Understanding Symbolic LinksFri, Jun 5, 2009
What are they?
Symbolic links are like shortcuts or references to the actual file or directory. Most of the time these links are transparent when working with them through other programs. For example you could have a link to a directory to one hard drive inside a directory that lies on a different hard drive and an application will still treat it the same.
Symbolic links are used all the time to link libraries and make sure files are in consistent places without moving or copying the original. Links are often used to “store” multiple copies of the same file in different places but still reference to one file.
Creating a Symbolic Link
To create a symbolic link in Linux we use this syntax:
ln -s /path/to/original/ /path/to/linkName
What happens if I edit the link? Any modifications to the linked file will be changed on the original file. What happens if I delete the link? If you delete the link the original file is unchanged. It will still exist. What happens if I delete the original file but not the link? The link will remain but will point to a file that does not exist. This is called an orphaned or dangling link.
Creating a Hard Link
You can create a hard link like so:
ln /path/to/original.file /path/to/link.file
The Difference Between Soft and Hard Links
- Only link to a file not a directory
- Can not reference a file on a different disk/volume
- Links will reference a file even if it is moved
- Links reference inode/physical locations on the disk
Symbolic (soft) links
- Can link to directories
- Can reference a file/folder on a different hard disk/volume
- Links remain if the original file is deleted
- Links will NOT reference the file anymore if it is moved
- Links reference abstract filenames/directories and NOT physical locations. They are given their own inode
Practice Makes Perfect
To really grasp the concept of symbolic links lets give it a shot.
Go into the tmp directory:
Make a directory
Copy over the host.conf file or any file to test with.
cd original cp /etc/host.conf .
List the contents and take note of the inode (first column)
Create a symbolic link to host.conf called linkhost.conf
ln -s host.conf linkhost.conf
Now do list out the inodes
Notice how the inode for the link is different.
Now create a hard link to the same file
ln host.conf hardhost.conf
Now list the inoes one more time
Notice how the inode numbers are exactly the same for the hard link and the actual file.
Lets try some file operations now
Open up linkhost.conf and edit it and save it
Now look in host.conf and notice that the changes were made
Lets move host.conf now and see if it causes any problems
mv host.conf ../
Uh oh, now when we list the directory our link turned red lets see what is in side it
cat linkhost.conf cat: linkhost.conf: No such file or directory
It looks like our symbolic link is now broken as it linked to a file name and not the inode. What about our hard link?
Looks like our hard link still works even though we moved the original file. This is because the hard link was linked to the inode the physical reference on the hard drive where the file resides. The soft link (symbolic link) on the other hand was linked to the abstract file name and was broken the moment we moved the file.
This leads to an interesting question. What happens if I delete a hard link? Even though the hard link is referenced to the physical location of the file on the hard drive though an inode, removing a hard link will not delete the original file.
Symbolic links will remain intact but will point to a non existent file.