Tip 20: Hard/Soft Links
Create a soft link:
inductor:foo mrg$ ln -s ../test.sp test2.sp
Create a hard link:
inductor:foo mrg$ ln ../test.sp test3.sp
Now look at the directory:
inductor:foo mrg$ ls -al total 16 drwxr-xr-x 4 mrg staff 136 Aug 18 16:36 . drwxr-xr-x+ 97 mrg staff 3298 Aug 18 16:31 .. lrwxr-xr-x 1 mrg staff 10 Aug 18 16:36 test2.sp -> ../test.sp -rw-r--r-- 2 mrg staff 815 Jul 17 12:44 test3.sp
Note that the soft link is designated with a "->" while the hard link just looks like a file. However, the hard link is actually the SAME FILE:
inductor:foo mrg$ ls -al ../test.sp -rw-r--r-- 2 mrg staff 815 Jul 17 12:44 ../test.sp
which shows the same date, permissions, etc as the link test3.sp. You can confirm this by seeing what the filesystem inode is for each of these using the -i options to ls:
inductor:foo mrg$ ls -li ../test.sp 18906739 -rw-r--r-- 2 mrg staff 815 Jul 17 12:44 ../test.sp inductor:foo mrg$ ls -li test3.sp 18906739 -rw-r--r-- 2 mrg staff 815 Jul 17 12:44 test3.sp inductor:foo mrg$ ls -li test2.sp 19237135 lrwxr-xr-x 1 mrg staff 10 Aug 18 16:36 test2.sp -> ../test.sp
The first two are identical where as the symbolic link is a separate inode.
So, why use hard links? Well, what if you remove the original file?
inductor:foo mrg$ rm ../test.sp inductor:foo mrg$ ls -al total 16 drwxr-xr-x 4 mrg staff 136 Aug 18 16:36 . drwxr-xr-x+ 96 mrg staff 3264 Aug 18 16:39 .. lrwxr-xr-x 1 mrg staff 10 Aug 18 16:36 test2.sp -> ../test.sp -rw-r--r-- 1 mrg staff 815 Jul 17 12:44 test3.sp inductor:foo mrg$ more test2.sp test2.sp: No such file or directory inductor:foo mrg$ more test3.sp * File: invx1.pex.netlist ...
The soft link is pointing to nothing now. The hard link still has the file. Files are only "removed" when there are no more hard links to a file.
One example where I use hard links is when I do backups. You can tell rsync, for example, to use hard links rather than make new copies when files don't change. That's beyond the scope of this though.
