ln copies a file instead of setting a hard link [SOLVED]

cyberpatrol · Last edited by cyberpatrol on Tue Nov 01, 2005 5:09 am; edited 1 time in total

I have a problem with ln.

I tried to set a hard link to a file by giving this command:
ln /usr/dir1/file /usr/dir2/file
or
cd /usr/dir2 && ln /usr/dir1/file

Both /usr/dir1 and /usr/dir2 are on the same partition and therefore on the same filesystem.
The filesystem is reiserfs.

Instead of setting a hard link in /usr/dir2 to /usr/dir1/file it copies file from /usr/dir1 to /usr/dir2.

Why does this happen and how I can set a hard link?

paul555 · l33t Joined: 22 Nov 2004 Posts: 612 Location: Greece

Jake · Veteran Joined: 31 Jul 2003 Posts: 1132

Are you sure you understand how a hard link is supposed to behave? It will look like the original in every way, except it won't store a copy of the data. If you delete the original, a hard link will still point to the data. For example:

i92guboj · Posted: Tue Nov 01, 2005 12:44 am Post subject:

Jake said it fine For all purposes, the file and the hardlink are the same, since both point to the data on the hd, instead of the link pointing to the file. So, if you remove one the other will stay. They are both valid names for the same data on the disk. The symbolic or soft links are different. In a symlink the link points to the original file (the name, to explain it on an easy way) but not to the data itself.

@Paul, -s is for a soft (or symbolic) link, hence the '-s', also called 'symlink'.

cyberpatrol · Posted: Tue Nov 01, 2005 12:48 am Post subject:

In this case I indeed need a hard link and not a soft link.
And I think I understand how a hard link is supposed to behave.

The problem is if I edit the file to which I set a hard link the hard link should contain the same changes and vice versa.
E.g. if I set a hard link with `ln /usr/dir1/file /usr/dir2/file` and I edit /usr/dir2/file (the link) then `cat /usr/dir1/file` and `cat /usr/dir2/file` should show the same content.
Or am I wrong in this point and this is only the behaviour of symlinks?

I have to admit that I'm not sure right now?

Or let me reword it. How can I find out if a "file" is a copy or a hard link?

Jake · Veteran Joined: 31 Jul 2003 Posts: 1132

So you mean if you did

i92guboj · Posted: Tue Nov 01, 2005 1:36 am Post subject:

cyberpatrol · Posted: Tue Nov 01, 2005 1:46 am Post subject:

This is an idea. I still have an empty partition which I could format with ext2 and test it.
I could have thought of by myself.

Btw, after setting the hard link I edited the "link" but the "link" was changed and the "original" file was not. So it must be copy instead of a link.
I will test it on ext2 and maybe ext3.

Jake · Veteran Joined: 31 Jul 2003 Posts: 1132

The second column in the "ls -l" output is the number of hard links to a file. If two of your files show link counts of "2", you won't know which is the original, but you'll know that each has another link.

Just an idea, but have you tried running "sync" after editing the link?

And if possible, please post a complete comand sequence producing this behavior.

cyberpatrol · Posted: Tue Nov 01, 2005 4:17 am Post subject:

I've found the error. It was my fault.

ln hasn't copied the file. In fact it has set a hard link.

But because I forgot the thing with the link count of 2 in the ls -l output I tried to edit the file(s) and looked if both files have been changed.
The mistake was that I used emacs to edit the files and that emacs automatically saves a backup. It renames the original file to <filename>~ - which I often delete at once - and saves the edited file as a copy and names it <filename>.

So after editing the hard link points to <filename>~ and not to <filename> anymore. This is why the files were different.

But thank you all for explaining the hard link stuff.
I new it once but have forgotten it again because I usually don't use hard links.
Now it's clear again and I hope I won't forget it again. :-)

Thanks again!

Jake · Veteran Joined: 31 Jul 2003 Posts: 1132

I would never have thought emacs would screw up hard links. *cough* vi *cough*

Remember to append [SOLVED] to the subject of your original post.

cyberpatrol · Posted: Tue Nov 01, 2005 5:19 am Post subject:

Done.

Maybe I should send a bug report to the emacs developers. But a bit later.
If emacs would first copy the original file <filename> to <filename>~ and then save the edited file to <filename> then it shouldn't screw up hard links again.

But I'd never thought that, too.

Btw, nano is a small but also a quite nice and easy editor which can also handle hard links.