[ILUG] How to copy some files

[kevin lyda]

On Sat, Jul 20, 2002 at 08:24:47PM +0100, Aidan Kehoe wrote:
>  Ar an 20u la de mi 7, scriobh kevin lyda :
>  > actaully i think soft links were invented because you can't hard link
>  > directories. 
> But you could hard link directories, back when soft links were
> being invented, AFAIK. 

that was before my time.  all unix systems i've used didn't allow hard
links to directories, or if they did they were restricted to root.
the reason why is because you could cause infinite loops in the kernel -
usually a bad place for infinite loops.

>  > apparently some systems limited soft links to the same device but
>  > gave up after a while.  
> Why?

to make them consistent with hard links.

> A better way of doing it would be a) have global unique filesystem
> identifiers for every FS created (such that the chance of two of them
> clashing is miniscule; 64 bits creatively used would do it, I'd say),
> and b) implement the target info for the soft link as a {FSID, inode}
> pair; the OS can work out if the thing linked to is now on a different
> mount point, or has been moved. (HFS fans, is that what's done? Or are
> aliases implemented differently?)

let's call these super-soft-links.  ln -ss

    % ln -ss foo bar
    % ls -i foo
    111 foo
    % mv floyd foo
    % ls -i foo
    222 foo

and now bar no longer points to foo.

    % ln -ss foo bar
    % ls -i foo
    111 foo
    % rm foo
    % touch floyd
    % ls -i floyd
    111 floyd

the fs would need to maintain a table of links going the other direction.
so when the move command unlinks foo in the first example, it could
check the table and mark that bar is now disconnected.  the same would
be true for the second example - and even more important since bar points
to floyd if no table is consulted.

and this all fails to handle nfs mounted file systems or filesystems
that have dynamic inodes (the fat fs's and reiser lacks inodes i think).

kevin

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