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# 1  
Old 08-05-2010
Hard Link Examples

Hello,
Please move this if I chose the wrong forum category. This question pertains to Unix and Linux I believe. I google the difference between hard and symbolic/soft links and I understand the difference. What I am trying to find is a real example of a hard link being used in a Operating system.

e.g.

RedHat uses softlinks from:

Code:
lrwxrwxrwx  1 root root   19 Jul 16 11:14 S98haldaemon -> ../init.d/haldaemon
lrwxrwxrwx  1 root root   19 Jul 16 11:14 S99firstboot -> ../init.d/firstboot
lrwxrwxrwx  1 root root   11 Jul 19 15:39 S99local -> ../rc.local
lrwxrwxrwx  1 root root   16 Jul 16 11:12 S99smartd -> ../init.d/smartd

I would like to find a hard link example. Can some one show me one please?

Thanks,
jaysunn

Last edited by jaysunn; 08-05-2010 at 02:21 PM..
# 2  
Old 08-05-2010
Basically, a soft link is a special kind of file that points to a different file somewhere on the available (mounted) filesystems. A hard link is a regular file entry in a directory, only that it points to the same inode as a different directory entry, and cannot cross to other file systems.

If you remove the file a soft link points to the link itself becomes invalid. If you remove the file entry that points to the same inode as a different entry (the hard link), the hard link still is fully valid. Also, both entries for an inode share the same access permission (UID, GID, permission bits).
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# 3  
Old 08-05-2010
@pludi,
Thanks for the reply. However I understand the differneces. I am just wondering why one would use a hard link? Or is there a example of a hard link that I can view in a shipped O/S version?

Thanks,

jaysunn
# 4  
Old 08-05-2010
Common example is the "ls" command.

Code:
First find the inode <number> of the ls command.

ls -lisad /usr/bin/ls

The inode number is in the first column.


Then use "find" to find every file with the same inode <number>.

find /usr/ -xdev -inum <number> -exec ls -lisad {} \;


The "ls" command will behave differently according to the name by which it was called.


Now try the same sequence for the "vi" command and be amazed.
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# 5  
Old 08-05-2010
Not sure what I am doing wrong or not understanding. I thought I would see more output. I am on RHEL5:


Code:
[root@jralph-linux ~]# ls -lisad /bin/ls
15237133 96 -rwxr-xr-x 1 root root 91240 Feb 23 04:54 /bin/ls
[root@jralph-linux ~]# find / -xdev -inum 15237133 -exec ls -lisad {} \;
15237133 96 -rwxr-xr-x 1 root root 91240 Feb 23 04:54 /bin/ls
[root@jralph-linux ~]# ls -lisad /bin/vi
15237163 624 -rwxr-xr-x 1 root root 632912 Jun 12  2009 /bin/vi
[root@jralph-linux ~]# find / -xdev -inum 15237163 -exec ls -lisad {} \;
15237163 624 -rwxr-xr-x 1 root root 632912 Jun 12  2009 /bin/vi
[root@jralph-linux ~]#

# 6  
Old 08-05-2010
Interesting. Obviously a bad example for that strain of Linux, but still a valid example for mainstream unix.
In unix when the links count for a file is greater than value 1 this signifies a hard link. Possible to try some likely directories (like /bin) and sort by links count to see if there are any:

Code:
ls -la|grep \^\-|sort -n -r +1|more

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# 7  
Old 08-05-2010
Try this on RHEL5
Code:
ls -al /bin/ed /bin/red

You should see that the link count is 2
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