Creating a hard link using ln creates a copy of file

jeffshen · 03-11-2010, 08:20 PM

Dear all,

I am trying to use ln to create a hard link to file a and whenever I do it, it creates a copy of the file instead. After having edited file a, when opening the link, it shows the old information and opening file a shows the new information.

The command I am using is

Code:

ln /home/user/file

within the new directory i am trying to link from.

I am using centos 5.4.

Thank you very much in advance,

Jeff

Tinkster · 03-11-2010, 08:28 PM

Are those directories on the same mountpoint? Otherwise hard links can't
be done to begin with; even so, the behaviour you're describing doesn't
really gel ... what file-systems are you using, how are they mounted, can
you see anything odd if you try that ln under strace?

Cheers,
Tink

paulsm4 · 03-11-2010, 08:30 PM

Hi -

I'm not sure what you're trying to do, or what you're actually seeing, but:

Quote:

# Here's an example file:
$ cat a
AAAAAAAA
AAAAAAAA
AAAAAAAA

$ ls -l a
-rw-r--r-- 1 paulsm users 27 Mar 11 19:39 a

# Let's create a hard link:
$ ln a b

# Finally, let's check the results:
$ ls -l a b
-rw-r--r-- 2 paulsm users 27 Mar 11 19:39 a
-rw-r--r-- 2 paulsm users 27 Mar 11 19:39 b

$ cat b
AAAAAAAA
AAAAAAAA
AAAAAAAA

So what does this mean?

All you're doing when you create a file is:
a) assign some properties (like a meaningful name and a directory location) to a set of "inodes".

All you're doing when you create a hard link is:
b) assign a NEW "name" to the SAME set of "inodes"

Neither name is the "real" name. You can delete one link (for example, "a") or the other link ("b"); the file itself remains - untouched - until you remove the last link. All names are "equal" to the operating system.

Interestingly, the OS-level API for "delete a file" is "unlink()".

Make sense?

'Hope that helps .. PSM

fpmurphy · 03-11-2010, 09:22 PM

Quote:

Originally Posted by jeffshen

Dear all,

I am trying to use ln to create a hard link to file a and whenever I do it, it creates a copy of the file instead. After having edited file a, when opening the link, it shows the old information and opening file a shows the new information.

The command I am using is

Code:

ln /home/user/file

within the new directory i am trying to link from.

If the command that you have shown us is the actual command that you are using, then you are invoking ln incorrectly. ln requires two arguments - target file and linkname.

Do a full directory listing of the old file. If the link count is 1, then you have not successfully created the hard link.

pixellany · 03-11-2010, 09:50 PM

Code:

[mherring@Ath stuff2]$ ls -l
total 4
-rw-r--r-- 1 mherring users 200 Mar 11 19:43 alph
[mherring@Ath stuff2]$ ln alph alphlink
[mherring@Ath stuff2]$ ls -l
total 8
-rw-r--r-- 2 mherring users 200 Mar 11 19:43 alph
-rw-r--r-- 2 mherring users 200 Mar 11 19:43 alphlink
[mherring@Ath stuff2]$ rm alph
[mherring@Ath stuff2]$ ls -l
total 4
-rw-r--r-- 1 mherring users 200 Mar 11 19:43 alphlink
[mherring@Ath stuff2]$ ^C
[mherring@Ath stuff2]$

Note the following:
1. The new hard link "alphlink" is indistinguishable from alph. This is because the hard link is simply another name for the same inode.
2. Deleting alph leaves alphlink intact. (same reason as above)

Excercise for the student: "When and how is a hard link different from a copy?"

Sayan Acharjee · 03-11-2010, 09:52 PM

You can't create hard links if the two files exists in different file system.

jeffshen · 03-12-2010, 05:03 AM

Thank you for the very quick replies.

I am not sure whether the directories are in the same hard drive as this is a hosting service provided by a company, but the file is within /home/usera/file and i am trying to access it from /home/userb/

I have tried using

Code:

ln /home/usera/file /home/userb/file

as well.

doing ls -li gives me 2 different inode numbers for the file in the different directories, but are usually within a few digits of each other, ie. 12156936 and 12156932.

The file i am trying to link has permissions of 777 so I cant think of why it cant link.

Thank you very much in advance!

arashi256 · 03-12-2010, 05:05 AM

Hard links cannot cross filesystems - I'm assuming this is the problem. Do soft links work?