Congrats for this wonderful explanation of hard/symlinks.
Patrick
On Tue, 27 Jul 1999, you wrote:
> On Tue, 27 Jul 1999, Richard Myers wrote:
> > On Tue, 27 Jul 1999, Andy Goth wrote:
> > > I was about to make that statement earlier, but I then thought it wasn't
> > > tre since everything that has been said indicated that hard links point
> > > to a single file and when all hard links die the file does as well (and
> > > that kinda invalidated what I thought). It's a good thing this isn't
> > > the case! It's waaay too complicated and unwieldly. Imagine making a
> > > hard link and changing your mind about it... if this was true, I
> > > couldn't delete it! The truth is much better. Yeah, that is a good
> > > idea about deletion protection. If I want to make sure that some data
> > > cannot be deleted, I can keep a hard link.
> >
> > I think that you misunderstand.
> >
> > A hard link is the way that you access a file. BUT, there is only one
> > file.
> >
> > Suppose that we have a file named... well, lets create a file:
> >
> > $ echo put this in a file > hardlink_1
> > $
> >
> > We have created a file the "quick" way, and we gave it the name
> > hardlink_1.
> >
> > And then we "cat" the file, which shows what is in the file.
> >
> > $ cat hardlink_1
> > put this in a file
> > $
> >
> > Hardlink_1 is a text file which has the contents, "put this in a file".
> >
> > OK, lets say it a different way. The echo command sends (we say it
> > re-directs) "put this in a file" into the contents of a file named
> > hardlink_1. The ">" character is the nifty command that does this
> > redirection.
> >
> > Lets look at the long display of this file:
> >
> > $ ls -l
> > total 2
> > -rw-rw-r-- 1 rtm 465 19 Jul 27 02:09 hardlink_1
> > $
> >
> > OK, notice that it has a file size of 19 (just before the date).
> >
> > Count the characters in the "string" of characters:
> >
> > 123456789012345678
> > put this in a file
> >
> > 18. One extra byte to store this string gives us the 19.
> >
> > Now we will add another hard link:
> >
> > $ ln hardlink_1 hardlink_2
> > $
> >
> > ...and if we look at our directory again:
> >
> > $ ls -l
> > total 4
> > -rw-rw-r-- 2 rtm 465 19 Jul 27 02:09 hardlink_1
> > -rw-rw-r-- 2 rtm 465 19 Jul 27 02:09 hardlink_2
> > $
> >
> > Both filenames point to the same file. We can display the contents of
> > both files:
> >
> > $ cat *
> > put this in a file
> > put this in a file
> > $
> >
> > Well, actually, we displayed the contents of ONE file twice. Once using
> > each filename. The * is a wildcard that matches all filenames in the
> > directory.
> >
> > OK, now for the test.
> >
> > We are going to redirect "nothing" to the first hardlink.
> >
> > $ > hardlink_1
> > $
> >
> > We have replaced the contents of hardlink_1 with "nothing". Since there is
> > nothing in front of the ">", nothing is put into the file, replacing
> > whatever had been there. Notice that we don't even need the echo command
> > to do this.
> >
> > And, ls -l tells the story:
> >
> > $ ls -l
> > total 0
> > -rw-rw-r-- 2 rtm 465 0 Jul 27 02:22 hardlink_1
> > -rw-rw-r-- 2 rtm 465 0 Jul 27 02:22 hardlink_2
> > $
> >
> > For further proof, try to cat the contents of the two files (or more
> > accurately, the contents of our ONE file twice):
> >
> > $ cat *
> > $
> >
> > Nothing there.
> >
> > Soooooo, a second hard link doesn't "protect" the contents of a file. It
> > only offers another way to access the contents of a file.
> >
> > If we use either hard link to change that file, then the contents are
> > changed. Period.
> >
> > Now, lets create a soft link and put something back into the file:
> >
> > $ ln -s hardlink_1 softlink_1
> > $
> >
> > The -s "switch" to the link command ln makes this a symbolic, or "soft"
> > link, instead of a hard link.
> >
> > $ ls -l
> > total 2
> > -rw-rw-r-- 2 rtm 465 0 Jul 27 02:22 hardlink_1
> > -rw-rw-r-- 2 rtm 465 0 Jul 27 02:22 hardlink_2
> > lrwxrwxrwx 1 rtm 465 10 Jul 27 02:35 softlink_1 -> hardlink_1
> >
> > Whoa! Notice that our soft link is created with WORLD WRITE permissions
> > (the 3rd "w" in the group of "rwx's" in the line above). Doesn't matter,
> > the symlink REALLY inherits the permissions of whatever it points to.
> >
> > Also notice that the symlink has 10 bytes. Why is that?
> >
> > Now we will put something into the file that the symlink points to:
> >
> > $ echo another string > softlink_1
> > $
> >
> > So, what happens if we cat the contents of all the files in the directory?
> >
> > $ cat *
> > another string
> > another string
> > another string
> > $
> >
> > $ ls -l
> > total 6
> > -rw-rw-r-- 2 rtm 465 15 Jul 27 02:35 hardlink_1
> > -rw-rw-r-- 2 rtm 465 15 Jul 27 02:35 hardlink_2
> > lrwxrwxrwx 1 rtm 465 10 Jul 27 02:35 softlink_1 -> hardlink_1
> > $
> >
> > All of our files now contain 15 bytes-- 14 for the string, plus one. Well,
> > EXCEPT for the symlink. It still contains 10 bytes. That is because it is
> > just a pointer to another filename (whether that filename exists or not).
> >
> > If we remove the first hard link, see what happens:
> >
> > $
> > $ rm hardlink_1
> > $
> > $ ls -l
> > total 4
> > -rw-rw-r-- 1 rtm 465 15 Jul 27 02:35 hardlink_2
> > lrwxrwxrwx 1 rtm 465 10 Jul 27 02:35 softlink_1 -> hardlink_1
> > $
> > $ cat *
> > another string
> > cat: cannot open softlink_1: No such file or directory
> > $
> >
> > Since our symlink points to a non-existant filename, we cannot display
> > contents. BUT, the contents still exist under a different filename.
> >
> > BUT-- the symlink still points to that filename. If we create that file
> > anew:
> >
> > $ echo some other NEW stuff in our file > hardlink_1
> > $
> > $ cat *
> > some other NEW stuff in our file
> > another string
> > some other NEW stuff in our file
> > $
> >
> > Neat stuff, huh? This is Unix.
> >
> >
> > best wishes,
> >
> > richard myers
>
> : ) Ever thought about teaching?
> I always had trouble grasping the diferrence between hard links and soft
> (symbolic) links, untill now. And I was'nt even the poster of the message.
> cool , thanks alot, you really have patients and should consider teaching as a
> career. Great stuff indeed, Unix has always facinated me but I thought it too
> hard for me to grasp. More lessons like that and there might be hope for me yet.
> I shure am glad I make it a habbit to at least skim through each and every post.
>
> Thanks alot,
>
> John Love
> [EMAIL PROTECTED]
