?Hard links, Soft links, & Aliases--Explain

[Fred Moore]

Hi All,

Could some knowledgeable UNIX type please explain the differences between
hard links, soft links, and traditional Mac aliases. Responses which
include the important ramifications of the differences would be most
appreciated.

I'm pretty familiar with the ins and outs of Mac aliases; but the other
day I repaired a dual boot Mac using Norton booted into 9.2.2 and it
generated a file called Lost and Found containing a whole bunch of zero KB
items labeled 'Hardlink [some long numeric string]'.

Any info will be appreciated; tia.

--Fred

[Jeremy]

Fred Moore <fmoore@gcfn.org> wrote:

> Could some knowledgeable UNIX type please explain the differences between
> hard links, soft links, and traditional Mac aliases. Responses which
> include the important ramifications of the differences would be most
> appreciated.

A "hard link" is a name for a file. When you make a hard link to a file,
you are adding a name for it in the filesystem, which is on equal standing
with the original name -- nether of the two is the "real" filename, they
both are now equally names for the same file.

Technically, every file has at least one hard link -- what you think of as
the filename. It points to the actual data on the disk. If you have more
than one, it's just more than one filesystem entry pointing to the same
data.

A soft link, or "symbolic link", points to another file by name. If you
change the name of the target file, the link becomes invalid, because the
reference it was using was the filename. However, completely changing
the underlying file (including what disk it's on) will not invalidate
the symlink as long as the path name stays the same.

A Mac alias points to another file's data, regardless of the name. Since
it's pointing at the disk data, it becomes invalid if you move the original
file to a different filesystem (a different disk, for example). It will
technically also become invalid if you replace the underlying file but keep
the name the same, but they've worked around that -- if the alias is invalid
but there is a new file in place of the old file, the alias will now refer
to the new file.

This can actually cause confusion; if you have an alias to a file, and you
move the original file to the trash and replace it with a new file (for
example, installing the new version of a program), but you don't empty
the trash, the alias still points to the *old* file, now in the trash,
even though there is a new file with the same name in its place (because
an alias points to the file, not the name). But if you empty the trash,
then use the alias, the Mac will see that its file is gone but a new one
is in its place, and start pointing to that instead.

> I'm pretty familiar with the ins and outs of Mac aliases; but the other
> day I repaired a dual boot Mac using Norton booted into 9.2.2 and it
> generated a file called Lost and Found containing a whole bunch of zero KB
> items labeled 'Hardlink [some long numeric string]'.

Those would be files that lost their "link" to the filesystem. What you
have is the data, but none of the metadata, including the filename itself.
It recovered the files themselves but had no idea what files they were, so
they ended up in Lost and Found with random-looking names.

--
Jeremy | [email]jeremy@exit109.com[/email]

[Tom Stiller]

In article <bdcpd5$m2n$1@acme.gcfn.org>, [email]fmoore@gcfn.org[/email] (Fred Moore)
wrote:

> Hi All,
>
> Could some knowledgeable UNIX type please explain the differences between
> hard links, soft links, and traditional Mac aliases. Responses which
> include the important ramifications of the differences would be most
> appreciated.

From the man page for 'ln':
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
normally refer to directories and may not span file systems.

A symbolic link contains the name of the file to which it is linked.

A Mac alias contains both the name of the file to which it points and
its volume reference number.

You can move a hard link to another subdirectory in the same filesystem
and will still reference the (one and only) file (inode) to which it
refers.

If you move the target of a soft link, you break the link.

You can move a Mac Alias to another subdirectory on the same volume and
it will still reference the (one and only) file (VolRefNo) to which it
refers.

> I'm pretty familiar with the ins and outs of Mac aliases; but the other
> day I repaired a dual boot Mac using Norton booted into 9.2.2 and it
> generated a file called Lost and Found containing a whole bunch of zero KB
> items labeled 'Hardlink [some long numeric string]'.
>
> Any info will be appreciated; tia.
>
> --Fred

--
Tom Stiller

PGP fingerprint = 5108 DDB2 9761 EDE5 E7E3
7BDA 71ED 6496 99C0 C7CF

[Fred Moore]

Thanks, Jeremy and Tom! Just what I needed to know. Certainly didn't
understand about multiple hard links being equal. Now I do.

Fred Moore (fmoore@gcfn.org) wrote:
: I'm pretty familiar with the ins and outs of Mac aliases; but the other
: day I repaired a dual boot Mac using Norton booted into 9.2.2 and it
: generated a file called Lost and Found containing a whole bunch of zero KB
: items labeled 'Hardlink [some long numeric string]'.

To follow up on these Lost and Found hardlinks, should I delete them? Is
the file system accessing them w/o names? How did they lose their names?
And what might be in them?

Thanks again,

--Fred

[Anno Siegel]

Fred Moore <fmoore@gcfn.org> wrote in comp.sys.mac.system:

> Thanks, Jeremy and Tom! Just what I needed to know. Certainly didn't
> understand about multiple hard links being equal. Now I do.
>
> Fred Moore (fmoore@gcfn.org) wrote:
> : I'm pretty familiar with the ins and outs of Mac aliases; but the other
> : day I repaired a dual boot Mac using Norton booted into 9.2.2 and it
> : generated a file called Lost and Found containing a whole bunch of zero KB
> : items labeled 'Hardlink [some long numeric string]'.

The numeric part is usually the inode number of the file.

> To follow up on these Lost and Found hardlinks, should I delete them?

If you're sure *you* don't want them, delete them (but not the
lost+found directory itself).

> Is the file system accessing them w/o names?

No.

> How did they lose their names?

Some file system malfunction. Could be hardware or software.

> And what might be in them?

Anything. If a disk repair program (fschk, most notably) finds a file
with no directory entry pointing to it, it places the file into the
lost+found directory under a made-up name (the "Hardlink..." you saw).
It is up to you to inspect the content (if any) and do with it as you
please. If you happen to be able to identify a file in lost+found (a
rare occurrence) you may be able to put it back where it belongs and
save yourself some work.

If you can't identify them you may as well delete them. The files are
lost to the system anyhow, you won't make things worse than they already
are.

Anno

[Fred Moore]

Anno Siegel (anno4000@lublin.zrz.tu-berlin.de) wrote:
[lots of informative stuff about hardlinks and lost and found]

Thanks, Anno. Really helps me get a grip on this. I'll start inspecting
the files to see what's in them.

--Fred