Linux administration

• A symbolic link (or symlink) is a pseudo-file which behaves as an alternative name for some other file or directory
• The 'contents' of the symlink are the real name pointed to
• When you try to use a file name including a symlink, the kernel replaces the symlink component with its 'contents' and starts again
• Symlinks allow you to keep a file (or directory) in one place, but pretend it lives in another
  • For example, to ensure that an obsolete name continues to work for older software
  • Or to spread data from a single filesystem hierarchy over multiple disk partitions

• ls -l shows where a symbolic link points to:

$ ls -l /usr/tmp
lrwxrwxrwx 1 root root 30 Sep 26 2000 /usr/tmp -> /var/tmp

• ls can also be made to list symlinks in a different colour to other files, or to suffix their names with @
• A symlink is created with the ln -s command
• Its syntax is similar to cp - the original name comes first, then the name you want to create:

$ ln -s real-file file-link
$ ln -s real-dir dir-link
$ ls -l file-link dir-link
lrwxrwxrwx 1 bob bob 9 Jan 11 15:22 file-link -> real-file
lrwxrwxrwx 1 bob bob 8 Jan 11 15:22 dir-link -> real-dir

• Where symlinks refer to other files by name, a hard link refers to another file by inode number
  • An inode is the data structure that describes a file on disk
  • It contains information about the file, including its type (file/directory/device), modification time, permissions, etc.
• A directory entry contains a name and an inode number
  • So a file's name is not considered to be part of the file itself
• You get a hard link when different directory entries on a filesystem refer to the same inode number

• A symbolic link refers to filename, which in turn refers to an inode:

• A hard link is a normal directory entry, referring directly to an inode:

• Use the ln command to create a hard link
• Don't use the -s option when creating hard links
• As when creating symlinks, the order of the arguments to ln mimics cp:

$ ls -l *.dtd
-rw-r--r-   1 anna  anna    11170   Dec 9   14:11   module.dtd
$ ln module.dtd chapter.dtd
$ ls -l *.dtd
-rw-r--r-   2 anna  anna    11170   Dec 9   14:11   module.dtd
-rw-r--r-   2 anna  anna    11170   Dec 9   14:11   chapter.dtd

• Notice that the link count in the listing increases to 2
• The two names are now indistinguishable
  • Deleting or renaming one doesn't affect the other

• Commands that operate on files often take options to specify whether links are followed
• The tar command notices when two files it's archiving are hard links to each other, and stores that fact correctly
• By default tar also stores symlinks in archives
  • Use the -h option (--dereference) to instead store the file pointed to
• The cp command by default ignores both hard links and symlinks
  • Use the -d option (--no-dereference) to preserve all links
  • Use the -R option (--recursive) when copying recursively to ensure that symlinks are preserved
  • The -a option (--archive) implies both -d and -R

• The find command has a -lname option which searches for symbolic links containing some text:

$ find / -lname '*file' -printf '%p -> %l\n'

• This command prints the names and destinations of all symbolic links whose destination ends in file
• Be aware that running find over the entire filesystem is very disk-intensive!

• Hard links can be found by searching for directory entries with a given inode number
• First, identify the filesystem and inode number of the file you're interested in:

$ df module.dtd
Filesystem  1k-blocks   Used    Available   Use%    Mounted on
/dev/sdb3   13647416    5241196 7712972     40% /home
$ ls -i module.dtd
245713 module.dtd

• Then use find's -inum option to look for directory entries in that filesystem with that inode number:

$ find /home -xdev -inum 245713

• The -xdev option prevents find from recursing down into other filesystems

• a. Make a temporary directory and change into it.
• b. Make some test files as follows:

$ echo "oranges and lemons" > fruit
$ echo spuds > veg

• c. Make a symbolic link called starch to the veg file.
• d. Make a hard link called citrus to the appropriate file, and check that it has the same inode number.
• e. Delete the original fruit file and check that citrus still contains the text.
• f. Delete the original veg file and try to look at the contents of starch. Use ls to check the symlink.

• a. Try to see what the following loop does, and then create some .htm files and try it:

$ for htm in *.htm; do
> ln -s $htm ${htm}l;
> done

• b. Make a symlink called dir to a directory (such as /etc).
• c. Try the following commands to display the link and compare the results:

$ ls -l dir
$ ls -l dir/