Links Software Tools

Lecture 4 / Slide 2 Links l A link is a pointer to a file. l 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/ l 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/

Lecture 4 / Slide 3 Links ln creates a new link, not a new file. The new link and the original filename are equivalent pointers to the file. l The last argument is the link destination, and can be: n A pathname of a new regular file $ ln names NAMES n A pathname of an existing directory (a link with the same basename as the original file is created in the directory) $ ln names secret n No second argument (same as giving a second argument of “.”) $ ln secret/letter1

Lecture 4 / Slide 4 Links l A link has two pieces of information n A name n An inode number l 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 jbond NAMESnamesletter3 007 Golden Eye Tomorrow Never Dies inode: user: 4501 group: 1501 address:... system table file contents

Lecture 4 / Slide 5 Links You can use ls -i to see if two links point to the same inode: $ ls -li total rw-r--r-- 3 jbond cs 64 Feb 6 18:36 NAMES rw-r--r-- 1 jbond cs 34 Feb 4 15:00 letter rw-r--r-- 3 jbond cs 64 Feb 4 15:00 names 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.

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

Lecture 4 / Slide 7 Differences Between Hard and Soft Links l 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’ : hardlink not allowed for directory $ ln -s secret secretslink $ ls -li total rw-r--r-- 3 jbond cs 64 Feb 6 18:36 NAMES rw-r--r-- 1 jbond cs 34 Feb 4 15:00 letter rw-r--r-- 3 jbond cs 64 Feb 4 15:00 names drwxr-xr-x 2 jbond cs 512 Feb 4 17:10 secret/ lrwxrwxrwx 1 jbond cs 6 Feb 8 17:21 secretslink -> secret/ lrwxrwxrwx 1 jbond cs 5 Feb 8 17:09 snames -> names $ cd secretslink $ pwd /homes/jbond/secret

Lecture 4 / Slide 8 Differences Between Hard and Soft Links You can also make symbolic links across file systems. $ pwd /homes/jbond/secret $ ls -l /tmp total 26 -rw-rw-r-- 1 root sys 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 rw-r--r-- 1 jbond cs 154 Feb 4 16:38 letter lrwxrwxrwx 1 jbond cs 12 Feb 8 17:39 ps_data -> /tmp/ps_data l There is no way to tell how many symbolic links there are to a file.

Lecture 4 / Slide 9 Biggest Difference Between Hard and Soft Links l The most important difference between hard and symbolic links occur when a link is removed. n For a hard link: $ echo 123 > first $ ln first second $ rm first $ cat second 123 $ echo 456 > first $ cat first 456 $ cat second 123

Lecture 4 / Slide 10 Biggest Difference Between Hard and Soft Links n 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

Lecture 4 / Slide 11 Appending and Pattern Matching We have seen input redirection ( cat file ). We can also append to a file using >> $ date > file $ who >> file l Simple file pattern matching The * pattern matches any number of characters: $ ls -l letter* lists all files in the working directory that start with “letter” The ? pattern matches any single character: $ ls -l letter? lists all files in the working directory that start with “letter” followed by exactly one character.