilities/ c-utils
**** TIP: It's a good idea to include the trailing slash when
creating links to directories, so when you look at the link with
'ls -l', you'll know it's pointing to a directory.
Hope that helps you to start using links right away.
BEST TIP OF ALL: Forget registries, forget Windows, forget DOS,
and you'll have a much easier time learning Linux!! Start fresh;
after all, UNIX came first, then MS-DOS borrowed heavily from UNIX
and ended up as a very watered down proprietary sub-set with some
subtle (proprietary?).
Many of the commands in UNIX were never replicated in DOS, so they
will be completely new to you. New concepts and ways of thinking
to grasp. While DOS had piping and redirection, without a rich
command set you were still very limited in comparison to UNIX.
John
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