Linux – Difference Between /var/mail/XXX and /var/spool/mail/XXX

A hard link traditionally shares the same file system structures (inode in unixspeak), while a soft-link is a pathname redirect.

• Hardlinks must be on the same filesystem, softlinks can cross filesystems.
• Hardlinked files stay linked even if you move either of them (unless you move one to another file system triggering the copy-and-delete mechanism). Softlinked files break if you move the target (original), and sometimes when you move the link (Did you use an absolute or relative path? Is it still valid?).
• Hardlinked files are co-equal, while the original is special in softlinks, and deleting the original deletes the data. The data does not go away until all hardlinks are deleted.
• Softlinks can point at any target, but most OS/filesystems disallow hardlinking directories to prevent cycles in the filesystem graph (with the exception of the . and .. entries in unix directories which are hard links).
• Softlinks can require special support from filesystem walking tools. Read up on readlink (2).

(Some details brought back to mind by mat1t. Thanks.)

aptitude is a wrapper for dpkg just like apt-get/apt-cache, but it is a one-stop-shop tool for searching/installing/removing/querying. A few examples that apt might not supply:

$ aptitude why libc6
i   w64codecs Depends libc6 (>= 2.3.2)
$ aptitude why-not libc6
Unable to find a reason to remove libc6.

$ aptitude show libc6
Package: libc6
State: installed
Automatically installed: no
Version: 2.9-4ubuntu6
Priority: required
Section: libs
Maintainer: Ubuntu Core developers <ubuntu-devel-discuss@lists.ubuntu.com>
Uncompressed Size: 12.1M
Depends: libgcc1, findutils (>= 4.4.0-2ubuntu2)
Suggests: locales, glibc-doc
Conflicts: libterm-readline-gnu-perl (< 1.15-2), 
tzdata (< 2007k-1), tzdata-etch, nscd (< 2.9)
Replaces: belocs-locales-bin
Provides: glibc-2.9-1
Description: GNU C Library: Shared libraries
 Contains the standard libraries that are used by nearly all programs 
 on the system. This package includes shared versions of the standard 
 C library and the standard math library, as well as many others.