Paths under Linux
~~~~~~~~~~~~~~~~~

Every program and package installed on a Linux system is installed into a 
particular 'prefix'.  The main places are:

  /               This contains the base system.  The basic tools needed to 
                  boot the system and bring it up and running are stored here.
                  The idea here is that you can have a "boot" partition
                  which is separate from the rest of the system.  If 
                  something catastrophic happens, there are enough tools 
                  here to boot up and recover the system.

  /usr            This contains the rest of the system.  Most programs get
                  installed in here.  Note that /usr is for programs installed
                  by the package management system (rpm, apt-get, dpkg); if 
                  you compile something yourself, it is put in /usr/local.

  /usr/X11R6      The X Window System is installed here.  This only applies to
                  X itself; most graphical applications are installed in /usr.
                  This is becoming less relevant over time, and some 
                  distributions now just install X in /usr.

  /usr/local      This contains programs that have been manually installed
                  (not by the package manager).  If you compile and install
                  something yourself, it is usually put here.  The idea here 
                  is that you can backup your system by saving the contents
                  of /usr/local, and ignore the rest of /usr to save space.

  /opt/(pkgname)  Packages sometimes want their own completely separate prefix
                  and are installed here.

For each prefix, there are a number of subdirectories:

  (prefix)/bin        This contains programs that normal users can run (.exe 
                      files under Windows)

  (prefix)/games      Games are installed in a separate directory to other
                      programs.  Some systems just put everything into bin.

  (prefix)/sbin       This contains programs that are intended for only the 
                      system administrator to run.  Some of these can still be
                      run by normal users, though (like /sbin/ifconfig)

  (prefix)/include    C header files.

  (prefix)/lib        This contains library files (.dll files under Windows)

  (prefix)/lib64      64-bit libraries.  This allows 32-bit and 64-bit 
                      programs to co-exist on a system together.

  (prefix)/share      (prefix)/share/(name) contains data files used by the
                      program/package named (name).  Various other data files
                      are also stored here: /usr/share/pixmaps often contains
                      shared image files, for example.

  (prefix)/share/doc  Similar to (prefix)/share, documentation for a program
                      named 'progname' will be stored in 
                      (prefix)/share/doc/progname.

Extra directories do not belong in a prefix and are:

  /boot           OS kernels and bootloader configuration files are  
                  stored here.

  /dev            Device files representing physical devices on the system.

  /etc            System configuration files

  /home           Users have home directories in /home/(user name) in which 
                  they can store their documents, etc.

  /lost+found     If the filesystem is corrupted, the fsck(8) tool can be
                  used to recover the system.  If a file is recovered but
                  has no name, it is placed here.

  /media          Removable drives (floppy drives, CD-ROM drives) are mounted
                  in here.

  /mnt            Other mounts are mounted here (hard drives, NFS shares, etc)

  /proc           Virtual process filesystem.  There is a directory here 
                  for each running process on the system, along with other
                  files with useful information about the system.

  /root           root (the super user) has its home directory in /root, rather
                  than in /home with everyone else.  The idea here is that 
                  /home can be mounted separately from the rest of the system,
                  but root's home directory is on the boot partition and is 
                  therefore always available in case it is neccessary to 
                  recover the system.

  /sys            Information about kernel objects is available in here under
                  Linux 2.6.

  /tmp            Temporary files are stored in here.

  /usr/src        Program source code.

  /var            Files which may change are stored in here.  The idea here is 
                  that /usr can be mounted as read-only (eg. from an NFS share)
                  to make the system more secure.  Examples of things found
                  in here are log files (/var/log), program cache files
                  (/var/cache) and mail files (/var/mail).

Windows Analogies (roughly):

  /etc            The registry

  /home           C:\Documents and Settings

  /lib
  /usr/lib        C:\Windows\System32
  /usr/local/lib
 
  /media          Different drive letters
  /mnt

  /opt            C:\Program Files

  /tmp            C:\Windows\Temp