The Debian Alternatives system manages groups of symlinks, which can be modified en-masse.

For each program that it has to manage, Alternatives expects there to be a symlink from /usr/bin to /etc/alternatives/[PROGRAM NAME].

For example, with vi, on my system:

$ which vi
/usr/bin/vi

$ ls -ll /usr/bin/vi
lrwxrwxrwx 1 root root 20 2009-11-03 15:33 /usr/bin/vi -> /etc/alternatives/vi

$ ls -l /etc/alternatives/vi
lrwxrwxrwx 1 root root 16 2009-11-03 15:33 /etc/alternatives/vi -> /usr/bin/vim.nox

$ ls -l /usr/bin/vim.nox
-rwxr-xr-x 1 root root 1825312 2009-09-21 13:23 /usr/bin/vim.nox

So, on my system, the command vi starts /usr/bin/vim.nox via 2 symlinks.

Commands

List Main Programs

$ update-alternatives --get-selections

Show setup for a certain program

$ update-alternatives --display vi
vi - auto mode
 link currently points to /usr/bin/vim.nox
/usr/bin/vim.nox - priority 40
 slave vi.1.gz: /usr/share/man/man1/vim.1.gz
 slave vi.fr.1.gz: /usr/share/man/fr/man1/vim.1.gz
 slave vi.it.1.gz: /usr/share/man/it/man1/vim.1.gz
 slave vi.pl.1.gz: /usr/share/man/pl/man1/vim.1.gz
 slave vi.ru.1.gz: /usr/share/man/ru/man1/vim.1.gz
/usr/bin/vim.tiny - priority 10
 slave vi.1.gz: /usr/share/man/man1/vim.1.gz
 slave vi.fr.1.gz: /usr/share/man/fr/man1/vim.1.gz
 slave vi.it.1.gz: /usr/share/man/it/man1/vim.1.gz
 slave vi.pl.1.gz: /usr/share/man/pl/man1/vim.1.gz
 slave vi.ru.1.gz: /usr/share/man/ru/man1/vim.1.gz
 Current `best' version is /usr/bin/vim.nox.

Set Up a Program

$ sudo update-alternatives --install /usr/bin/[SYMLINK] [TITLE] /PATH/TO/PROGRAM 10