Difference between revisions of "Tip 20: Hard/Soft Links"

Revision as of 23:44, 18 August 2014

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

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.