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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).
These 2 Users Gave Thanks to pludi For This Post:
jaysunn thegeek
# 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.
This User Gave Thanks to methyl For This Post:
jaysunn
# 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

This User Gave Thanks to methyl For This Post:
jaysunn
# 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
This User Gave Thanks to fpmurphy For This Post:
methyl
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