Links

1
Software Tools

• Links

2
Links

• 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
  1500 letter3
• -rw-r--r-- 1 jbond cs 64 Feb 4
  1500 names
• drwxr-xr-x 2 jbond cs 512 Feb 4
  1500 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
  1836 NAMES
• -rw-r--r-- 1 jbond cs 154 Feb 4
  1500 letter3
• -rw-r--r-- 2 jbond cs 64 Feb 4
  1500 names
• drwxr-xr-x 2 jbond cs 512 Feb 4
  1500 secret/

3
Links

• 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 secret/letter1

4
Links

• 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
5
Links

• 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
  1836 NAMES
• 42976 -rw-r--r-- 1 jbond cs 34 Feb 4
  1500 letter3
• 42979 -rw-r--r-- 3 jbond cs 64 Feb 4
  1500 names
• 59980 drwxr-xr-x 2 jbond cs 512 Feb 4
  1710 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.

6
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 1836
  NAMES
• 42976 -rw-r--r-- 1 jbond cs 34 Feb 4 1500
  letter3
• 42979 -rw-r--r-- 3 jbond cs 64 Feb 4 1500
  names
• 59980 drwxr-xr-x 2 jbond cs 512 Feb 4 1710
  secret/
• 42916 lrwxrwxrwx 1 jbond cs 5 Feb 8 1709
  snames -gt names
• Symbolic links are sometimes called soft links,
  and regular links are sometimes called hard
  links.

7
Differences Between Hard and Soft Links

• You cant 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 12
• 42979 -rw-r--r-- 3 jbond cs 64 Feb 6 1836
  NAMES
• 42976 -rw-r--r-- 1 jbond cs 34 Feb 4 1500
  letter3
• 42979 -rw-r--r-- 3 jbond cs 64 Feb 4 1500
  names
• 59980 drwxr-xr-x 2 jbond cs 512 Feb 4 1710
  secret/
• 42917 lrwxrwxrwx 1 jbond cs 6 Feb 8 1721
  secretslink -gt secret/
• 42916 lrwxrwxrwx 1 jbond cs 5 Feb 8 1709
  snames -gt names
• cd secretslink
• pwd
• /homes/jbond/secret

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 13636 Feb 2 0141
  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 1638
  letter1
• 59597 lrwxrwxrwx 1 jbond cs 12 Feb 8 1739
  ps_data -gt /tmp/ps_data
• There is no way to tell how many symbolic links
  there are to a file.

9
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 gt first
• ln first second
• rm first
• cat second
• 123
• echo 456 gt first
• cat first
• 456
• cat second
• 123

10
Biggest Difference Between Hard and Soft Links

• For a symbolic link
• echo 123 gt first
• ln -s first second
• rm first
• cat second
• cat cannot open second
• echo 456 gt first
• cat first
• 456
• cat second
• 456

11
Appending and Pattern Matching

• We have seen input redirection (cat ltfile) and
  output redirection (cat gtfile). We can also
  append to a file using gtgt
• date gt file
• who gtgt file
• 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.