Links

1. Links Learning Objectives: To understand the basics of links & its usage To learn the construction / removal of different types of links To distinguish the differences between hard & soft links

2. Links Table of Content • Links • Symbolic Links • Differences between Hard & Soft Links • Biggest Difference between Hard & Soft Links • Appending & Pattern Matching

3. Links (1) • A link is a pointer to a file. • In fact, in UNIX all filenames are just links to a file. Most files only have one link. -rw-r--r-- 1 jbond cs 154 Feb 4 15:00 letter3 -rw-r--r-- 1 jbond cs 64 Feb 4 15:00 names drwxr-xr-x 2 jbond cs 512 Feb 4 15:00 secret/ • Additional links to a file allow the file to be shared. • The ln command creates new links. $ ln names NAMES $ ls -l total 8 -rw-r--r-- 2 jbond cs 64 Feb 6 18:36 NAMES -rw-r--r-- 1 jbond cs 154 Feb 4 15:00 letter3 -rw-r--r-- 2 jbond cs 64 Feb 4 15:00 names drwxr-xr-x 2 jbond cs 512 Feb 4 15:00 secret/

4. Links (2) • ln creates a new link, not a new file. The new link and the original filename are equivalent pointers to the file. • The last argument is the link destination, and can be: • A pathname of a new regular file $ ln names NAMES • A pathname of an existing directory (a link with the same basename as the original file is created in the directory) $ ln names secret • No second argument (same as giving a second argument of “.”) $ ln /bin/cat

5. jbond NAMES letter3 names Links (3) • A link has two pieces of information • A name • An inode number • An inode number is an index into a system table that has all the information about the file (e.g., owner, size). $ ln names NAMES system table file contents inode: 42979 user: 4501 group: 1501 address: ... 007 Golden Eye Tomorrow Never Dies

6. Links (4) • You can use ls -i to see if two links point to the same inode: $ ls -li total 8 42979 -rw-r--r-- 3 jbond cs 64 Feb 6 18:36 NAMES 42976 -rw-r--r-- 1 jbond cs 34 Feb 4 15:00 letter3 42979 -rw-r--r-- 3 jbond cs 64 Feb 4 15:00 names 59980 drwxr-xr-x 2 jbond cs 512 Feb 4 17:10 secret/ • So, using rm actually only removes a link. When the last link to a file is removed, the operating system actually removes the file.

7. Symbolic Links • A symbolic link is a pointer to a pathname, not a pointer to the file itself. • ln -s original target creates a symbolic link. • A symbolic link is not equivalent to a hard link. The symbolic link has a different inode. $ ln -s names snames $ ls -li total 10 42979 -rw-r--r-- 3 jbond cs 64 Feb 6 18:36 NAMES 42976 -rw-r--r-- 1 jbond cs 34 Feb 4 15:00 letter3 42979 -rw-r--r-- 3 jbond cs 64 Feb 4 15:00 names 59980 drwxr-xr-x 2 jbond cs 512 Feb 4 17:10 secret/ 42916 lrwxrwxrwx 1 jbond cs 5 Feb 8 17:09 snames -> names • Symbolic links are sometimes called soft links, and “regular” links are sometimes called hard links.

8. Differences Between Hard and Soft Links (1) • You can’t make a hard link to a directory, but you can make a symbolic link to a directory. $ ln secret secrethlink ln: secret is a directory $ ln -s secret secretslink $ ls -li total 12 42979 -rw-r--r-- 3 jbond cs 64 Feb 6 18:36 NAMES 42976 -rw-r--r-- 1 jbond cs 34 Feb 4 15:00 letter3 42979 -rw-r--r-- 3 jbond cs 64 Feb 4 15:00 names 59980 drwxr-xr-x 2 jbond cs 512 Feb 4 17:10 secret/ 42917 lrwxrwxrwx 1 jbond cs 6 Feb 8 17:21 secretslink -> secret/ 42916 lrwxrwxrwx 1 jbond cs 5 Feb 8 17:09 snames -> names $ cd secretslink $ pwd /homes/jbond/secret

9. Differences Between Hard and Soft Links (2) • You can also make symbolic links across file systems. $ pwd /homes/jbond/secret $ ls -l /tmp total 26 -rw-rw-r-- 1 root sys 13636 Feb 2 01:41 ps_data $ ln /tmp/ps_data ps_data ln: ps_data is on a different file system $ ln -s /tmp/ps_data ps_data $ ls -li total 4 59944 -rw-r--r-- 1 jbond cs 154 Feb 4 16:38 letter1 59597 lrwxrwxrwx 1 jbond cs 12 Feb 8 17:39 ps_data -> /tmp/ps_data • There is no way to tell how many symbolic links there are to a file.

10. Biggest Difference Between Hard and Soft Links • The most important difference between hard and symbolic links occur when a link is removed. • For a hard link: $ echo 123 > first $ ln first second $ rm first $ cat second 123 $ echo 456 > first $ cat first 456 $ cat second 123 • For a symbolic link: $ echo 123 > first $ ln -s first second $ rm first $ cat second cat: cannot open second $ echo 456 > first $ cat first 456 $ cat second 456

11. Difference Between Symbolic and Hard Links secret symlink1 hardlink1 hardlink2 >cat hardlink1 • 1 2 3 [return] • ln hardlink1 hardlink2 • ln –s hardlink2 symlink1 • rm hardlink2 • cat > hardlink2 • 4 5 6 [return] • cat symlink1 • result? 1 2 3 4 5 6

12. Links in Directories • ras.cs.ust.hk:qyang:63> ls -l secret • total 16 • drwx------ 2 qyang 96 Jun 18 10:32 mysecret • -rwxrwxrwx 1 qyang 56 Sep 10 2001 names • -rw------- 1 qyang 4 Jun 18 11:41 names2 • ras.cs.ust.hk:qyang:64> • Why does mysecret has a link number 2? • It contains two links • One to itself: “.” • One to its parent: “..”